Ducted heatpump sharing ducts with Wood/oil combo

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southshore

New Member
Apr 11, 2022
4
Nova Scotia
Hello everyone!

I'm new to this forum and new to running a house on wood heat in general. I took a look at the forums for a while but none answer my question in particular so here goes:

We bought a house last summer that ran off a Newmac Wood/oil combination furnace. When we got our mortgage, we opted to get a ducted heatpump install into the ductwork. We live in an area fairly prone to power outages caused by storms, and so not wanting to freeze to death in case of power outages, we decided to keep the Newmac system in place and hook up the heat pump to the ductwork. The HVAC guy pulled some of his hair out rigging up the dual system, but we did end up with the setup in the picture attached (drawing is mine).

There are two manual dampers for switching over the systems: #1 at the heat pump's output duct and #2 at the furnace output duct (labelled "manual" on the picture). The hatched area is the output duct from the heat pump as it would meet the output duct from the furnace if both dampers were open. The input and output ducts are side by side in actuality, but it was easier to depict them as shown (I think).

Is it possible to run both heat pump and wood furnace concurrently? I notice that on really cold winter days where the temp is in the single digits F, the auxiliary heater in the heat pump kicks on and with it, my electric bill rises significantly. The wood furnace by itself does not seem sufficient as a sole system to heat the house (not sure if previous owner used oil in conjunction with wood or just oil) but ideally I would like to supplement the heat pump with the wood furnace at the same time, but I am worried about damaging the heatpump somehow or voiding my warranty. Can I run both systems with both dampers open or am I risking damaging something?

Grateful for any insight whatsoever since I'm a complete newbie with all of this.

Thanks,
Mike

Heat pump furnace diagram.jpg
 

begreen

Mooderator
Staff member
Nov 18, 2005
94,669
South Puget Sound, WA
In that drawing there is no return air entering the wood/oil furnace. That is important. Kuuma has a good diagram for the proper flow. The oil/ga/electric in your case is the heat pump.

kuuma vf100 duct connect.JPG
 
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southshore

New Member
Apr 11, 2022
4
Nova Scotia
In that drawing there is no return air entering the wood/oil furnace. That is important. Kuuma has a good diagram for the proper flow. The oil/ga/electric in your case is the heat pump.
i should
View attachment 294635
There is return air to the furnace, I think my drawing is just unclear. The return air going into the furnace is junctioned to the heatpump and to the furnace.

That's similar to my setup, except I have manual dampers instead of check flaps (important?) and my damper for the heat pump is on the output side of the HP not at the intake like in Kuuma's diagram.
 

begreen

Mooderator
Staff member
Nov 18, 2005
94,669
South Puget Sound, WA
There has to be return air. Can you correct the drawing to show what is actually there? Or post a picture showing the ducting. The return needs to be directed to one heat source or the other. The check valve on the intake (return) serves this function. The check valve on the output serves to prevent a closed loop where the hot air goes back into the return.
 

southshore

New Member
Apr 11, 2022
4
Nova Scotia
There has to be return air. Can you correct the drawing to show what is actually there? Or post a picture showing the ducting. The return needs to be directed to one heat source or the other. The check valve on the intake (return) serves this function. The check valve on the output serves to prevent a closed loop where the hot air goes back into the return.
So in other words, the #1 damper (located at the air output from the heatpump) needs to be relocated to the return air side, where the heatpump would draw return air from? That is the only meaningful difference it seems to me from my actual setup besides the use of check flaps.

Also, from Kuuma's drawing, does it matter which is the primary (year-round system)? For most of the year, I would like to isolate the furnace as it will likely needlessly lose/gain heat from the heat pump system if the heat pump is running as a sole source heater/airconditioner.

Is this setup somewhat common or completely unorthodox? I'm not sure if I'm trying to reinvent the wheel (by having both systems sharing ducting in this way) or if this was some far-fetched idea to have both that is rare for a good reason.
 

begreen

Mooderator
Staff member
Nov 18, 2005
94,669
South Puget Sound, WA
Did you mean for the drawing post to be deleted? It's helpful.

