Use HX in hot air plenum to produce DHW?

[RockyMtnGriz]

First post here - but I have appreciated being able to use this site as a resource for years. Thank you all!

I couldn't find a post that spoke to this, so I'm asking directly:

Does anyone have any experience with using a fin and tube HX to create DHW? I'm pretty much wanting to reverse the more common process of using boiler water to produce hot air for forced air heat.

My situation is that I am considering installing a diverter in the discharge plenum of my hot air furnace that would allow me to direct all, a portion, or none, of the hot air created through a tube and fin HX depending on how much DHW vs. house heat I wish to produce. The DHW would flow through the HX and recirculate, heating an 80 gallon tank next to the furnace.

I'm currently using an older furnace with two 24" in-firebox loop coils to heat all of my DHW. I'm in the process of replacing that furnace with a modern unit that can't use in-firebox coils, but am looking to duplicate the DHW delivery. Using a variable air diverter and air to water HX would allow me better control over the ratio of DHW (the demand for which is fairly constant) to home heat (which obviously varies greatly with outside temps).

If anyone has done anything like this, I would appreciate learning about your experience. My biggest question to answer at this point is what kind of output I might expect from a particular size HX. Ideally, I would like this setup to thermosiphon, rather than using a pumped water side, so that will reduce the heat transfer potential quite a bit compared to the usual boiler heating air setup. I'm not afraid of HX math, but I think the reality of the situation requires one to make a lot of assumptions about flow, and the vagaries of wood heat output, such that someone's practical experience is likely to be much more accurate than a careful calculation using guesses as inputs.

Thanks!
Griz

 

[E Yoder]

Never done what you're describing but thinking about thermosiphoning. I'd think the 80 gallon tank would need to be higher than the heat exchanger?

 

[maple1]

I'm not sure how well that would work.

If you consider a DHW temp of say 120 to be a reasonable temperature, you would need quite a big DT in order to transfer enough heat to the water. How hot would your hot air temps be coming off the furnace? A lot less than something in the firebox would see. Supply temps say in sidearm or plenum HXs (not the same application I know) are typically pretty hot - 160 or more.

Also any diverting you do will be taking away heat from the house.

I went to a fair bit of trouble and some expense to do my system to be able to heat DHW with wood pretty efficiently during the non-heating season. I could do it with one firing weekly (with a re-load or two during the burn). I now don't bother, and just let our ordinary electric tank heater do it for a monthly cost of around $25. But when I do do it when heating, I don't rely on convection (I started out doing it that way with a sidearm, but it wasn't working for me when heating from storage). I use a small B&G Ecocirc to circulate to the tank. Variable adjustable speed and uses next to no electricity and hasn't skipped a beat going on 7 years now.

You might be able to make something work if you use a HX that doesn't mess with the flow of air to the house too much, circulate (at slow flow) through it to the tank whenever the duct fan is running, and have a big enough tank that you could heat up a big enough reserve during periods of non-use (night time).

 

[PassionForFire&Water]

What you are trying to do is against physics.
These coils are designed so the aluminum fins suck the heat out of the copper pipe (that's what aluminum is good for)
Using it the other way around will result in almost nothing; wild guess would be 10% efficiency

 

[RockyMtnGriz]

Thank you for the thoughtful replies.

I agree with you Yoder, the stove coils need to be below, or at least the outlet of the stove coil needs to be somewhat below the top of the storage tank. The lower, the better. Ideally, the circuit should form a cold trap, with the low point significantly below the bottom of the tank to prevent reverse circulation when the stove is cold. My current setup has the lowest stove coil slightly above the bottom of the tank, and no cold trap for space reasons. Slight reverse circulation does occur, but not enough to present a problem. Since the only source of heat to that tank is the woodstove, there's normally a layer of very cold water that has come into the tank from water use since last firing that seems to inhibit the reverse circulation. If that tank had another source of heat, I think I would have to stop the reverse circulation somehow.

