I would say if you have a make-up air vent as large as your furnance fule, you would be doing great!

'I would say if you have a make-up air vent as large as your furnance fule, you would be doing great!"

I don't believe that's how it works.

I think for gas burning appliances in a closed space, there's a formula that's probably in the local code. It depends on the BTUs of the appliances. My experience is limited to installations where air vents supply a closed space and the space supplies the appliances, both at atmospheric pressure, but the input vents in that case are quite a bit larger than the exhaust.

Here's the rule for Minneapolis: If you have less than 600 cfm kitchen exhaust you don't need makeup air.

A number of years ago I got a furnance replaced, the work passed inspection. The inspector told me that the fresh air combustion vent must be the same size as the furnance flue pipe.

The OP's concern is inadequate air supply for his furnace and water heater, not his kitchen exhaust.

OP, as you increase the combustion air supply to the mechanical room itself, the room will have outdoor air and temperatures. The entry to it becomes more like an exterior door than an interior one. A good step is to add a threshold and weatherstripping to the door frame. That might also lead to less back-drafting of your combustion venting when air exhaust devices are in use in the house.

Your fresh air to cold return setup is like leaving a window permanently open - is that what you want?

There's the conundrum - I am not thrilled about the 'open window' system, but I don't see what choice I have.

Currently, the mechanical room doors are 4 louvred bifolds so I'd have to replace them with solid doors before the weatherstripping thing became an issue. The idea of a damper like the Broan one here is appealing:

http://www.kitchensource.com/range-hoods/pdf/br_mdt_installations.pdf

In a sense I don't care about the combustion air, it's the fact that with more than one exhaust fan running, air is being drawn back down the flue.

There is already a traditional fresh air/make up air run to the cold air return. I don't know if I'm being paranoid or if this is normal and technically acceptable, or what?

I've done my best to draught-proof the house and upgraded windows...so permanently opening it up sounds like a drag, but so does dying of CO poisoning does also.

Here is a link that might be useful: Broan make-up air supply

The Broan device will close a damper when the exhaust fan is off and open it when the fan is turned on. When the damper opens you'll have the exact same outside air supply to the living space that you have now, I don't see why there would be any change insofar as the backdraft condition is concerned.

I'm just a fellow homeowner and not an expert, but it seems to me that If you had better ventilation to and closed up the mechanical room (to isolate it from the rest of the house), the issue would be over.

It's good you were alert enough to recognize the problem, you need a competent person to end your conundrum. I don't think the solution is all that complicated, It may be that a carpenter or handyman is more useful to you than an HVAC guy. Find out what your local codes require before you do anything.

Remember that air is dumb - it doesn't necessarily follow the path you want it to, it follows the one that is shortest and offers the least resistance.