Optimal Pump Speed for Heat Pump

[scdaren]

Hello,

I have a Pentair 140 heat pump. I'm wondering if there is an optimal speed to run my pool pump when heating to get the most overall efficiency. If I run the pool pump slower, will the water gain more heat as it moves through the heat pump? Or does that not happen, and the faster I move water through it the better? Or is it 6 of one half dozen of the other?

I have been running the pool pump around 80 to 90 percent when heating so as to reduce the power consumption of my pool pump during that time. Would there be an overall heating advantage to running the pump at 100%? Or would I be better off somewhere slower like in the 60 to 70% range? Obviously the heat pump uses WAY more power than my pool pump, so whatever reduces my heat pump run time would be a benefit.

Or am I just way overthinking this???

Thanks!

 

[jblizzle]

You have competing "optimals" I think.

I think optimal heating you want to get as much water through the heater as you can as you have lower heat losses when you raise a lot of water a few degrees vs. raising a little water many degrees (this is true for solar, and I am pretty sure it carries over to other heaters).

But, running at the higher speeds is going to make the pump use a lot more electricity (although as you state, still much less than the heat pump).

My gut says runs the pump up near the maximum flow rate for your heater (before bypass starts) to reduce the heat pump run time, but it is obviously a complicated physics problem

 

[scdaren]

You have competing "optimals" I think.

YES. This is what is making my head swim and the OCD in me get agitated.

I think optimal heating you want to get as much water through the heater as you can as you have lower heat losses when you raise a lot of water a few degrees vs. raising a little water many degrees (this is true for solar, and I am pretty sure it carries over to other heaters).

Right -- so I think what I'm doing is probably right. My pool pump running at 80% uses like half of the electricity at 100%. So probably running the heat pump at 80-90% will still produce close to optimal heating and save a kw on the pool pump side.

Thanks!!

 

[JamesW]

The main thing is to keep the flow rate in the range set by the manufacturer.

Once you get the flow rate above the minimum, the internal bypass begins to open and bypass the exchanger anyway.

The heat transfer depends on the temperature differential between the water and the hot gas in the exchanger.

The gas gets to around 200 degrees Fahrenheit.

For example, if the water enters the heater at 64 gpm and 80 Fahrenheit and exists at 82 Fahrenheit, that's a 2 degree rise. The temperature differential between the hot gas and the water is 120 degrees at enter, 118 degrees at exit and 119 degrees on average.

Dropping the flow to 32 gpm causes the temperature rise to double from 2 degrees to 4 degrees. That's an exit of 84 degrees. That's a temperature differential of 120 degrees at enter, 116 degrees at exit and 118 degrees on average.

So, the average temperature differential is only 1 degree which is a small fraction of the total. So, heat transfer is almost identical.

The efficiency at 64 gpm is slightly better than at 32 gpm, but by less than 1% difference.

I would suggest that you keep the flow rate closer to the lower end of the range to reduce pump power and to reduce erosion in the heat exchanger.

Since water begins bypassing the exchanger pretty close to the lower end of the range, going higher wouldn't have any benefit.

Going closer to the bottom of the range should capture at least 99.5% of the energy of going higher.

 

[scdaren]

The main thing is to keep the flow rate in the range set by the manufacturer.

Once you get the flow rate above the minimum, the internal bypass begins to open and bypass the exchanger anyway.

The heat transfer depends on the temperature differential between the water and the hot gas in the exchanger.

The gas gets to around 200 degrees Fahrenheit.

For example, if the water enters the heater at 64 gpm and 80 Fahrenheit and exists at 82 Fahrenheit, that's a 2 degree rise. The temperature differential between the hot gas and the water is 120 degrees at enter, 118 degrees at exit and 119 degrees on average.

Dropping the flow to 32 gpm causes the temperature rise to double from 2 degrees to 4 degrees. That's an exit of 84 degrees. That's a temperature differential of 120 degrees at enter, 116 degrees at exit and 118 degrees on average.

So, the average temperature differential is only 1 degree which is a small fraction of the total. So, heat transfer is almost identical.

The efficiency at 64 gpm is slightly better than at 32 gpm, but by less than 1% difference.

I would suggest that you keep the flow rate closer to the lower end of the range to reduce pump power and to reduce erosion in the heat exchanger.

Since water begins bypassing the exchanger pretty close to the lower end of the range, going higher wouldn't have any benefit.

Going closer to the bottom of the range should capture at least 99.5% of the energy of going higher.

Thanks! This is the info I was looking for. It's a heat pump, not gas, and I did not realize there is a bypass inside. I'll try to find out what that flow limit is and keep it toward the lower end.

 

[scdaren]

Hmmm.... okay looked into this. Heat pump docs say max flow is 120 gpm, otherwise a bypass valve is needed. My pump's max flow at 3450 rpm (assuming zero feet of head) is right at 120, so it won't ever bypass and I wouldn't need a bypass valve, and I'm pretty sure I don't have one. I'm thinking running at around 85% is still probably a good point to be.

Amazing what you learn from reading the manual... I have been heating up on weekends, but it's gotten almost impractical to raise the pool up 15 degrees on demand. It says it's much more efficient to just set the thermostat and have the pump maintain a temperature.

 

[JamesW]

The maximum flow that you can send to the heater is 120 gpm. It has an internal bypass. I think that the range is 30 to 120 gpm.

At some point over 30 gpm, the check valve will begin to open and begin to bypass some of the water.

The more flow sent to the heater, the more goes through the bypass.

Ideally, the flow through the exchanger should always be at the optimum.

The spring on the check valve in the bypass is listed as 12 lb, which seems high to me.

I think that I would target about 40 gpm.

You can see the internal bypass in the manual where it shows the parts diagram.

The heat pump has a pressure switch that needs to trip to make the heat pump work. So, you have to send at least send enough water to trip the pressure switch.

You can get an idea of the flow rate by taking the (filter pressure x 1.4 x 2.31) = tdh in feet of head and then checking that against the pump curve at the speed.

85% might be about right for the pump to hit 40 gpm depending on the pressure.

 

[scdaren]

Thanks!