Use HX in hot air plenum to produce DHW?

E Yoder

Feeling the Heat
Jan 27, 2017
307
Floyd, VA
Sounds great, definitely post back when you have the hot water thing running.
 

electrathon

Minister of Fire
Sep 17, 2015
523
Gresham, OR
And, the drumroll - I think from what I've seen, I CAN heat my water with the output air. I ordered a heat exchanger today, and once the many inevitable delays have run !their course, I should have info for you on how it worked out (going where no-one has gone before - and all that!).
How hot is the air in the plenum that you are going to draw heat from? I think that is the biggest issue. If you have 120 degree plenum air, over time, lots of time, the water in the heat exchanger would eventually get there. If the bottom of the tank is higher than the top of the exchanger you will have decent circulation, but this process would be very slow. The tank would also need to be very well insulated to retain the precious little heat you can capture.

My guess: If the plenum air is running continuously at about 120 degree and you inlet water is about 50 degrees and the system is very well balanced with the tank high enough to allow natural circulation you will be able to produce about 20 gallons of 105 degree water a day. Less if the room is cool the tank is in or if the plenum air drops lower than 120 degrees. There are a lot of variables that affect this, most would lower the amount of water or the temp of the water. A few would increase it.

Post your results, please. I would like to be wrong. I personally had a coil screwed against the outside of my woodstove, with a pump that was controlled by a solar temperature differential controller, and when the stove was real hot I got less than 5 degree rise in my 50 gallon tempering tank.
 

RockyMtnGriz

New Member
Apr 19, 2019
23
SW Montana
…My guess: If the plenum air is running continuously at about 120 degree and you inlet water is about 50 degrees and the system is very well balanced with the tank high enough to allow natural circulation you will be able to produce about 20 gallons of 105 degree water a day. Less if the room is cool the tank is in or if the plenum air drops lower than 120 degrees. There are a lot of variables that affect this, most would lower the amount of water or the temp of the water. A few would increase it.

Post your results, please. I would like to be wrong. I personally had a coil screwed against the outside of my woodstove, with a pump that was controlled by a solar temperature differential controller, and when the stove was real hot I got less than 5 degree rise in my 50 gallon tempering tank.
Timely reply, as it's time for an update. After a bit of time to get everything installed, the last few days have been snowy, giving a good opportunity for some testing.

Here's a photo of the rough install:
stove water heater.JPG

I have a 22x22 heat exchanger in the 24x24 plenum of the VF100, plumbed with mostly 3/4 pipe, to an 80 gallon water tank. The silver flex line at the bottom center drops near the floor for a cold trap, then back up to the exchanger. The tank thermometer is installed at about the top third line of the tank, pretty much at the level of the return line from the heat exchanger.

So, how does it work??
The first test was done exactly as shown. Loaded the tank full of 48 degree water, and started up the Kuuma. Kuuma throttle set at Med., and house thermostat set at full cold (low speed blower only). House temp was about 59, and outside temp about 32, so the return air temp was probably close to winter conditions. In one hour the tank thermometer had gained 10 degrees to 58. I loaded a partial load of wood into the stove at this point, turned it to low, and went to bed. 9 hours later, the tank temp had risen to 97 degrees. The house had risen to about 63 degrees by bedtime, and stayed there until morning.

After a day of being otherwise busy, I wrapped the output line with a bit of insulation, and started the stove again. The top part of the tank had lost some of it's heat to the cool water at the bottom, and now read 93 degrees. Four hours later, it was 102. Again, I went to bed, this time turning the computer off on a small load of coals. The next morning the water temp was 108, and the house still about 61.

A small fire in the morning and evening of the third day showed a peak tank temp of 111.

So, what do I think?
Well the heat exchanger is certainly not in an optimal vertical relationship to the tank, but it clearly thermosiphons. The tank position is as it was for the old stove that had the heat coils much lower. I could move the water tank to a bedroom closet directly above the stove on the floor above, and I think that would help a bunch. Right now, I don't think I'm going to do that, but I'll sleep on it some. Something about keeping all of the plumbing clutter in the room with the concrete floor and the floor drain just kinda appeals to me!

There's clearly plenty of heat available. I think the main problem with the current setup is the poor thermosiphoning with the heat exchanger so high relative to the tank. I'm sure pumping the loop would quickly cure that, and I'm starting to explore my circ pump options. Advice on pumps is welcome. In the bit of looking I've done, I see I'm going to have to avoid built in check valves and the like.

