New Heat Commander Install - Looking for Tips and Troubleshooting

[thewhaleshark]

Hey all! Been lurking on these forums for a while as I've been researching upgrades to my heating and HVAC. Currently have an LP primary furnace (Trane XR95), and previously had a Hot Blast furnace installed as an add-on.

House stats: I have a ~1700 square foot single-story ranch (built circa 1959) in New York's Capital Region (Glenville, if anyone knows it), and all the furnaces are located in the basement (~1400 square feet).

After a lot of reading and research, and a nicely-timed 26% federal tax credit, I bought a Heat Commander and traded the Hot Blast to an HVAC guy for the cost of installing the new furnace. Pretty good deal, I think. I have it wired up in the interlock configuration depicted in the manual for a parallel add-on.

I'm having a couple of issues with it, and I think I know the solutions, but wanted to make sure I've got my understanding correct. I'm consulting with the HVAC guy, but figured there's a lot of expertise here, so more opinions is more gooder.

1) I've read that, ideally, you want backdraft dampers between the furnaces and their entry point into the main trunk of the ducting. We didn't do that on the install so I'm going to have to refit it, but I wanted to double-check what exact kind of damper I'd be looking for. Seems like these round gravity dampers are the most common:

Dampers

Greenheck Fan is the worldwide leader in manufacturing and distributing air movement, air conditioning and air control equipment.

www.greenheck.com

But I was wondering if a rectangular type configuration would also be workable? I have rectangular ducts, so I think it'd be easier to fit:

Dampers

Greenheck Fan is the worldwide leader in manufacturing and distributing air movement, air conditioning and air control equipment.

www.greenheck.com

It's aimed at roof-mounted exhaust fans so my assumption is "no," but it also seems like it might work?

2) As you might imagine, because there are no backdraft dampers, the systems push air through each other - when either blower is running, it pushes air backwards through the other furnace. This is probably not great? In particular, when the LP blower is running, it stops the wood furnace from building meaningful heat in the plenum, and puts it into the basement.

The obvious answer here is "backdraft dampers for the furnaces," but I've been reading about folks who have backdraft dampers in their return air ducts as well. Can anyone speak to that? Necessary? What exactly are they doing?

3) Currently, I don't have it ducted to the cold air return either, my assumption being that my basement air volume is more than sufficient. However, seems like I'm having trouble pushing heat from the furnace to the far corners of the house, so it's pretty clear that the air volume in the basement is insufficient. I'm also assuming that a return duct will do a substantially better job of maintaining the correct static pressure in the system. Any particular tips I should know about running a new cold air return? Size it to be at least as large as the hot air outlet?

4) I think, but I'm not 100% sure, that I might not be getting enough combustion air. I didn't attach the additional combustion air supply adapter, once again assuming the basement air supply was more than enough, but I'm wondering if the combination of no air return and no combustion air is just leaving the thing starved for air. That would track with performance, because when the fire got low last night, I wound up getting acrid/smokey air in the house, like a fire that was being strangled.

Has anyone attached the additional combustion air? Any advice on that?

Thanks in advance!

 

[laynes69]

I added a outdoor air intake on our Caddy Advanced (same firebox) and removed it. Not having your furnace tied into the cold air return will take in cooler air and not utilize the system. Without backdraft dampers, you will build zero pressure in the ducting and little flow.

As for the old unit, it probably burnt much hotter, but shorter. What size is your chimney and liner? Also are you burning seasoned wood? There's a significant difference between the old and new.

Not tying into the cold air return can cause a negative pressure in the basement affecting burn and heat output.

 

[thewhaleshark]

I added a outdoor air intake on our Caddy Advanced (same firebox) and removed it. Not having your furnace tied into the cold air return will take in cooler air and not utilize the system. Without backdraft dampers, you will build zero pressure in the ducting and little flow.

As for the old unit, it probably burnt much hotter, but shorter. What size is your chimney and liner? Also are you burning seasoned wood? There's a significant difference between the old and new.

Not tying into the cold air return can cause a negative pressure in the basement affecting burn and heat output.

Yup to seasoned wood, made that mistake on my first burn by using some wood that the Hot Blast used to just eat. HC didn't like that one bit. Using much drier stuff now that's reading ~15% on a fresh split, plus some kiln-dried stuff and some bioblock-type solid fuel bricks. It's much happier with that burn.

The Hot Blast definitely burned hotter, and also ate wood like nobody's business. The HC is definitely slow and steady in comparison, but does a good job.

I'm not sure what diameter my chimney pipe is off the top of my head. I believe it's 6", but it might actually be 7". It's a masonry chimney with a clay liner and a new stainless pipe - just had it rebuilt last year when I had roof work done. I should check on that, because I know if the diameter is too large that causes issues.

"Little flow" does actually sound familiar, and might be a problem I've been having in this house. The Hot Blast didn't have backdraft dampers either, and so had the same issue of LP air getting pushed out of the unit when the blower was going.

 

[brenndatomu]

I don't care those spring loaded butterfly dampers for wood furnaces...if the power goes out right after you load 'er up and go to work...you really need to be able to gravity flow some heat away from the furnace for a while until the fire dies down some (which may not happen for a while if you have good dry wood and the fire has progressed to the "cruise" part of the burn) and the spring loaded dampers stop gravity flow...honestly even the gravity dampers will stop gravity flow...they are fine for the LP furnace though...and all you have to do is have the HVAC dude make up a rectangular gravity flapper for your square/rectangular ducts off the LP...but for the wood furnace side, I prefer a spring open/power closed damper...that way when the power is on and the LP needs to kick on, you aren't back feeding the HC...but when the HC is running it will be open...even if the power goes out.
Hooking up the return air and fixing these back feed issues will very likely help straighten things out a lot

 

[thewhaleshark]

Looks like the cold air return has solidly solved the acrid gas and static pressure issue. Still need to do dampers but we ran out of time today. Baby steps.

The tricky part now is that we're getting smokey air coming through the vents, so either the blower is sucking in smokey air from the basement (plausible since you do have to have the door open a bit to fire it up and to reload it), or there's a defect that's allowing smoke to escape the combustion chamber into the plenum space. Right? Is there another possibility I'm overlooking here?

My ducting is certainly leaky in many places, as is my garage, and so it seems entirely plausible to me that smoke is coming in either from the outside (it's cold enough that smoke can fall to the space in front of my garage and enter the basement if I give it a path), or getting sucked in from the basement when I open up the door. I think I can solve that with basement ventilation, we'll see.

One other question: I think my interlock is not wired correctly. If I understand correctly, it's supposed to shut off the main furnace when the HC blower kicks on, right? Operating it today, both furnace blowers were operating simultaneously at a couple of points. I have it attached to the LP furnace control board, but I might try running it directly to the thermostat as per option 2 in the interlock section, just to cut out the possibility that the LP furnace board is being weird.

I've also noticed the damper making a strange whirring noise when the firebox temperature is low, and it appears that the automated damper arm is actuating in a kind of stuttery manner when that happens. Thoughts?