Fresh air directly connected to supply plenum

[piperboy72]

I've been reading the bbs here for a few months and have learned quite a bit. Thanks for the help in advance!

I meant to write return plenum instead of supply.

I just moved into a 22 year old home about a month ago and the previous homeowner has a single fresh air duct directly connected to the return plenum (just before the air filter). Is this a good practice? There is no combustion air duct, this is the only outside air supplied into the house. I am planning to replace the furnace later this year (with 80% AFUE model) and wanted to know if I should keep this as is, or disconnect the line from the return plenum and leave it open to serve combustion air for the furnace, water heater, and gas dryer. During heating I would assume that more air is forced into the home than is needed for all appliances and the excess is vented through the flue or fireplace chimney. When the furnace is idle I would assume that fresh air could be drawn through the furnace and return plenum (and filter) as needed by the other appliances.

I'm located in southern Minnesota, the house built in 83 has original windows and seems well insulated. The furnace is original, a 110kBTU Lennox. I have had a couple of estimates to replace the system and the heat load calc determined a loss of 52kBTU for a 2400 sq ft split level home. The fresh air duct is about 3 inches in diameter with an insulation jacket and feeds outside from about 4' above the ground.

Thanks Again,
piper

[Edited by piperboy72 on 02-01-2005 at 10:13 AM]

[mayguy]

For here in MN, I would remove it, and just drop it into the furnace room like I did with mine.

You can leave it on if you want to, but the down fall of it is.

-Adding more load to your furnace, and A/C.

-If you run your fan on all times, the air will feel cool and drafty in the winter, and humid in the summer.

-In the winter, when you bring that cold air in, it'll dry out the house more.

Only plus side of it I see, It'll bring fresh air into the house when the fan is running.

Where in southern MN are you?

Have you thought about 90% insted of 80%?

[baffin789]

site Buildingscience.com Go to the literature/articles page and look up HVAC articles. The previous owner was dealing with moisture build up in the house during the winter months. It's probably a pretty tightly built house. There should be a way to control the amount of outside air you bring in by turning a lever. I'd leave it hooked up. Just plug up the outside air inlet. If you have any problems down the road with moisture you can always use it.

[mayguy]

Baffin,

Most area here by code you can't put a damper on fresh air intake.. It has to provide fresh air for air that is lost from venting from the dryer/fans, and water heater, furanace.

[piperboy72]

Thanks for the help mayguy!

I'm actually in Saint Paul on the lower east side.

I understand the points you made and I think I would prefer to leave it on if it will meet code when I have the new furnace installed. I don't run the fan alone, the only time would be when heat or a/c is on. I will look into installing a bypass humidifier when the new furnace is put in to help with the extra dryness in the winter.

My main concern is: will the 3" supply provide sufficient fresh air for the water heater and dryer when the fan in not running? I have tested this configuration and there is a noticeable draft through the furnace and no spillage around the B-vent on the water heater that I could detect. I was using a match to determine this.

I would prefer it if I did not have to add more exterior penetrations to the home for an additional combustion air supply. This is also part of the reason why I am considering a new 80% furnace over a condensing model. The easiest run for direct venting would be to the back of the house but that would be under a deck. The front and side of the house would be too close to the ground. I would maybe consider vertical concentric venting through the existing 6" flue, but then I would also need to replace the water heater with a direct vent and find a good spot to route that as well. It just seems like too much at this point for what I would consider marginal benefit. Maybe 10 years from now when I'm looking for another furnace the direct venting and condensing furnace technology will be sorted out. Some of the newer homes that I have seen with direct vents lead me to think that installers also may still have a bit to learn. I saw one that was sloping downwards to the exterior and had an ice sculpture nearly 3 feet off the ground. Most of the homes also look like they have bazookas poking out at the sides. Anyway, that's just my opinion on it, I'm sure there are also many qualified compentent installers as well. I don't think furnaces are as durable as they used to be, so I expect that anything I buy now could need replacement in another 10 years or so. Maybe by that point condensing furnaces will be required by law.