Tankless water heater issue

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.

Good one Steve…!

Thanks for the info!

Thanks Stephen, I’ll definitely keep that in mind.

Thanks for the education! I think we will see more tankless in the future and this info will come in handy.

Good info.

Most tankless type water heaters will require a stainless steel vent kit designed specifically for that unit. I don’t know of any (although there may be some) that are compatible with galvanized “B” vents. Most of the current crop of higher efficiency units produce condensate in the vent which is corrosive and must be drained off similar to a HE furnace.

The switch over to instantaneous may not save that much energy and can be bit of a nuisance each time you draw water (it takes longer to heat up from cold). There are actually some storage type hot water heaters that are more efficient than instantaneous!!


Here’s a website with independently tested efficiencies. For example, Bosch has a range of Energy Factors (EF…equates to efficiency) of from .69 to .93 with only one being over .90 and most being in the low .8’s!!

ps: It was a bit late (4:11 AM) when I posted…here’s the address for the energy efficiency directory for appliances:

Good info Brian. There is also another issue with tankless units. There is a minimum flow required before the unit turns on. If you are washing your hands using a newer low flow faucet, there may not be enough flow to start the heater. Same thing can happen with the new ultra low flow showerheads. They do not draw enough water to activate the system.

Thanks!!! Good information.

very nice


Here is one I put on my web site for tank-less systems. It has charts and math problems to consider when switching systems. Basically, if you have three or more bathrooms in the home, you may need two tank-less systems. This requires perhaps two gas meters and lines. Pricing would then be out of sight.

Now THATS a good thread, Thanks for all of that info…

Most if not all that info is for electric tankless. Gas units are more efficient and produce higher temps and higher GPM flow rates. Noritz has a 98% efficient unit but the cost is through the roof.


I would want to compare a tankless to one of these. Put it in your garage and heat your garage at the same time!

It takes heat from the garage and transfers it to the water.
Garage will be cooler.

Well, that will feel good in the summer! :smiley:

I have read about those units for some time and I can’t wait to see one installed so I can give it the run through. If you look at the spec sheets they are designed to work best in a specific climatic zone.


Be a real man. Get one of these!

That Rheem is a good price…have a look at this:


Definitely have a look at the recommended climate zones!!