Thread: Inlet Duct

This duct is attached to an attic vent. It pulls air from the outside to help with the HVAC system. It has a duct for interior air also. Is this correct?

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You'd have to look at the installation specs to be sure, but I've seen similar things several times before.

Thank you. I have been doing this a long time and have never seen a duct pull air from the outside like this. It does not seem to be a problem. Just more air for the unit to work.

How does the duct know to suck air in from the outside or blow air to the outside?

Combustion air needs to be supplied from at or below the appliance. HVAC installers often place a duct into the attic and have it terminate below the attic near the appliance. They act as chimneys not combusion air supply.

The duct in the picture is acting as a chimney and is not supplying combustion air if it is above the appliance.

It is on the intake side of the furnace. I do not see how it could release air on that side. Can you please explain?

It's a fresh air intake. The more sophisticated installs have motorized dampers and some have heat exchangers. The part you need to look for is air filtering up-stream of the air-handler.

Originally Posted by Ken Milton

...... It pulls air from the outside to help with the HVAC system. ...

How does pulling exterior air help with the HVAC system? In the winter you're pulling colder air than inside, and trying to heat it. In the summer you're pulling hotter air than inside, and trying to cool it. I don't get it.

Also, if this duct is connected with an inside return duct, your inside heat could go up the return, and then up and out of this duct, when the furnace is not running.

Originally Posted by Brent Crouse

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How does pulling exterior air help with the HVAC system? .

.
Combustion Air.
.

Originally Posted by Billy Stephens

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Combustion Air.
.

I don't think this is combustion or make-up air. It looks like fresh oxygen rich air to be mixed into the conditioned space.

Originally Posted by Vern Heiler

I don't think this is combustion or make-up air. It looks like fresh oxygen rich air to be mixed into the conditioned space.

Exactly....which doesn't exactly "help" with the efficiency of the HVAC system.

Originally Posted by Brent Crouse

Exactly....which doesn't exactly "help" with the efficiency of the HVAC system.

Nothing is free .

It is there to allow fresh air into the home. With today's bulding wraps there are not as many leaks as with an older home that breaths. Usually there is an electonic controller at the air handler that you can set the frequency that the vent will pull in outside air to mix with that air coming in from your return registers. Helps prevent "sick" houses. All the new builds around here have had them for a couple of years.

Looks like a fairly large duct to be a fresh air return for an AC system. In Sundays Houston Barnacle (Chronicle). Our local radio show house guy had an article in the paper explaining how to install a fresh air return for an AC system to help keep the house from being negatively pressured. He suggests putting a hole in your intake air duct/plenum, running dryer vent hose to the ridge vent or other vent in the roof decking with a piece of filter over the dryer vent. Idea is to bring in a small amount of fresh air.

What your showing looks a little too large for that though.

Originally Posted by Gary Burnett

It is there to allow fresh air into the home. With today's bulding wraps there are not as many leaks as with an older home that breaths. Usually there is an electonic controller at the air handler that you can set the frequency that the vent will pull in outside air to mix with that air coming in from your return registers. Helps prevent "sick" houses. All the new builds around here have had them for a couple of years.

Build it tight and ventilate right.

Other ways to ventilate "right" (to provide needed CFM of fresh air as calculated per ASHRAE 62.2) are:

Exhaust fan on a timer. Depressurizes house. Entry points of fresh air are uncontrolled so some fresh air may come from undesireable locations (e.g., crawlspace, attic, wall cavities).

Intake fan on a timer. Fresh air can be drawn from outside. Pressurizes house.

HRV or ERV. Balanced air flow (so no pressurization or depressurization). More energy efficient by recovering a large portion of heat (and moisture with ERV) in exchanged air.

Fresh make up air for the interior, not for combustion air.

The duct looks that large as it is insulated, the actual duct inside is not that large.

The duct actually is not doing what it is intended to do, partially, yes, but it is intended to pull in fresh air from outside, not air from the attic which is full of fiberglass fibers, insecticides dusted in the attic are any other contaminates in the attic.

That looks like trusses with blocking between them to support the roof sheathing, and closer than every 4' for the roof sheathing joints - what was the spacing of that blocking and why do you suspect it was there?

You guys can sure see a lot more than I can in that one pic.

This concept always makes me laugh.... the houses are so tight that we need to introduce air into the house. In the old days it was accomplished by gaps around the windows and doors and virtually everywhere else.

Talk about coming "full circle"

Originally Posted by Matt Fellman

This concept always makes me laugh.... the houses are so tight that we need to introduce air into the house. In the old days it was accomplished by gaps around the windows and doors and virtually everywhere else.

Talk about coming "full circle"

It's about planned, controlled air infiltration rather than unplanned, uncontrolled air infiltration. Most would rather not have the latter.

Originally Posted by Matt Fellman

This concept always makes me laugh.... the houses are so tight that we need to introduce air into the house. In the old days it was accomplished by gaps around the windows and doors and virtually everywhere else.

Talk about coming "full circle"

I know what you mean and agree, BUT I don't want to pay to heat and cool one of those old "self ventilating" models.

Most environmental guys recommend 50 cfm outside air per person in commercial buildings, residential is a whole other story, I have seen homes in a negative that almost killed its occupants with CO, I have seen homes with so much outside air that the evaporator coil had feathers in it. The bottom line is, If you exhaust it, bring it in, If you bring it in, let it out!