Filtering fresh air furnace intake

[ryan paulsen]

I have an outside-air line hooked into the cold air returns of my furnaces. I also live in an area where we can burn leaves. Is there a way to add a filter (HEPA filter?) to this fresh air line to eliminate the furnace from filling the house with smoke? One of the furnaces is in the attic, so a manual damper would be a no-go, plus I would like to keep the positive pressure to keep the smoke from seeping in. Any ideas?

[Dan Hintz]

How are you not destroying the efficiency of your furnace by doing that? WHY would you do that?

[Ken Fitzgerald]

The easiest method I can think of is to turn the furnace off while burning leaves.

[ryan paulsen]

How are you not destroying the efficiency of your furnace by doing that? WHY would you do that?

Code required installation of the fresh-air intakes to displace the accumulation of indoor pollutants/toxins.

The easiest method I can think of is to turn the furnace off while burning leaves.

Thanks for the reply, this works in the short term but sometimes these fires can go on from dawn to dusk and into the next day. I have talked to the neighbor and requested that he play the wind while burning, but was looking for a solution I have control over on my end if possible.

[Jerome Stanek]

Isn't open burning illegal there

[Ken Fitzgerald]

Jerome,

If you look at his opening statement, the OP says it's legal to burn leaves.

[Michael Weber]

Learn to live with a few indoor pollutants and block that thing off.

[Myk Rian]

How are you not destroying the efficiency of your furnace by doing that? WHY would you do that?

By not creating a negative pressure throughout the house, thus not allowing cold air to come in through any cracks, openings, windows, etc..

Normally, a fresh air intake is controlled by a skuttle damper. It shuts the duct when the furnace is off. It works by a counter-balance on the damper shaft.
I have never heard of putting a filter in-line, but don't see why not.

[Dan Hintz]

By not creating a negative pressure throughout the house, thus not allowing cold air to come in through any cracks, openings, windows, etc.

But a house is normally a closed system. The only way to create a negative pressure is via the exhaust on a fire-heated furnace, but that has nothing to do with the cold air returns (and any of today's systems would use a pipe-in-pipe setup to bring in fresh air for combustion only... which still leaves it as a closed system, just two of them).

[Lee Schierer]

But a house is normally a closed system. The only way to create a negative pressure is via the exhaust on a fire-heated furnace, but that has nothing to do with the cold air returns (and any of today's systems would use a pipe-in-pipe setup to bring in fresh air for combustion only... which still leaves it as a closed system, just two of them).

His system is a forced air system and the fresh air duct is probably on the cold air return to the furnace from the house. You would need to reduce the size of the cold air return so that the fan inlet is slightly starved so that some outside air is drawn in. I would think that an air to air heat exchanger would be more efficient than using the main recirculating system to suck in cold outside air while forcing warmed air out through cracks and leaks elsewhere in the house. The air to air heat exchanger would at least recover some of the heat.

To answer the OP's question. I don't think a Hepa filter is going to remove the smoke. I would think you would need some sort of air scrubber system like those used by industry.

[Myk Rian]

I have an outside-air line hooked into the cold air returns of my furnaces.

His system is a forced air system and the fresh air duct is probably on the cold air return to the furnace from the house..

This is what is being discussed. The skuttle ties into the return air duct. Many newer systems have 2 of them. The other provides fresh air for the furnace and water heater burners, and exits near the floor.

[Karl Andersson]

To expand on Lee's comment: the leaf smoke will have SOME particulate component, like ash (which is what a HEPA filter will block), but a HEPA filter will not block any of the vapors and very small-particle fumes produced by the combustion. I used to work a lot in HEPA tight-fitting respirators on asbestos removal jobs, and you could always smell the smoke if some nitwit tried to hide and have a smoke (yes, with the respirator off inside the enclosure.)

You'd need something like the HEPA filter plus activated charcoal beds on the intake to remove the smoke vapors and odors, which would be custom-made and probably very expensive. Depending on how air-tight your house is, the better bet might be to shut your fresh air intake during burning season, but then you'll invite other indoor air quality issues like potential humidity build-up and subsequent mold.

[Brian Kent]

One reason not to. Our AC was working very poorly - running for hours and only changing the temp 1 or 2°. We had a technician come out. The unit was fine and charged. He was checking out the ductwork when I saw a grate over a wall. We took it off and it wasn't a wall. It was many years of accumulation of stuff on an air filter that had been installed long before we bought the house. We took off the air filter and it turned out this was the main intake. The system sprung to life and works well.

If you decide to do this, find a way that you can remember, and inform future owners, so they can change the filter. This one was on a high wall over the stairs so we never saw it from normal lines of sight.

[George Bokros]

Years ago houses were not any where near as tightly sealed as they are today. This combined with the fact that the newer furnaces draw their combustion air from outside thus not creating a negative pressure in the house. With the reduced negative pressure inside less fresh air enters when you open a door so the air in the house is not refreshed. The vent bringing the fresh air into the supply air keeps the air in the house fresher with out introducing cold air as you would get if you opened a window. If you introduce this fresh air upstream of the normal house air filter it will be filtered for particulate but not odors. You may have to change your HVAC filter more often though.

Do you know how nice it smells in the house when you open a window for the first time in the spring?

[ryan paulsen]

Thanks for all the responses so far. As mentioned above, I now realize that a charcoal filter would be more what I need as opposed to a HEPA. Curiously enough, google searches for fresh air intake filtration lead to many more responses dealing to people with more, er, horticultural interests than my particular issue...

The only other issue with merely closing off the damper is not maintaining positive pressure on he house, thereby allowing smoke to seep in. Lesser of 2 evils maybe?