Maple 1, yes, delta T is the biggie that I'm scrapping with at the moment. I'm hoping to get firm data from Kuuma on the output temps at different settings and CFMs, but don't have it at the moment. 500 cfm at 140 degrees seems a reasonable guess to me in the meantime. The math gets fuzzy when I don't really know what the water flow rate is. Flow rate is going to be a function of the amount of heat from the heat exchanger (which gets badly circular), and I think I have to allow for the fact that tube heat exchangers don't like to be run at low flows like the thermosiphon would create. I understand that that they tend to have a laminar flow at low rates where the fluid touching the tube gets hot while the fluid in the center stays colder as it moves through the tube. I'm not a heat transfer engineer, or I'd already have my answers, so I'm not sure how that really affects water with it's great heat transfer ability and stable viscosity, but it's a huge problem with oil, etc. Cooled oil, for instance, forms a thickened layer along the surface of a cooler keeping the hot oil from contacting it. Some turbulence is your friend when transferring heat!

I'm trying to design this thing to thermosiphon, not use a water pump, for a few reasons:

1. If I can avoid (cost effectively) another energy input to run the pump, why not do it?
2. Less moving parts is always better, if feasible.
   
3. I'm not off-grid, but where I live, the grid is frequently off. A pump is just one more load to provide back-up power for.
4. Most importantly - a system designed to thermosiphon is inherently resistant to meltdown should a power or pump failure occur. My backup power is not automatic, and I wouldn't trust it 100% even if it was.
5. I'm ashamed to admit it, but I have been trying to save the addition of a circ pump as an ace-in-the-hole. If, after my best efforts to build this right, it doesn't put out quite enough hot water, I can add a pump to hopefully salvage the situation, while keeping the thermosiphon function as meltdown protection.

I'll throw out a bit of detail about what I've been doing with my old setup. It might help you understand where I'm coming from and trying to get to with the new furnace (VF100), but more importantly, it might help somebody with their project or decisions:

Seven or so years ago I bought a home with the smaller model Clayton furnace (#1600 - IIRC) added on to the central propane furnace (which I don't use). I understand that the previous owners had quite some challenges using it. They had the power air inlet on it, and there's three stories plus of straight up metalbestos pipe above the stove. The thing could draft hard enough to empty the ash pan onto the roof! If you lit it, it was pretty much in runaway mode the whole time. It did an excellent job of heating the chimney cap, and not much else, except for some firebox parts that it melted. Ignoring the instructions, I ditched the power inlet blower for a duct control that would shut it down when the power goes out, and added a manual damper in the stovepipe. The Beast, as we call it, was tamed!

I added the 24" firebox DHW coil option from the stove manufacturer, along the side of the firebox, using an 80 gallon electric water heater tank right next to the tank, on a stand as tall as the floor joists would allow. I wanted more hot water, so I added another of these coils, in a custom location, below the top baffle, directly above the fire. This approximately tripled the hot water delivered. It then provided all the hot water my wife and I need to live a normal suburban style life, so I went from using it for tempering, to our normal water heater, with the addition of a temp mixing valve.

Because we have the unusual situation of a heating season that is 46-50 weeks long (we don't usually get measurable snow in August), the Clayton provides all of our DHW for all but 2 weeks to a month of the year.

In winter, I run the fan control to turn on at about 80 degrees, or use a manual always-on blower switch. Even though the DHW tubes are inside the firebox, isolated and insulated from the forced heating air, running the blower significantly reduces the DHW generated. There are two other woodburners in the house I can supplement with, as well, to help keep the house warm, but they have their own issues which I won't go into here.

Conversely, in summer, I bump the fan control up to 200 degrees, and build a small fire periodically out of woodcutting scraps and choke the air down a bit. The stove produces a lot of DHW and little hot air. On a cold summer day, I can use the always-on switch to pull more hot air.

