Heat Commander in Series as Add-on Furnace?

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sleewok

New Member
Jan 27, 2021
19
USA
The manual for the heat commander states that it is not authorized for usage in series with an existing furnace. From my understanding that is how you would typically run an add-on Wood furnace, correct (hot air going to the return of the existing furnace)?

Does this mean it is unsafe to run in series, or just that it doesn't get the efficiency rating in that configuration?

If it is not safe, what are my other options? Any other stoves that can run as an add-on? Kuuma is way out of my price range...

Thanks!
 

andym

Feeling the Heat
Feb 6, 2020
434
Hicksville, Ohio
Having the wood furnace push hot air through your existing furnace will overheat the electrical components inside. (Circuit boards, sensors, etc) this is the biggest reason as I understand.
You could turn it around and have the existing furnace push through the wood furnace. That's how many older wood furnaces were designed. The main drawback here is that you can't run both furnaces simultaneously. Your wood furnace would run too hot and close the combustion air supply.
The proper way (and best) is to do a parallel install. Both furnaces are connected to the same supply plenum, with a dackdraft damper by each furnace. The cold air return can be shared also but it is often easier to install a dedicated return for the wood furnace.
The manual should state the minimum square inches of main duct required. For the Heatmax 2 i have it required at least 170 square inches. Return side should be larger. In addition you want to connect it in a way that allows heat to rise naturally as much as possible.
I would not hesitate to purchase a Heat Commander. It appears to offer nearly the performance of a Kuuma at an entry level price point. Wish there was a way to convert my furnace!
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,400
NE Ohio
do a parallel install.
This ^ ^ ^...or install separate ducts, which may or may not be possible depending on your house...keep in mind you don't neccasarily need ducts to every room like most fossil fueled furnaces do, just depends on your homes layout, and what you are expecting the wood furnace to do...every room 72*? Or the common living areas 72* and the bedrooms/etc cooler...for example.
 
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laynes69

Minister of Fire
Oct 2, 2006
2,599
Ashland OH
I agree. The heat commander is not meant to be installed as a series install. The only way would be to remove the blower and install the plenum of the central furnace into the intake of the woodfurnace. The HC is not authorized for this type of install. The HC has a decent blower and plenum, for a parallel install there may be an interlock to stop the gas from firing when the woodfurnace is in use, but not sure. The Caddy had an interlock, maybe the HC does also. To share a cold air is the option.
 

sleewok

New Member
Jan 27, 2021
19
USA
I ended up calling Drolet to get more info. The customer service was extremely helpful and knowledgeable. If I want to run into the existing ducting then I would definitely need to install backdraft dampers and the primary system could not run at the same time. This would mess with the static pressure too much as the systems would be fighting against each other.

I really like the idea of doing a parallel setup by running supply duct to the areas that I want heat.. That way I don't have to worry too much about having a proper return setup on the HC as I can run the fan on my main unit.

To start I plan to run supply to each upstairs bedroom (4 total), the kitchen, living room, family room and basement common area, bedroom, and office. I have a couple of concerns and things that I need to figure out with the system. First, I have a large ranch style home. Some of the areas that need supply are quite far (30-40') and the maximum run for a main duct is 35'. Rigid duct is expensive and I don't think we can afford to run rigid to all the locations. I'm wondering how effective semi-rigid flexible would be? If I would have too much restriction for the main ducts, perhaps I could use rigid for some of the secondary ducts (branches off main). The maximum length of all the main ducts is 180’ and I believe I would still be way under that and perhaps that would allow for flexible ducting...

Hopefully there is a way to test the system blower without a fire actually burning. I would like to be able to check CFM and static pressure for different ducting configurations to see what will work...
 

sleewok

New Member
Jan 27, 2021
19
USA
Follow-up question for parallel install. Does it matter where I connect to the existing supply? There are two branches near the HC that I could connect to, and then I could run another couple supplies 25' to the main of the existing supply. Will that work?

