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Richwho 01-11-2009 09:17 PM

Cold Air Return Question

I've read through some of the threads on this forum and it looks like there are a lot of knowledgable people who read these posts. I'm hoping that someone can take a look at my issue and give me their opinion. Here we go:

I've got a 95% efficiency propane forced air furnace. This of course means that it's vented through PVC and has a dedicated PVC line to bring in combustion air. So far, so good. But there is also a cold air run that is connected to an exterior vent to pull cold outside air into the cold air return when the system is running. While I understand that there is some benefit to bringing in fresh air, I have to think this is hurting the efficiency of the system. This week temperatures here in Michigan are headed for single digits or lower, that's some serious cold air being introduced to the system. I'm considering closing off this return path.

My question is: is this a bad idea? Why or why not?

Thanks in advance for any advice that anyone can provide.

triple D 01-12-2009 12:36 AM

Is it cloder in house?
I dont think this is inefficient. The furnace has no problem tempering that air before it is introduced to the home. If the outside air was not introduced in the cold air return, but rather directly into house, the furnace would have to run longer to heat all the air in house. Plugging this could make for some stuffy, musty, possibly moldy conditions. Good luck....

agatto2a 01-12-2009 04:50 AM

Winter time is pretty dry in Michigan. I would think to install a very good damper so you can close that off in the colder months. How big is the hole 6" ? You are very correct in saying it lowers your efficiency.

Richwho 01-12-2009 06:17 AM

One thing I forgot to mention is that this a 125 year old, 2500 sqft home that has new windows and such, but I'd hardly call it "tight".

TripleD - My problem is the opposite of musty and moldy. I have to run a humidifier in the winter or else my indoor humidity stays around 20% during the coldest parts of the season. I'm sure the furnace can temper the incoming air, and when the outside temperature is 40 and above I don't have an issue with this, but this week we'll be seeing lows of 0-5 degrees. I keep the house at 68, that's quite a difference in the return air from the house and the outside air. Do you think with my low humidity problems I'd really cause mustiness and moldy if I closed that vent of during the coldest parts of the year?

Agatto2a - Yep, it's a 6 inch opening that has a diffuser attached outside, but no type of door that closes when the furnace isn't running. So right now with my furnace not running cold air is getting into the cold air return. I never feel it through the return registers when the furnace is off, but when the furnace turns on I get really cold air out of the supply registers for 2-3 minutes while the system pushes the cold air out of the ductwork. I'll have fun installing a damper because they enclosed a joist space directly to the outside vent. I may just have to come up with a solution to block that vent part of the year.

Thanks for the replies folks, if anyone else has any experience with this feel free to weigh in. So far one vote for closing it off and one against.

Hube 01-12-2009 08:26 AM

Richwho; if the home, as you said is not "tight", then this outside air intake is not really needed, especially in the winter time.
If it is necessary to have this intake, it should have a 'ventamatic" flap type damper installed at the entry point into the return duct. This can be be installed by just attaching a piece of stiff cloth type material over the entry into the duct. Attach this flap to the top of the opening so when fan is off it will block the entry hole. Whenever the fan is on, the flap will be sucked open.
Now, whenever the fan is off, there will be no admittance of outside (cold) air into the duct system.
To install this flap, you may have to cut in an access opening into the side or bottom of the duct in order to install it. Then screw a metal cover plate onto the access opening when finished.
If you have any more questions on this just ask. Hube

glennjanie 01-12-2009 11:21 AM

Welcome RichWho:
Your installer used the extra pipe for 'make-up' air to keep a constant pressure in the house. That keeps the heat pushing out at the cracks insetad of cold air coming in.
I would recommend a damper in the pipe that can be manually closed off in extreme temps. A metal damper in a metal pipe is rather simple when you use a damper kit from the HVAC supply store.

Hube 01-12-2009 01:24 PM

Bringing in "make-up" air to aid in combustion thru a Return air duct is not a choice place for it.Use a return only as a last resort.

One of the best ways to bring in air for "make-up air is to bring it in via a separate pipe that terminates close to the floor (basement) and right next to the heating appliance (furnace)
A 4" or 6" diameter pipe will do, depending on the amount of total btu's the heating appliance's have.
By installing 2- 90 * elbows at the bottom of this pipe (as in a "U" ) it will eliminate any cold 'downdraft' effect coming into the home as this "U" formation method will only allow in air as it is required for combustion purposes.

Another method to avoid unwanted air without using any elbows is to terminate the bottom of the pipe approx 1"-2" above the floor and within a bucket/pail.
These 2 methods are referred to as the "anti-spill" method
Hope this helps, good luck

inspectorD 01-12-2009 02:13 PM

You can also install a heat recovery ventilator for makeup air, it heats the incoming air with the cold air.

Richwho 01-12-2009 04:39 PM

Hey all, thanks for the replies and advice. I've blocked this inlet (but it's easy enough to open it again), we'll see what happens from here.

I used a probe to take the temparature at the supply register closest to the furnace, while the outside vent was open the supply was at 98 degrees, after closing the outside vent the supply is running 110. That should mean the furnace doesn't run as long to bring the house up to temperature.

I also went to the farthest cold air return register from the furnace and performed the highly scientific toilet paper square test for air draw. The tp square didn't even move. After blocking the outside supply there was a noticable increase in draw at the same register.

Hopefully I notice some difference in the amount of propane I'm using!

Thanks again, now I know where to turn to with questions!

Hube 01-12-2009 04:47 PM


Originally Posted by inspectorD (Post 26757)
You can also install a heat recovery ventilator for makeup air, it heats the incoming air with the cold air.

Yes, a Hrv will not only bring in fresh air, it will also exhaust the home's stale air out at the same time. The Hrv has to be installed,ducted and set up with an Magnehelic instrument so that the home does not experience a negative or positive effect .In other words, it has to be air "balanced" ,usually by a Pro.
But an Hrv can cost several hundred dollars, while a couple of 4" elbows and 8 ft. of pipe may only cost $10.
Big difference for just some make-up air, eh?

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