Installing multiple heat pumps in parallel—advisable?

[RubiconValley]

Hi, all, I understand from reading the “Pentair UltraTemp Heat Pump Further Reading” page that it is possible to install multiple heat pumps in parallel (with each heat pump getting an equal flow of water and confirming that my plumbing system can provide the minimum flow of 30 gpm to each heat pump). My question is whether this is advisable? It seems to (layperson me) to be a good idea, especially since the cost of energy is so high, relative to the one-time cost of an additional heat pump, here in Silicon Valley. Has any one done this? Would appreciate insight and advice. Thank you!
Edited to change "cost of electricity" to "cost of energy," with thanks to ajw22.

 

[ajw22]

It seems to (layperson me) to be a good idea, especially since the cost of electricity is so high, relative to the one-time cost of an additional heat pump, here in Silicon Valley. Has any one does this?

Explain why you think multiple heat pumps and the cost of electricity are related?

Your cost per BTU of heat will be the same with one or multiple heat pumps.

If you want more heating capacity to heat a pool faster then multiple heat pumps may be the answer.

 

[RubiconValley]

Explain why you think multiple heat pumps and the cost of electricity are related?

Your cost per BTU of heat will be the same with one or multiple heat pumps.

If you want more heating capacity to heat a pool faster then multiple heat pumps may be the answer.

Sorry, was typing too fast--I should have said "cost of energy" instead of "cost of electricity" =p I will have a Raypak 407K natural gas heater for when the ambient temperature is not warm enough for the heat pump to work. But, of course, am hoping the heat pump will do most of the heavy lifting. Sounds like, from your answer, it would make sense to spring for two heat pumps and install them in parallel--instead of relying on just 1 heat pump and 1 gas heater to pick up the slack.

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[ajw22]

If you have a spa I think you should keep a gas heater. A gas heater will heat the spa faster and a gas heater will heat the spa at any outside temperature.

The HP gives max BTU's at 80F and declines as the air temp declines until it shuts down around 50F. HP's are not going to work well to heat your spa during cool evenings.

 

[RubiconValley]

If you have a spa I think you should keep a gas heater. A gas heater will heat the spa faster and a gas heater will heat the spa at any outside temperature.

The HP gives max BTU's at 80F and declines as the air temp declines until it shuts down around 50F. HP's are not going to work well to heat your spa during cool evenings.

Got it, thank you. Current thought is 2 heat pumps and 1 gas heater for exactly the reasons you mention =)

 

[ajw22]

Tell me why you think 2 heat pumps will be better then 1?

You think you will need more BTUs then 1 heat pump can give you to MAINTAIN the pool temp?

Gas heater will raise pool temp; HP will maintain pool temp.

 

[RubiconValley]

Tell me why you think 2 heat pumps will be better then 1?

You think you will need more BTUs then 1 heat pump can give you to MAINTAIN the pool temp?

Gas heater will raise pool temp; HP will maintain pool temp.

Hmm, that was the advice of the gentleman I spoke with at AquaCal. Two units of the AquaCal Heatwave Superquiet SQ166R (126K BTU; heat and cool) installed in parallel, instead of just one unit of the SQ225 (143K BTU; heat only)--apparently, the heat pumps that can heat and cool are able to continue to operate at lower temps? Does that sound reasonable? For a 43K gallon lap pool than I'm hoping to keep at 79 degrees Fahrenheit and swim in 365 days/year.

 

[ajw22]

OK, 43K pool maintaining at 79F 24/365 requires BTUs. I can see needing two HP's for that. You will not get 126K BTUs from those units in colder weather which is why you need at least 2.

Will the pool be covered when not in use?

 

[RubiconValley]

OK, 43K pool maintaining at 79F 24/365 requires BTUs. I can see needing two HP's for that. You will not get 126K BTUs from those units in colder weather which is why you need at least 2.

Will the pool be covered when not in use?

Thank you, that is reassuring to hear from you. Yes, we have an automatic pool cover, so the pool will always be covered when not in use. We also put in a massive Tesla solar system--and ran out of roof space for pool solar doing that. The gentleman at AquaCal said that with two of the heat/chill heat pumps, and the solar panels, we no longer need the pool solar--does that ring right to you?

