Bricks, Masonry, Asphalt and Concrete - Brick chimney

I have a 49 year old home with an exterior brick chimney which runs up the side of the house. During the past few winters (I live in MN)some of the brick faces have popped off of the bricks. My guess is this is happening because moisture is getting into cracks, then expanding when freezing. The furnace and water heater vent through the chimney, so it is continually going from warm to cold during the winter. I generally don't use the fireplace. Is there a product I can use to seal the bricks to prevent further damage to the bricks? Thanks.

Corrective maintenance is long past due. A subsurface breathable type of sealer (opposed to topical types that block moisture and vapor transmission) might be used after replacing the spalled brick plus inspecting and affecting repairs to the remaining component's. A sealer alone is not the solution to the problem, and neglecting maintenance because the fireplace is rarely used is not sound logic.

Your guess is on target, but the source of moisture can be from the exterior or the interior. It's time to call a licensed sweep, mason or masonry contractor, and have things inspected and repaired.