Heaters need water


Gold Supporter
May 27, 2019
Houston, TX
Pool Size
Salt Water Generator
SWG Type
Hayward Aqua Rite Pro (T-15)
I'm playing around with my VSP speed and had adjusted it to 65 GPM. I turned the heater on and it was heating the pool very very slowly. This morning, I pressed one of my preset speed buttons (3450 RPM for 6 hours) and ran the heater. Pool temp rose more than a degree per hour. Lesson of the day: Pool heaters need maximum water flow to be most efficient.

Really need to setup my pump speed to be controlled by my automation system. I just need 3 speeds: SWG (min 40 GPM), Skim (65-70 GPM) and Max (120-130 GPM). Just need 3 spare relays...


TFP Expert
Mar 2, 2011
Heaters need about 10 gpm per 100,000 btu/hr.

For a 400,000 btu/hr heater, you need a minimum of 40 gpm. Anything more will not make much difference in heating speed or efficiency. I would not exceed 20 gpm per 100,000 btu/hr in any case.

At 40 gpm for a 336,000 btu/hr heater (400,000 btu/hr x 84% efficiency), the temperature rise from inlet to outlet will be 16.8 degrees. At 80 gpm, the temperature rise will be 8.4 degrees. For example inlet = 80, outlet = 96.8 or 88.4 degrees.

The heat transfer efficiency from the hot exhaust gas to the water depends partly on the temperature differential between the hot exhaust gas and the water temperature. With the exhaust being about 1,474 degrees Fahrenheit before it hits the heat exchanger, there is a difference of 1,377.2 or 1,385.6 degrees. 1,377.2/1,385.6 = 99.39%. So, you can estimate that the higher flow will be almost exactly the same heat transfer as the lower flow rate, especially when you account for the internal bypass, which will bypass most flow above 40 gpm around the heat exchanger anyway.

The heat gain from a 336,000 btu/hr heater for a 25,000 gallon pool will be about 1.6 degrees Fahrenheit per hour, not accounting for heat loss.

SWGs need about 15 to 20 gpm.

Skimmers need about 10 to 20 gpm per skimmer to get good action.

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