 

[ajw22]

HP with PV solar will heat a lot better then pool solar heating. Especially in the winter when you have a low sun angle and short days.

You have the Tesla battery wall?

 

[RubiconValley]

HP with PV solar will heat a lot better then pool solar heating. Especially in the winter when you have a low sun angle and short days.

You have the Tesla battery wall?

A relief! Yes, we put in 4 Tesla Powerwalls. Getting my husband to accept lots of pool solar all over the yard would have been an uphill challenge. Thank you so much for your guidance!

 

[ajw22]

Sounds like you are set.

It will be interesting to hear how it all works out.

 

[RubiconValley]

Sounds like you are set.

It will be interesting to hear how it all works out.

I will report back when it's all finally done (fingers crossed).

 

[Homebrewale]

Why are the heat pumps being installed in parallel? You would be splitting the water flow into equal parts. To maintain the proper GPM flowrate, you would need to run your pump (assuming VSP) at a higher rate. Why wouldn't they be installed in series? That way you wouldn't need to split your water flow and could run the pump at a lower speed.

 

[ajw22]

Why are the heat pumps being installed in parallel?

The first HP in series will output hot water and the remaining HPs downstream will input hot water and never turn on.

 

[Homebrewale]

The first HP in series will output hot water and the remaining HPs downstream will input hot water and never turn on.

Thanks. I forgot about that. I guess if the output temperature from the first is higher than the set point on the second, it wouldn't turn on.

 

[scdaren]

Is there a reason you are looking at the 126k heater/chillers versus two 143k heaters? I don't think you need the chiller, you're not going to use it. I'm in the Fresno area, which gets WAY hotter than where you are. The only time of year that the pool is so hot we feel a chiller might be good is when we start getting into the 110's -- but the problem is at that point, the chiller just can't keep up. We have a Pentair 140 with a 15k gallon pool.

I just think don't it's going to be hot enough there for you to need the chillers, especially with the size of that pool. Better off having more BTU's for the rest of the year, and equipment with less complexity.

 

[RubiconValley]

Is there a reason you are looking at the 126k heater/chillers versus two 143k heaters? I don't think you need the chiller, you're not going to use it. I'm in the Fresno area, which gets WAY hotter than where you are. The only time of year that the pool is so hot we feel a chiller might be good is when we start getting into the 110's -- but the problem is at that point, the chiller just can't keep up. We have a Pentair 140 with a 15k gallon pool.

I just think don't it's going to be hot enough there for you to need the chillers, especially with the size of that pool. Better off having more BTU's for the rest of the year, and equipment with less complexity.

Valid point. I was told that the heat pumps that chill + heat (as opposed to just heat), operate better when the ambient temp drops (which it does for a few months in the year, and the hope is to swim at 79 degrees Fahrenheit 365 days a year)...

 

[scdaren]

Valid point. I was told that the heat pumps that chill + heat (as opposed to just heat), operate better when the ambient temp drops (which it does for a few months in the year, and the hope is to swim at 79 degrees Fahrenheit 365 days a year)...

That's interesting, I didn't know that, but I'm no heat pump expert. This year I did start experimenting with mine running at night due to much lower electricity costs, when it got down to the mid-50's, and it did seem to heat fine. I had been told that under 60 degrees it would stop working.

I suppose you might get over a 79 degree water temperature there in a heat wave, so at that point the chiller would be good to have for lap swimming purposes.

Figuring out the most cost-effective way to run these things is no simple matter -- the PG&E time of use rates play a really big role. One might assume that the best time to run them is when the solar is generating. But the problem is, at least for us on our electric vehicle rate plan, energy is more expensive during those hours, so it makes more sense to have the solar panels generating back into the grid at the higher rates, and then running the heater on grid power when cost of energy is lower. Lots of variables and moving parts, including in your case trying to figure out at what point your gas becomes more efficient to heat with than the hp's.

 

[JamesW]

You can do series if the heaters are controlled by automation because the automation tells the heaters to run, not the heaters own temperature sensors.

At 126,000 btu/hr, the temperature rise will be 6.3 degrees at 40 gpm.

Assuming the temperature going into the first heater is 78 degrees, the temperature going into the second heater will be 84.3 degrees, which will lower the efficiency a little bit, but the energy difference is offset by the lower pump speed required.