Kuuma Vapor Fire 100 - Setup Help

[Pustiki]

Hello, I have been following these boards for quite a while now. Hoping the hive-mind can help me get the new Kuuma running to its full potential. I upgraded to the Kuuma this past September from a 1960’s era Shenandoah wood furnace. The shoulder season through New Years was awesome with the Kuuma, but this week we’ve been hit by a real cold spell (single digits) and I’m struggling. Through New Years, the Kuuma was keeping the first floor of my house at 72-73 degrees consistently on two loadings per day. With this cold snap, I’m struggling to maintain 66-67 degrees. I believe the furnace is working properly and based on my reading of the forums, my setup is the likely culprit. Any advice will be much appreciated, so here goes:

With the Shenandoah, I was going through 9-10 cord in a season, constantly loading and manually adjusting a thermo-spring air intake damper. I was aiming to keep the house at 68, but would’ve preferred 70.

I got the Kuuma hooked up in September, using the same ductwork. The ductwork has a roughly 25’ long trunk with 8 registers into the first floor. The ductwork also services a propane furnace/heat pump. There are two cold air returns from upstairs, but only one effectively services the Kuuma. My installer set up the return ducting to cover the bottom half of the Kuuma, while the top half of the Kuuma is open to the basement. The basement door has a grate to allow return air. I have had better luck blocking off the top half of the Kuuma.

The furnace is located in a block-wall basement, unfinished and below ground. The furnace is located at one end of the house. The chimney is an 8” flue, brick with terracotta liner, located on the exterior of the house. I’ve never had a problem with draft (even had to slow it down with the Shenandoah), but I don’t have a manometer. One is arriving today and I will measure the draft. The 6 inch exhaust from the Kuuma is ”necked-up” to 8 inches at the wall. I am considering a chimney liner, but draft seems to be okay (pending manometer reading). The barometric damper is set at the lowest setting and it flaps only slightly. Any higher of a setting and it will not move. There is a clean out door in the chimney below the thimble. I will usually start a draft for the first burn by lighting a small fire at the bottom of the clean out. Once going, the draft sounds like a freight train with that door open.

The house is 1980s construction, R-19 insulation in 2x6 walls. Conditioned space is 2400 sq. feet (1200 each floor). I had cellulose blown in the attic this year over top of the fiberglass batts that were there. It is two stories above the basement. The furnace and ductwork only service the first floor. There is an electric heat pump located in the attic to heat and cool the second floor. In the past, I relied on the furnace downstairs to heat the whole house with the upstairs being 3-4 degrees cooler than downstairs. Over the past year, I have worked on buttoning up the house pretty good. I have one door with a small air leak that I’m working on and another door with barely anything of a leak. New windows throughout the home.

We did an addition on the first floor this year, 800 sq. feet with a 13 foot cathedral ceiling. I did not bring any ductwork into that space as it’s full width is open to the first floor anyway. I will install a Fisher wood stove in there for space heating and ambience, not intended for primary heat of the house. The walls and ceiling of the addition were insulated with closed cell foam. The floor of the addition is not yet insulated and it is built on a crawl space foundation. Insulators couldn’t do the floor on the day they did the walls due to a moisture issue, but they are set to finish that this week. I am hopeful that insulating the floor will make a big difference, but I’m finding it hard to imagine quite this much heat loss or additional heat load from that floor.

For example, I left the house this morning with a full load in the Kuuma and it was on “c.” The computer is set on Medium. Until this week, it lived on Low. I brought the home up to 68 degrees using the propane furnace and re-loaded the Kuuma. When I left, my Nest thermostat read 67 degrees. Less than an hour later, it’s reading 66. Upstairs is reading 65. Based on yesterday, I anticipate it will drop to 64 within the next two hours, but the Kuuma will still be doing “c-1-c-1.” Up to New Years, a full load of wood in the morning would still be burning when I return home at 5pm. The propane furnace will kick on at 62 degrees. Last night, I did a full load at 11pm. At 7am, the propane furnace was on and the Kuuma was at “3.”

Using a laser thermometer, the single wall flue pipe out of the back of the Kuuma is reading about 200-225 surface temp. I have the low-limit fan switch currently set at 115F. Prior to New Years, it was set at 105F. The plenum is warm to the touch and the fan is no longer running constantly through the burn. The registers closest to the furnace upstairs will show a supply temp of about 100 degrees, dropping to 90 or less within 3-4 minutes of the fan kicking on.

