View Full Version : What to do with the fresh air intake

11-22-2011, 10:11 AM
I have a 95% furnace with a two 3" pvc pipes (exhaust and combustion supply) and my water heater is the same (two pvc pipes (exhaust and combustion). Fireplace is a direct vent as well (not power venting). So in terms of combustion make up air, I'm ok.

The delema is what to do with the 6" fresh air intake on the cold air return of my furnace. I want to try and minimize the cold air that leaks into the basement when the furnace is not running. Right now there is manual damper at the furnace connection that is about half way open. I also run my thermostat on "CIRC" mode which kicks the fan on for 10 minutes if there has not been a A/C or heat call.

I'm very interested in hearing thoughts on the pros/cons of a motorized damper vs barometric damper vs the old fashion P trap.

My current thought is to use the "Aldes 6" Backdraft Damper - BDD6" - has anyone had any success with these?

The motorized damper didn't really make sense cost wise when I'd be opening the damper on each "FAN" call.

11-22-2011, 01:06 PM
If I understand what you have, I'd block that vent off and install a heat recovery ventilation device. This uses heat exchangers to precondition the air you bring in from outside, works to maintain humidity levels, too. I don't see how what you have is very efficient.

11-22-2011, 01:37 PM
Rare is the house that is so tight that it actually NEEDs mechanical ventilation, but a 6" air intake unbalances the system and FORCES exfiltration (paths unknown) which can cause moisture accumulation issues along the exfitration paths in a MI winter climate. Unless your indoor humidity in winter is excessive (>>35% RH @ 70F) pulling in outdoor air is simply pushing heating-dollars out the cracks. The recirculation mode may help balance the humidity and temperatures within the building, but that too is more often than not an unnecessary not-very-useful energy expenditure.

If your place IS that tight, as jadnashua suggests, heat recovery ventilation (HRV) or energy recovery ventilation (ERV- which does a moisture exchange as well as sensible-heat) is a much better approach. These are balanced systems when properly installed, and don't pressurize/depressurize the house creating exfiltration/infiltration issues.

11-22-2011, 02:01 PM
Interesting comments guys, please keep them coming. I believe that the intent of the fresh air intake was to control the bringing in of fresh air (lost from fans, dryers, etc) to mix with stale air in order to improve air quality. Humidity is not a problem - I run a humidifier in order to keep the humidity at 45%. I like the idea of the HRV, but the investment dollars outwiegh the benefit at this time.

11-22-2011, 04:24 PM
You might be surprised what what you have is costing you! Dealing with it properly might quickly recover the costs. In the interim, I think I'd just block that vent off and see if you notice any comfort or air quality difference.

11-22-2011, 05:09 PM
I've been in hundreds of older houses in the Midwest and never do I recall one that brought in outside air as part of the ventilation system. You might as well be throwing your money in the fireplace.

11-23-2011, 07:53 AM
Thanks guys, sounds like I should cap it this weekend. Anyone out there support the use of a fresh air intake and care to submit final arguements?

11-23-2011, 10:02 AM
In a really tight house, you need an engineered air exchange, not an arbitrary, uncontrolled air intrusion. What you have is like a 6" diameter hole in the wall...not very efficient. It's like leaving a window open a couple of inches all winter/summer long. Many older houses leak so much, that fresh air isn't really an issue.

11-23-2011, 02:42 PM
Interesting comments guys, please keep them coming. I believe that the intent of the fresh air intake was to control the bringing in of fresh air (lost from fans, dryers, etc) to mix with stale air in order to improve air quality. Humidity is not a problem - I run a humidifier in order to keep the humidity at 45%. I like the idea of the HRV, but the investment dollars outwiegh the benefit at this time.

The dew point of 70F, 45% RH the air is ~48F, and by keeping the humidity that high in a MI climate you're putting the building materials at risk of reaching rot or mold-inducing levels, even from vapor permeation through kraft paper vapor retarders on batts, not to mention any exfiltration paths. Keeping it between 30-35% is completely healthy & comfortable for the humans & hamsters in the house, but the dew point of that air is about 10F cooler, which means the number of hours over a winter that the sheathing & roof deck are below the indoor dew point are reduced by an order of magnitude, making them much less likely to reach critical moisture levels.

Medical & health pros recommend keeping indoor air between 30% (winter low) to 50% (summer high) for optimal health. Building science pros few anything over 40% in winter a very-high-humidity interior that requires changes in the insulation &/or structural design to make it resiliant to the moisture drives. If your house was built with an interior polyethylene vapor barrier under the gypsum and it's air-tight every where it needs to be you're probably OK. But the consequences of a single picture-hanger nail hole in the poly raises the mold potential in that stud-bay by quite a bit with a 45%RH interior.

If nobody has dust mite allergies it's OK to let the summertime RH rise to as high as 60% @ 75F but at 65%+ the mold and fungus hazard skyrockets.

If you have vented siding (either a rainscreen gap between the sheathing & siding, a brick veneer cavity wall, or vinyl siding), you may have sufficient drying to the exterior that it would keep up with vapor diffusion through kraft facers on batts, but not air leaks. Or, if you have fiber-gypsum or fiber-cement sheathing that isn't susceptible to moisture you'll be OK too. Otherwise, hold the line at 35% on the humidity.

In a reasonably tight house with more than 2 people showering, cooking and breathing the indoor RH will creep up during the winter, but it's easy to control that humidity by the ventilation rate on an ERV/HRV, or even a bathroom fan. If not ERV/HRV, ventilating via exhaust-only is recommended, since depressurizing the house brings in drier outdoor air in rather than forcing humid indoor out through unknown paths to possibly condense on susceptible material. In summer in MI you'd need to air condition or run a dehumidifier to keep the humidity down, as the summertime dew points are often too high to dehumidify with ventilation. Pulling in outdoor air when dew points are above ~55F only adds to the latent cooling load.

11-23-2011, 09:24 PM
A scholarly dissertation, but one must account for his geographical location. In Maine or Michigan, things are fairlly alike year around.
In California we have desert, ocean fog zones, rain forest and mountains ranging from mild to Mt. Everest sort of weather.
And Los Angeles or San Diego that have no weather to speak of whatsoever.

It would take a real genius to design a heating and vapor barrier and venting system for each location and zone to correctly address the locales conditions. Here, within 20 miles we can have 3' of snow lingering for a week and another with 1/2" of snow that remains overnight.

The local codes only consider insulation R values and roof loads. The rest is up for luck. And the designers that hang their shingles out don't seem to give a [s] whit about the micro-climates here.

I would certainly close out that 6" hole immediately, especially as when you circulate on a no heat call - a bit odd - you are really throwing out the money.

Just rebuilt a Goodman 80% "UPSIDE DOWN" furnace for a rental. [made for the burners to be on top or bottom of the pathetic heat exchanger] ... On bench inspection and operation, [the unit of course need have a draft inducer due to this reversible use] I took sad note of the fact that the outlet combustion air could easily light a cigarette or heat another home. A 4" steel pipe on the exhaust run 30' through the house with fins on it could likely bring it up to a 90% furnace.

Its a very sad fact that a few more feet of heat exchanger with more surface area could save the renters half their propane bill. I noted that this unit is 40" tall, and the new 80% goodmans are about 33" tall, meaning that they are more likely 40% efficient furnaces. Sad comment on this theft of our money and precious natural resources for the sake of a another dollar or 3 profit per furnace to the Mfgr.