Another issue that I think is a bit more challenging to solve is attaining a temperature in the tank that is safe for not growing nasty things like Legionella. I'm thinking that solving that will involve controlling the temperature and airflow through the heat exchanger and managing the pump with a controller that would allow me to maximize the temperature of the water moved to the tank. Ideally, I'd like to get the tank to 130-150 degrees at some point daily. If anyone would like to provide a hint as to a good controller or controller brand, I would appreciate it.

One thing that I learned in the process of tinkering with this, that I didn't expect, is that there is a huge range of air temps and even directions of airflow in the discharge plenum above the stove. Toward the rear of the plenum, there seems to be an eddy of cool air that has come up the back of the stove and across just a few inches of the stove top, that may be as low as 80 degrees, and the airflow in the back few inches of the plenum is actually downward. Conversely, at the very front, it might be 200+ with a strong upward blast. At some point all this mixes together to provide the 100-120 air that is provided to the ducting. In the testing that I've done so far, I've tried to take advantage of this hotter air, by mounting the HX with the tubes running E/W, so the back of the HX can be dead, with cool air and water, while the front tubes supply the hot water that exits to the tank.

Since the HX is 22 inches, and the plenum is 24, I have two inches of space at the rear of the plenum that is currently blocked off. I plan to remove this block off plate and install a divider between the front half of the plenum where the really hot air is, and lead the cooler air from the back half of the stove to bypass the HX through the gap at the rear of the plenum. I hope this may help in attaining the peak tank temps that I'm looking for.

So, at the beginning of official summer, that's where I'm at with this project. I think that a pump will give it the recovery ability for heating a real volume of water, and I'm hoping that some tweaking of the airflow and getting the right pump control will allow me to get the peak temps I need.

Happy woodcutting season!
 
Last edited:
  • Like
Reactions: DoubleB

electrathon

Minister of Fire
Sep 17, 2015
523
Gresham, OR
Where are you taking the temp readings in the tank? Near the top or bottom? I am surprised that you have 200 degree air temperature coming off the furnace. If that is being produced I would agree there is plenty of heat available. Are you using the water in your system currently? Were you able to get this gain during normal household use? I am curious about the drop pipe in the cold loop, if that helping or impeading heat gain. My understanding is that any part of the tank or piping below the heat source will impede the curculation.
 

RockyMtnGriz

New Member
Apr 19, 2019
23
SW Montana
Where are you taking the temp readings in the tank? Near the top or bottom? I am surprised that you have 200 degree air temperature coming off the furnace. If that is being produced I would agree there is plenty of heat available. Are you using the water in your system currently? Were you able to get this gain during normal household use? I am curious about the drop pipe in the cold loop, if that helping or impeading heat gain. My understanding is that any part of the tank or piping below the heat source will impede the curculation.
The temp reading for the tank is at about the top third line. I've found that readings taken near the bottom of the tank are somewhat meaningless in this application, as the 30-40 something degree water that comes in with usage causes huge variations at the lower levels.

For this first test, I had the storage tank isolated from the water system. I wanted to see what it was doing in terms of heat gain, and having it connected to the kitchen sink would give my wife far too much power to frustrate me :oops:. As it sits, it obviously wouldn't keep up. But it's just a test to get some idea of what I have to work with. I've far too many hot air leaks before the HX that I'm going to get to work on sealing up, and clearly, with the temperatures I'm working with, having the HX so high relative to the tank won't work. I think I'm going to go the pump route, and I think that once there's significant water circulating through the rather huge HX, capturing the heat won't be a problem.

The thing that's really weighing on me right now is the complexities of manipulating the airflow inside the plenum to get the cooler air properly bypassing the HX. I'm afraid there's going to be some serious fiddling around required there.

I've probably thought too much about the cold loop, and admit that it still perplexes me a bit. I picked up the idea from a number of designs that use it, and they say it prevents reverse circulation. Does it impede forward circulation? I'm trying to stop thinking about that, but I haven't been able to. While it's something to occupy my brain, in my case it's a moot point. I can see that it's not going to matter. I need to kick the heat capture way up to have what I want, and even if it was hindering, getting rid of it isn't going to be enough. I will say that it appears to work in limiting reverse circulation. My old setup didn't have the cold loop, and would reverse somewhat when the furnace was cold. This one shows no sign of doing that. That's a good thing for me, as regardless, I don't think I can afford to lose what I'm able to capture, during the times that the stove would not be hot enough to contribute to water heating, and I don't trust a check valve not to inhibit circulation in a pump (power) failure situation. I'm not comfortable with the idea of that stove trying to heat that exchanger up to say 400 degrees when the power goes out. So, for my application, I'm pretty sure that the cold loop will remain, though probably with a pump mounted in the bottom of it.