I wondered a lot about the thermosiphon effectiveness before I tried. My results show it works amazingly well! My cold water from the tank drain goes into the bottom of the bottom loop, exits the top, then is plumbed into the horizontal loop above the fire. Exiting the top loop, it travels over and a couple of feet up, to the port where I removed the top heating element on the tank. The stainless heating coils are 3/4 ID, and the plumbing to them is mostly 3/4 stainless flex, with some 1" fittings at the tank. I have a thermometer probe on the tank a few inches away from this port, and the difference is visible within a couple of minutes of starting a fire. It usually drops a degree or two as the slug of cold water in the pipes is pushed out, then begins to climb.

The last couple of days have been near summer weather, and the wife and I have been away from the house a lot, so it's been showers only, pretty much. With the fan control in the winter setting, I've had to burn 2-3 6x19 lodgepole pine logs per day to keep up with the DHW, and gotten a fair bit of hot air to keep the house from cooling too much. It runs the probe temp from say 114 to 124 in an hour or two. So, yeah, it can really heat water, and knock on wood, I not had a single problem doing it.

Why would I screw with a good thing? Well, to keep this very long story short(er), while it's great for heating water, it's not so good at heating the house. Burning the only wood I have in quantity - lodgepole pine, I have to tend the thing every couple of hours. The more wood I put in it, the more heat I get, but the efficiency gets even worse, and the stove actually needs reloaded more often. A cold house in the morning is a given. I'm about to freak some of you completely out, but my target daytime temp in the house is 54-60 depending on the weather, so a cold morning house temp can be 50 down to I've seen 36 on the bedroom thermometer. 50 is ok with me, if I could give it a quick bump when I'm ready to get up, but the Clayton will neither keep burning for more than 3 hours after I go to sleep, nor does it have the horsepower to warm things up with any speed when I get up. So, I'm hoping to improve both of those with the new Kuuma. And I wouldn't mind only feeding it every 4-6 hours during the day, either.

I hope someone finds something useful in my experience, and I'm still hoping to benefit from other's experiences with finned heat exchangers and water heating. Thanks everybody!

​

 

[RockyMtnGriz]

Passion, I will respectfully disagree with you, and point out that there are commercial air conditioning type units that use cold water from a central water chiller to cool the air in a room using the type of thing I'm contemplating. The air is cooled, and the water is warmed in the process. Some earth source cooling schemes have used a similar process. Cooling air / heating water and dumping the heat into the cooler ground or a lake or such.

 

[E Yoder]

I agree, heat conducts either way equally. Not saying the math will necessarily work tho.

 

[sloeffle]

Maple 1, yes, delta T is the biggie that I'm scrapping with at the moment. I'm hoping to get firm data from Kuuma on the output temps at different settings and CFMs, but don't have it at the moment. 500 cfm at 140 degrees seems a reasonable guess to me in the meantime.

Paging @JRHAWK9. I am sure Mr Hawk can answer your question. He has all kinds of sensors rigged up to Space Hawk 9000 ( JR's Kuuma).

 

[JRHAWK9]

IMO, the plenum does not get hot enough for that to work, at least effectively. I think it would work better when using the copper water coil bolted to the rear of the firebox, they way Lamppa used to do it before they became PH2 certified. This was very effective at heating DHW and could very easily be added to a new VF100 if you chose to do so. My plenum temps, at the highest when running the furnace full bore, are mid 120's. These are higher than most others with a Kuuma because of how I slowed my blower down. Most of the times, when the furnace is running on low, my plenum temps are 115°-117°. Like others have said, I don't think the T is high enough for you to effectively thermocycle and heat DHW using a radiator style HX in the plenum. JMHO

 

[RockyMtnGriz]

Thanks JRHAWK9! After Sloeffle gave me the idea, I spent quite a bit of time searching back through your posts, and gained quite a bit from it. Something tells me you might have more data than Kuuma! Glad to have you on this!

So, I think I know the answer, but just to make certain, when you say you're running flat out, is your custom blower control running the blower at a lower speed than the stock low speed?

