Dampers for PF100 in Parallel with Oil Furnace

[Corksterman]

I recently had a Harman PF100 installed next to an existing oil furnace. I am wondering if the dampers were installed correctly. I am new to home ownership and the HVAC world so I don't know much about it but I don't think it was done properly. I am hoping for some answers as researching it didn't get me too far.

The installer placed dampers in the ductwork that connects the hot air plenum of the pellet furnace to the hot air plenum of the oil furnace. This way if the oil furnace did need to kick on it wasn't backfeeding air into the pellet furnace. I get that, it makes sense.

However, I am uncomfortable with this only because I felt like hot air was backfeeding into the oil furnace. I know this to be true because my oil furnace's blower will short cycle on and off as I assume it is trying to cool down.

Should there be a damper in my oil furnace's pelnum to stop the PF100's air from backfeeding into it, or does that not matter?

 

[begreen]

This depends on how the pellet furnace was connected. It is possible to put the back-draft damper in the trunk duct too. There should be a damper on the return air plenum if this is so. If there is one already installed, check to see if the hi/lo limit controller is after the damper and on the pellet furnace side. If so, that could be the issue? Here's a basic diagram from a wood furnace tie-in that shows typical damper locations. I'll see if I can find an alternative diagram that shows the damper in the hot air plenum too.



PS: Harman's manual is woefully lacking in illustrations for tie-in details. It might be helpful to check other pellet furnace manuals to see if they do a better job.

 

[kinsmanstoves]

Yes you need a back draft damper on the PF100. It is real important if there is a/c.

Eric

 

[Corksterman]

Here are some pictures of my dual furnace setup.



This is a full frontal with pellet furnace on the left and oil furnace on the right. A good view of the two hot air plenums.



This is also in the front but an angle from below shooting upward to show the second 10" hot air hidden behind the flexible tube in the first picture. This section of pipe between the two furnace plenums has a damper to stop air from flowing back into the pellet furnace. The one with the flexible hose in front does not have a damper. It is my understanding that this should and I'm not sure why the installer did not put it in. It seems like a very easy fix and I'm sure I could do it myself.



This picture shows the back of the furnaces where the cold air returns are located. No dampers anywhere back here. Notice one cold air return (right of pic) is installed to the duct work and the other (left of pic) is installed directly to the cold air plenum of the oil furnace.


A little more information as well. When my pellet furnace was nice and hot and the blower had been running for a while I took the back panel off the oil furnace... this is where the blower and the filters are located. I could feel the warm air coming out of the blower and the filters were warm as well. So I know the warm air from the pellet furnace is back-drafting into the oil furnace. Basically, the pellet stove is pushing hot air into the oil furnace and is recycling some of the hot air through the cold air returns seen just above. The cold air plenum of the oil furnace was also warm to the touch. Not hot, but noticeably warm.

Am I being overly OCD here about efficiency? Or, should additional dampers be installed to stop that hot air from basically being wasted? If so, where do you think the damper(s) should be installed?

 

[skibumm100]

Wow. Whatever happened to craftsmanship? I do not have a pellet furnace in my house but I think the best way to have it installed would be to have a backdraft damper on the outlet duct of each appliance so that either can be run without recirculating through the other. I would be ashamed to install something that looked like that. If I do a furnace I will probably do the install myself and have the ductwork fabricated by my local sheet metal shop. It's more expensive than flex and should be less restrictive. You see an awful lot of flex duct these days because it's quick and easy and often out of sight for the most part. If it were me, I'd get the installer back and have him do it right. Just my $0.02.....

 

[Corksterman]

I appreciate your input but it's a bit off topic. I don't personally have a problem with the installer using flex duct work. Maybe if this were to be installed in a living space I may have a different point of view... but it is in an unfinished basement, so aesthetics is the least of my concerns. I am however looking for some input on the functionality. Right now my problem is with the hot air backfeeding into my oil furnace. Or at least I think it is.

 

[skibumm100]

I do not have a pellet furnace in my house but I think the best way to have it installed would be to have a backdraft damper on the outlet duct of each appliance so that either can be run without recirculating through the other.....

^^^^^^^^^^^ Better?

Each heating source should be isolated so it doesn't recirculate through the other one. It doesn't sound like yours is installed that way.

 

[kinsmanstoves]

There is way to much restriction! You took a Lamborghini and put a 35 mph governor on the motor. I am speechless!

Eric

 

[moey]

Wow. Whatever happened to craftsmanship? I do not have a pellet furnace in my house but I think the best way to have it installed would be to have a backdraft damper on the outlet duct of each appliance so that either can be run without recirculating through the other. I would be ashamed to install something that looked like that. If I do a furnace I will probably do the install myself and have the ductwork fabricated by my local sheet metal shop. It's more expensive than flex and should be less restrictive. You see an awful lot of flex duct these days because it's quick and easy and often out of sight for the most part. If it were me, I'd get the installer back and have him do it right. Just my $0.02.....

Flex duct works great for short straight runs opposite of what people use it for. Gets rid of any fan noise and pulled straight over a short distance offers not much more resistance then a regular round duct. OP installed it in that manner hes got metal elbows with flex duct connecting them although that duct tape looks a little crappy.

I cant comment on the actual install design.

 

[Corksterman]

Unfortunately, I'm at a loss. I don't know what to do or how to fix it. I completely trusted this installer since he has been doing this for 30+ years. I don't know where to start or what to do. This situation has me completely frustrated as I did put quite a bit of money into the new furnace and installation.

 

[kinsmanstoves]

Unfortunately, I'm at a loss. I don't know what to do or how to fix it. I completely trusted this installer since he has been doing this for 30+ years. I don't know where to start or what to do. This situation has me completely frustrated as I did put quite a bit of money into the new furnace and installation.

If he is an HVAC tech or professional he will have an air flow calculator. The minimum dimension of the duct work can be determined with blower size and a few other things (I am not an HVAC Tech but I have them on the payroll). As a rule of thumb look at the size of the plenum and you reduced it by how much? That is not good!

Eric

 

[tjnamtiw]

begreen's right hand photo clearly shows two dampers, one on each source. Y'all can fight over the duct work.