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Old 08-11-2019, 12:42 PM   #1
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1991 34' Limited
beecher , Illinois
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Tankless Heater GPM?

Hello everyone,

I'm installing a tankless heater in my renovation. I bought a 4.8 GPM heater and realized that I only have a 3 GPM pump to go with it. I know that using more than 4.8 GPM will stress the heater, but will I have a problem with the 3 GPM pump being too small for the 4.8 GPM heater?

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Old 08-11-2019, 06:35 PM   #2
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2007 22' International CCD
Corona , California
Join Date: Jul 2013
Posts: 8,404
Tankless Heater GPM?

Depends on the tankless heater. Check itís specification for its range of flow rates necessary for proper functioning. 4.8 gpm is maximum rate for some rated minimum water temperature.

Lower flow rates are normal, but there is a minimum flow to get it to heating water.

I have a PrecisionTemp 500 series. The directions say turn on hot water to max, then add in cold water to adjust temperature. See what your user/install manual says about flow rates.
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Old 08-11-2019, 10:03 PM   #3
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1974 27' Overlander
Baltimore , Maryland
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I don't expect you'll have a problem. 3 GPM would be astoundingly high for minimum flow to trip the switch. But certainly check your water heater manual to find the minimum flow rate.

Our Precisiontemp RV-550 requires a bit of massaging to get it to behave with our 1.5 GPM shower head if you use the pause valve. Have to listen carefully for the heater kicking on every re-start, or just shut the cold off for a second. Once it's on, it stays on at a much lower flow rate than required to trip the switch. Works fine once you're used to it.
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