Heater Outlet Temperature

[icepop456]

I have a 30k gallon in ground pool (20x40 ft). I updated to a VS pump running about 45 GPM and a Rapack 406A heater (~399Kbtus).

While the pool generally heats up at around 1.4 F/hour, the return jet temperature does not seem that high. I get a rise of about 8 F at 45 GPM. I think that is roughly half of the expected value.

Is this normal or is my internal bypass diverting too much water? The UG is working fine but I did not toy with the header to figure out the bypass valve.

Any suggestions for raising the outlet temperature? We enjoy the hot jets on the stairs.

 

[PoolStored]

Welcome to TFP!!!

 

[PoolStored]

The UG is working fine but I did not toy with the header to figure out the bypass valve.

What is UG?

 

[icepop456]

What is UG?

Unitherm Governor

 

[ajw22]

Welcome to TFP.

A pool heater puts out constant BTUs. The heat is either going out the exhaust or into the pool. IF too much heat is going out the exhaust the heater will overtemp and shutdown.

Look at the rate your house gas meter is spinning. If it is recording 400 cu ft of gas/hour then your heater is putting out the right amount of heat. If your heater is not getting sufficient gas flow then it will not put out the expected amount of heat.

I suspect your heater is operating fine. But if you are going to look for a reason a heater will put out insufficient heat without shutting down it will be due to low gas flow.

How do you know your heater is getting a flow of 45 GPM?

 

[1poolman1]

I have a 30k gallon in ground pool (20x40 ft). I updated to a VS pump running about 45 GPM and a Rapack 406A heater (~399Kbtus).

While the pool generally heats up at around 1.4 F/hour, the return jet temperature does not seem that high. I get a rise of about 8 F at 45 GPM. I think that is roughly half of the expected value.

Is this normal or is my internal bypass diverting too much water? The UG is working fine but I did not toy with the header to figure out the bypass valve.

Any suggestions for raising the outlet temperature? We enjoy the hot jets on the stairs.

Maximum heat rise per pass through most heaters is about 16 degrees, but by the time it mixes with the water that is already in the plumbing and enters the pool where it gives up its heat faster, 8 degrees at the pool isn't bad. If you really want to know the heat rise of your pool, a Pete's plug can be put in port for the pressure/temperature release and a thermometer inserted. Any thermometer with a metal rod, like an a/c one.

 

[icepop456]

I will look at gas consumption. That’s a good idea.

For GPM, I installed a flow meter. The data makes sense for TDH and pump size. I can’t say it is 100% accurate but seems readable.

 

[icepop456]

Maximum heat rise per pass through most heaters is about 16 degrees, but by the time it mixes with the water that is already in the plumbing and enters the pool where it gives up its heat faster, 8 degrees at the pool isn't bad. If you really want to know the heat rise of your pool, a Pete's plug can be put in port for the pressure/temperature release and a thermometer inserted. Any thermometer with a metal rod, like an a/c one.

I have a temperature probed inserted into the PVC pimple before and after the heater. I am measuring about 2ft of pipe before the heater (pool temp) and about 1ft after the heater. That delta is about 8 deg F. Not much mixing besides 1.5in pipe volume.

 

[1poolman1]

I will look at gas consumption. That’s a good idea.

For GPM, I installed a flow meter. The data makes sense for TDH and pump size. I can’t say it is 100% accurate but seems readable.

Slow the water a bit and the delta will increase. Still takes the same amount of BTU to heat your body of water to temperature. Heating the pool at 1.4 degree/hour is what a 406A is expected to do. 240,000 pounds of water (30K gallons) getting +/-330,000 BTU per hour works out to that number.

 

[JamesW]

400,000 btu/hr at 84% efficiency is 336,000 btu/hr.

45 gpm is 2,700 gallons per hour or 22,518 lb of water per hour.

Temp rise should be 336,000/22,518 = 14.92 degrees.

 

[JamesW]

336,000 btu/hr/250,200 lb water = 1.34 degrees per hour whole pool temp rise.

 

[1poolman1]

I have a temperature probed inserted into the PVC pimple before and after the heater. I am measuring about 2ft of pipe before the heater (pool temp) and about 1ft after the heater. That delta is about 8 deg F. Not much mixing besides 1.5in pipe volume.

Remember that while the flow through the plumbing may be 45gpm, the flow through the heat exchanger is less. There's mixing in the manifold as well. There is an internal bypass that limits the amount in the tubes to be heated at any one time. It is controlled by both the bypass valve between the inlet and outlet ports and the Unitherm governor that keeps the heat exchanger temperature a little above 100 degrees, so not all that 45gpm is being heated, but only a small portion. The temperature rise in the pool indicates all is well.

 

[JamesW]

While the pool generally heats up at around 1.4 F/hour

That is close to the expected rise, so the heat output seems accurate.

So, maybe the flow meter is off or the temperature sensors are off.

Can you show the system?

 

[icepop456]

Slow the water a bit and the delta will increase. Still takes the same amount of BTU to heat your body of water to temperature. Heating the pool at 1.4 degree/hour is what a 406A is expected to do. 240,000 pounds of water (30K gallons) getting +/-330,000 BTU per hour works out to that number.

I was trying that today. When I get down to 35-40 GPM, I do get 9-10 deg F rise but then I start tripping Hi Limit 2 (trips and restarts).

Thanks @JamesW I was using your calculations from other posts that’s shows something is not adding up with my system.

 

[JamesW]

All of the 45 GPM gets heated once it mixes at the heater outlet.

 

[JamesW]

Never run below the minimum required flow rate.

https://s3.amazonaws.com/AWSProd/sites/raypakcom/documents/241236.pdf

 

[JamesW]

What pump do you have?

What speed for 45 GPM?

What is the filter pressure at 45 GPM?

What temperature sensors do you have?

What is the before and after temperature?

Does the before temperature match the heater reading?

 

[icepop456]

Setup as I was prepping for the pump change.

The flow meter is shown in horizontal pipe. I recently added that so I cannot say for sure on accuracy.

Amazon:
Blue-White Residential Pool Flow Meter for 1.50" Pipe https://a.co/d/gTpuhpM

I use ds18b20 temperature sensors and confirmed “accuracy”. When heat is off, the delta is < 0.1 F change. I’m confident in the temperature readings at least relative.

Pool temp is measured just after the flow meter. The heater outlet temperature sensor can be seen on my measurement shot on the right pipe.

 

[icepop456]

What pump do you have?

Hayward VS700 SP2670020VSP. Previously had a Pentair 1HP single speed and got similar results (7 F rise due to higher flow)

What speed for 45 GPM?

Speed is about 2875 I believe.

What is the filter pressure at 45 GPM?

Just cleaned the DE filter today. I believe just under 10 PSI but need to check.

What temperature sensors do you have?

DS18B20 + ESP8266 WiFi

What is the before and after temperature?

Today I has 80 F pool and 88 F outlet at 45 GPM

Does the before temperature match the heater reading?

The heater reports about 1 F higher pool temp. I’m not aware of how to get the outlet temp from the heater. The SWG is ~1F higher than my probe. The delta rise matches well.

 

[JamesW]

Y = Temp Rise.

X = GPM.