Draw furnace return air through furnace closet

[Tim D]

Hi,

I have a Ducane forced air natural gas furnace with sealed combustion (PVC intake and exhaust pipes that vent directly to the roof), and the system was installed with a very undersized return duct, resulting in low airflow. One option a contractor recommended to fix it is to remove the old return duct, cover the hole where the return air enters the machine with a filter, and cut a louver in the door of the furnace closet. This would allow the system to draw air from the main living space of the unit, through the louver, into the closet, and directly into the return of the machine through the filter (no return ductwork).

I think this would solve the low airflow problem, but is there any issue with drawing return air through the furnace closet? The combustion is sealed as I mentioned, but my concern is that if there ever were an exhaust leak, carbon monoxide could get sucked into the return and be spread throughout the house.

Would this setup be a code violation? I'm in Massachusetts.

Thanks in advance,
-Tim

 

[cda]

Welcome

Not a hvac person

Mine is not sealed unit but draws like your hvac person is suggesting

I thinking there are several reasons you can have co pumped into your house

A good co alarm should be installed

 

[cda]

Should have asked??

One story house or more??

Just make sure there is enough vent return opening, so your unit is not damaged

I have to about 18 x 8 inch vents

 

[Tim D]

Thanks very much for your response, cda. I have a CO alarm outside the furnace closet, but it might not hurt to install one in the closet as well. It's a two-unit condo building (building is basement plus three floors, my condo is the top two floors). My HVAC guy said that the system requires 1,600 cfm, and the current return is 20"x8" with a few bottlenecks before it reaches the furnace (which results in about 650 cfm). The plan would be to widen the hole where the return air enters the machine to 20"x20" or so and make sure that the louver in the door is at least that big.

Thanks again for the help and please let me know if you see any issues with this plan. I saw this post on another site (https://www.nachi.org/bbsystem/viewtopic.php?t=17394), which explicitly says, "A furnace can not draw return air from the same room/closet that the furnace is installed," which is why I was concerned.

 

[cda]

I guess my concern is if you are occupying two floors

How is it going to get good return from all portions of your unit?

I have one return in a bedroom and when that door is closed, you can hear the a/c struggle

 

[Tim D]

Yes, drawing return air from all portions of the unit has been a challenge since we moved in. The short story is that the developer clearly cut some corners on the HVAC system to save a few bucks. The single return is not ideal, but the bigger issue so far has been the low volume of airflow.

 

[cda]

How old is the hvac unit??

 

[Tim D]

Installed in 2009.

 

[cda]

Did they suggest ADD

More return ducts??

 

[Francis Vineyard]

Installed in 2009.

Is this a new unit or a replacement?

Also direct vent appliance it is not a violation to draw return air through a louvered door but is must be sized accordingly for the amount of air flow.

We are also assuming the original return air duct is undersized or is the appliance over-sized?

 

[steveray]

Central return system sucks (pun intended) but it is the cheap way to do it, which makes it the common way (but so does a poorly designed duct system). You could try cutting a small return at the unit in the closet and in the door to get some more return air back...Are there gaps under the doors in all of the rooms? If there are supplies in all of the rooms but no returns, door undercut is important to keep from trying to "inflate" the room and allow the return to work....

 

[Tim D]

Did they suggest ADD

More return ducts??

Hi cda. Yes, the contractor looked into adding more return ducts but it won't be possible without ripping the whole condo apart (which I'm trying to avoid for cost reasons). Given how tight the space is, options are very limited unfortunately. They should have done it right the first time.

 

[Tim D]

Is this a new unit or a replacement?

Also direct vent appliance it is not a violation to draw return air through a louvered door but is must be sized accordingly for the amount of air flow.

We are also assuming the original return air duct is undersized or is the appliance over-sized?

Hi Francis. Thanks for your response. The condo was gutted and redone in 2009 (before I bought it), and I believe the furnace was installed new at that point. The appliance is the right size (not oversized) -- my contractor confirmed this. There is a 20"x20" return grille that feeds the system, but when you take it off, you can see that the actual return duct is only 20"x8". When the duct reaches the furnace closet, it gets down to about 4" wide to fit the space (the closet is very tight). My high-level research combined with my HVAC contractor's assessment indicate that this is way too small (like breathing through a straw).

My building was the first one the developer ever rebuilt, and I think the HVAC part was an after-thought that he didn't plan for appropriately. Lesson learned for me.

