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Thread: Will low pump speed limit heating?

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    coog61's Avatar
    Join Date
    Nov 2009
    Richmond, TX

    Will low pump speed limit heating?

    Up to now I've had no problem with the Max-E-Therm heater quickly warming up the spa. However I have turn down the VS pump from 3000 to 2000 rpm and the heater would not go above 82 deg. It showed it was still heating up, but the temp did not change. Starting water temp was 75. I turned the pump back up to the max (3450), and the heater went to the set temp.
    Is there a cause and effect with the pump speed?
    17,500 Gallon, SWG 33' x 21' freeform w/ Sunstone Cobalt Plaster. Attached 6 X 6 Spa raised 12" w/ 6 Jets with Moss Rock Spillover. 25' Moss Rock Waterfall. Pentair Intelliflo VS-3050 for Pool, 2 HP Whisperflo for Waterfall, Pentair Clean & Clear Cartridge 320, Pentair 400K BTU Max-E-Therm heater, Pentair Intellichor, Pentair EasyTouch 4P, Pentair Legend Platinum Cleaner w/ 3/4 hp pump, autofill system, nat gas fire pit. BBB convert

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    Re: Will low pump speed limit heating?

    You have to have enough pressure on the flow switch to get the heater to run. Sounds like you are too low on pressure at low speed to fully close the switch.


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    mas985's Avatar
    Join Date
    May 2007
    Pleasanton, CA

    Re: Will low pump speed limit heating?

    Most heaters these days contain both a pressure switch and a thermal limit switch. The low speed pressure may be enough to keep the pressure switch closed so the heater will turn on. However, because the water is flowing slower through the heat exchanger it will heat up to a higher temperature at the outlet. This may be enough to trip the thermal limit switch and turnoff the heater until it can cool down. But it should give some sort of error condition that requires a manual reset. If not, it could be cycling on and off.
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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