Your diagram looks like it would work if the manual dampers seal reasonably well when closed. How tight or sloppy are the manual dampers? Do the close against a firm stop that has a gasket seal?

To answer your question, it would not be a good idea to run both the heat pump and the wood furnace concurrently with both dampers open. The reason being the supply of one system would try to feedback into the return of the other, creating a loop that would possibly lead to overheating and much less heat into the house.
 

southshore

New Member
Apr 11, 2022
4
Nova Scotia
Did I delete it? Not sure how, but here it is again:

The dampers are pretty sloppy it seems, just a flap that blocks off the majority of the air from passing (the heatpump fan starts to run hard if the #1 is closed). #2 is especially poor since the ducting bulges around it.

Is there a way to have the best of both worlds and run the system so that the wood heat acts as the auxiliary heater and the heat pump runs when the weather is more mild? If I were to switch to something like Kuuma has depicted, would this heat my house with primarily wood heat on especially cold nights and heat pump on warmer nights? How about if I were to place backdraft dampers instead; would that work?

furnace drawing based on Kuuma.JPG
 

EbS-P

Minister of Fire
Jan 19, 2019
3,039
SE North Carolina
Gravity check flaps are much better In My option. I Have have many days started a fire in my stove early in the morning and let it got out and then once it warms up kicked the heatpump on. You forget or someone else forgets the manual dampers you could overheat the heat exchanger(is that even possible I’m not a wood furnace guy) or send hot refrigerant somewhere isn’t not supposed to be. You don’t want to liquid refrigerant to find it’s way to the compressor.

Simply put the dampers need to be installed in a way that prevents one blower being able to blow through the other appliance in the wrong direction. You ducting is sized only for a single blower at a time. So I would only run one at a time.

New diagram position of dampers looks ok to me.
 

begreen

Mooderator
Staff member
Nov 18, 2005
94,669
South Puget Sound, WA
Yes, that drawing is better, but the install is not the best. My guess is that the mechanic just inserted a simple manual flap in the existing ductwork. The dampers will be somewhat leaky due to gaps around the periphery.

The Kuuma setup is with backdraft dampers that have stops. To be done properly, these backdraft damper assemblies would be prefabricated in a rigid section that then gets spliced into the existing duct plenums. It can be done on-site too, but usually takes a good sheet metal shop to do this properly so that the dampers are balanced correctly.

Is there a way to have the best of both worlds and run the system so that the wood heat acts as the auxiliary heater and the heat pump runs when the weather is more mild? If I were to switch to something like Kuuma has depicted, would this heat my house with primarily wood heat on especially cold nights and heat pump on warmer nights? How about if I were to place backdraft dampers instead; would that work?
That's more of a controls question. The wood furnace must be fired manually. It needs someone to feed it. If this was a pellet furnace then yes, with a 3 stage thermostat then the wood could be set up as the secondary or tertiary heat source if the gravity backdraft dampers were installed per the Kuuma diagram. However, that would need power still and my understanding is that the main concern is heat in a power outage which is going to be a manual operation.
 

brenndatomu

Minister of Fire
Aug 21, 2013
7,333
NE Ohio
The only thing that I don't like about gravity dampers on forced air wood or coal heat is what happens when the power goes out right after you just loaded the firebox for the day, and then left for work? That furnace is gonna get REAL hot...probably the ductwork too (have proper CTC on it?) because the gravity dampers will block the gravity air flow that you would normally get with an open duct (assuming there is some rise on the runs)
What I prefer (and did) is to install a combo setup...I have a gravity damper on my oil furnace supply plenum, that keeps things from backfeeding through the oil furnace when the Kuuma is running (which is almost all the time) and then the return air duct to the Kuuma has a power closed/spring open louvered damper in it that closes when the oil furnace (or AC) runs...that keeps things from backfeeding through the Kuuma when the oil or AC is running...and with this setup the supply and return air ducts for the Kuuma are wide open if the power fails (plus I have an emergency heat dump door on it too...but with Charmaster and Yukon out of business now, it seems nobody makes those anymore...at least not commercially...might have heard some scuttlebutt of an individual doing low volume custom orders though)