Wiring for Kuuma and LP furnace sharing ductwork

[motoguy]

So, after the first week of Kuuma usage, I'm very impressed. Easy to use, does a good job heating our ~4k sq ft home (probably tied to good insulation in the home as well), low flue temps, long burn times, just overall very satisfied. I'm still concerned to see how it does when the temps drop down near 0, but time will tell. It was mid 40's yesterday, high 20's last night. 71 inside when I went to bed, 70 when I woke up, and the knob on the Kuuma was in the "medium heat" position. I have not hooked up the Kuuma remote thermostat.

So, now I'm looking at how to integrate the Kuuma and the LP, so that the LP can serve as an emergency backup. The Kuuma is going to be our primary heat source, but if we're gone/away from the house/whatever, I'd like the LP to kick on when the temp gets below a certain low-limit (say, 60 or so when we're home...probably 55 if we're gone for an extended time).

The LP has a backflow preventer in the plenum, between the LP furnace and the main trunk. The Kuuma has a long "connector trunk", between the Kuuma and the main duct trunk. There is a backflow preventer in this connector line, as well. The LP has return air tied to the main house return air ducting. The Kuuma has no return air setup; it just pulls from the filtered area at the motor housing, and we leave our main stairwell door open for return air from upstairs.

Really, I think my question is a wiring issue. Specifically, probably just a switch/relay issue. How do I ensure that only 1 blower can be energized at the same time? If I can do that, I think everything else will be fine, given the backflow dampers and their installed areas.

Thanks!

 

[laynes69]

If you have backdraft dampets installed and the return air not tied together, there's no need for a relay. Just set your backup furnace lower and keep the woodfurnace where you want it. If the woodfurnace goes cold and the house drops, the central furnace will take over. If they were side by side, and sharing everything, an interlock would prevent the central furnace from operating when the woodfurnace was in operation .

 

[JRHAWK9]

This thread may/maynot help you. https://www.hearth.com/talk/threads/electrical-gurus-please-come-in.140944/

My Kuuma shares the supply ducts with my LP furnace. Just like in your situation, it does not share return ducts with my LP furnace. I use an open staircase in the middle of the house for return air. I have a gravity damper in the plenum of my LP furnace to protect back flow though the LP furnace. I also have a spring open/powered closed 120V electric damper in the Kuuma supply duct between the Kuuma plenum and main house supply duct. I use the electric air cleaner circuit on my LP furnace for the feed, as it's powered whenever the LP blower is on. I have it wired in such a way the powered damper will remain open in all situations except for when the LP blower is the only one running (in summer and winter), then it will close to prevent back feeding the Kuuma. In all other situations it remains open. I needed it to remain open with no power in case of a power outage. This allows me to run both the LP blower and Kuuma blower at the same time if I ever need to...or at the end of a burn cycle on a very cold morning when the Kuuma blower is cycling and the LP thermostat calls for heat. Having the ability to do this allows me to keep the LP thermostat set at whatever temp I want (we keep it at 68°) and not have to worry as both furnaces can run in harmony with each other.

 

[motoguy]

My concern is the temp dropping, but Kuuma still putting out heat, and the blower is still running. Kuuma trying, buy unable, to raise house temp, and the LP blower kicking on while Kuuma blower is still functioning.

The LP blower is probably stronger than the Kuuma, so Kuuma BD should stay closed. Then you have all the heat stuck in the closed ductwork (same concerns as powet off failure), in addition to the waste/wear of one damper fighting the other.

Or am I overthinking things?

 

[JRHAWK9]

I know what you are saying and it is a legitimate concern, IMO. This is why I went with a powered damper in between the Kuuma and main supply duct and have it wired like shown in the link above. Doing so allows the blowers from both furnaces to run at the same time if need be.

