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.

http://i938.photobucket.com/albums/ad222/MykRian/misc/20151121_124618_zpsqug0kcoi.jpg

[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?

[Dan Hintz]

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?

Combustion air coming from outside is fine, and it allows the house to remain as sealed as you want... but I still can't comprehend a (efficient) system that expect fresh air on the intake side. It just defeats the entire purpose of a closed system. It would be like standing in front of a window-style A/C unit while you're in the middle of a park... you'll get cool, but only as long as you stand directly in front of the unit. It's not going to cool the park around you.

[Myk Rian]

I have 35 years experience with HVAC. If you want to argue the topic, it's not going to be with me.

[Lee Schierer]

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

Any chance you can get a copy of the code section that requires this type of vent?

[Bill Orbine]

Move the fire (burning leaves) to another area away from the intake. :D

[Myk Rian]

Move the fire (burning leaves) to another area away from the intake. :D
It's his neighbor that is burning leaves.

[Brad Adams]

Myk is correct. I have twenty years hvac experience. A lot of local codes require a 4" fresh air vent on the return air. We wire them with a damper to shut it off when the furnace isn't running. If an air exchanger is installed the vent isn't required. As much as you don't understand it Dan, fresh air is required in a building.

[Brian Elfert]

I know here in Minnesota either a fresh air intake or an air exchanger is required. It seems silly to me to have a high efficiency furnace, but then run a 4" or 6" duct to to the outside that brings in cold air in the winter. Everyone says to insulate and seal against air infiltration, but then the utility room has an intentional air leak that defeats all the other air sealing. A friend of mine had a new furnace installed in a really old house and the installers put in a fresh air intake. My friend plugged the intake as soon as the installer left. He said the house was fine without it for 60+ years and the house has plenty of air leaks.

I had a complete forced air system put in last year to replace electric baseboard heat. The HVAC company said I was right on the edge of needing a air exchanger.

[Joe Tilson]

Brad & Myk,
Are we applying the 5% fresh air intake to home systems like the commercial systems have been doing for years?

[Rod Sheridan]

Myk is correct. I have twenty years hvac experience. A lot of local codes require a 4" fresh air vent on the return air. We wire them with a damper to shut it off when the furnace isn't running. If an air exchanger is installed the vent isn't required. As much as you don't understand it Dan, fresh air is required in a building.

Very interesting Brad.

I've only seen combustion air vents that open into the furnace room, not the return air duct.........Thanks for the info..........Rod.

[Dan Hintz]

I have 35 years experience with HVAC. If you want to argue the topic, it's not going to be with me.

I'm not trying to argue the topic, but so far no one has offered a logical explanation as to why you would want a link between the inside and outside air (claiming a health benefit is too homeopathic-like in its reasoning). Claiming many local codes require it is also not an explanation of what problem it solves, just an explanation of why its required on a build. Providing such a vent near a combustion source (propane, oil, etc.) makes complete sense to avoid asphyxiaton, but I'm not seeing a valid reason yet for making a connection between inside and outside. Following the reasoning provided so far (it's required in certain jurisdictions) can be easily applied on the other side... it's NOT required in many jurisdictions and it's NOT used. I know my 3-year-old house doesn't have it on any of the three A/C system in the house (one of which is propane), and the system in the last 40-year-old home didn't, either.

[Tom Stenzel]

First off, no, I don't know a solution to Ryan's problem. I don't see a HEPA filter keeping smoke out.

The reason the inlet is there is is because of code.

With houses being sealed tighter now the problem wasn't just a negative pressure, it was also mold. According to a Minnesota seminar (I didn't go to the seminar, I just saw the PowerPoint) that stated that mold went from essentially zero to 5% of all housing failures. No, nothing was cited to support that claim, don't ask me what they mean by housing failure or how they conjured the 5% figure.

What Ryan has is a passive air inlet. If he had a exhaust fan larger than a certain size an active make up air system would be required. That is an fan interlocked with the exhaust fan. As far as size the number 400 CFM and larger comes up a lot.

