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We moved into our 19th century townhouse last spring and, not having had occasion to use our heating system yet, just had our National Grid yearly check-up done. A tremendous amount of sand-like corrosion had collected on the ground under the burner tubes and the coils inside the boiler were caked with the same corrosion. A referred cleaning company came and scrubbed down the coils and explained that it was likely caused by improper ventilation – if the exhaust and vapor is not being pulled up the flue, it would linger around the coils and be re-burned or carbonized. Made sense to me. Chimney Company 1 came and inspected the boiler flue and told me the cement holding the clay flue liner sections together had eroded and the gaps and spaces would cause a lack of convection. They quoted $100 a foot to fit a stainless steel liner inside the clay liner (which everyone agrees is wider than necessary for the boiler output). Chimney Company 2 came and said hogwash – there are certainly some cracks and small gaps in the liner seams, but nothing that’s going to affect convection, certainly not given the diameter of the flue to the output of the boiler. If anything, there may have been some blockage at the bottom of the flue that the National Grid people would have removed (they didn’t mention whether they did) but there were no obstructions anymore. Given that CC2 was passing up a $3k+ job I’m apt to trust him, but I don’t want to risk re-carbonizing the boiler or worse, having exhaust back up into the basement or seeping through the clay flue liner seams if there is insufficient convection. Do I go for a third opinion? Any thoughts? Any idea what would cause that level of corrosion if the flue is okay and there wasn’t an obstruction? Just normal wear? Many many thanks for any tips or referrals.