Something to remember when you run into a home that has replaced their old gas water heater with a new tankless model. There could be an issue with the amount of gas delivered to the home.
Tank water heaters use an average of 36k btu on a standard 50 gallon model up to 88k btu on a standard 100 gallon model. They are supplied with a 1/2" gas line to the valve. Not a problem.
Now the owner comes in and replaces this with a tankless gas water heater. These units operate at 199k btu, quite a bit higher, and needs to be supplied by a 3/4" gas line.
There are numerous things you need to check when this change is done. Gas is delivered to a normal home through a meter that normally supplies 250 cfh, or 250k btu. The meter must deliver enough gas to supply all appliances operating at the same time. So let’s take a look at an all gas house.
New tankless water heater = 199,000 btu
Standard Furnace = 48,000 btu and up
Oven / Stove = 48,000 btu
Fireplace = 39,000 btu
Total 334,000 btu.
By exchanging the old tank style water heater with the new tankless, you have exceeded the standard gas meter output and also the capacity of the initial 3/4" gas line into the home.
The only way to take care of this issue is to upgrade the gas meter to a higher capacity model that can also operate at a higher pressure. But that means a pressure reducer is needed at each appliance. These are not inexpensive upgrades from the gas company. They can be pretty spendy.
So if you see a tankless water heater, start by checking the capacity of the gas meter and how many appliances use gas. This could be an issue for your client.
There is also the issue of the b-vent, which could be undersized for the additional btu output, especially if it is combined with the furnace.