 

[motoguy]

This thread may/maynot help you. https://www.hearth.com/talk/threads/electrical-gurus-please-come-in.140944/

My Kuuma shares the supply ducts with my LP furnace. Just like in your situation, it does not share return ducts with my LP furnace. I use an open staircase in the middle of the house for return air. I have a gravity damper in the plenum of my LP furnace to protect back flow though the LP furnace. I also have a spring open/powered closed 120V electric damper in the Kuuma supply duct between the Kuuma plenum and main house supply duct. I use the electric air cleaner circuit on my LP furnace for the feed, as it's powered whenever the LP blower is on. I have it wired in such a way the powered damper will remain open in all situations except for when the LP blower is the only one running (in summer and winter), then it will close to prevent back feeding the Kuuma. In all other situations it remains open. I needed it to remain open with no power in case of a power outage. This allows me to run both the LP blower and Kuuma blower at the same time if I ever need to...or at the end of a burn cycle on a very cold morning when the Kuuma blower is cycling and the LP thermostat calls for heat. Having the ability to do this allows me to keep the LP thermostat set at whatever temp I want (we keep it at 68°) and not have to worry as both furnaces can run in harmony with each other.

OK, this is exactly what I'm looking for. The LP BD is gravity close. The Kuuma is spring/gravity closed, with a setscrew to hop it open in power outage. Sounds like what I need is a spring/normally open, electric closed setup, like you have.

 

[JRHAWK9]

OK, this is exactly what I'm looking for. The LP BD is gravity close. The Kuuma is spring/gravity closed, with a setscrew to hop it open in power outage. Sounds like what I need is a spring/normally open, electric closed setup, like you have.

I would tend to agree. I love how I have ours setup. No having to sacrifice house temp to keep you from worrying about backfeeding one or the other. Just set your LP thermostat where you want the house kept and forget about it. The two furnaces will seamlessly work together whenever needed to keep the house at that set temp.

 

[motoguy]

I'll do a google search when I'm inside, but do you have a link suggestiom where to look for such a damper?

 

[JRHAWK9]

I got mine from Retrozone. Give them a call. I talked to the owner when I bought mine and he set me up with exactly what I needed. I opted for the better Belimo motor.

https://www.retrozone.com/

 

[brenndatomu]

My first thought here is that it would be a very unlikely situation that the LP would kick on while the Kuuma blower is on "high" (especially since you do not have the tstat hooked up)
And just one additional thought here, what about a heat dump mounted in the Kuuma plenum...it can never overheat that way, no matter what the circumstance.

 

[STIHLY DAN]

My first thought here is that it would be a very unlikely situation that the LP would kick on while the Kuuma blower is on "high" (especially since you do not have the tstat hooked up)
And just one additional thought here, what about a heat dump mounted in the Kuuma plenum...it can never overheat that way, no matter what the circumstance.

No T stat means no high speed blower on the Kuuma, unless it hits a high limit situation. Also a heat dump will not allow you to heat your house with a power outage. Lose it all to the basement.

 

[STIHLY DAN]

The Kuuma is spring/gravity closed, with a setscrew to hop it open in power outage.

How does this work? Can't picture it in my head.

Sounds like what I need is a spring/normally open, electric closed setup, like you have.

This is the only way to do it in my eyes.

 

[motoguy]

How does this work? Can't picture it in my head. [/quote]

Horizontal ducting, with a spring-tensioned vertical flap inside the ductwork. Gravity alone should pull it closed, but there's some spring tension to pull it down, as well. The air pressure from the Kuuma blower overcomes the spring tension, and opens it up. I have to manually lock it open in a power outage situation.

This is the only way to do it in my eyes.

That sounds like EXACTLY what I'm looking for.

And...btw...does this mean the blower function I've been getting is the low speed, 250cfm only? I've not yet been introduced to the high speed (1500cfm) blower? T-stat is not hooked up, just using the dial on the side of the Kuuma (low-med-high) to control temps.

 

[JRHAWK9]

And...btw...does this mean the blower function I've been getting is the low speed, 250cfm only? I've not yet been introduced to the high speed (1500cfm) blower? T-stat is not hooked up, just using the dial on the side of the Kuuma (low-med-high) to control temps.

yes, you need the thermostat hooked up and calling for heat for it to kick the blower on high...........or if the high limit switch reaches it's threshold then it will also turn the high speed on.