I first found out about this when a relative built a house. He needed the vent. Plus the bathroom was on an outside corner but he couldn't have any windows in it and still have the windows he wanted in the living and bedrooms. He couldn't have a window to open but he had to have a vent.

And don't bother asking why after paying dearly to carefully seal up a house that you're then required to poke big holes in it. That's already been done on many of the builder's forums, in triplicate.

-Tom

"Well meaning idiots are still idiots." -By too many to list.

[ryan paulsen]

Thanks everyone for the information, and the lively discussion regarding the merits of a fresh air intake. I think I am going to try an inline filter box and use a charcoal filter to see if it helps. Good news is the leaves are done falling, so burning season is about over, so I have some time. Any thoughts on a product like this:

http://www.amazon.com/dp/B003E5U8BO/?ref=cm_sw_r_pi_dp_7w1uwb19VNN84#Ask

with a filter like this:

http://www.airfilterusa.com/astro-sorb-plus-carbon-merv-11-expanded-metal-pleated-10x20x1.html

[Pat Barry]

I know here in Minnesota either a fresh air intake or an air exchanger is required. It seems silly to me to have a high efficiency furnace, but then run a 4" or 6" duct to to the outside that brings in cold air in the winter. Everyone says to insulate and seal against air infiltration, but then the utility room has an intentional air leak that defeats all the other air sealing. A friend of mine had a new furnace installed in a really old house and the installers put in a fresh air intake. My friend plugged the intake as soon as the installer left. He said the house was fine without it for 60+ years and the house has plenty of air leaks.

I had a complete forced air system put in last year to replace electric baseboard heat. The HVAC company said I was right on the edge of needing a air exchanger.
I had a high efficiency furnace installed last year and they vent in outside air through a 1 1/2" PVC pipe, not a 4 or 5" duct. This is combustion air, not going in to the cold air return AFAIK

[Pat Barry]

Thanks everyone for the information, and the lively discussion regarding the merits of a fresh air intake. I think I am going to try an inline filter box and use a charcoal filter to see if it helps. Good news is the leaves are done falling, so burning season is about over, so I have some time. Any thoughts on a product like this:

http://www.amazon.com/dp/B003E5U8BO/?ref=cm_sw_r_pi_dp_7w1uwb19VNN84#Ask

with a filter like this:

http://www.airfilterusa.com/astro-sorb-plus-carbon-merv-11-expanded-metal-pleated-10x20x1.html

Yes it will work, yes it will also hamper the flow of air into the furnace from outside. Your fresh air intake may be oversized in which case it won't be a problem to put restriction inn the line. Just keep in mind that the more you filter the incoming air the less air you will get and you could end up causing problems

[ryan paulsen]

I'm not trying to argue the topic, but so far no one has offered a logical explanation as to why you would want a link between the inside and outside air (claiming a health benefit is too homeopathic-like in its reasoning). Claiming many local codes require it is also not an explanation of what problem it solves, just an explanation of why its required on a build. Providing such a vent near a combustion source (propane, oil, etc.) makes complete sense to avoid asphyxiaton, but I'm not seeing a valid reason yet for making a connection between inside and outside. Following the reasoning provided so far (it's required in certain jurisdictions) can be easily applied on the other side... it's NOT required in many jurisdictions and it's NOT used. I know my 3-year-old house doesn't have it on any of the three A/C system in the house (one of which is propane), and the system in the last 40-year-old home didn't, either.

Here is my understanding, based solely on the research I've done as well as discussing with my builder and HVAC contractor. The house is typically in a condition of negative pressure, due to appliances, hoods, and exhaust fans venting to the outside. So, outside air will go from higher pressure (outside) to lower pressure (inside) either through gaps/failed seals around penetrations, or whenever doors or windows are opened. This is unconditioned air entering the house. By adding a fresh-air line to the return plenum, the air is conditioned before entering the house. This air will also put the house in a position of positive pressure, so conditioned air will push out of the house to the outside as opposed to the other way around.

I have also read the benefits of replacing the indoor air to decrease the buildup of contaminated air via off-gassing, CO buildup, etc but as far as that goes I'd just as soon open a window.