Considering that the min area of supplies is 175" (I think) I'm trying to figure out how this should be done without running six supplies into the main supply.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,400
NE Ohio
parallel install. Does it matter where I connect to the existing supply? There are two branches near the HC that I could connect to, and then I could run another couple supplies 25' to the main of the existing supply. Will that work?
Have some pics of your system...or at least a drawing?
 

sleewok

New Member
Jan 27, 2021
19
USA
Here is a rough sketch of the layout for the existing system.

Currently we have about 2 registers in each room to supply air. The largest two rooms are roughly 350sqft. The rest are < 200sqft. The branches on the sides closest to the HC are 9" rectangular ducts. I believe that means I could potentially run two 6" supplies from the HC to each branch? If I run the other two into the main then that would point directly into the other branches hopefully pushing more air to that side of the system... so many variables.
IMG_20211014_131426.jpg
 
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brenndatomu

Minister of Fire
Aug 21, 2013
6,400
NE Ohio
So what are you thinking...tie into that main duct that is 8' away from the HC?
What size is it?
 

sleewok

New Member
Jan 27, 2021
19
USA
So what are you thinking...tie into that main duct that is 8' away from the HC?
What size is it?
Both of the branching ducts are 9" rectangular. I was considering running two 6" supplies to each branch (the 8' and 15' one. I don't think they are large enough to handle three and I'm thinking I need to run at least two up to the main. Thoughts?

One tag on question. Will it make any difference if I have a single backdraft damper on the HC supply plenum vs one on each 6" duct? My hope that once the system pressurizes it wouldn't make any difference once the supplies from the HC are filled with air?

I appreciate your time and thoughts!
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,400
NE Ohio

trx250r87

Burning Hunk
Nov 30, 2012
169
NE Wisconsin
I did three 8" take-offs from the HC into my furnace plenum. Wherever you tie into will likely be the warmest room, keep that location in mind.

Eric
 
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JRHAWK9

Minister of Fire
Jan 8, 2014
1,685
Wisconsin Dells, WI
I, personally, would try to tie into the main central supply, otherwise you will probably have uneven heating and poor results. Ducts are not sized to be running air backwards through them with the input at the smallest duct.
 

sleewok

New Member
Jan 27, 2021
19
USA
I did three 8" take-offs from the HC into my furnace plenum. Wherever you tie into will likely be the warmest room, keep that location in mind.

Eric
Where did you end up putting your backdraft damper for the HC? Did you have one for each take-off, or one large one at the plenum? Does it matter?
 

trx250r87

Burning Hunk
Nov 30, 2012
169
NE Wisconsin
Where did you end up putting your backdraft damper for the HC? Did you have one for each take-off, or one large one at the plenum? Does it matter?
I put one in each 8" take-off. I believe they are around $30-$35 a piece at Menards.

Eric
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,400
NE Ohio
Both of the branching ducts are 9" rectangular.
Sorry, 9"x9"
9x9 is square... ::-)
Yeah if its that small, I gotta agree with JR, run a 180-200 sq in duct over to the main furnace plenum...I assume the HC needs to be where its at due to a chimney connection?
 

sleewok

New Member
Jan 27, 2021
19
USA
9x9 is square... ::-)
Yeah if its that small, I gotta agree with JR, run a 180-200 sq in duct over to the main furnace plenum...I assume the HC needs to be where its at due to a chimney connection?
This is what I am going to do. I'll have my local HVAC guy get me the duct that I need.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,400
NE Ohio
This is what I am going to do. I'll have my local HVAC guy get me the duct that I need.
If you have a sheet metal shop nearby, they can make it too...a lot of the HVAC guys do that...getting to be less and less that make their own ducts/fittings...
 

KC Matt

Burning Hunk
Oct 29, 2016
151
Kansas City
Manufacturers and safety commissions have to accommodate the least talented among us so their advice assumes we are all morons. Yes, you can add a wood furnace in your primary ductwork. I've been running a Tundra1 (sn 1100ish) for years in this way and it works great because it's installed correctly. Question is, can you do it correctly?