I am burning mixed hardwoods that were seasoned for over a year in log length, then cut-split-stacked this past spring. Moisture on a fresh split reads about 21%. I should have 2-3 cords left over from this stack going into next season, so hopefully much dryer by next October.

My thoughts on what to do next:
1) Insulate the floor of the addition
2) Measure the draft and adjust as necessary. Chimney liner possible.
3) Change the return air setup
4) Increase the low-limit switch to 120-125.
5) ?

It seems to me the supply temps are too low and the plenum should be much warmer on a fire that gets to “c” as quickly as it does. Wondering if I’m losing a lot of heat up the chimney, but don’t know how to stop that.

Long post, thanks for reading. Any tips, hints, advice much appreciated.

 

[Pustiki]

My thoughts on what to do next:
1) Insulate the floor of the addition
2) Measure the draft and adjust as necessary. Chimney liner possible.
3) Change the return air setup
4) Increase the low-limit switch to 120-125.
5) ?

6)Slow down the fan speed somehow.

 

[brenndatomu]

Hi, congrats on the Kuuma!
Have any pics of your setup?

 

[brenndatomu]

pending manometer reading). The barometric damper is set at the lowest setting and it flaps only slightly. Any higher of a setting and it will not move.

You have low draft then...get that manometer hooked up to verify...and order that liner...the chimney is the ENGINE that powers the furnace...and the Kuuma does not waste a lot of heat up the chimney to provide plenty of draft like the old furnace did...you have to keep the heat that is there, so you need an insulated 6" liner.

 

[brenndatomu]

It seems to me the supply temps are too low and the plenum should be much warmer on a fire that gets to “c” as quickly as it does.

Do you have the limit switches mounted as Lamppa requests? (low limit high in the plenum)

 

[brenndatomu]

Oh, also do you know if the baffle(s) in the HX are properly in place?

 

[Pustiki]

Hey, thanks for responding. I've followed your other posts about this machine.

Hi, congrats on the Kuuma!
Have any pics of your setup?

I will get some pics posted this evening.

You have low draft then...get that manometer hooked up to verify...and order that liner...the chimney is the ENGINE that powers the furnace...and the Kuuma does not waste a lot of heat up the chimney to provide plenty of draft like the old furnace did...you have to keep the heat that is there, so you need an insulated 6" liner.

Makes sense. I will get a liner ordered. I did try the "foil over the damper" trick, but no real improvement.

Do you have the limit switches mounted as Lamppa requests? (low limit high in the plenum)

No, the low limit is just above the computer. I didn't do the install myself, so I'm not sure if the installer chose that spot or it came from the factory located there. Do you know if there's a good writeup on relocating? I imagine I would need to plug the current hole somehow and re-route the wiring with a new hole drilled into the plenum?

Oh, also do you know if the baffle(s) in the HX are properly in place?

Yes, I believe they are. As part of this diagnostic, I cleaned off the HX yesterday (wasn't really much to clean) and inspected the baffles. The baffle with the insulation material is behind the opening of the firebox, up against the four tubes that enter the plenum area. The angled sheet metal baffle is set in front of that one, angled up toward the front of the machine and over the top of the fire box opening.

 

[brenndatomu]

I didn't do the install myself, so I'm not sure if the installer chose that spot or it came from the factory located there. Do you know if there's a good writeup on relocating? I imagine I would need to plug the current hole somehow and re-route the wiring with a new hole drilled into the plenum?

I think there is, should have come with it...I'll see if I can find it...basically they want the low limit as high on the side of the plenum as it can go...toward the front IIRC...they only mount it because they are required to mount it, but it really needs to be in the top of the plenum, which it does not come with, otherwise they'd have it mounted there already, or at least a hole for it.
As far as turning the low limit up more, I would not...my experience is that when the blower stops, the temp drops (house) so you want the blower running as much as possible (within reason...no sense in circulating cold air)
On your return air, I think you'd be better off with it hooked up to the whole filter box...if you wanna pull some air from the basement just cut in an adjustable louver vent somewheres.

 

[brenndatomu]

Here it is...

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[Pustiki]

Great, thank you. Watching the video makes me think my plenum is over sized. The top of my plenum goes directly into the trunk line. Essentially, it's a huge plenum. I wonder now if I actually need some sort of restriction in between. In that video, he has the opposite problem (too much restriction).