It seems strange to me that the Kuuma plenum temps would be so low, because they are so much higher than that on the old furnace I'm running right now. A modest fire yields 120 - 130, and anything that's doing real burning is DON'T TOUCH! I don't know if the blower was replaced on this Clayton, but in any case, it sure doesn't put out anything near the 1400 cfm that the manual suggests - 400 maybe. My gas furnace says it does 1750, and the difference in the two blowers is night and day. Unlike your place, my place seems to have a preference for more air circulation, rather than less. Maybe we should have worked a trade - definitely kidding!

It's crossed my mind that someone with super free flowing ductwork might see lower plenum temps, but when you say that others are experiencing even lower temps than you, hmmm. I know that I can get the temps where I would need them, say a constant 130, even the manual suggests turning the fan control to 145 to help tame it for warmer weather, but if you're normally running quite a bit lower than I need, I would obviously be giving up some stack efficiency.

The heat exchanger would see some radiant, being directly above the top of the stove, but my head scratching is leaning toward that being a slight benefit with large delta T, and about nothing due to the cooling effect of the air passing through it as the water approaches the air temp. I could be wrong, though.

One thing I've learned about heating DHW, is there's a huge variation of heat transfer in the process. Water can come into my house very near freezing. When run through the stove, it really sucks up the heat. The first 50 degrees comes quick, the next 50 takes a fair bit more doing, and the 50 after that is pretty slow - which is good, because I consider 180 to be the speed limit. If my setup would produce a lot of 115 degree water anytime (by using a lot of heat exchanger), then grind all day or night producing say 125 degree water from that (because of the diminishing delta T), it would probably do the job. It would likely be better than what I have now in the winter, because it would be self limiting, though it might give up some ability to do the job in the summer.

I'm willing to have a little longer conventional water heating period in the summer, if I have a winter heater that I don't have to monitor the water temps, does a better job heating the house, and maybe even produces more 115 degree water for the washer and such, though it needs to get warmer than that at some point daily for showers and safety from nasty stuff growing in the heater. Worst case, I could create a separate appliance strictly for heating water, which would take care of the shoulder season and summer duties, but they would be trading the flue, so I need to create something that will certainly take care of the winter, and I'd like to get as much out of this unit as I can.

I've talked with Darryl about this, he and I are of the same opinion that there is no way that the coils on the back of the stove will produce anything near what I'm looking for. That's partly because the stove burns front to back, and mostly at the top of the firebox, so the back is not very hot except for when the fire gets to the end of the load. The other issue I see is, that coil is directly in front of the cold air coming in from the blower. I do have a coil in that area of the stove in my tentative plans, but I was planning on it mostly serving the function of a cold loop to prevent reverse circulation when the heated water in the tank is warmer than the heat exchangers.

We talked about adding coils to the top, etc., which I'm somewhat reluctant to jump into, as I think that in order to be really effective, they would interfere with heat transfer from the stove when the water temp is satisfied. I would have to burn it some before I would feel comfortable screwing with Darryl's finely tuned machine! Darryl didn't have an objection to it, though.

I'd be interested in any other thoughts you, or anyone else have on this project. Thanks!

 

[maple1]

Do you have any idea what your hot water usage would be?

I don't really find DHW heating to be that big a load or deal. As mentioned above, I do ours for $25/mo the 'old fashioned electric' way. And it's a very easy cheap setup/install.

 

[Fred61]

I think you'll be disappointed. Low mass heating high mass. How many cubic feet of warm air does it take to heat one cubic foot of water to a useable temperature and what will the temperature of the air emitted from the register? Comfortable?
.

 

[JRHAWK9]

So, I think I know the answer, but just to make certain, when you say you're running flat out, is your custom blower control running the blower at a lower speed than the stock low speed?

Yes it is. Going off of memory here, but I believe I was seeing plenum temps around 110° back when I was running the blower on low with the Kuuma on high.

It seems strange to me that the Kuuma plenum temps would be so low, because they are so much higher than that on the old furnace I'm running right now. A modest fire yields 120 - 130, and anything that's doing real burning is DON'T TOUCH!

