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Old 09-27-2009, 01:46 PM   #1
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Make up ventilation through return on furnace question


Hello. Last winter we had problems with high humidity and condensation on windows. We ran a dehumidifier almost 24/7 to keep the RH lowered. I would like to provide a fresh air intake to the house to bring in fresh, dry air and hopefully this will help with the RH problem. We have a forced air natural gas furnace made by Heil. I cant seem to find a model number or year that it was made on the furnace. It has no fresh air intake or combustion air intake. The house is a ranch house 1100 sq ft with an unfinished basement. We dont heat the basement directly yet we leave the basement door open during the winter.

I would like to add a duct to the return on the furnace connected to a 4" duct with some type of damper/baffle connected through the wall to the outside. Anyone have any ideas or concerns related to doing this? The duct run from the wall to the return will be about 10 ft and I will connect it to the far end of my return to allow the outdoor air to mix with indoor air prior to entering the furnace. Im wondering what type of damper/baffle to use to allow the outdoor air in, should it be electric or passive, what are the pros/cons for each? Also should I add in a fresh air intake duct that ends near the furnace to allow for combustion air intake?

I think we have issues with negative indoor air pressure as our new windows were leaking around the bottom of the seams and the window tech looked at them and said I had installed them correctly. With a gas furnace, weater heater and dryer, I think the make up air is infiltrating through my windows and walls, id rather have it come from a controlled system that can be filtered by my furnace.

Ive looked at HRV systems but we cant afford it due to all the other projects that need to be done.

Thanks for any input.

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Old 09-27-2009, 01:59 PM   #2
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Make up ventilation through return on furnace question


I would use at least a 5" pipe, 4" may be too small. We use a motor operated damper:http://www.hoyme.com/
interlocked with the furnace to allow air in only when the furnace is running. You can also use that for a combustion air pipe which is different than a fresh air pipe to the return duct. Not sure if the Hoyme is available where you are. May be a different brand that does the same job. There may be energy audit companies who can come test your house to see how airtight/leaky it is and give you some good advice before you make big changes to your system. Spend a few $$ on advice and save more in energy/fuel later.

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Old 09-27-2009, 02:36 PM   #3
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Make up ventilation through return on furnace question


Are you close to the lakes?

Are there water stains in your basement.
Is your furnace vented with plastic pipe, or metal pipe.
Most of your moisture trouble is probably coming from leaving your door to the basement open.

Aprilaire also has a fresh air vent system, that is easy to install and set up.
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Old 09-27-2009, 03:00 PM   #4
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Make up ventilation through return on furnace question


We live about 20 miles inland from Lake Ontario. The humidity problem is likely contributed from the basement but we tried closing the door and it didnt help. No water stains or mold in the basement. The furnace is not vented, that is part of the problem I think. I did find a sticker on it saying it was installed new in 1993. Basically the house has no fresh make up air ventilation so it comes in wherever it can squeeze through and I dont want this so thats why im looking into bringing in fresh air in a more controlled manner and looking for tips.

Thanks so far for the replies.
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Old 09-27-2009, 03:10 PM   #5
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Make up ventilation through return on furnace question


By vented. I meant the flue pipe.
Is it metal or plastic?
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Old 09-27-2009, 04:44 PM   #6
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Make up ventilation through return on furnace question


If you do decide to add a fresh air intake to your return system, make sure to insulate it well. Every time we dip below zero to -10F mark, we get condensation on the air intake from the outdoors.
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Old 09-27-2009, 05:34 PM   #7
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Make up ventilation through return on furnace question


Is your furnace sized to heat -10 or -20 air?
Back in the 80s, our local natural gas provider mandated that these 'fresh air returns' be installed in every home.
So, there we were, installing a 4" insulated duct from outside to the return air on each furnace.
This policy was quickly canceled because of a whole range of issues.


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Old 09-27-2009, 05:40 PM   #8
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Make up ventilation through return on furnace question


We still use them here and it gets to be -40. All our new homes have a central bathroom exhaust fan system (one fan and ducting for the whole house). Have to bring in replacement fresh air and it goes into the return duct ideally 3' away from the furnace. Exhaust fan is interlocked with the furnace fan. No problems with ruined heat exchangers etc like we thought would happen. I took my fan unit out and replaced it with a Lifebreath HRV, much better idea.

Last edited by yuri; 09-27-2009 at 05:42 PM.
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Old 09-27-2009, 05:53 PM   #9
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Make up ventilation through return on furnace question


We do the central exhaust system here as well, however, it is not interlocked with the furnace fan.
The law states that the central exhaust fan must have it's own switch, centrally located and labeled.
Make up air is introduced via the old 'Saskatoon loop'.
As far as the OP goes, high humidity is due to lack of ventilation.

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