 

[brenndatomu]

Great, thank you. Watching the video makes me think my plenum is over sized. The top of my plenum goes directly into the trunk line. Essentially, it's a huge plenum. I wonder now if I actually need some sort of restriction in between. In that video, he has the opposite problem (too much restriction).

You might call Lamppa to ask about this, see what they think...I know when I had my plenum made the ole feller that did it had been making ductwork and plenums, etc. for the local HVAC place for many years, he said you aren't supposed to come directly off the top of the plenum...something about that being "bad practice"...I dunno, I just figured, hey, he knows a lot more about it than me!

 

[Pustiki]

Here are some pics. I did just finish moving the low limit switch though. Please don’t mock the flue pipe… I didn’t do it.

 

[brenndatomu]

Yeah I bet you are lowering your return air temp a good bit there...with sucking the cool air toward the floor, and also with that return duct running on the floor and against the wall, it will be cooling the air in there too.
You can have a 10* difference in temp between the floor and the ceiling...and for every degree gained going into the blower, equals another degree going into the supply plenum too.
Is that flue pipe just stuck into the chimney breech and fiberglass stuffed around it? Just FYI, fiberglass is not fireproof...and if that is not stuffed tight you could be loosing a lil draft there too...also, make sure that cleanout closes tight/seals...for the same reason.

 

[Pustiki]

The ductwork down there is admittedly a mess. Whoever did that install made some crazy choices. I’m kind of retrofitting and I’ll address that over time. Air temp at the ceiling is around 65F. I just moved my thermometer to see where it’s at on that return. Wondering if it’s worth abandoning the return and grabbing air from ceiling height.

The flue has a 6” to 8” adapter on it inside the clay thimble. That adapter is tight to the walls of the clay and much further in than that fiberglass. Was a temporary test to see if it improved draft in case of a leak. It’ll go away when I get the new liner. It’s not fireproof, but I believe combustion temp is at least 1000F. I just didn’t have rock wool on hand.

I moved the low limit this evening and re-set it to 105. Anything higher than that and the fan will cycle on off. The temp in the living space is holding 68, but it’s “only” 21F outside now.

 

[brenndatomu]

The temp in the living space is holding 68, but it’s “only” 21F outside now.

Its about the same temp here right now...its 71 inside and I'm down to pretty much just a few coals in the firebox at this point.
One thing that I've found is that I do better with loading 3 smaller loads per day when it gets real cold out, rather than 2 larger loads...or in warmer weather I even go down to as little as 1 small load/day.
If you have any dry pine, or other low coaling "softwoods" those will actually put more heat to the house than hardwoods will...assuming that someone is around to load more often...reason being is that you don't have to wait long for the coals to burn down.

 

[RockyMtnGriz]

I'd say that the OPs description of how his Kuuma is operating lines up with how mine operates.

Pusitki, before you pull your hair out, consider that since I'm sure your winter has been a lot like mine, ie it just got here, your heating needs have increased drastically. I have gone from 1 load every 2 days to 3 loads a day, plus a supplemental stove during the -40 cold this week.

If the Kuuma is not keeping up, and the thermostat is drifting down, you need to turn the control knob up. Remember that the Kuuma pays no attention to the reading on the thermostat. It's going to go "c" to "1" to "c" at the same rate all else being equal according to the output knob setting regardless of whether the house temp is 40 or 80. Yes, there's the hi / low blower thing with the thermostat, but IMO that is irrelevant to heat output and I'd say most users are in that camp. Setting the Kuuma thermostat to a very high temperature will keep its blower on hi all the time (though you'll have to keep the control turned up or it will just cycle continuously, which is probably counterproductive), though that will cause more cycling at the end of the burn. It probably depends on the specifics of your house as to whether that makes a difference or not. I keep my thermostat set full low, for low fan speed, and only turn it up (to the highest temp) when running flat out, and then usually only for a while to move cool air from distant rooms faster. Yes, I'm just using the thermostat as a fancy round switch, but hey, it was installed when I bought the house, so it's the laziest option. You most likely just need to turn the control knob up and load it more often.

