Make up air?
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.