Fresh Air Makeup ?

I inspected two houses this week that did not have a combustion air make up vent in the utility room with the gas furnace and water heater but did have a fresh air vent from the exterior to the cold air return. Both of these house’s were using 80% furnaces and in one house the utility room was very small with the gas water heater next to it. I called both of them out and recommend have a combustion air vent installed but wanted to get some more in site into this.

What are your thoughts about running a fresh air vent directly into the cold air return? Is this a good practice?

See it all the time.

What is your concern?

This helps explain it.

Minnesota State Mechanical Code no longer allows the combustion air duct to be connected to the return plenum. Section 708.1, #3 says:[Combustion air ducts shall:] Terminate in an unobstructed space allowing free movement of combustion air to the appliances.

My concern is if running the vent into the cold air return is a proper way of providing combustion air to the gas burning appliances.

Thanks for the link, Reuben does a really good job on his blog posts.

It was an accepted practice for many years.

I explain to clients this way.

If you don’t have the combustion air source the system will get its combustion air from somewhere including every crack bad door or window seal.

CAT IV systems(sealed combustion chambers) generally have their own source of combustion air from the outside side but that still leaves gas water heaters and gas dryers as well as gas fireplace in need of combustion make up air.

Ducting it directly to the cold air return is extremely common.

I am not sure I agree with Mr. Saltzman’s article. Others can chime in.

Well he thinks he does.:shock:

Centerpoint energy still lists this method in their informational brochures.[1].pdf

Do you have any idea how each one works and why it becomes an issue (if ever)?

Before you talk about it your report, it would be a good idea to know the real answer. Not just opine from this place.

Thanks Mike, I was mainly looking for a better way to comment about this in my reporting. I should have worded my original post better.

If the room has a door and can be shut it needs make up air in that room. The duct to the furnace is not going to help much if you can close a door. The house has enough volume to not need combustion air usually by code.

I do not like feeding outside air into the return for a number of reasons. First it probably bypasses the filter. Second it can be a high temperature. The coil normally has intake air between 70-80 not 90 - 100. Third it can contain more humidity than the interior air.

As far as combustion air if it is in a closet it should have a make-up air opening 12" from the floor and 12" from the ceiling.

How does it bypass the filter?

Yea that would be pretty tough to plumb it in after the filter.

Anyone know why you have a filter?

Hint: It’s not to clean your indoor air…

If you don’t intake into the return, how do you deal with the water vapor in the make up air?

High temperature? Do you know how to conduct air mixture calculations?

Is it any hotter than the air pulled around the windows and doors because you didn’t control building pressure?

No, it is actually cooler with the latent heat removed (even if it is the same temperature).

Sorry, had a long day with a con-tractor that can’t seem to figure out that you have to deal with the same volume of air that you pump into a crawlspace without an outside (of the building envelope) air source to break the vacuum in the living space of the building.

Also can not figure the fluid dynamics that weep holes can allow more moisture in, when they are for letting it out…

His crawlspace was sealed and didn’t let air out, but at the same time crawlspace air leaks into the house because you can’t seal floor leakage…

In south Texas most filters are in the grill in th ereturn boot in the hallway. Therefore, when air is entered directly into the return air plenum it bypasses the filter. Yes, David I know the function of a filter.

That’s good because most people do not.