Turning the low limit switch back down should help and moving it up into the plenum should also help. Especially with big duct work, you want the fan on whenever the stove is warm enough to avoid a condensation problem. The cycling fan turns my big ducts into a version of in-floor heat, with most of the air being delivered to distant rooms being cold from sitting in the duct for a while. I replaced my snap switch with another fan control like the one provided for high limit. It does a much better job of starting and stopping the fan at the appropriate time. I have read of people using the same fan control for both with the proper wiring setup, but I kinda like the redundancy of the second switch (I actually added a third for further redundancy, and easy switching between settings appropriate for the winter vs. shoulder seasons, but that's another story).

There's a second, sort of "secret", range setting for the control behind the plastic plug on the face. Turning the screw CCW increases the temperature range, if you will, of the control knob settings. Turning it CCW makes low hotter and high hotter. I found that I didn't like the amount of deposits I was getting in the HX and pipe with the stock setting running the controller at lowest. After I did a cleaning and experimented with running with the knob set above minimum for a test, I adjusted the range setting up so that low now matched the setting that gave me a cleaner running stove. That had the side benefit of having a little more horsepower at max setting. If I was struggling to keep warm I'd turn it up some more, or maybe use separate shoulder and winter settings so as to keep the longest burn time in the shoulder seasons and have more horsepower on tap in the winter. That control screw seems delicate though, so I've so far avoided adjusting it again. If you don't find yourself using the lowest setting, or you're running a max a lot and still not getting by, it would make sense to turn the range screw up.

I marked the setting I like on the screw and made a note on the controller which way to turn the screw if I'm ever desperately freezing my butt off. Like marking the screws on the chainsaw carb!

My observation is that the low to high setting on the control knob represents a doubling of the burn rate, so there's going to be more of a limit as to how much wood you can run through the Kuuma compared to your old heater I'm sure. So at some point it becomes a matter of keeping it running as hot as possible for as much time as possible with the control maxed out if your house and weather is at the upper end of its capacity as mine is. With my wood, I have to turn it down quite a bit at night or face a middle of the night reload. I can avoid the reload by adding a second stove to the mix when it's really cold, but not quite severely cold. And, if it's severely cold, I don't mind the house temp dropping substantially. If a few degrees are a problem, one should really question trying to use 100% wood heat.

I seriously improved the seal between the front face cover and the stove and that provided a big increase in heat delivered to the house ducts vs. the stove room. If your stove room is a bunch warmer than the rest of the house, that would be a good place to start.

Hopefully something here helps. Give yourself a little time, you've just started learning in actual winter conditions.

 

[brenndatomu]

have gone from 1 load every 2 days to 3 loads a day, plus a supplemental stove during the -40 cold this week.

Yeah but you are much colder than over here, and have a larger home IIRC...he is heating about the same space as me (little less) and about the same climate, and my VF plays with this load, most of the year is a little big honestly...just right when it gets real cold.
I'd not start fooling around with the pot on the control board until everything else is optimized first...and from the OPs description, I can almost guarantee that it won't be needed for this install.

 

[Pustiki]

And, if it's severely cold, I don't mind the house temp dropping substantially. If a few degrees are a problem, one should really question trying to use 100% wood heat.

I seriously improved the seal between the front face cover and the stove and that provided a big increase in heat delivered to the house ducts vs. the stove room. If your stove room is a bunch warmer than the rest of the house, that would be a good place to start.

Thank you for your reply and tips. I’m not so much as pulling my hair out as I am a tinkerer. A few degree swing is fine. The Kuuma is performing far better than the old furnace and I am chewing through much less wood. We enjoy the feel of wood heat over the propane and collecting firewood is a good hobby. We got spoiled with how easy it was to maintain 72 degree interior temps before this cold snap. The old furnace would not have been able to do that, even before real winter arrived. Still, with the Kuuma, I was surprised that the cold snap took me from a steady 72F down to a steady 65F even with an additional loading. As brenn said, I’m just looking to eke out all the performance potential of the machine.

The seal between the face cover and the stove does leak a lot. I had it on and off to clean the HX the other day, but I will seal it with tape or something.

Yesterday’s performance went something like this: Full load before I left in the morning. Maintained a steady 65F on the first floor until around 5pm when I used the propane to bring it to 68F as the wife and kids got home. I did a couple smaller loads when I got home shortly after and moved the low-limit switch to the top of the plenum. The interior temps maintained at 68F the rest of the evening.
I did a full load at 11pm. At 6:30am, the first floor was 67F, computer on ”3,” and the blower was off. The blower kicked on before I opened the door to load up. There were a nice pile of red coals and the computer went back to ”1” within a couple minutes of loading.