In this case, I can't see where the cold loop would hinder the flow, as there's normally some very cold water at the bottom of the tank. It might even be helping, as the cold water can sink down the tank side of the loop, be slightly warmed as it passes across the bottom, and further warmed as is rises up the pipe to the HX. I haven't attempted to measure the temperatures, but I can feel the slight rise in temps as the water passes through the loop. Of course, this also points to a rather low flow rate.

As an aside, while with the pump I'm adding, that cold loop line will have to be insulated, I'm not sure that it would if one was doing pure thermosiphon, like my old system. Again, it's the cold water that enters the bottom of the tank. The line from the bottom of the tank was often quite cold, and with the thermosiphon, when the water at the bottom of the tank is warm, the thermosiphon effect slows way down - and at that point, you have plenty of hot water anyway. As at least MY furnace room stays quite warm, not insulating that line might actually help, though probably not significantly. Which leads to another thought, if a guy already had a heat pump water heater ($), and a reliable supply of electricity to run it (me, not so much), he might be ahead to just use that in the furnace room, and skip these mods with a Kuuma. It would probably be pretty efficient in a hot room, and if you had real summers (again, me, not so much), what would be wrong with cooling the house a little when the stove isn't running?

I need to get off my butt, and go work on the stove! Stay tuned, it's getting more interesting now!!
 

maple1

Minister of Fire
Sep 15, 2011
9,936
Nova Scotia
Adding a pump may not help much. If you increase flow, your DT will decrease, and you might not see temps as warm. But sometimes things like this are hard to accurately predict.
 

electrathon

Minister of Fire
Sep 17, 2015
523
Gresham, OR
Adding a pump may not help much. If you increase flow, your DT will decrease, and you might not see temps as warm. But sometimes things like this are hard to accurately predict.
The main reason I see for a pump is because his heat exchanger is sitting too high. It should be lower than the bottom of the tank for proper thermosiphoning. If he adds a pump the return to tank loop needs to be moved to the cold water inlet so that it is not adding "cool" water 2/3 of the way up the tank as it is being pumped. Is should be pumped in near the bottom, moving in the tank and the heat will then rise up. If the pump is moving the warm water out of the plenum faster, drawing in cooler water it would raise the delta T allowing the water to absorb BTUs
 

maple1

Minister of Fire
Sep 15, 2011
9,936
Nova Scotia
I don't think it could draw in cooler water than what it is now? Also not sure the advantage of outputting the heated water into the bottom of the tank? Ideally it should be inputted into the top area, since that is where the hot water is being used from and I would think you'd want he hottest water in the spot it would be used from?

Might need more temp readings to get a full handle on how useful this will be - mutliple tank temps at various levels, and/or temp readings in & out. It would at least supplement the 'other' water heating some, so there should be some value to it, but not sure about cost vs. benefit here. The biggest uncertainty might be how will that big coil in the airstream affect the house heating & furnace perfomance in heating season. I don't think any design & engineering work on it was done with something like that in the airstream, but not sure what effect it might have.
 

electrathon

Minister of Fire
Sep 17, 2015
523
Gresham, OR
I don't think it could draw in cooler water than what it is now? Also not sure the advantage of outputting the heated water into the bottom of the tank? Ideally it should be inputted into the top area, since that is where the hot water is being used from and I would think you'd want he hottest water in the spot it would be used from?

Might need more temp readings to get a full handle on how useful this will be - mutliple tank temps at various levels, and/or temp readings in & out. It would at least supplement the 'other' water heating some, so there should be some value to it, but not sure about cost vs. benefit here. The biggest uncertainty might be how will that big coil in the airstream affect the house heating & furnace perfomance in heating season. I don't think any design & engineering work on it was done with something like that in the airstream, but not sure what effect it might have.
The coolest water is going to be at the lowest point in the system (without a pump). If the coil is half way up the tank the coldest the water will be is as cold as the water in the middle of the tank. There will be a cold bubble of water sitting at the lowest portion of the tank. A pump will fix this issue because you are not relying on convection to move the water.