Yes it will work, yes it will also hamper the flow of air into the furnace from outside. Your fresh air intake may be oversized in which case it won't be a problem to put restriction inn the line. Just keep in mind that the more you filter the incoming air the less air you will get and you could end up causing problems

Thank you. The furnace system was balanced before the fresh-air lines were added, they were added during final inspection. I am not opposed the restricting those lines slightly, so hopefully this does the trick.

[Dan Hintz]

[QUOTE=ryan paulsen;2497403]Here is my understanding, based solely on the research I've done as well as discussing with my builder and HVAC contractor. The house is typically in a condition of negative pressure, due to appliances, hoods, and exhaust fans venting to the outside. So, outside air will go from higher pressure (outside) to lower pressure (inside) either through gaps/failed seals around penetrations, or whenever doors or windows are opened. This is unconditioned air entering the house. By adding a fresh-air line to the return plenum, the air is conditioned before entering the house. This air will also put the house in a position of positive pressure, so conditioned air will push out of the house to the outside as opposed to the other way around. [\QUOTE]

Finally, a logical reason! Thank you.

[George Bokros]

[QUOTE=ryan paulsen;2497403]Here is my understanding, based solely on the research I've done as well as discussing with my builder and HVAC contractor. The house is typically in a condition of negative pressure, due to appliances, hoods, and exhaust fans venting to the outside. So, outside air will go from higher pressure (outside) to lower pressure (inside) either through gaps/failed seals around penetrations, or whenever doors or windows are opened. This is unconditioned air entering the house. By adding a fresh-air line to the return plenum, the air is conditioned before entering the house. This air will also put the house in a position of positive pressure, so conditioned air will push out of the house to the outside as opposed to the other way around. [\QUOTE]

Finally, a logical reason! Thank you.


This is basically what I said in my response, post #14 in this thread.

[ryan paulsen]

[QUOTE=Dan Hintz;2497414]


This is basically what I said in my response, post #14 in this thread.

Yes it is, thank you for that.

[Brian Elfert]

I had a high efficiency furnace installed last year and they vent in outside air through a 1 1/2" PVC pipe, not a 4 or 5" duct. This is combustion air, not going in to the cold air return AFAIK

That is the combustion air intake for the furnace itself.

I had my house converted from electric baseboard heat to a forced air furnace last year. I have two PVC pipes for intake and exhaust on the furnace. In addition I have a duct about 5" in diameter that brings fresh air into the utility room from the outside.

[Dan Hintz]

[QUOTE=Dan Hintz;2497414]


This is basically what I said in my response, post #14 in this thread.

Rereading your post, I see it now, George. My problem with most of the posts has been they tend to emphasize the combustion of the furnace being the cause of the negative pressure, but that's not the case for mine (and I know many others like it). The combustion air is brought in through a separate source pipe, allowing it to remain a sealed (and separate) system, having zero effect on the house's pressure. Now, other sources of negative pressure (bathroom fans, kitchen exhausts, etc.) are sources I didn't consider in my original notion.

[Pat Barry]

That is the combustion air intake for the furnace itself.

I had my house converted from electric baseboard heat to a forced air furnace last year. I have two PVC pipes for intake and exhaust on the furnace. In addition I have a duct about 5" in diameter that brings fresh air into the utility room from the outside.

They never installed one of those. Maybe the installer figured my old house was leaky enough

[Darcy Forman]

A neighbour four doors down the road from me decided to block off the cold air intake in his house. It was winter and I assume he felt it would be more efficient to do so. Little did he know his natural gas furnace had a cracked heat plenum. That night he went to bed. He never woke. Carbon monoxide. Guys never block off a fresh air intake. There is a reason it is a building code requirement.

[George Bokros]

[QUOTE=George Bokros;2497419]

Rereading your post, I see it now, George.

No problem Dan. My post probably could have been written better. I got this concept from an article several years ago about bringing in outside air into the HVAC return air. On first take it seems to be defeating heating your house. Now you are bringing in air substantially colder than the air in the house and it takes more to heat it. My guess is the amount of outside air coming in must be minimal.

Interestingly I have never seen it done.