Our house has an electric AC and natural gas furnace heater setup. The AC is getting old and having issues, so we want to replace it, and are thinking of upgrading to an air source heat pump. We talked to a few contractors and one option they offered is to replace the AC and furnace with a heat pump and air handler. However, one contractor stated that this would not be the most cost efficient option, because the gas furnace is in a common vent configuration with the natural gas water heater next to it, and if we disconnect the gas furnace, we would have to shrink the common vent as it will be too large (diameter-wise) for just the water heater.
This has me a bit confused. The other contractors did not mention this as a problem. What would the problem be that an oversized vent could cause? Also, if it is a problem, then in the summer months when the furnace is off and the water heater is still running, wouldn't it already be a problem now?
Edit: We're looking at air source heat pumps.