With a pump (this is different than without a pump) the water will continually loop from tank to the heating coil. If there is not a thermal controller, rather a simple temperature switch to run the pump, the cold water is drawn off the tank bottom and heats a few degrees as it passes the coil. It then returns to the tank slightly warmer than it left (this is actually the best way to get the highest delta T, but there is an energy waste of running the pump constantly). If you return the slightly warm water to the top of the tank it can mix with the hot water at the top and cool it down. Return it near the bottom and the heat will naturally, slowly, rise to the top. Also if there is demand usage you will find the cool water dumped in to the top being drawn out into the home instead of the warmest water. There is a thread about a rear ago in here about a guy with a pump that was having that exact problem He could not understand why he was getting cold water to the home when the pump was running.

As to the restriction caused by the hydronic coil placed in the upper plenum airstream, it should be a similar restrictive effect as an air-conditioning coil placed there. There will be some sort of restriction, but not enough to worry about it.
 

RockyMtnGriz

New Member
Apr 19, 2019
23
SW Montana
So it continues... I did get off my butt and work on the stove yesterday. I figured I'd tackle the least pleasant part first. I cut a big hole in the plenum under the heat exchanger, and removed the 2" block off in the plenum behind the heat exchanger above the rear of the stove. I then fabbed up a kind of a scoop/baffle to hopefully collect the cooler air that moves up the back of the stove and channel it through the 24x2 gap behind the heat exchanger. I sealed the plenum back up, and lit her up. It's a sample size of 1, and the weather has warmed up to the point where it's ranging from the mid 30s to about 60, so I didn't fire the stove to the extent I did a few days before when it was snowing off and on all day, so I wouldn't really call it a fair test, but I was happy with the results.

The air now streaming through the gap behind the heat exchanger is about 80 degrees, the crucial part is that the air moving up through the heat exchanger is now around 130 to 150ish. The nice thing is the air is pretty consistent across the area of the exchanger, where before it would be 80 -100 at the back, and 150-200 at the front. The delivered air temp dropped around 5 degrees, but there might be a slight increase in delivered air volume. That's pretty hard to estimate accurately at the vents... Regardless, the heat is going into the air, or the water, as it doesn't seem to be going up the stack. I wasn't expecting hot water without burning more wood, so I can live with that - I have lots of wood.

If I can harvest the heat from that consistently hot air, eliminating part of the HX trying to cool, while the other part tries to heat, I just might have it. If I can get 130 water from this, over time, to make me feel good about the safety aspect, and produce all the 115 water I can use, for periods of high demand - home run!

If you're worried about me screwing the heat recovery, or cooking the stove somehow, yeah, I was definitely on the lookout for that. I wouldn't want to give up the efficiency that I have put all this work into to get, and of course, overheating is not good either. So here's the thing - again with a sample size of one summer firing - the flue gasses are showing 10-20 degrees cooler than with the unmodified setup. Could be the stove is just not as hot, but at least it's not showing a move in the wrong direction. The real hot air spot in the plenum toward the front of the stove is not there anymore - weather that is due to more air going around the sides and the front of the stove, and/or a strong eddy above the stove and below the HX mixing it out is beyond my testing to answer. And, at this point, I can't see where it matters.

I do have a bit of relief, that I at least got close on the first try with the baffle to separate the hot air from the cold. Installing or fiddling with a baffle "arthroscopically" inside of a plenum is unpleasant work. Getting the temps I needed, and thought I could get, without causing other unforeseen problems - at least ones I've seen at this point, is more than justification for the beverage I'm currently consuming.

One great side benefit, is the behavior of the fan is much improved. By keeping the blast of relatively unheated air out of the area of the fan control sensors, the fan now operates in a much more consistent manner that I think more accurately reflects the temperature of the stove. The short cycling while warming up and cooling off is greatly reduced. The original fan control that allowed for high fan if the stove was hot enough, and the house thermostat was calling for heat, now seems to have some consistent judgment about the temp of the stove, rather than just the time the fan had already been running on high. In a normal install, I think this kind of thing is handled by putting the controls well up in the plenum, to sense the homogenized output air. With my setup, I don't want to be sensing the output air to the house, as a cold HX and a hot stove could offset. BTW, I ditched the stock snap switch activating the low fan for another Honeywell fan control early on. The original snap switch didn't work well. The one I had laying around didn't impress me either, and I decided that the real fan control is what should have been used for this in the first place. It's working for me.