Kuuma's, and all EPA furnaces, burn wood in a much better controlled manner in order to get that efficiency. You will not see the raw BTU's out of any newer EPA furnace when comparing them to the older ones.

It's crossed my mind that someone with super free flowing ductwork might see lower plenum temps, but when you say that others are experiencing even lower temps than you, hmmm.

This is exactly what I chalk my situation up to. Back with my stock blower setup, I did have the lowest plenum temps of all the people I talked to who had VF100's. When I mentioned others having lower plenum temps than I do, I meant compared to how I have my system currently setup.


Back when I was running a water coil on the rear of my VF100 and pre-heating the water feeding our water heater, we were pretty much getting free DHW during winter. The water heater would finally kick in well into a 4th shower in a row. I know this because we had company over one weekend and I wanted to see how long it took before the heater kicked in. Normally it's just two of us in the house, so taking two showers in the morning was not enough to kick the water heater in. Last spring my tempering tank sprung a leak so I removed it. The house seemed to heat easier as soon as it was removed so I don't think I'm going to go back to using it. The system worked very good, however this house needs as many BTU's as possible when it drops below zero outside. I would rather the BTU's go towards the house than heating our water. I will know this fall when we do our yearly tank top-off how much more LP we burned compared to the previous handful of years when we were heating our DHW with the Kuuma during winter. Judging by where our tank is now, I'm not expecting to see -that- much of a difference.

IMO, the luke warm water you may get out of doing things the way you are thinking will not be worth the drop in plenum temps/decreased house heating ability you will be seeing. I could be wrong, but this is how I see things happening. Heck, I have an April Aire system attached to my Kuuma plenum and when it's turned on there is a noticeable drop in plenum temps. All that is doing is taking in heated plenum air and passing it through a ~12"x8" panel with cold water running down it. Just that is enough to knock down my plenum temps a good 3-5°.

 

[RockyMtnGriz]

Do you have any idea what your hot water usage would be?

I don't really find DHW heating to be that big a load or deal. As mentioned above, I do ours for $25/mo the 'old fashioned electric' way. And it's a very easy cheap setup/install.

First, let me say that I'm not arguing with you on the real economic terms. I think what I'm doing is somewhere between sorta ok and wrong on that basis. Though if I spend 500 extra on the project, at $25 a month, it's still a less than 2 year payback. The PV energy guys would kill for something like that! I have a number of motivations to avoid conventional heating of water, and some of those might similarly be sorta ok or wrong.

1. The setup I have now works great (water wise), so I'm interested in replicating it as best I can, if I l can. I spent around $500 for the extra stuff to heat water, and it's been giving me what I consider to be free water for more than 11 months of the year for 7 years. If I scrapped everything I'm using today (which I'm not), it's been paying me about $20 a month, using your $25 a month expense. Not really a needle mover, but positive at least. Yeah, I have to cut the wood, but I would have to cut the dead trees anyway, and it keeps me calm and in shape.
2. My plumbing setup requires the use of a power vent heater (which I have) or would require the install of an instant heater (prefer not to go there) due to venting issues if I stay propane. I hate just about everything about my power vent heater. Between combustion air draw (wood burners), stack effect, and prevailing wind, all the pilot smell ends up in the house. It's noisy. It sucks a ton of air out when the power vent is in operation. It vents to an area where it creates an ice problem, and I can't really change that. In the summer, the vented exhaust is a nuisance, either in a cloud where I often work, or if there's a breeze, it gets sucked into open windows. I have more, but that's enough. I like it ok when it's drained and unplugged though!
3. Electric is nice, but it's off too much here, so I can't really depend on that.
4. Burning the too weird to stack and other scraps during the cutting season actually has proven to be a nice way to get rid of that stuff.
5. The routine of building a fire is not a negative for me.
6. I sometimes do stuff because I think it can be done, but since nobody has done it (or will admit to the failure!), I need to find out for myself. I've had a pretty good track record on that kind of thing. A very bad economic argument, however!

