Cold air pushed through registers when furnace is off

[colinml]

Bought this house last year. Furnace works fine, but it is old and inefficient, so my wife and I are looking at replacing, but we also want get a sense of whether the whole system is designed correctly first.

When we use the fireplace, it does a pretty good job of heating the main areas of the house, and the furnace doesn't kick on all day. However, cold air leaks from the registers...well, more than leaks: it comes out hard enough to lift a tissue laid across the register. This, needless to say, this creates cold drafts. The fireplace has it's own makeup air, so it doesn't pull air from the living space. So I don't think it is the direct source of the problem, in that sense. The return is placed in the highest spot in the house, and the furnace is in the unheated garage. Warm air, presumably, enters the return and cools on it's drop to the furnace, and this is "pushing" the cold air out the registers. I haven't found any leaks, and everything is insulated. Of course, I don't have testing equipment. I've just looked for obvious leaks in the flex duct connections.

So, is this a normal condition, or does this indicate a design flaw?

[genduct]

It sounds to me like you have already rounded up most of the likely suspects!
Cold air in retuns dropping into the space etc. but I think the fireplace and it's chimney are creating a good deal of the pressure diff that has the leakage/ infiltration rate of your home creating this situation.
If you can get a piece of board insulation like an iso board or a piece of ductboard that can be cut to fit the hearth opening and effectively block the fireplace, I would like to hear if that had an effect on your situation before you started looking deeper.
Just a thought

[colinml]

Sorry. I wasn't clear about the fireplace. Should have said "woodstove." It's brand new, and the door is already sealed. The air is drawn from the crawlspace, not the living space. But, I will add that the cold air problem also happens when the fireplace isn't in use. That is, between the times that the furnace kicks on, the cold air comes out of the registers.

[genduct]

Having a unit in the garage is not a good start! beside the fact that garages are NEVER consider inside, the potential for fumes is a very great concern.
This would be a good time to take a good look at where you are and figure where you want to be (duct design/ system wise. If you figure your in this home for a while, then sooner is better than later since the situation you describe has got to be costing you $$$$.00
Fix the insulation/ sealing of the building envelope with the duct system as part of the 'big picture plan" Once you agree the garage location is not good you'll want to get a new heater anyway (wouldn't pay to move the old dog)
Email me (found in profile) if you need more detail

[colinml]

Everyone I know has the furnace in the garage, and, until now, it's never occurred to me there was any problem with that. Let's see: I paid a home inspector to "inspect" prior to purchase, and then I paid an HVAC contractor to "double check" the furnace, just to be "sure." Neither of them mentioned that there was anything even slightly less than optimum.

Oh well.

Thanks for the heads up. I've got some research to do.