Thread: Combustion Air Intake Vent

Inspected this 1986 80,000 BTU'S Trane furnace today. Can anyone tell me if this air intake vent is correctly installed on the floor. Thank You

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that's the return and it depends on where that furnace is located.
cheap style install at best.
did you pull the filter and check the coil?

The coils did have some dust on them. Furnace was located in a closed off room 12x10, doors were open when i went down to basement/ laundry room. Does the height of this vent have to be extended outside

Originally Posted by wayne soper

that's the return

Originally Posted by Sam Morris

Furnace was located in a closed off room 12x10, doors were open when i went down to basement/ laundry room.
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Does the height of this vent have to be extended outside

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As Wayne said this is return air.

Has nothing to do with the combustion air ( it's only drawing air from this unconditioned space.)

Get competent , licensed, HVAC Company to correct .
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That's a return louver thats too close to the draft hood (thats not a fan assisted unit is it?).

A duct needs to be installed so the opening is more than 10 feet from the draft hood.

Why does it have a return in a laundry room / basement? HVAC guy wanted to replace unit or and install A PVC pipe outside.

I am having a bit of a problem keeping up....can you provide more details that explains your question better for us slow learners? Thanks!

Return air vent openings should be located no less than 10 feet from an open combustion source for safety. The mechanical draw of the fan blower on the return air side may overpower the natural draft of the furnace and pull combustion gases into the supply air for the house.

Like others have said, the vented opening on the bottom furnace panel for the fan blower of the furnace is not a combustion air intake vent. It is a return air vent......and wrong in its current configuration.

Yes, this is return air not combustion. Beyond what others have said, NOT pulling return air from the actual spaces being heated/cooled reduces comfort levels, air quality and can increase utility costs.
You got the lack of air gap and trap at the AC drain, right? Also how close is that PVC line to the flue?

Originally Posted by Darren Miller

That's a return louver thats too close to the draft hood (thats not a fan assisted unit is it?).

A duct needs to be installed so the opening is more than 10 feet from the draft hood.

Even with a "fan assisted" furnace, if it's a Cat 1 it would be wrong. Cat 1 furnaces are considered as natural drafting units. If it were a high efficiency furnace with only venting to the exterior it would be wrong. Only if it were a direct vent (unit pulled combustion air from the exterior) might it work. But if a water heater or clothes dryer were in close proximity it would again be wrong. In general, an open return at the furnace is really not a smart idea. Even it things appear to vent/draft correctly the burners may not be getting the oxygen they need and therefore will have incomplete combustion of the fossil fuel which promotes fouling of the system and increased carbon monoxide generation.

Can anyone explain how the electric combustion air intake vent works? I am assuming it opens up when the furnace heat is activated. I have only seen two of these. The duct from the vent is always connected to the cold return. Are there concerns doing this with one of these electric combustion air intake vents? What exactly are they called so can do some "Googling" to find out more? Thanks!

Any pics of the type of vent you're talking about Dave?

Originally Posted by Dave Taurinskas

Can anyone explain how the electric combustion air intake vent works? I am assuming it opens up when the furnace heat is activated. I have only seen two of these. The duct from the vent is always connected to the cold return. Are there concerns doing this with one of these electric combustion air intake vents? What exactly are they called so can do some "Googling" to find out more? Thanks!

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I do not have any pictures. I may be out there again tomorrow and I will get a picture if I can. I have only seen a couple of these this year. The vent hood on the exterior wall has a mechanical damper that opens when the furnace is turned to "heat".
I would like to read up more on these. I called three HVAC guys and two did not know of them but one said he has seen many in recent years but has never installed any himself and did not know exactly what they were called. He did not see any problem with them,

I have always called out a combustion air duct connected to a cold return...which I could not verify in this last case due to a finished ceiling in the furnace room. I think can understand the logic of this product but also it's short comings. Any additional thoughts on this would be welcome.

On a call for heat the damper opens, once open it triggers an end switch allowing burner to start.

It sounds like you're describing a power venter.
It works just as described. On a call for heat, the damper opens and a small blower at the power venter also starts, to pull products of combustion from the vent system for discharging to the outside. If wired correctly, after the damper is fully opened and the blower in the power venter starts, the heat call (W signal) is passed to the furnace control allowing the furnace to start and operate normally.

There are tons of applications. Some gas boilers have a damper installed at the top of the boiler to help heat stay in the boiler during off cycles. Some units have a fan in a can system that have to prove combustion air pressure before a burner will come on. The bottom line is, with the technology available there is no reason to not install systems correctly and safely.

This is rich! I went back out there today. The owner was there and I asked if I could move some stuff so I could investigate some things I could not do on the first inspection. Well...this guy claims and I believe him, that he did not do the following. Someone took a Jenair mechanical exhaust duct and wired it to the furnace. It opens and close with the heat controls. It is the only combustible air or fresh air coming into the home. The furnace is a 90+ and the water heater is power vented as well. With 4 bathroom exhaust fans, a kitchen range hood exhaust fan and no fresh air intake in a pretty tight house, I suggested an install of a fresh air duct to help reduce negative pressure. Also, the area is known for high Radon levels. I was glad he let me move some stuff so I could get a closer look. I called the city building inspector and told him what a had run across and he was dumbfounded. He said" and exterior mechanical combustion air vent? in Minnesota? No, no, no,!"

Originally Posted by Dave Taurinskas

Can anyone explain how the electric combustion air intake vent works? I am assuming it opens up when the furnace heat is activated. I have only seen two of these. The duct from the vent is always connected to the cold return. Are there concerns doing this with one of these electric combustion air intake vents? What exactly are they called so can do some "Googling" to find out more? Thanks!

I have never seen combustion air taken from a return vent. Here is the closest thing I can find in the NC Gas code:

304.9 Mechanical combustion air supply.

Where all combustion
air is provided by a mechanical air supply system, the combustion air shall be supplied from the outdoors at a rate not less than 0.35 cubic feet per minute per 1,000 Btu/h (0.034 m 3/min per kW) of total input rating of all appliances located within the space.

304.9.1 Makeup air.

Where exhaust fans are installed,
makeup air shall be provided to replace the exhausted air.

304.9.2 Appliance interlock.

Each of the appliances served
shall be interlocked with the mechanical air supply system

to prevent main burner operation when the mechanical air
supply system is not in operation.

304.9.3 Combined combustion air and ventilation air
system.

Where combustion air is provided by the building's
mechanical ventilation system, the system shall provide the
specified combustion air rate in addition to the required ventilation air.

Many homes here in Minnesota had the fresh air/combustible air ducted into the cold return. I guess to keep the furnace floor from getting so cold. It is not common practice but I see 30 or so every year.

I honestly think you are confusing a ventillator for fresh air intake/make-up air supply...

With a Combustion air supply for a Direct Venting or HE furnace.

Combustion air for a furnace doesn't need to be tempered; HRV for fresh air ventillation of the home or make-up air often is; especially in colder winter climate regions.

P.S.

Where are the pictures from your return visit today?

Sorry ...since I was able to sort this thing out on the return visit I did not take pictures. What was confusing about this whole configuration is the furnace had it's own combustible air supply (PVC to the furnace cabinet) which I may not have made clear. In addition, when the furnace was activated the mechanical damper on a separate 6" duct opened as well but it was only open when the furnace was on. This duct was not visible initially and I thought it must be going to the return above the finished ceiling. With no open register in the plenum, the other appliances needing combustion air or make-up air would not have a source.