Heating AGP with a Heat Pump - best flow rate?

[archjeb]

Hi Folks,

Its a little earlier than I usually open the pool; but given the COVID-19 stay at home and the kids chomping at the bit to swim, I opened it up a month earlier than I normally do. But our evening temps are pretty low with a heat pump - like 40ish and so we have no heating at night for the most part. Daytime is upper 60's - will be in the 70's later in the week. Currently (as I type this) outside temp is 67degrees and water temp at 1PM is a 80degrees.

So this has lead me to think about improving heat transfer and general heat retention.

What we have today:
-Solar cover
-Foam around metal sides (internally between liner and pool wall) 1/8" thick
-Pentair 90 heat pump


Some questions about fine tuning flow rates that I couldn't find anywhere.
The Heat Pump has a spec'd minimum flow rate of 30GPM and my current pump is delivering 62GPM. If I increase backpressure on the return side to *slow* down the flow rate, the water does get hotter. But does that really improve heating? I see conflicting info on the internet about this. Some say, slow it down so the heat exchanger has more *time* with the water to transfer energy; others say improve flow to get more efficient transfer. Is there a sweet spot for flow rates that has the best heat transfer?

What direction (pointed down or towards surface) is best for the return jet for mixing water?

Normally when the outside air temp is warmer, I'm not as overly concerned about this; but given I'm riding the edge of temperature operation on this heat pump (at least in the evening and early morning hours) I'm trying to squeak out every little bit more of efficiency.

-J

 

[mas985]

If I increase backpressure on the return side to *slow* down the flow rate, the water does get hotter. But does that really improve heating? I see conflicting info on the internet about this. Some say, slow it down so the heat exchanger has more *time* with the water to transfer energy; others say improve flow to get more efficient transfer. Is there a sweet spot for flow rates that has the best heat transfer?

Heat transfer is proportional to GPM * dT where dT is the temperature difference caused by the heater. With a constant heater BTU output, halving the flow rate, doubles dT. So the net effect should be that the amount of heat remains about the same. However, this ignores heat loss. Heat loss is proportional to the temperature difference between the water and the surrounding environment. So if dT increases, one should expect the heat loss to increase and thus the heater efficiency to drop. So keeping the flow rate high will maximize heater efficiency. Anyone who tells you anything different does not understand thermodynamics.

What direction (pointed down or towards surface) is best for the return jet for mixing water?

Depends on the objective. For skimming, the surface is probably best. However for heat retention, sending it to the bottom would probably be best. However, for higher flow rates, it probably doesn't matter all that much because water gets mixed up pretty well either way.

 

[archjeb]

Thanks for the explanation. So the higher the GPM the better. I wonder if there is a calculator (I'm sure there is somewhere) to see GPM impact of 90degree elbows vs 45degree, etc. to minimize any friction loss.
Or maybe its not worth worrying about it for such a short run and just keep the filter clean, etc and call it good?

-J

 

[mas985]

Not really worth the effort. Most plumbing changes that you could actually make, won't change the flow rate that much to make any measurable difference in heat efficiency.

 

[jseyfert3]

Thanks for the explanation. So the higher the GPM the better. I wonder if there is a calculator (I'm sure there is somewhere) to see GPM impact of 90degree elbows vs 45degree, etc. to minimize any friction loss.
Or maybe its not worth worrying about it for such a short run and just keep the filter clean, etc and call it good?

-J

Well, within reason. You'd have to do a complete system lifecycle analysis to say for sure. If you want to get technical with it, then you have to compare costs for pumping water vs costs due to heat lost due to increased piping temps. My gut feeling is that your pump is wasting more energy than you save by having a higher than minimum flow rate, and you'd save money with a variable speed pump pumping less GPM. But my brain is shot right now. I started to do some rough back of the envelope calculations and went "nope". I'm going to go have a beer and take my dog out instead.

Really, once it comes to paying to heat a pool, the amount of energy we're talking about here is dwarfed by the amount of energy it takes to run the heater. So, my advise? Call your flow good and go have a beer. Because to get the the bottom of your original question about eking out maximum efficiency for heating the pool faster, well, that won't happen to any noticeable extent by changing pump speeds or reducing pipe friction, at least in your case.

 

[archjeb]

Another question. Normally I leave the pump on 24/7 (not worry about the timer).

At night when the heat pump is not running because of being below 50degrees, is it better for heat retention to shutdown the pump? Or leave it running. Which if either would reduce heat loss?

-J

 

[mas985]

Turning off the pump should reduce heat loss. Heat is lost mostly at the pool surface and with the pump running, it brings warmer water to the surface more quickly than convection would.