tankless gas water heater

bob eck

Most tankless gas water heaters require 140,000 up to 199,000 BTU on residential models. In a lot of homes the gas line is 3/4". I believe on most jobs when totaling up all BTU needed to run all gas appliances the 3/4" line will not be big enough so if the home owner wants to have a tankless gas water heater installed they need to up grade the gas line and that is a major expense when trying to sell TGWH. Does any tankless gas water heater manufacturer have the capability of making the TGWH have priority over the gas furnace or gas boiler. When the TGWH turns on the power from the thermostat to the gas furnace or boiler would be turned off this way the original gas line in the house would be big enough to supply the TGWH and you the contractor would not have to up grade the gas line to a larger size.  

Big Will

I have never seen it.

I don't know about there but even if they did the inspector would not go along with it. The make us size the gas line for 100% duty.  That means all loads at high fire. Because of the cost of running the gas line and the stainless flues and the low flow rate I usually talk the customer into the Vertex from A.O. smith. It usually costs less and it is more efficient than a standard tank-less water heater.

JK_3

Gas pipe sizing

Gas pipe sizing is based on all appliances being able to be run at the same time.

Many times I find that only a short section of pipe near the meter is all that needs to be changed to make it work.

I usually do the sizing starting at the appliance and work back toward the meter.

3/4 can be run upto 20 if only feeding the tankless( elbows and tee are 5' each)

If you need help sizing provide all btus and the lenghts and pipe and fittings from the meter to the last appliance. Example= 10' of 1 1/4" to first tee(gas dryer 40,000 btu) then 15' of 1" to tee for stove 54,000 btu etc etc.

with this I can help with sizing.