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Thread: Heating pool, does reducing flow velocity help

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    Heating pool, does reducing flow velocity help


    Briefly, last year we took ownership of a new "old" pool with our home purchase. The owner at the time did not have an eyeball on the closest jet to the pump, and the two times I heated the pool last fall i noted that the water from this "open" return was nice and warm if not a little hot. This year I heated the pool for the first time last night. The pool was covered with the solar, starting water temp was 57, air was 48. I started heating at 11pm. At 6am the water was only 62, but by 11am it climbed to 66, so the heater did its job of the 1degree per hour rule even fighting an uphill battle.
    So the open jet from last year, I did put a 3/4 inch eyeball there. It is my most powerful jet and also the only original (still with the old black pipe) I think this may leak under the concrete deck and perhaps the owner did not use an eyeball to reduce the pressure. Anyways feeling the water with the heat on, it did not seem as "warm" as it did last year. So I removed a different eyeball and viola - the water felt really warm. So is this a perception thing where the force from an eyeball just blows the heated water by so fast drawing in the cold surrounding water where you can't tell by hand and removing the eyeball slowed it down where you "can feel it" or in my case would there be a benefit to slowing down the water and thereby allowing more time spent in the heater core even if milliseconds?

    New to all of this so thanks in advance, and BTW thanks to TFP so far I have sparkling water after reading here all winter.
    16 X 32 wooden frame with aluminum braces in ground pool. 3.6ft shallow end, 8.5ft deep end. 22,000 gallons. Pool constructed in 1976 in Chicago area. Mechanics: 1HP hayward super pump, Jaccuzi sand filter, Hayward SWG, and Raypak 2100 heater. No main drain (capped years ago) 3 jets, 1 skimmer. Liner replaced May 2013.

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    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Heating pool, does reducing flow velocity help

    This is almost certainly a perceptual issue. Higher flow rates, which mean lower incoming water temperatures, are more efficient. However with a gas heater the difference is tiny and shouldn't make any real difference.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    JesseWV's Avatar
    Join Date
    Apr 2011
    North Central WV

    Re: Heating pool, does reducing flow velocity help

    Yes, low flow rate causes a higher temperature rise. It feels hotter but counter intuitively that's not what you want.

    Heat loss is proportional to temperature difference. The hotter the water is, the less efficient it is at transferring the heat from the exchanger. You want a high flow rate for maximum efficiency.
    16k gal, 28'x3.5', Vinyl A/G, 1hp Pentair Dynamo 2-speed Pump, Hayward S160T Sand Filter, Intermatic HB800RCL Digital Timer, Intex 8110 SWG, TF-100 Test Kit, SpeedStir Author: Jesse's Graphical Pool Testing Log

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    Join Date
    Jul 2012
    Cary, NC (Raleigh)

    Re: Heating pool, does reducing flow velocity help

    I too was wondering something similar. We just installed a heat pump this year and it needs at least 15gpm to be operating. I tend to run my pumps on low speed mostly and it seems that the heater will work on low speed but I believe its only marginally above its flow limit requirements.

    As I have forgotten all my thermodynamic and heat transfer classes from college there has to be some type of cross-over in terms of efficiency and cost of running the heat pump but using the low speed setting on the pump versus perhaps a greater heater efficiency but having to run the pump at high speed?
    18x30 Freeform In-ground Vinyl Lined. 11,500 gallons.
    4 returns, 2 skimmers, 2 main drains.
    Hayward TriStar 1hp 2-Speed Pump. Hayward S244T Filter.Hayward Goldline Panel + SWG.
    Polaris 280 cleaner. AquaComfort 75k BTU Heatpump

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