You need powered dampers on the return side on your gas furnace and on the supply side on the wood burner and you need to interrupt the "W"wire to your gas furnace when the wood burner is calling for the blower. I did this with a series of RIB relays and Johnson Controls digital temperature controllers because I don't know how to build a circuit board. You can look at my post history I'm sure I posted pics of the control board. Yeah it's not fancy like the new models but it's reliable and serviceable. SBI could fold tomorrow and it wont affect me one bit. Seeing the feedback for this newest SBI furnace has me loving my setup more by the minute. I wouldn't trade my T1 for the new one.
 

trx250r87

Burning Hunk
Nov 30, 2012
169
NE Wisconsin
I had an early, original Tundra and I'm not sure I would agree. The T1 served me well but the Heat Commander delivers more heat compared to the Tundra and I don't have to babysit the Heat Commander like I did the Tundra.
The HC also has terminals to wire an interlock for a gas furnace.

Eric
 
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KC Matt

Burning Hunk
Oct 29, 2016
151
Kansas City
I had an early, original Tundra and I'm not sure I would agree. The T1 served me well but the Heat Commander delivers more heat compared to the Tundra and I don't have to babysit the Heat Commander like I did the Tundra.
The HC also has terminals to wire an interlock for a gas furnace.

Eric
I have followed your posts for years and enjoy your insight and wisdom. No doubt you have earned your stripes through this Drolet adventure. To be sure when they hit the market their product was undeveloped and unprepared to be released to the public.

Mine has never cracked and is on the original fire brick. I never installed the updated fire brick for the face plate. No doubt the fire box on the T1 is small for my application. Since install and sorting through the modifications I made, I've had exactly zero maintenance or repairs. As far as babysitting, I load mine and walk away. Takes maybe a minute. I have an "interlock" also. That's the part where a relay interrupts the "w" wire on the furnace. It's actually quite easy to do.

And when one of my relays or control boxes goes bad 10 years from now I will just replace it for 15 or 80 dollars, less if I wanted cheaper parts. What will the owners of the new units do when their circuit boards die and they can't buy another?
 
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sleewok

New Member
Jan 27, 2021
19
USA
Manufacturers and safety commissions have to accommodate the least talented among us so their advice assumes we are all morons. Yes, you can add a wood furnace in your primary ductwork. I've been running a Tundra1 (sn 1100ish) for years in this way and it works great because it's installed correctly. Question is, can you do it correctly?

You need powered dampers on the return side on your gas furnace and on the supply side on the wood burner and you need to interrupt the "W"wire to your gas furnace when the wood burner is calling for the blower. I did this with a series of RIB relays and Johnson Controls digital temperature controllers because I don't know how to build a circuit board. You can look at my post history I'm sure I posted pics of the control board. Yeah it's not fancy like the new models but it's reliable and serviceable. SBI could fold tomorrow and it wont affect me one bit. Seeing the feedback for this newest SBI furnace has me loving my setup more by the minute. I wouldn't trade my T1 for the new one.

Absolutely agree that you need dampers on the return side. They don't need to be powered though. I found some Dayton vertical install dampers that work great and have a spring to adjust the tension. They open with minimal air flow from the air handler on the furnace, and close when the HC kicks on. The interlock makes sure that both blowers don't turn on at the same time. Thankfully this makes for a simplified install. Although I really like the configuration you have. If I had to go that route I would probably get an arduino board setup with relays to control everything. I have done a couple projects like that. I have an arduino that controls a solid state relay for my beer brewing electric setup :D.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,400
NE Ohio
Absolutely agree that you need dampers on the return side. They don't need to be powered though
This works for some...but the way some systems are laid out it can't be done (correctly) without going to powered dampers...mine was one of them.
The other thing people need to keep in mind is keeping air flow available to the wood furnace when the power goes out...especially if it happens right after loading a big load and the fire is well established, to the point of being "on cruise control"...the furnace is still gonna get hot, but without some gravity air flow things can get extremely hot, including the ductwork...that's why the CTC specs on ductwork.
Anyways, gravity dampers can block gravity air flow...again, depends on your system though.
 
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