 

[motoguy]

I see retrozone offers dampers in 24v and 120v. Is there a reason, given my situation, I'd choose one over thevother?

 

[sloeffle]

I got mine from Retrozone. Give them a call. I talked to the owner when I bought mine and he set me up with exactly what I needed. I opted for the better Belimo motor.

https://www.retrozone.com/

I couldn't agree more. I bought mine with the Honeywell motors and they lasted only a few years. The Belimo motors are much better.

 

[JRHAWK9]

I see retrozone offers dampers in 24v and 120v. Is there a reason, given my situation, I'd choose one over thevother?

All depends on how you are wiring it. I chose 120V because my EAC circuit on my furnace was 120V and I was using this as the control.

 

[motoguy]

Fully loaded, heat knob on "high", with high speed blower hard wired "on", it was all the Kuuma could do to hold house temps today from between 67 and 65. Temps were in the mid teens outside.

I let the Kuuma burn low this evening, in order to make sure I could give it a full load at bedtime. Upon making it home tonight (about 9:30 pm), house temp was down to 61. 21 outside right now. Kuuma temps were VERY low. Dark coals, no low speed blower, lots of ash.

I turned the LP furnace on, so it could get the house to temp while the Kuuma spooled up. I think below 20, the LP may have to help the Kuuma, or vice versa. Still much better than the $$$ of all LP heat, or the hot/cold of LP cycling. I think the Kuuma can handle 90% of the work, if the LP can help when needed...I'm going to let them run together for now.

I noticed a bit of backfeed through the Kuuma when the LP was blowing, and the Kuuma was not. Not a ton, but enough to push the filters away from the port on the Kuuma, and pin them against the fresh air side. A noticeable amount of heat, too. This means that the bd on the Kuuma side isn't sealing perfectly, but i suspect that is normal. Is a bit of backfeed ok, or something to be addressed?

It's like having heated leafblowers in the registers when both blowers are running!

 

[STIHLY DAN]

Do you have an insulated basement? what temp is your basement normally.

 

[motoguy]

Do you have an insulated basement? what temp is your basement normally.

It's a walk-out basement, partially finished. Concrete on 3 sides (insulated at top, between slab and floor joists on exterior walls), 4th side is insulated. Quite a bit of window area along the non-concrete wall, though. Not sure about the normal temp down here, a few degrees above the upstairs temp, due to the Kuuma radiant heat. I'm showing it's 64 at upstairs t-stat, 72 down here right now.

 

[JRHAWK9]

yikes. non-insulated basement walls. Concrete is not a good insulator...lol Once the frost sets in it may get even worse for you, as the walls will be subjected to even cooler temps.

Do you have 10' ceilings? 4,000SF is a lot of area, now take 10' ceilings in account, you are talking a buttload of volume. Does this 4,000SF include your basement?

Suggestion, seeing your basement is warmer than upstairs. I believe you are taking your return air near the bottom of the cold concrete basement floor. I was originally doing the same thing, makes for some very cool return air temps which don't seem to warm up much. For starters, I would build a cold air plenum to within 6"-12" or so of the bottom of your floor joists in order to take the warmer air off the ceiling of the basement. I did this and it increased my return air temps, which also increased my supply temps. I then took it one step further and built a duct system which sucks some of the radiant heat off the face of the Kuuma and introduces it into my cold air plenum right before it enters the blower box. This increased my return/supply air even further. I went from having about 65° return air temps when at the basement floor to around 85°+....with no real loss of basement heating ability. Re-heating 85°+ air makes from some decent supply temps. Basement still stays roughly the same temp as the rest of the house, without having any open registers down stairs. Just thought I'd share what I found out worked great for me.

 

[brenndatomu]

I went from having about 65° return air temps when at the basement floor to around 85°+....with no real loss of basement heating ability. Re-heating 85°+ air makes from some decent supply temps.

