More make up air questions - rules I guess
I posted a little while ago about backdraft coming through my B-vent when I run more than one exhaust fan in the house.
Built in the mid-80's in British Columbia, the house has a 4" duct from outside to the cold air return, call it "make up air" if that's the correct term or fresh air intake. I'm thinking it's inadequate or does not work sufficiently when the fan is not running in the furnace.
An HVAC guy I was speaking with agreed I should probably run a combustion air intake into my mechanical room. Currently I have a mid-efficiency furnace with an older style hot water tank, so the b-vent has two devices.
My question this time is, what do the rules state in terms of make up air? Is the worst case assumed for exhaust fans, ie, all running at once?
I have two bathrooms with exhaust fans, kitchen exhaust vented outside, central vac vented outside, and electric clothes dryer vented outside. Theoretically, if we and house guests had been out in the snow all day, we could all be taking showers, running the dryer and cooking dinner whilst the maid was vacuuming. That would be a lot of negative pressure.
When I did a small test, I could feel warm combustion gases coming back down the b-vent when I had both bathroom and kitchen exhaust running.
I'm guessing a 5 inch combustion air vent would be more desirable, since its larger diameter would create an easier air flow than the B-vent would?