My wife has confirmed for me that if I move the water tank to the closet above the stove, I should be on the lookout for a new place to live. Though, she did offer that if I wanted to build a new closet in the house, I would be welcome to put the tank there. Offering to build said new closet inside of the existing closet directly above the stove, where I need the tank to be, was not seen as a compromise. I'm not married to the thermosiphon-only mode, I am, however, married to her. I kind of like the tank in the equipment room anyway. So - I ordered a Taco 006e3 circ pump this morning. Not the cheapest option, but one I think I can get turned down to the 2-3gpm area, so I don't screw up the stratification in the storage tank - as mentioned in the last few posts.

To add my 2c to the last few posts, I observed with my previous setup - with the coils inside of the firebox, and capable of creating some REALLY HOT water, with rapid variations in temp: When the water is gently returned partway up the tank, it makes a choice. If it's hotter than the water in the tank, it travels up and collects at the top of the tank. If it's colder than the water above it, it sinks until it finds the equivalent temp level, and the cooler water below is forced into the coil. I could actually feel the warm vs. cold level in the tank by running my hand over the outside surface, and feel how it moved up and down with big heat or cold water inputs. Because the hottest water available went straight to the top of the tank, even with a cold tank, if the stove was crankin', some seriously hot water could be delivered at the faucet - hence the tempering valve in the picture. I probably won't really need the tempering valve anymore, but I will say it's nice to be able to stumble to the shower, set the shower valve to the position you always use, and just have the water come out at the temperature you always like.

So, now I wait for the circ pump to be delivered. If that works out as expected, the next step should be working out some type of aquastat control to encourage the tank to peak out the temps as much as possible. I have plenty of air leaks to seal up, and lots of stuff that could be neatened in the next few days before it arrives. Plus, it IS woodcutting season - it'd be great to get out of the basement more!

One aside, for those who are really into it - I did hook the storage tank into my hot water supply a couple of days ago - in tempering tank mode. At that point it had risen to 111 at the level of the thermometer, though the bottom was still rather cool despite no cool water being added. I took a couple of longish hot showers, but otherwise usage has been very light. I ran a light 4 hour test firing 24 hours ago, and the tank thermometer is now at 85. The gas heater hasn't fired for the last couple of days - pilot light only doing the final heating. It will certainly fire when the timer lets it tomorrow morning, though. But the furnace is doing at least a little useful work beyond heating the house with the small pile of cutting scraps I brought in to feed it, even at this crude point of it's evolution.
 
  • Like
Reactions: sloeffle

RockyMtnGriz

New Member
Apr 19, 2019
23
SW Montana
Well the results are in.... (the summer results, anyway): For the last week, the Kuuma has been supplying all the hot water for our home, with no problems. Since you last heard from me, I've added a Taco 006e3 circ pump to the heat exchanger loop, rather than moving the water storage tank to the closet on the floor above the heater. I think this has to work better, keeps the plumbing, tanks, and potential leaks in the room with the concrete floor and drain, and has the side effect of preserving my marriage through closet space. An interesting thing - with the pump set at it's lowest setting, which supplies plenty of water flow, without negatively affecting the hot water stratified at the top of the tank, that pump draws a total of 4 watts! I wouldn't thing you could do anything with water with 4 watts of energy, but it does and makes the whole thing work! Not the cheapest pump you could buy, but an impressive piece. It feels like quality too!

Besides adding some temporary, crude insulation to the whole circ loop, another thing I've done in the time is to seal up all the leaks of really hot air. Many were in my plenum connection to the stove, and the heat exchanger insertion point, but one that I think every owner should look at is the front removable cover area. From the factory the stove had an 1/8 gap there that shot out a lot of 200 degree air that is better used elsewhere unless you really need to heat the room the furnace is in. Plugging the leaks made a HUGE difference in the heat in the furnace room, and seemed to make a real difference in the amount heat delivered to the water. Being the middle of July, heating the house is not really a thing, much, so I can't really tell from observation, about the delivered air, but it has to make a substantial difference there too.