Forced hot air heating is kinda funny sometimes, huh?! Things that you wouldn't think amount to squat makes a big difference

 

[motoguy]

Do you have 10' ceilings? 4,000SF is a lot of area, now take 10' ceilings in account, you are talking a buttload of volume. Does this 4,000SF include your basement?

The 4k area does include both floors. ~2k up, ~2k down. 10' ceiling downstairs (unfinished basement, no actual 'ceiling', just exposed laminate beams for upstairs floor joists). Living room and one bedroom upstairs have high, vaulted ceilings. The other ceilings are 8'. It's currently 20 outside, and the Kuuma alone seems to be holding temps at 64. I set the LP tstat for 64 last night, and it was cycling on and off throughout the night. I turned the LP tstat down to 63 about an hour ago, and house temps are still holding at 64. It appears that the low-mid 20's are the breakover point for the Kuuma being able to handle it all on it's own.

FWIW, last night is the shortest burn time I've had with the Kuuma. I'm now accustomed to going 10-12 hours before reloading, but that has been in low-mid 30's weather. I loaded the Kuuma last night around 11pm. Full load, temp to high, outside temp between 9 and 14 degrees. Woke up this morning around 5:30, and the Kuuma blower was not running. Came downstairs to check. Still had a large volume of coals (would normally have let them burn down), but not enough heat to trigger the low speed blower. I loaded it back up at that time.

Suggestion, seeing your basement is warmer than upstairs. I believe you are taking your return air near the bottom of the cold concrete basement floor. I was originally doing the same thing, makes for some very cool return air temps which don't seem to warm up much. For starters, I would build a cold air plenum to within 6"-12" or so of the bottom of your floor joists in order to take the warmer air off the ceiling of the basement. I did this and it increased my return air temps, which also increased my supply temps. I then took it one step further and built a duct system which sucks some of the radiant heat off the face of the Kuuma and introduces it into my cold air plenum right before it enters the blower box. This increased my return/supply air even further. I went from having about 65° return air temps when at the basement floor to around 85°+....with no real loss of basement heating ability. Re-heating 85°+ air makes from some decent supply temps. Basement still stays roughly the same temp as the rest of the house, without having any open registers down stairs. Just thought I'd share what I found out worked great for me.

That's a good idea. I'll give that a try. I've got the Kuuma elevated ~1 ft off the floor on concrete blocks. So it's not picking air directly off the concrete, but pretty close. It appears that I'm on the very edge of the Kuuma's capabilities. Just a little bit of help, and it might be able to take care of the house on it's own. While not actually necessary, it would be nice.

The basement is plenty warm, with all registers closed.

 

[JRHAWK9]

How can you guys down south be colder than us northern guys?! Just not fair Although next week they are talking highs in the single digits and lows below zero. I PM'd you a couple pics.


oh, another thing. If your low limit switch is still mounted on the side of the jacket housing, I would move it, per Daryl, to somewhere up in your supply bonnet for more accurate and consistent on/off temps. Daryl said he had to put it where it was because they did not include a plenum and it had to to be mounted in order to be UL listed. If you do move it, then you also should increase the temp to at least 110°, but you can play around with that to see what works best for you. I actually have mine set at 125°, but will probably be soon moving it down to 120° when it gets real cold out in order to keep the blower on longer at the end of the burn cycle. Having warmer return temps allows me to keep the furnace air jacket nice and warm while still keeping the blower running. My plenum temps, according to my HVAC thermometer probe, go from 160° (when the blower kicks on) to 120° (when the blower kicks off).

 

[laynes69]

I don't have a Kuuma, but operating on high should put out a good deal of heat. A modern home should be much easier to heat than an old farm home. It sounds like a few things will move you into a better direction. Moto, have you had a blower door test done on your home? The only reason why I ask is our neighbor had a new 3200 sqft home built, just to find it leaker than our 150 year old victorian. My inlaws have over 2 feet of insulation in their 8 year old home's attic, and suffer snow melting from the lack of airsealing.