Wood Furnace ducted into with Heat Pump and unsure of dampers

[DerrickH]

Hello all. I'm going into the second winter in our home with a wood furnace and heat pump combo sharing the duct system. I spent my first winter coming to better understand our solid, but nearly antique King wood furnace from U.S. Stoves / Martin Industries. This time, I'm trying to ensure that the way the two furnaces are incorporated into the system is safe and up to modern standards.

As you might all imagine, the original owner/builder who passed many years ago, didn't leave behind any instructions for the house, so I'm left with making reasonable deductions as I go.

Here's my main question. I believe there is a reverse flow gravity style damper above the wood furnace helping to prevent air from the heat pump blower from continuing into the wood furnace. I can't visually see evidence of it, but I can hear slight metal squeak/swing sound when the wood furnace blower kicks on, and a very light thud when it turns back off. So, to me that's a strong audible indicator of a damper on that end. However, I have no indication whatsoever of a damper near the heat pump protecting it from the wood furnace.

I would add just two more pieces of information:

1. Just a few months after moving in last year, we had to have the whole heat pump system replaced due to the familiar story of a condenser going/gone bad and the repair bill would have been a significant percentage of getting a system with a warranty. So... I really want to ensure the existing duct set up is protecting my brand new investment. (And no, I didn't think to ask this damper question to the installers when the updated the system. I just wasn't far enough along in my learning curve at that point.)

2. The wood furnace is actual located quite a distance away from the heat pump, with approximately 20 feet of duct work in between the two. Which makes me wonder if the original owner/builder (who was himself a contractor for what it's worth) believed that distance between the two units allowed there not be a need for damper protecting the heat pump, believing possibly the heat from the wood furnace would perhaps be considerably less intense over that distance as to not endanger the heat pump. Of course, I don't know whether or not that theory holds any water at all. It's just something I thought of when trying to piece together why there seems to be a damper on the wood furnace side and not on the other – at least as far as I can tell.

 

[brenndatomu]

Have any pics?

 

[DerrickH]

Sure thing. The red box I drew indicates where the heat pump is behind the wall relative to the wood furnace you can see to the right.

 

[DerrickH]

If my question is confusing or not detailed enough please let me know. Happy to clarify.

 

[brenndatomu]

Warm air wants to go up, so as long as it can, it will...as long as there is not too much restriction to going up, if there is, and going down is easier, then it will do that. Bottom line is that you really can't tell what's what from the pics...you'd have to get someone in there with a scope to know for sure...or just pop things open for a lil look around. If you are not familiar with how to do that then get someone in there to do it for you, shouldn't take too long.
I'm not sure how much you plan to use that furnace, but it is probably going to use lots of wood, not the most efficient for sure.
If you plan on using it a lot, a modern wood furnace would be a great investment.

 

[DerrickH]

Warm air wants to go up, so as long as it can, it will...as long as there is not too much restriction to going up, if there is, and going down is easier, then it will do that. Bottom line is that you really can't tell what's what from the pics...you'd have to get someone in there with a scope to know for sure...or just pop things open for a lil look around. If you are not familiar with how to do that then get someone in there to do it for you, shouldn't take too long.
I'm not sure how much you plan to use that furnace, but it is probably going to use lots of wood, not the most efficient for sure.
If you plan on using it a lot, a modern wood furnace would be a great investment.

Thanks a bunch. I didn’t even think about putting a scope up there. I have one and will do that.

As for the efficiency of this antique burner, the good nose is that here in Tennessee it’s pretty mild. As of now I’m considering on using it around Christmas through February, and only on the weekends when I’m at the house and that’s about it.

 

[DerrickH]

I was able to put a scope in the ductwork and determine there is a gravity, backflow damper above the wood furnace,. But as far as I can tell, there is no damper to protect the heat pump side.

So assuming that is, indeed, the case is there anyway the distance that I originally referred to between the wood, furnace and heat pump/air handler would be sufficient as to not necessitate a damper or is that an absolute must that should be installed ASAP.

 

[brenndatomu]

I was able to put a scope in the ductwork and determine there is a gravity, backflow damper above the wood furnace,. But as far as I can tell, there is no damper to protect the heat pump side.

So assuming that is, indeed, the case is there anyway the distance that I originally referred to between the wood, furnace and heat pump/air handler would be sufficient as to not necessitate a damper or is that an absolute must that should be installed ASAP.

Its hard to tell without some temp readings...and even then, that still doesn't tell you what things would be like in a worst case/SHTF situation...

 

[DerrickH]

I had an idea.

I just ran the blower on the wood furnace without a fire in it and tapped in to the plenum just above the heat pump to see if observe airflow at that point.

All of the registers throughout the house are producing noticeable air, obviously as they should but where I made an access point in the ductwork above the heat pump I could barely feel any air at all.

So to me that actually suggests there is some type of a check valve damper somewhere in the line above the heat pump. I can’t see it, but that certainly suggest to me that there’s something in the system designed to block air from the wood furnace to the heat pump. The next time, my HVAC guys come to do their quarterly inspection I’ll have them try to validate this theory, but in the meantime, does this sound like reasonable conclusion to you?

 

[brenndatomu]

It could be that the air is just taking the path of least resistance...