OK, I know many of you are wanting more detail on the results, so here, at the risk of boring some, is more of that: If somebody wants even more detail, please ask. I've been firing the stove usually for about 4 hours an evening, every other day. That has worked to supply all our hot water needs, using a diet of dry but ugly, rotten, short, or misshapen wood debris that gets produced during the summer cutting season. For those who are going to guess that we're living some kind of minimalist lifestyle - no - it's just my wife and myself, and we have a reasonably low flow shower head, but a 20 year old dishwasher that runs frequently, since restaurants and take-out are not options in the middle of nowhere, plus a super capacity top load washer, that almost always washes on warm, so though we give some thought to conservation of hot water, we use our share. I've been learning to adjust the wood I put in the stove according to the starting temperature of the water tank. I'm getting this figured out pretty well now. With the circ pump, the heat exchanger is so efficient at transferring heat, that most of the heat from the stove ends up in the water tank. If there's 60 degree water in the bottom of the tank, the air coming from the duct is not more than 70 degrees, even though the air measured below the heat exchanger might be 150. Of course, at the end of the heating, if the tank is 125, the output air is also about that, not much hotter, as some of the cooler air is bypassed around the heat exchanger.

Today for example, was a large use day. I last fired the stove yesterday morning, topping the tank out at 125 after another heavy use time. I pounded the hot water supply with a load in the dishwasher, showers, and two super loads of laundry since, and to top it off, I also used a lot of somewhat warm water to wash my super sized smelly German Shepard. The last load of laundry left the sensor at the 2/3 tank height reading 64 degrees. The tap water was still hot, but I was too lazy to get an exact temp reading. I fired the stove 2 hours and 15 minutes ago, putting my guess at the right amount of wood in, and leaving the temp control on HI. When I checked a few minutes ago, the water temp was up to 100. The nice thing, is it is really a set an forget process, once you learn how much wood you need. Because I have yet to decide how best to control the circ pump, I still have to unplug it at the end of the heating cycle, but that's temporary. A possibly interesting side note - while I've tested this process with consistently 60 degree return air, and closed ducting, in preparation for winter; for the summer, I've found that opening the discharge and return air ducts in the furnace room makes the process faster an more efficient for just heating water. Because the heat transfer to the water is so efficient, tonight the temp of the furnace room had risen from 62 when I started the stove, to just 88 a few minutes ago.

I am using a fan control, rather than the factory snap switch to control my fan. And with the ducting open, allowing for maximum convection, the fan cycles somewhat even with the stove on high. To primarily heat water, I'm using a bit higher settings on the fan control than would be desirable to get the most hot air out of the stove. But, I'm not using temperatures any higher than I observed in firing the stove in the bone stock configuration, as the standard snap switch's inaccuracies allowed for some pretty large variations. I've smoothed out the peaks and valleys, and that, combined with taking the air from the hot parts of the stove, has given me the temps I need to heat water. The only thing that I've been able to see that's changed noticeably is the exhaust right at the stove collar rises about 15-20 degrees when I'm running the stove in the extra hot furnace room. I attribute that to the warmer return air not being quite as good at the final cooling of the gasses, plus the somewhat higher fan setting. It still tops out at 220 though - cool enough to touch very briefly.

I just checked again, and the water temp has risen from 100 to 108 in the last 45 minutes I've been sitting here, and the furnace room is still sitting at 88 with the door cracked open. I did decide (probably wrongly!) that I had slightly underestimated the wood, and tossed another small scrap in on the coals for good measure.

So, in the end, I'm really happy with the results! If I can get enough hot water to run my home in the middle of summer (today is July 21) firing every other day, about, I'm sure this will work in the winter. Oh, I guess I forgot to mention, I've been maintaining the house temperature between 60 and 65 degrees the whole time, so no, I'm not overheating the house to do this (no, I don't have A/C). And, yes, it is the freakishly cold climate I live in that allows me to do this through the summer, but I would think it would also work well for someone in a more normal climate through the shoulder seasons. One of my concerns was to be able to get temps high enough for safety (legionella, etc), and I've proven I can get that. It's wait and see what I get in the winter, once I'm running the fan to get every ounce of hot air, but I'm reassured that I could at least switch to a higher fan setting say during the overnight hours to get the water temp up regularly to ensure safety. And, clearly, when it comes to raw output of warm enough (100-115) water for a marathon laundry session, this system is far superior in recovery rate to the old one. This makes lots of 100 degree water fast, it just peaks out a lot cooler.

