I just recently had to run new 4″ furnace pipe down through my old chimney because there was too much condensation forming inside of the chimney and getting into my house. My hot water heater is also ran into that chimney. My question is, can I connect my 3″ hot water heater line into my furnace pipe just outside of the chimney? The water heater requires a quarter of an inch in every foot and I can exceed that by an inch.
If you have a 4″ furnace vent pipe / liner inside the chimney, it would be too small to support your WH and furnace venting. The basic requirement for two appliances venting into a common chimney is that the diameter of the chimney flue must be at least largest vent pipe + 50% of the smaller diameter. In your case this would be 5.5″ (6″ because they don’t make half sizes).
So, the proper way would be to have a 6″ diameter chimney flue instead of the installed 4″ and then both appliances connected to it – You can do it on the outside of the chimney, but the connector has to be 6” x 4” x 3” (“Y” if furnace has an induced draft motor) or “T” if both natural draft appliances. That 6” end would have to tie into the 6” liner. If you have a brick chimney with 4” hole, you’d have to make the opening larger to accommodate 6” diameter pipe… If it’s easier, run both pipes separately into the new 6” liner. You can use single wall if there are sufficient clearances to combustibles – at least 6”, and if the area pipe runs through is conditioned and fully accessible (not inside the walls, ceiling, etc.)
If your WH vents into that unlined chimney at this moment (next to the furnace pipe), this might also prevent proper drafting and cause more condensation and furnace pipe deterioration… especially during the cold season. Increasing WH vent pipe pitch will not help in this case…