Make up air?

alan_s_thefirstSeptember 10, 2012

Here in BC, when I've been working on new construction I often see the HVAC people will run an air supply into the mechanical room as well as the dedicated air supply that runs to the HE furnace. I've also seen where they don't run an air supply to the HE furnace, but that's on the very cheap end. I'm not sure if this is what you really call "make-up air."

Anyway, this supplementary air's often a flex insulated duct, and has the slotted canister over it, and usually runs to floor level.

My house is mid '80s, poly vapour/air but not around electrical outlets etc, so it was done when the rules were more lax. I have been insulating and sealing better wherever I can, and recently installed a kitchen fan/microwave that's vented outside.

I'm mindful of negative pressure and have done a couple of tests with a candle next to the flue hat on the HW heater - it's a flued one, not power vented. The plastic covers over the water inlet/outlets has melted back a bit so I was concerned I was getting backdraft, but the candle test showed air going the right way. When it's windy may be when it happens, I suspect, although I've seen the same melting on other HW heaters of the same model, the inlet/outlets are right next to the flue.

I'm wondering if I should run a make-up air vent into my already-crowded mechanical room? I've run a good part of my alarm and media wires so I have an idea of what space is free. I'm trying to also make allowances to upgrade the furnace (mid-efficiency flued)and have an existing chase that a lineset and air/exhaust can be run through (no ac at present)

I read somewhere, apropos of the make-up vent bringing cold air in, that it only draws air when it's needed, ie when all the extraction fans, central vac (vented outside) etc are running.

I'm wondering firstly, is there a damper/flapper that restricts airflow til it's needed, should I have one in case, and is there any downside to having one? In the future, we may consider putting in a gas cooktop.

It's around 1800 sf, two bathrooms with outside vented fans, one kitchen fan, central vac. I've replaced all but two windows with higher efficiency vinyl and have done a lot of sealing and weatherstripping.

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I assume you have a gas fuurnace and hot water heater. What is the input BTU to each appliance? Where is you mechanical room located?

If the mechanical room has a door, then it must have louvers in order to allow sufficient air flow for combustion. Depending on your layout and the tightness of your house, you may need to bring in additional air from the outside.

I have never seen a damper on a fresh air supply. If they did exist it would have to controlled by the furnace so it will open when it fires. The drawback is if the damper fails then the furnace should automatically should down for safety reasons.

    Bookmark   September 11, 2012 at 12:00PM
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first you have to determine air leakage of your home.
blower door testing will do this.
from air seal and then blower door test
to verify how tight house is, then add fresh air
according to ashrae 62.2 guidelines.

best of luck.

    Bookmark   September 11, 2012 at 12:18PM
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Both appliances are gas.

Mechanical room is in the middle of the basement and it does have louvered doors. Not sure of the BTUs and I guess I could get a blower door test but they're quite expensive, whereas if there's no DOWNSIDE to the supplemental venting, I can probably do it myself for I was also wondering about whether it would be a prerequisite if I upgrade my furnace, because I want to do as much prep work as I can, to save money - pour my own concrete pad for a/c, run lineset, etc.

I'll have to look into the local codes, but I'm not working right now so am not bumping into HVAC guys much right now.

    Bookmark   September 11, 2012 at 2:04PM
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always look into local codes, but keep
in mind that code is the legal safe minimim.

investing in sealing house leakage is for the life
of the material you use to seal the leaks.
otherwise you always pay utilities to heat and
cool. a tight house is a healthy, efficient
and comfortable house. not exactly priceless..LOL
but a worthwhile investment.

some utilities do a blow and go energy "audit",
not my cup of tea..but if you know where the house
leaks, we can walk you through most of the sealing.
the benefit of knowing amount of leakage is
to let you know when you are getting tight
enough to add make up air.

best of luck.

best of luck.

    Bookmark   September 11, 2012 at 3:44PM
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Thanks. Unless you're living in a tent, I can't see how you could NOT have a problem when you have doors and windows shut, and 2 bathroom fans, kitchen fan, central vac AND dryer running.

Presumably the rules assume this as a worst-case scenario.

    Bookmark   September 12, 2012 at 2:55PM
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The downside is having conditioned air leave your house.

If you upgrade to 90%+ furnace then you mostly likely be exhausting out the side of the house with PVC. Air venting for the furnace will be installed at the same time. Depending on where the vent exists your house it can be either a one pipe or two pipe venting and exhaust set up.

    Bookmark   September 12, 2012 at 7:03PM
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One problem with passive "flappers" on ducts admitting outdoor air is that they can freeze shut in cold climates under some conditions. I believe that they are not allowed if make-up air is truly needed. Why has no one mentioned interlocked combustion air intake systems like Tjernlund? They will work with the furnace, but I am not sure how they can be controlled by your water heater.

    Bookmark   September 13, 2012 at 2:05PM
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Certainly it's a downside, losing that heated air, but we don't really have a choice. I suppose an HRV unit takes up some of the reasons for exhausting bathroom air, I believe some can dehumidify, but otherwise, we want that stale/greasy/smokey air out.

Apropos of fans, I was in a wholesaler today, they have a Broan variable fan up to 80cfm (when you increase the back pressure on it, it ramps up) that produces only .3 Sone. I don't know how to convert that to db, but it's incredibly quiet in its basic mode. Just air moving.

Ionized, I believe you're right about the flappers, I've seen some kitchen range hoods that require dedicated make-up air.

Cool gadget you referenced there. I don't think it would fit in my tiny mechanical room.

I'll have to research local rules here. I'd just as soon put in a passive vent and be safe, and ready if necessary if new hardware requires it.

I see these vents right next to a HE furnace with dedicated PVC vent and air intake, and a power vented hw heater. Are they just covering themselves?

Rules may be different here in Canada, but we seal houses pretty tight now.

Upon delivery, most houses have one bathroom fan permanently wired on, to comply with some reg or another, in such a way the fan can easily be returned to switched use.

If the house is insulated with sprayfoam, HRVs are mandatory. I don't know what happens with a power outage, the available supply of breathable air would decline, I suppose, especially if you're burning a boatload of candles.

    Bookmark   September 14, 2012 at 2:27AM
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