Raypak 406A Pool Heater question


Active member
Jul 22, 2020
Folks, just sitting here waiting for a small snow storm to roll in and I was thinking about things that need to be addressed with the pool this coming season.

This past summer I had a lot of issues with the pool that came with our house back in June. The pump that came with the pool was a Hayward Super Pump 1.5hp with 1.5" plumbing throughout. I replaced it with a Waterway 1.4hp variable speed pump (like it a lot) and replumbed everything on the pad with 2" piping and Hydro Seal valves.

My wife isn't really into the pool all that much as she like *really* warm water. Whenever she gets in with the kids I usually turn on the heater and the fountains features so that she can sit under them and enjoy a 110 degree spa treatment. After I swapped out the pump I noticed that water returning from the heater doesn't seem to be as warm, it does eventually get pretty warm, but not as hot as when we had the Super Pump. I've played with the pump's rpm settings and found the sweet spot that seems to move about the same amount of water as the Super Pump but the water still doesn't seem as warm (the spousal unit points this out *every* time she is in the pool).

I've check the "uni-form governor" and it was clean as a whistle and worked perfect when I tested it in a glass of water. I know there is some sort of "by pass valve" inside the unit but I don't exactly understand what it does compared to the governor. Anyway, help or suggestions would be most appreciated.

Just a note, I did purchase a new pressure sensor which I intend to install this Spring, however as long as the filter is reasonably clean and the pump rpms over 2000 the heater doesn't seem to have an issue with the burner shutting off so I suspect it's some sort of pressure or flow issue.



Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Run the VS pump at the lowest flow where the heater operates properly for the warmest water. Heater output temperature is inversely proportional to the water flow.

Heater Outlet Temperature Rise
Heater outlet temperature rise will depend on the water flow rate. Slower water flow will have greater outlet temperature since the same BTUs go into less water volume.

Changing the pump speed can vary the return temperature from just warm at high RPM to very hot at minimum flow for the heater.

For example, a 400,000 btu/hr heater with an efficiency of 84% transfers 336,000 btu per hour to the water.

At a minimum required flow of 40 gpm, there will be 2,400 gallons of water (20,000 lbs.) that receives the heat.

Each btu raises the temperature of one pound of water by one degree Fahrenheit.

336,000 btu into 20,000 pounds of water is 16.8 degrees of temperature rise (336,000/20,000).

So, the maximum temperature rise you should ever have is 16.8 degrees Fahrenheit.

As shown below, higher flow results in a lower temperature rise.

  • 40 gpm = 16.8 degrees temperature rise
  • 50 gpm = 13.44 degrees temperature rise
  • 60 gpm = 11.2 degrees temperature rise
  • 70 gpm = 9.6 degrees.
  • 80 gpm = 8.4 degrees.


Active member
Jul 22, 2020
Thanks Allen,

That makes a lot of sense, I'm going to go build a spread sheet to track it all.

I guess I knew the flow rate would affect the temperature, but I didn't really grasp how much. Your bulleted table really put it into perspective.

Now I'm going to go buy a salt water generator. :)
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