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Tankless Water Heater: GPM and Incoming Water Temp Questions

Wayne Reibold
14 years ago

I received my first estimate today for a Noritz gas tankless water heater and am a bit confused.

The salesperson was trying to figure out GPM I'd need simultaneously and confused me a bit. He'd talk about 2.5 GPM for average shower heads then would throw around 2.1 GPM for average shower heads and when I'd ask him if the 2.1 GPM figure was due to the fact that both hot and cold water are mixed together in the mixer at the shower so that 2.1 GPM is hot water and 0.4 GPM is cold water he couldn't give me a clear answer. So when figuring GPM for a tankless is 2.1 GPM of hot water reasonable for a 2.5 GPM shower head/body spray?

Also, he talked about tankless water heater mfr's published GPM ratings being for warm climates, which I'm not in, and that the GPM in a cold climate area where incoming water is cold would be significantly lower GPM but was tossing around reduced GPM figures that didn't seem consistent. For example, the Noritz 841 tankless published rating is 8.4 GPM but he was throwing around reduced figures of 5-6 GPM for where I live where he said incoming water temp is in range of 45-55 degrees depending on season. I'm in Seattle Washington. Does a reduction of published GPM of 8.4 GPM down to 5 or 6 GPM for an incoming temp of 45-55 degree water seem correct? If 5 GPM would be a worst case scenario based on an incoming temp of 45 degrees then that's the figure I'll work with to figure out if the Noritz 841 will power what I need to be running simultaneously.

Thanks in advance!

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