A benefit I've seen, besides the set and forget burn of the Kuuma, is with the circ pump, the water tank is an honest temp, top to bottom. Before, during the summer, I might fire the old stove until I had about 150 at the temp sensor. But using just thermosiphon, the bottom of the tank was somewhat cooler. So, I seem to be getting more mileage out of the 125ish full tank, than I was getting out of the 150 degree old system, which I typically fired daily in the summer. And I think the old stove produced more sometimes unneeded and unwanted hot air to the house during these summer firings.

A big benefit I will see in the winter, again, besides the set and forget nature of the Kuuma, is that the lower temps of the plenum scavenged system can never try to overheat my water tank. I used to have to choose when it was really cold, to either fire another stove, or think of a way to use hot water, as my storage tank climbed above 180 degrees (not that a hot bath on a sub-zero day is a terrible thing to be forced into - just at some point, it's not so productive).

So, as I end this novel, I just checked again. 3 hours and 45 minutes after starting the fire, the fire is waning, with a couple of big red glowing chunks in there, and the air control now on 3. The water tank is at 115 degrees, and the furnace room is 90. I shut the Kuuma computer off. The rest of the house is at 62 and cooling with the doors and windows open and a breeze blowing the 58 degree outside air in. I'll vent the heat from the furnace room in an hour or so when I unplug the circ pump. I'd just let it warm the house, typically, but we're expecting a small heat wave - supposed to get to around 80 the next couple of days and I don't have A/C, so I'm storing some cold :).

It's been a bit of an uncertain road, and I still have some tidying up to do (insulation is a very good thing - and I need to do more of that), but I have to say that I'm happy and reassured that the work has been worth it. I'm looking forward to a somewhat warmer and easier winter this year (inside the house, anyway). For now, life is good!
 
Last edited:

RockyMtnGriz

New Member
Apr 19, 2019
23
SW Montana
I'm putting this in a separate post, as the last one was long enough, and it doesn't really have to do with results.

To confirm that the change one thing, need to change another, upgrade cycle is still in effect, I learned last week while the power was out, that the blower on my new Kuuma will not reliably start off the tiny Honda 650 generator that reliably powered my old stove blower. This is a bit of a surprise, as the new blower uses significantly less power when operating than the old, smaller blower, but results are results. Yes, I have an 8000 watt generator (and it just howls!) that I can fire up when I need 220, or just need to run everything at once, for some reason, but I also need a quiet little one that can carry the small normal house load, plus the furnace, unnoticed through the night on a sip of fuel. Yes, I could probably get elaborate with soft start schemes, or putting a UPS inline, and get the furnace to work on the little generator, but I'm not going for those headaches. I'm going to face the upgrade! There are other side benefits to having a bit larger generator - I was just getting by with what I already had - until now.

So if it's not considered too far off topic, I've been looking at the Honda EU2200i, vs the EG/EB2800I vs. the similar Yamahas, vs. of course the Champions, and the like. It seems like the EB2800i Hondas are trading somewhat cheaper than the EGs, and the main difference is the GFCI outlets on the EB. It seems like it would be pretty easy to delete one GFCI on the EB for home powering use. Anyway, if it's not a violation of protocol, and someone would like to chime in on the topic, I'd appreciate hearing from experience.
 
Last edited:

sloeffle

Minister of Fire
Mar 1, 2012
556
Central Ohio
The rest of the house is at 62 and cooling with the doors and windows open and a breeze blowing the 58 degree outside air in. I'll vent the heat from the furnace room in an hour or so when I unplug the circ pump. I'd just let it warm the house, typically, but we're expecting a small heat wave - supposed to get to around 80 the next couple of days and I don't have A/C, so I'm storing some cold :).
How cold does it get where you are at in the winter ? It was 106F at our house yesterday with the heat index. :(
 

brenndatomu

Minister of Fire
Aug 21, 2013
4,147
NE Ohio
I can't imagine having the furnace lit in the summer...just cooking dinner can raise the house temp 2*!
And yeah, I got rid of the snap switch too...original didn't work, the replacement was different, but not right, so I installed a Honeywell fan/limit control, much better.
 

RockyMtnGriz

New Member
Apr 19, 2019
23
SW Montana
How cold does it get where you are at in the winter ? It was 106F at our house yesterday with the heat index. :(
It can get pretty cool here in the winter. I guess I don't really know how cool because the digital thermometer I can watch from the safety of my warm bed stops working at -42 or -46 or something. After that it just displays OOR for out of range. If I had to know, there is a weather station here I can pull up online, but really, if it says OOR, that's all I need to know!

