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.

first you have to determine air leakage of your home.
blower door testing will do this.
from there...you 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.

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.

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.

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.

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.

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.

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.