The home owner had installed a tankless water heater in the attic hard piped through the existing 50 gal tank of the old water heater in the hallway. I’d never seen that. What would be the purpose of installing it this way?
Cost savings. Can’t use the existing b-vent. Probably didn’t have an exterior wall so they went to the attic where the stainless steel vent pipe would be short and less expensive. That and they wouldn’t have to replumb the hot distribution lines. Just a guess.
The question is, was the gas meter large enough to handle the new tankless unit? Most basic gas meters are not. How many other gas appliances were there and what were their Btu ratings?
Only the furnace: 88,000 BTU. he’d capped the gas line at the fireplace.
Another consideration is the gas-supply piping to the tankless unit. Most tank type heaters only use a 5/8" supply line, which is insufficient for most tankless heaters. The standard is 3/4".
Most basic gas meters are a 250K BTU capacity. If the furnace is 88k, the tankless is probably 190k, you are already over the meter capacity. No bbq pits, ovens/ranges? Each burner must be added in to the equation. Changing to a larger meter can be spendy depending on the utility company and as Jeff said, the lines may not be sized appropriately either. Of course the plumbers won’t tell the purchaser because the extra added cost would probably be enough to make them forget about the “upgrade”.
I saw this at one of my inspections last year. Installed the tankless right next to the old water heater. Was told by the owner that the plumber said that was the cheapest way to do it and not have to run a bunch of new piping.