Many years back, there was an official thermometer about 60 miles north of here, and at about the same elevation, that recorded the low temp record for the lower 48 states at -70.

But, summer here is glorious! And, we get 6-10 weeks of summer weather here, they're just not in a row! We just had a light frost Friday night, but we haven't gotten our usual July snow yet. We haven't had any snow since June 20.

Winter here keeps the place from getting crowded!

You have my sympathy, though. Having worked in weather from 117 to -50, I'll take the cold every time!

You also have my sympathy for making the effort to actually read all of my crazy long post!
 
Last edited:
  • Like
Reactions: brenndatomu

RockyMtnGriz

New Member
Apr 19, 2019
23
SW Montana
Brenndatomu, I know what you mean about the cooking dinner thing! I have a gas grille with a side burner on it that I used to figure out how to make everything on, when I lived where it got stinkin hot. Now the wife and I do the opposite. When it's stinkin cold, we come up with dinner ideas that maximize the amount of oven heat - like making a pizza!

I thought I recalled that you were using a variable fan control. If so, do you have the variable wired in series with the fan control switch?

I think a lot about the variable fan control, as I think that would be the most elegant way to run this setup, rather than letting the fan cycle. I'm not sure if more complexity is superior to having more wear and tear on a fan control, so I'm not diving in yet, but it definitely is on my mind. I also don't know if cycling is really hard on a blower motor. Cycling didn't seem to kill off either the fan control or motor on my old furnace. I guess I'm also hoping to get some cold weather experience with this furnace before I screw with EVERYTHING.
 

brenndatomu

Minister of Fire
Aug 21, 2013
4,147
NE Ohio
I thought I recalled that you were using a variable fan control. If so, do you have the variable wired in series with the fan control switch?
I've done it both ways...started out in series, then switched to just letting the speed control do its thing...I have it all torn apart right now (well, almost back together again now) to remove the old Yukon furnace and install the Kuuma in its permanent home (it was installed as an add-on last winter due to time, and the lack of) just need to finish the wiring on the Kuuma and install the stove pipes for the Kuuma and oil furnace both.
 

sloeffle

Minister of Fire
Mar 1, 2012
556
Central Ohio
It can get pretty cool here in the winter. I guess I don't really know how cool because the digital thermometer I can watch from the safety of my warm bed stops working at -42 or -46 or something. After that it just displays OOR for out of range. If I had to know, there is a weather station here I can pull up online, but really, if it says OOR, that's all I need to know!
That is cold. A couple years ago I had to feed the cattle in -10F with the windchill temps and I thought my face was going to freeze off. I even had a face mask on. I don't mind the heat nor the cold, I just don't like when the temps are extreme.

Winter here keeps the place from getting crowded!
That would be nice. Ohio is the 7th most populous state in the union. I could get use to less people for sure.
 

RockyMtnGriz

New Member
Apr 19, 2019
23
SW Montana
Edited out a ton of chit-chat, not really relevant to heating.

Last night, when I was ready to go to bed, I left the circ pump on, Kuuma warm but off, with the water tank at 116. After 7 or so hours, I returned to find the water at 123, and I unplugged the circ pump. This evening, after the day's use, the tank is again at 116. So, I have showers and normal use for tomorrow, and I'll see how warm it is to decide weather to fire it tomorrow evening. Since the gas heater is primed and warm, I might just wimp out and go to tempering tank mode on the Kuuma instead of working at it for a couple of days until it gets cool again.

I've rounded up a new generator off of CL. Yamaha EF3000iSEB. Nice and quiet! I haven't tried it yet, but that shouldn't have any problem starting up this blower, and it should make living on the small generator a lot easier too. The 500 watt budget of the old EM650 was sometimes a little inconvenient to live with.

I had firsthand proof of the need to de-rate for altitude. Being a Cl deal, I had taken 2 portable heaters with me to check out the generator. It pulled them (3,000 watts total) no problem at 3,000 feet, but once I unloaded it here at 7,000 feet - no go. About 2,300 watts is plenty for it up here - just about the 20% de-rate that the rule of thumb would suggest. It might improve a little if I can get it jetted right - another little project.

Have had the Kuuma water heater doing tempering the last week. We've had some hot periods, and sometimes I've just been busy, but it's still doing 80-90% of the water work I'd say, firing about every 3rd day. Got it going again tonight. So far, it seems to be doing as good a job as the old setup, while being a lot less demanding to tend, so I'm happy!
 
Last edited: