Heat pump confusion - colder ambient temp

[YonnyPiscinas]

Hi guys,

Sorry for my ignorance but I've always had a doubt about how a heat pump would work if the pool temp is still high but the ambient air temp is low.

Let's say my pool is at 30c already and the air temp is only 20c.

Can the heat pump still transfer heat to the water and keep it at 30c or even raise it more than that?
Do they have tables of ranges they work at?

My pool is 35c/36c right now (I know boiling and too hot for most) and next week we have colder weather hitting us.
20c max. On those cloudy cooler days would I be able to maintain this 36c? We have covers on the pool.

Thanks

 

[Joshii]

Yes it can work to transfer heat to the pool under those conditions. And it will almost certainly be able to maintain that temp if the pool is covered

 

[YonnyPiscinas]

Yes it can work to transfer heat to the pool under those conditions. And it will almost certainly be able to maintain that temp if the pool is covered

Thanks for the confirmation. With the current covers I only lose about 1c overnight with an overnight temp fo about 18-20c and pool temp of 36c.
Current solar panels are running for about 7 hours a day if the sun is shining.

We're enjoying the pool so much heated thinking about installing a heat pump for when the sun loses its energy during autumn when we can still have warm/hot days (and cloudy hot days).

It's only a small pool too with about 19,000 litres

 

[Joshii]

The way you keep it, it's not a small pool it's a large hot tub

 

[YonnyPiscinas]

The way you keep it, it's not a small pool it's a large hot tub

LOL very true indeed. You get in slowly to avoid the heat shock.
Sitting in there at 11:30pm with a cold beer and some music is just the best though. Not so great on a hot day when you want to refresh.

I'm sure once the heat wave is over we'll be back at 30c temps so just taking advantage.

 

[jseyfert3]

Of course a heat pump can do that. An air conditioner is a heat pump too, it's just not called that. An air conditioner takes heat from your cold house and pumps it into the much hotter outdoors. That's why the air blowing out of the outside air conditioner unit is hot.

Same thing works for a pool/spa heat pump, only they are now taking the heat from the "cold" outdoors and pumping it into the hot pool. In general, they stop being effective only when you get down close to freezing air temps (like 10 °C and below).

 

[ajw22]

. In general, they stop being effective only when you get down close to freezing air temps (like 10 °C and below).

Most pool heaters stop being effective at 50F.

Heat Pump Performance

• ajw22
• Feb 23, 2019

Pentair Ultratemp Heat Pump BTU Output by OAT

 

[YonnyPiscinas]

I've read that the SWG needs to be placed after the heat pump as the concentrated chlorine can damage the internals.
Is this true or can the heat pump be placed last in line?

Thanks

 

[Newdude]

I've read that the SWG needs to be placed after the heat pump as the concentrated chlorine can damage the internals.

The recommendation is for the SWG to be last for that reason.

 

[jseyfert3]

Most pool heaters stop being effective at 50F.

Heat Pump Performance

• ajw22
• Feb 23, 2019

Pentair Ultratemp Heat Pump BTU Output by OAT

Sounds like the 10 °C number I gave.

 

[YonnyPiscinas]

The recommendation is for the SWG to be last for that reason.

Ohh that complicates my piping a lot.
Really eager to add a heat pump now as everyone is loving the warmer pool.
Plan is just to use it on the cooler days when the panels can't maintain the heat.
I'll need to control 2 bypasses, one for the panels and one for the heat pump but as you say the SWG needs to go last. Hmmm

 

[Newdude]

Post a pic of the equiptment area and we can get some ideas going with the collective TFP brain. Also, dont forget that you can go up with the SWG. Many people have used vertical pipe when the space is limited and the SWG positioned with upward flow so gravity doesnt trigger the flow switch when the pump is off.

 

[YonnyPiscinas]

Thanks a lot guys.

What size of heat pump would you recommend for a 19,000 litre pool? 4180 gallons.
Would a 9kw pump be sufficient?

 

[YonnyPiscinas]

Set up is not the best. Bit embarrassing compared to most on this forum. Was adapted to an existing pipe system and moved to under an exterior kitchen worktop.
Guess the best would be to feed the solar collectors directly from the output of the filter with a bypass, then another bypass system of 3 valves to feed the heat pump outside the enclosure. Then back in to the SWG. That way I can have the solar collectors running in addition to the heat pump or one or the other..

Maybe go vertically up from the filter discharge and install the bypass valves against the back wall for the solar collectors first.
Or have all the valves for the bypasses outside next to the heat pump and just another pipe going back in to feed the SWG under the sink. I'm inclined to think this is the best solution

 

[YonnyPiscinas]

Place for the heat pump and bypass valves

 

[YonnyPiscinas]

Also I'd like to calculate the running cost.
Our electricity rate up until 5pm is 3.45 pesos kWh.
Which is about 9 cents at current exchange rate.

So a 9kw pump consumes about 1.54kw I see.

let's say 8 hours runtime. 13.86 cents an hour?
1.11 USD for 8 hours
This doesn't look right.

 

[jseyfert3]

Is the 9 kW input power or heating power? What specific pump are you looking at?

If that is output power, 1.54 kW input power assumes a COP of 5.8. While certainly not impossible, assuming ideal conditions, I very much doubt with your desired pool temps and an air temp of 20 °C that you would have a COP that high. My rough guess is a COP of ~3 or so.

Problem is a 9 kW (output) will be at best case COP, so the actual heating value would go down as the conditions diverged from ideal. You need manufacture charts to get actual heat output at specific temps.

 

[YonnyPiscinas]

HI these are the specs.
Cop of 5.8
9kw heating power

Haven't got my head around cop yet
Have to read around it tonight.
So the cop falls as the air temperature decreases as less heat is extracted per unit of input power.

 

[jseyfert3]

COP (actually C.O.P.) is coefficient of performance. How many kW of heat get moved for each kW of power input. Calculated as output/input.

A pump with 5 kW output than required 1.5 kW inout power has a COP of 3.33.

As the water temp increases and/or air temp decreases the COP goes down, as it depends on the temp difference between the two.

Say for the one you are getting that at your operating temps you get a COP of 3 instead of the optimal COP of 5.8. in this case, your input power will be mostly the same, but since the COP is lower due to non-ideal temps, your heating power is only 4.62 kW, instead of the rated maximum of 9 kW.

You need to size the heater based on the actual COP your temps will achieve, not the maximum possible COP.

 

[YonnyPiscinas]

COP (actually C.O.P.) is coefficient of performance. How many kW of heat get moved for each kW of power input. Calculated as output/input.

A pump with 5 kW output than required 1.5 kW inout power has a COP of 3.33.

As the water temp increases and/or air temp decreases the COP goes down, as it depends on the temp difference between the two.

Say for the one you are getting that at your operating temps you get a COP of 3 instead of the optimal COP of 5.8. in this case, your input power will be mostly the same, but since the COP is lower due to non-ideal temps, your heating power is only 4.62 kW, instead of the rated maximum of 9 kW.

You need to size the heater based on the actual COP your temps will achieve, not the maximum possible COP.

I see thanks a lot.
There are 12kw pumps that use 2kw.
I don't think I'd like to use more electricity than that.

We do use 2 covers also. A bubble type floating on the surface and a semi rigid cover across the top which both help.

Although my wife has not given me permission to spend 2k yet maybe next season