I do appreciate the rational advice, though!

 

[RockyMtnGriz]

Yes it is. Going off of memory here, but I believe I was seeing plenum temps around 110° back when I was running the blower on low with the Kuuma on high.

I had a conversation with Darryl today. He says he was running around 110 with the blower on high and the stove on high for the EPA test. That just makes more sense to me. Assuming that my old stove is around the 500 cfm of the low setting for the Kuuma, it wouldn't heat much of anything in the house at 110 on a cold day.


Back when I was running a water coil on the rear of my VF100 and pre-heating the water feeding our water heater, we were pretty much getting free DHW during winter. The water heater would finally kick in well into a 4th shower in a row. I know this because we had company over one weekend and I wanted to see how long it took before the heater kicked in. Normally it's just two of us in the house, so taking two showers in the morning was not enough to kick the water heater in. Last spring my tempering tank sprung a leak so I removed it. The house seemed to heat easier as soon as it was removed so I don't think I'm going to go back to using it. The system worked very good, however this house needs as many BTU's as possible when it drops below zero outside. I would rather the BTU's go towards the house than heating our water. I will know this fall when we do our yearly tank top-off how much more LP we burned compared to the previous handful of years when we were heating our DHW with the Kuuma during winter. Judging by where our tank is now, I'm not expecting to see -that- much of a difference.

I have motivations for doing this that go beyond economics, that I got into in the previous reply, but yeah, I understand!

I'm leery of comparing the water you get, with the water I would get, because I've read a lot of your posts. I think you're running your Kuuma a lot harder than I'll be running mine. It got to 56 here today (summer woo-hoo!), after snowing all day yesterday. By 11am, when it was up to about 43, my wife had a bunch of doors and windows open and was in spring cleaning mode. It's 57 in here right now, and everybody's happy with that. I'm usually aiming for somewhere around this temp in the winter. My house actually looks pretty similar to yours, and I'm sure it's not any better at holding heat. I haven't studied it, but I suspect our climate is pretty similar. You probably have slightly colder weather in the dead of winter, and I suspect I have longer cooler shoulder seasons. I think the difference in desired temperature is a biggie though.

Because all but some of July and August are typically the heating season here, I'm going to be building a fire in something anyway, though I don't need a lot of heat. Pulling BTUs to water might actually be a good thing in the not winter months, helping tame the output, and it might require just one more wood scrap in the firebox. In full on winter, the water would quit absorbing BTUs as soon as it reached plenum temperature, and would become a non-issue from there.



IMO, the luke warm water you may get out of doing things the way you are thinking will not be worth the drop in plenum temps/decreased house heating ability you will be seeing. I could be wrong, but this is how I see things happening. Heck, I have an April Aire system attached to my Kuuma plenum and when it's turned on there is a noticeable drop in plenum temps. All that is doing is taking in heated plenum air and passing it through a ~12"x8" panel with cold water running down it. Just that is enough to knock down my plenum temps a good 3-5°.

I had to look up AprilAire, but yeah, that makes sense. If you humidify the air, the temp will drop, just like a swamp cooler. The same heat is still there, it's just the air has a higher heat capacity with the water in it, so the temp will drop if you don't add heat at the same time..

Appreciating the conversations! It's looking more and more like I'm going to have to maybe put the furnace in stock and gather some more experience, then just take my best shot, and see how it turns out. It's always good to solicit ideas and reality checks, plus I was hoping to get somebody's direct results with something similar. My hope was to be able to do most of the work while it's still sitting in the middle of the garage and easy to work on, but...

Still in the market for thoughts - so keep 'em coming! Thanks everybody!

 

[RockyMtnGriz]

The above reply is a mess with the quotes - but it can still be followed by scrolling the quote section. Sorry - I'll try to do better next time!

 

[RockyMtnGriz]

JRHAWK9, I can't help but notice that you have your output plenum insulated on your "launch-ready" model. Would you be willing to elaborate on the insulation you're using?

