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Tankless DHW

MBean0302 Member Posts: 6
The HVAC company I work for as a service manager is transitioning into plumbing as well and while i am competent in boiler service and troubleshooting, I am not at all well versed in design and install of steam or hot water. It looks like we are going to possibly get on board with Rheem and install their tankless heaters for most of our water heater replacements.

With being the service manager, and concentrating mainly on service, this isn't a very big transition due to service and troubleshooting not being very much different than any other gas fired appliance, but I do a lot of replacement quotes and engineering when a system is determined to be too costly of a repair.

I have done quite a bit of reading on tankless heaters and I am a little worried about the average water temp here in my area vs the temp rise I am seeing on some of these spec sheets from Rheem.

From what I can tell, unless operating at a VERY LOW gpm flow, I'll at best be providing 100 degree water to my customers! Average water temp here is 45-55 degrees and a typical residence for one of our homeowners considering tankless would be a 3 bed 2 bath home. Assuming a washing machine and shower only running in tandem, you get well above 5gpm and the temp rise drops off to 40-45 degree!

Are we required in this situation to run two tankless heaters in series? Or add an electric booster at the hot water outlet with a relay to energize when the burners fire?

Looking for any input from the guys who install a lot of these... seems to me like a nightmare from a liability standpoint.


  • Zman
    Zman Member Posts: 7,574
    Forget about the charts.
    First take the BTU rating of the appliance and derate it for efficiency.
    If you take that number and divide it by the delta T and then again by 500, that is how many GPM's you will get.
    199,000 unit x 88% = 175,120
    175,000 / 80 (degree delta)/ 500 = 4.38 GPM

    Be sure to be clear about expectations with tankless heaters. It isn't a bad idea to have the techs measure flow at fixtures before selling a tankless.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • JohnNY
    JohnNY Member Posts: 3,231
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • MBean0302
    MBean0302 Member Posts: 6
    Gotcha Z-Man. I am, of course, familiar with that formula. Use it all the time to calculate HoE/HoR in a Geothermal system, but that doesn't really answer my question. That may let me know if i can expect a little higher rise than what is printed in manuals, but this was a fact-finding mission for information on what accessories are out there, and normally installed with demand hot water heaters to improve performance. if any. Can two be installed in series? Are 240v electric tank boosters a viable option for getting 5-10 more degrees hot water temperature in months where the city water is it's lowest temperature?

    These are all the kinds of information I was looking for.

    Your above comment was helpful though, sir. Just not exactly what I was looking for.