HVAC question

[rwhite]

I was in my attic and noticed my return air duct had a butterfly damper on it and it was sealed off with a plastic bag and duct tape. I think its a 18" return duct and the damper is 8". Kind of a backstory, I always felt my furnace pulled hard. I can't run my wood stove and furnace at the same time without sucking smoke in the house. Whenever the furnace comes on, it slams all the doors in the house. Anyway, this damper draws air from the attic space. Imagine a 18" duct running the length of the house, with air intakes at each end, drawing air from the inside and a butterfly damper right in the middle drawing air from the attic. So I unsealed it, the butterfly's are loose and basically open all the time. Unsealing it seemed to help the pressure issue when the furnace runs but I feel like I'm getting cold attic air in when it's off. I've noticed about a 5-6 degree temperature drop in house since I took it off. Not sure what I'm trying to answer here but any suggestions would be great. I didn't get a pic of the actual damper while I was up there but I attached a pic of what it looks like.

 

[EbS-P]

So this is a return that was sealed correct? You opened it and now it’s drawing attic air to the blower to blow into the house? Where is the house return?

Your house was a negative pressure zone. Some sucked in doors slammed. That would suggest to more air in the return and is being delivered via the the supply.

By opening the return in the attic you are now sucking attic air in to be blown into the house. This should cause a positive pressure in the house.

Never should you have a return drawing air from outside the insulated envelope unless is a small amount (100cfm or less) for fresh air ventilation.

If it were me I would be adding ceiling returns for those return ducts. If I thought the system was installed correctly. Really the best solution is to call out a pro and have them look the system over and balance it.

 

[rwhite]

So this is a return that was sealed correct? You opened it and now it’s drawing attic air to the blower to blow into the house? Where is the house return?

Your house was a negative pressure zone. Some sucked in doors slammed. That would suggest to more air in the return and is being delivered via the the supply.

By opening the return in the attic you are now sucking attic air in to be blown into the house. This should cause a positive pressure in the house.

Never should you have a return drawing air from outside the insulated envelope unless is a small amount (100cfm or less) for fresh air ventilation.

If it were me I would be adding ceiling returns for those return ducts. If I thought the system was installed correctly. Really the best solution is to call out a pro and have them look the system over and balance it.

Each end of the main 18" duct draws air from inside the house. The return air ducts are on both ends of the house. This 8" damper that was sealed (now open) is drawing air from the attic.

 

[EbS-P]

Each end of the main 18" duct draws air from inside the house. The return air ducts are on both ends of the house. This 8" damper that was sealed (now open) is drawing air from the attic.

check for supply side duct leaks, filters being clogged or really dirty coils.

I would seal the return back up. There is no reason to suck cold air into the house.

 

[Corey]

Ideally your HVAC would be "net zero" change in air pressure in the house. ...well, zero, except for a small draw up the flue if you are running natural gas/oil or other combustible fuel.

You say the furnace kicks on, blows doors closed and sucks air in from the wood stove? To do that, you would almost have to be blowing heated air outside somehow. Maybe a broken duct in an attic / crawl space? or possibly a register in a basement, garage or other room which is effectively 'cut off' from the rest of the house envelope? Guess dozens of small leaks in attic/crawl space ducting could do similar. But the effect is the same - furnace is blowing heated air outside the house envelope.

Sounds like someone possibly tried to fix this by adding a fresh air intake outside the house envelope. Bringing in fresh air from outside the house and exhausting heated air outside the house would restore the 'net zero' but as you say, at a large cost of pulling in cold outside air and exhausting a lot of heated air back outside.

 

[rwhite]

Ideally your HVAC would be "net zero" change in air pressure in the house. ...well, zero, except for a small draw up the flue if you are running natural gas/oil or other combustible fuel.

You say the furnace kicks on, blows doors closed and sucks air in from the wood stove? To do that, you would almost have to be blowing heated air outside somehow. Maybe a broken duct in an attic / crawl space? or possibly a register in a basement, garage or other room which is effectively 'cut off' from the rest of the house envelope? Guess dozens of small leaks in attic/crawl space ducting could do similar. But the effect is the same - furnace is blowing heated air outside the house envelope.

Sounds like someone possibly tried to fix this by adding a fresh air intake outside the house envelope. Bringing in fresh air from outside the house and exhausting heated air outside the house would restore the 'net zero' but as you say, at a large cost of pulling in cold outside air and exhausting a lot of heated air back outside.

I guess I really don't know if it is blowing the doors shut or sucking them shut. No broken or clogged ducts. The heating ducts are under the house and all sealed and insulated. Biggest problem is the only company I can get to come look is the same one that installed it for the previous owner. I doubt they'd find anything wrong with their work.

 

[gthomas785]

I'm not understanding how your system is laid out. The heating ducts are under the house but the return is in the attic? Where's the furnace?
Pulling return air from outside the envelope is just as inefficient as dumping supply air outside. All of your returns/supplies should go to the part of the house you want heated.

 

[rwhite]

I'm not understanding how your system is laid out. The heating ducts are under the house but the return is in the attic? Where's the furnace?
Pulling return air from outside the envelope is just as inefficient as dumping supply air outside. All of your returns/supplies should go to the part of the house you want heated.

Hopefully this explains the layout. Excuse the artwork but it's close. The top portion of the picture is looking down on the house. The bottom is what the furnace ducting would look like from the side. There is basically 1 hot air vent in every room, including bathrooms, with the exception of the master and the living room which have 2 in each.

 

[gthomas785]

That is certainly an unusual setup. I wonder if whoever installed the butterfly damper was trying to create some sort of fresh air intake? Anyway, if it were my house I would be sealing it up again.

 

[yooperdave]

Add a grille to the RA ductwork in the conditioned space. Not the attic.

Lower the blower motor speed to move less cfm.

Is the 18" attic trunkline made from insulated ductboard? Then it is only 16" due to the insulation. And where it connects to "drop" back into the conditioned space, does the trunkline increase in size to handle the cfm picked up from both ends of the house, or does it stay the same size all the way to the furnace? You may have undersized ductwork also.

With 5-6 degree lower temp difference since you opened up the attic damper, yeah....I'd get that sealed back up in a hurry!

 

[Highbeam]

Interior doors slamming shut does not indicate envelope leaks. Just indicates that you have a central return and then supplies in each room. This is typical. The rooms with only a supply are under pressure so the doors could slam shut.

You have a standard down flow furnace setup. Very common.

Seal off any leak (or open intake) in the attic.

 

[rwhite]

Sealed the damper off again. All systems back to normal.