Looks like I might come back for tips on variable fan speed before this is done, too.

 

[RockyMtnGriz]

I think you'll be disappointed. Low mass heating high mass. How many cubic feet of warm air does it take to heat one cubic foot of water to a useable temperature and what will the temperature of the air emitted from the register? Comfortable?
.

First off, jealous of your hydronics! Were I to build this house, it would have that. However, to add it to this house now, I might as well tear it down and re-build.

I think the equation is low mass, high volume, vs. high mass, low volume. It seems like it might be a fair fight. I have a feeling I'm going to be the one who finds out the outcome, fairly soon.

I'm loving this discussion because there's a range of opinions from, "you won't get much hot water," to "you won't get much heat in the house." Well, they can't both be right, because once the wood is loaded, and the match lit, the heat is going somewhere. I'm thinking that regardless of where this comes out on the scale, I'm going to have more heat to share around with the new furnace, because, let's face it, unless I'm really motivated (cold), I fail to adhere to the 2 hour tending routine required of the present setup. I just have to tweak the issues of getting the heat where and when I want it - especially when it's not screaming cold out.

Thanks for the reply! Keep 'em coming!

 

[JRHAWK9]

I'm leery of comparing the water you get, with the water I would get, because I've read a lot of your posts. I think you're running your Kuuma a lot harder than I'll be running mine. It got to 56 here today (summer woo-hoo!), after snowing all day yesterday. By 11am, when it was up to about 43, my wife had a bunch of doors and windows open and was in spring cleaning mode. It's 57 in here right now, and everybody's happy with that. I'm usually aiming for somewhere around this temp in the winter.

I do run it somewhat hard for a handful of days a winter when we get cold snaps. Damn, you guys keep it cold. Brings up another concern. If it's 55° in the house, what is the temp of your return air? Say it's 50°. You are not going to be seeing anywhere near 110° plenum temps. That would be a 60° T . I don't think you will ever be seeing that kind of T out of a unit rated for ~40,000 BTU's. I don't see a 60° T even with my blower slowed way down. My 75,000 BTU LP furnace doesn't either. It gets me a 42° T using the medium-low speed tap on the blower motor and a 48° T using the low speed tap. These were measured after the LP furnace was running for 15 minutes straight.

JRHAWK9, I can't help but notice that you have your output plenum insulated on your "launch-ready" model. Would you be willing to elaborate on the insulation you're using?

Sorry, I don't remember where I got it from. I'm sure it was online someplace. I'm also not really sure it's really doing much either.

 

[RockyMtnGriz]

Y'know, there's that old saying that when you're sure everybody else is crazy, - it's probably you! I haven't slept on this, but I'm about to, so I guess it's subject to change, but I'm thinking the best plan of action is to consider myself warned. I still think it would work, but when faced with a contrary opinion from a bunch of people whose experience and approach to things I respect, I think it's wise to slow down and consider further.

So my new tentative plan is to create a slightly enhanced OE style water heating coil, since it would be good to hook my tempering tank to something. I'll then run the thing some and see what it does. That should give me a much better idea of what I can use or modify to get what I'm looking for. I was hoping to install a setup that would be close,, so most of the work would be easier, but I think I can do some things to provide for an easier install of what I think I might eventually do. Should I come to the conclusion that it won't work, I will probably have a few hundred dollars in my pocket still thanks to you all.

Unfortunately, those who now have money on how badly this noob is going to screw this up are going to have to wait a bit longer to find out!

If I have to run that power vent water heater for a few months straight, I don't know what I'm going to do, but that power vent might show up on craigslist!

 

[RockyMtnGriz]

JRHAWK9, from here, a thousand miles away, that insulation looks like bubble wrap, but I've never seen anything like that capable of taking real heat. I'm not aware of anything I'd be comfortable with, except the HT stuff like is in the furnace, so I was curious. I know my conventional stoves can get to 700, so I'm nervous about any insulation in a meltdown mode situation.