Checked my thermometer at the return level in the basement. 62F. I would call the return ducting along the wall cold to the touch and the air coming into the back of the Kuuma is not what I would call warm. So, I’m convinced that the return air needs to be corrected. With an insulated chimney liner and a better return, I am hoping to achieve a steady 70F even through weather in the teens.

 

[brenndatomu]

With an insulated chimney liner and a better return, I am hoping to achieve a steady 70F even through weather in the teens.

I think you are on the right track.
Can you slide some insulation board between the return duct and the wall/floor?

 

[woodey]

One thing that I've found is that I do better with loading 3 smaller loads per day when it gets real cold out, rather than 2 larger loads..

👍

 

[Pustiki]

I think you are on the right track.
Can you slide some insulation board between the return duct and the wall/floor?

I am going to try that this weekend. The return is screwed into the wall, so I’ll have to adjust that to make a little room. Longer term, I’m just going to re-route the whole thing.

 

[RockyMtnGriz]

The seal between the face cover and the stove does leak a lot. I had it on and off to clean the HX the other day, but I will seal it with tape or something.

I studied the size of the gap along the sides of the cover, took the cover off and laid a bead of Ultra Black on the cover from the latching tabs to the top a little taller than the gap at each point (mine has a large gap at the top, and none at the very bottom). I covered that with strips of wax paper, and carefully replaced the cover. Let it cure for a day, took it off, and removed as much of the wax paper as I could. The remaining wax paper stuck to the silicone turns to dust pretty quickly in operation. You could substitute something else for the wax paper, but I find wax paper doesn't stink like other stuff when it gets hot.

Nice thing about this approach is you now have a permanent custom fit gasket that except for the house being warmer, you won't know it's there.

 

[brenndatomu]

I studied the size of the gap along the sides of the cover, took the cover off and laid a bead of Ultra Black on the cover from the latching tabs to the top a little taller than the gap at each point (mine has a large gap at the top, and none at the very bottom). I covered that with strips of wax paper, and carefully replaced the cover. Let it cure for a day, took it off, and removed as much of the wax paper as I could. The remaining wax paper stuck to the silicone turns to dust pretty quickly in operation. You could substitute something else for the wax paper, but I find wax paper doesn't stink like other stuff when it gets hot.

Nice thing about this approach is you now have a permanent custom fit gasket that except for the house being warmer, you won't know it's there.

I did something similar on my door gaskets...ultra high temp silicone (like 750*) smeared it on the gasket, and a strip of thin foil over it, lightly latch the door for 24 hours...then peeled the foil off, that would come off, bam, custom fit gasket! Next time (if there is one) I might try wax paper instead of foil...I was torn and just decided to use the foil.
I learned that some of the gasser boiler makers do this right from the factory, where I got the idea.

 

[Micdrew]

feeling lazy didn’t read through all the other comments but I’m sure someone mentioned about the return air being low to the floor, I’m pulling my return air from the basement too, I installed a manual damper this year on the plenum and saw a drastic increase in supply temps, when the furnace is rolling for a few days like it has been for the last week my return air is 80 degrees. I will say the kuuma has no problem heating my house when it’s 25 out but when it gets to 20 and below i have to work it hard, I’ve been burning 130lbs a day, computer on medium keeping the house at 72. pretty sure the builder used cardboard for insulation.

 

[RockyMtnGriz]

I did something similar on my door gaskets...ultra high temp silicone (like 750*) smeared it on the gasket, and a strip of thin foil over it, lightly latch the door for 24 hours...then peeled the foil off, that would come off, bam, custom fit gasket! Next time (if there is one) I might try wax paper instead of foil...I was torn and just decided to use the foil.
I learned that some of the gasser boiler makers do this right from the factory, where I got the idea.

At the risk of hijacking this into a gasket thread, I have a cat stove that likes to smell. The door lip is only about 3/16 wide where it creates a dent into the door rope seal. After replacing the gasket a couple of times and not stopping the creosote smell, I put a bead of silicone in the crease in the gasket, wax paper and lightly latched the door. Much better seal and it looks like the silicone impregnated door gasket will last about forever. I siliconed the window gasket too with good results. And the leaks in the stove pipe... I don't find the need to seek out the ultra high temp stuff, Ultra Black holds up, looks good, and mostly I like the way it flows and seals.