 

[Tim D]

Central return system sucks (pun intended) but it is the cheap way to do it, which makes it the common way (but so does a poorly designed duct system). You could try cutting a small return at the unit in the closet and in the door to get some more return air back...Are there gaps under the doors in all of the rooms? If there are supplies in all of the rooms but no returns, door undercut is important to keep from trying to "inflate" the room and allow the return to work....

Hi steveray. Thanks for your thoughts. Yes, central systems are not great. I got an Ecobee thermostat with remote sensors, which helps a bit, but it's still not ideal. The plan is as you described -- cut in a return at the machine and put a louver in the closet door so the machine can draw air in from the living space. This isn't great but probably the only option in my price range.

Yes, the space at the bottom of the bedroom doors combined with the low volume of airflow (due to the undersized return) make it so "inflation" is not much of a problem. This may change once I make the modification to the return to get more airflow -- we'll see.

 

[SCBO1]

Coach didn't put me in the game till now. Could an additional return air duct be added going through the floors, maybe through some closets where it's not such an eyesore?

 

[cda]

Hi cda. Yes, the contractor looked into adding more return ducts but it won't be possible without ripping the whole condo apart (which I'm trying to avoid for cost reasons). Given how tight the space is, options are very limited unfortunately. They should have done it right the first time.

I am wondering if they could at least do a floor vent from some of the top floor into ceiling of the floor below.

Just free flow opening

At least there is some circulation

 

[Tim D]

Coach didn't put me in the game till now. Could an additional return air duct be added going through the floors, maybe through some closets where it's not such an eyesore?

Glad coach put you in! I had a similar thought about adding another return, but I don't think there is a good way to do it. The unit backs up to the outside wall of the condo, so approaching from the back is out. The floor below belongs to our neighbors downstairs, so approaching from the bottom is out. The left side is where the current return enters (and the space is very tight). Through the wall on the right side is a bathroom (not a good spot for a return). The top is the supply plenum obviously. That basically just leaves the front of the machine.

The only other option we considered is penetrating the wall on the left side (where the current return is) so that the machine draws from the shared common space of the condo building stairwell. We could then put a transfer vent on the adjacent wall so that the machine pulls air from our living space, into the condo common space, and into the side of the furnace. This would work, but noise would likely be an issue (people in the common space would hear us through the transfer vent and vice versa, and the return grill would be inches from the fan, making it very loud).

 

[SCBO1]

Poster sez, "Low volume of air flow"? May have duct dampers in the supply that are partly closed? Just thinking.

 

[Tim D]

I am wondering if they could at least do a floor vent from some of the top floor into ceiling of the floor below.

Just free flow opening

At least there is some circulation

That's an option to consider. Thanks cda.

 

[Tim D]

Poster sez, "Low volume of air flow"? May have duct dampers in the supply that are partly closed? Just thinking.

Great call, Pcinspector1. The supply side of the system was undersized as well (no dampers, just bad design). My contractor fixed this as best he could, and improved air flow by more than 50% (we measured before and after). That was a great start -- now time to fix the return

It's incredible to me that they installed the system the way they did. My contractor said that it was actually dangerous in his opinion.

 

[SCBO1]

The furnace manufacture may have a trouble shooter that might fine a solution. It's worth
a shot to contact them IMO.

 

[cda]

Great call, Pcinspector1. The supply side of the system was undersized as well (no dampers, just bad design). My contractor fixed this as best he could, and improved air flow by more than 50% (we measured before and after). That was a great start -- now time to fix the return

It's incredible to me that they installed the system the way they did. My contractor said that it was actually dangerous in his opinion.

Do you have any flex duct you can see in the attic area, if you have an attic

 

[Tim D]

The furnace manufacture may have a trouble shooter that might fine a solution, it's worth a shot to contact them IMO.

Thanks Pcinspector1 -- I will give the manufacturer a try. I appreciate your help on this.

 

[Tim D]

Do you have any flex duct you can see in the attic area, if you have an attic

We don't have an attic unfortunately -- that's a big part of the challenge . All of the ductwork is behind drywall, so I would have to take down the ceiling to access it. There is flex coming off the main supply trunk, but I can see through the registers that that there are no kinks, bends, etc.

Before making any changes to the ductwork, I got 424 CFM through the system (goal per my contractor is 1,600). After fixing the supply-side issues, I got 653 CFM. Now, when I run the machine with the front of the fan cage taken off (so that the system is pulling air directly through the front -- not through the undersized return), I get 1,087 CFM. This is far less than 1,600, but a huge improvement. I'm hoping that fixing the return will provide similar results.

Thanks again to everyone for providing their thoughts. It has been super helpful and I really appreciate it. If you have any other comments (particularly on potential safety hazards or code violations with my plan to fix the return), please let me know.