Thanks for the time and knowledge you put into this thread for me. Not that it's over, I'm still building - just slower. If you or anyone has anything to share, I'm still listening.

 

[DoubleB]

If you or anyone has anything to share, I'm still listening.

@RockyMtnGriz , I'm late to your party, but in my opinion you are trying to get the do-all setup, whereas an 80/20 rig would probably have a better chance of being worthwhile. For example, when my power goes out, I can live without cold water--can you live without hot for that time? If so then a small circ pump is probably fine and might make your plan more feasible, which is also more likely you'll enjoy using it over the coming years.

Maybe I missed some details but that's the gist I got from skimming.

 

[RockyMtnGriz]

@RockyMtnGriz , I'm late to your party, but in my opinion you are trying to get the do-all setup, whereas an 80/20 rig would probably have a better chance of being worthwhile. For example, when my power goes out, I can live without cold water--can you live without hot for that time? If so then a small circ pump is probably fine and might make your plan more feasible, which is also more likely you'll enjoy using it over the coming years.

Maybe I missed some details but that's the gist I got from skimming.

Agree. I'm pretty much trying to replace the do-all setup I have now. I'm just hoping to improve it with a better woodburner that in order of importance, I don't have to tend every 2 hours on average, is more efficient, and smokes less. I can live without heat, plus water of either type, for a while. But my power goes off pretty frequently, and has been out for more than a week on more than one occasion in recent memory. Not talking natural disasters either, it's just life at the edge of civilization. As both of us work from home, it really helps to just get the work done and get through the day if it's as seamless a transition on and off of grid power as possible. 10 minutes of fiddling and switching stuff to make the on or off change really adds up if it happens to be off 3 times in a day for 2 hours at a time.

As I have learned and progressed in knowledge with the help of others in this thread, I do think a circ pump is probably going to be key to making this work (if it will work) in high demand periods, with the marginal plenum temps that this furnace will probably provide. I've never completely ruled out using a circ pump, it's just not my first choice, as I do have as an absolute requirement that the system work without meltdown issues in a power failure mode - which means it has to be able to thermosiphon well anyway.

In case anybody is reading this thread for data to apply to their own project of some sort, I've switched my Clayton furnace to the summer mode (min heat, max water) to deal with the high 20s to 50s days we're having lately. I also have to have the stove cold more than I usually would in this weather due to the work I'm doing in the furnace room, so though the heat is optional, the hot water need isn't. I've been building small evening fires only to prevent the house from getting too cold, and to heat water. I have the fan control with overheat in the stock setting at 200, with the on and off bumped as high as they will go without jamming all the triggers together in the control. Plenum temps are 148 at fan off and 190 at fan turn on. The fan is mostly off, so I don't get a bunch of heat to the house. A couple hours of burning like this recharges an average day's worth of hot water for the two of us. Just a note - the coils on this stove are in the firebox - not in the plenum. I'm just paying attention to plenum temps because of my pending project.

 

[maple1]

I've never completely ruled out using a circ pump, it's just not my first choice, as I do have as an absolute requirement that the system work without meltdown issues in a power failure mode - which means it has to be able to thermosiphon well anyway.

I'm doubtful that would be an issue with what you are thinking of doing - not likely plenum temps would get hot enough or heat transfer would be good enough to overheat the DHW stuff. That is, as long as the ducting is also designed for good thermal convection in a power outage. If not, then yes your DHW side might overheat - along with the ductwork and anything close to it.

 

[RockyMtnGriz]

Well folks, after the usual many delays, the Kuuma is in place. Fired it for the first time on Memorial Day (Monday 5/27/19 - for those of you in other places). I have some things to work on yet, but as far as the stove burning the wood and extracting the heat, I'm impressed. It doesn't smoke, so it's burning completely, and the stovepipe elbow right at the stove is around 200 degrees, so it's extracting the heat from what it burned. So far, it's just as advertised, which is as rare as it is good!

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!).
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Cheers!