Rebuilding shoddy boiler/hot water heater exhaust to chimney connection

[Mookie3333]

I replaced a hot water heater in a house we bought recently. Upon trying to connect the exhaust vent for the hot water heater, I noticed the concrete at the chimney connection cracking away and falling. I removed all of it, so I now have two exhaust ducts sitting at the entrance to the chimney, but wide air spaces all around it. How can I build back this connection? Do I use some sort of wire mesh/cement? High temperature cement? Thanks for the help.

 

[Dana]

First you have to make sure that the flue is sized correctly for the BTU ratings of the stuff you're venting into it. If it's oversized the crumbling may be due to excessive exhaust condensation. If it needs a narrowing liner, install one now or you risk more serious degradation of the chimney. Many flues in older houses are way oversized for right-sized heating equipment (and ridiculously oversized for a hot water heater on it's own.)

As for sealing the metal to the masonry, buy a tub of pre-mixed furnace cement aka "refractory cement". It's available at the usual box store outlets in half-gallon tubs or sometimes caulk gun tubes. If it's a bigger gap you may need to add some brick, or find bigger tubs (I haven't seen any 1 or 2 gallon tubs in box stores, but maybe...)