Gas Furnace: Line the Chimney or Add a Powervent?

I recently went through the process of having my 2 flue chimney rebuilt from the roofline up. I then lined one of the flues with a SS liner for a woodburning fireplace insert.
The 2nd flue is currently being used by my gas furnace and HWH. I noticed in the attic that some of the mortar is crumbing on that flue. (This is the original part of the chimney, not the new part, which is just from the roof up.) I plan to repoint these sections this spring. After reading some articles online & in magazines, I think the problem is that my boiler is causing too much condensation in the cold part of the chimney (in the attic). This is causing the deterioration of the mortar.
This said, I am thinking of either a) purchasing a power vent and redirecting the gas exhaust out the side of the house OR b) lining the 2nd flue with a gas-compatible liner.
The cost seems about comparable ($1000-$1500 if I do it myself).
Any pros/cons to consider?
TIA, Jonathan Nashua, NH
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Jonathan Royce wrote:

Only an opinion, - Good idea. Make sure that the liner will leave it with enough capacity for the furnace & HWH. udarrell
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The flue gases don't touch the brick. There's a clay type liner in there then surrounded by brick. The brick is structural only. Have it cleaned and serviced, a decent company will run a camera down there for cracks and voids.
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My chimney has no liner. Are you sure?
Bob
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He probably is sure, but he is not correct. Flue liners are required by many current building codes but go back a few decades or so and this was not the case. Older houses often had brick chimneys with no liner, and deterioration was/is a problem. High efficiency furnaces, which have lower flue temperatures, do tend to make it worse because of condensation, as some other psoters mentioned. The condensation is worse than plain water for masonry, because it is slightly acidic, which is generally harmful to mortar, and possibly the bricks themselves.
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How old are the furnace and water heater. Current efficient furnaces can us PVC to exhaust fumes because exhaust has little heat left in it. Water heaters can be equipped with failsafe double pipes that have intake and exhaust in one metal pipe. Spend money on these instead of chimney.
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