Pool heat pump installed!

[JP_]

So, how long should it take for a 4Kw heat pump to raise the temperature of a 9000L pool by 1 degree C on a day like today?

Temperate not moving. I am probably being impatient. Well, it went from 14 to 15, then back to 14 ... must have been on only an hour and a half ish ...

 

[StephenOak]

One joule of energy will raise the temperature of 1g of water 1ºC. 1L of water is 1kg =1,000g, so 9,000L will need 9 million joules to go up 1º.

The watt is a joule/second, so 4kW is 4,000 joules/second. So 9 million / 4,000 is 2,250 seconds or 37.5 minutes.

That assumes no loss of heat, which will not be true as soon as the pool water is warmer than its surroundings. Also the rate of loss of heat increases with the difference in temperature.

The actual rate of loss of heat (i.e. what you have to take off the heating effect) will depend on various factors
- Surface area of the pool - more means more losses.
- If it is covered.
- If it is in sunshine.
- If there is wind on the surface.

 

[JP_]

Hmmm, so I should have expected a bit more heating by now ... will check again. If it is on 16 I will stop worrying ... brb

 

[JP_]

It's on 16. But it is a sunny day, in the sun, with pool cover ...

When it is 20 I will be happy. Will check before bed. Or before...

 

[StephenOak]

Hmmm, so I should have expected a bit more heating by now

Theoretically yes. But it all depends on how much heat is being lost, and that depends on lots of factors.

9,000L is not that big, e.g. 3x3m by 1m deep. Is it above ground? If so it will loose more heat through the sides than an in-ground one would.

 

[Alec_t]

Doesn't the kW output of the heat-pump depend on the temperature of the air from which heat is being extracted?

 

[conny]

Have you checked what the thermostat is set to? We had an above ground pool, (18' x 12' x 4'), and from a morning temp of about 14C we could raise it to mid 30's by tea time. It was a 5.5kW air heat pump. Even in quite cold weather it raised the water temp by a few degrees each hour.

 

[JP_]

Thermostat set to 29 degrees.
Current temperature, 00.44, is 15 degrees!
Surface temperature had increased to 24 degrees earlier.
I am hoping that maybe it is because the warm water is sitting above the outlet and so the heater is still getting cold water.
But it has now been 12 hours, so maybe it is just not working!

I did contact the supplier this afternoon and they said ...

Thank you for getting back to me, your set up looks fine.
The air valve is not critical so you can keep air out of the system.
The 14 degrees is the temperature of the water so as long as you set the target temperature higher the heat pump should heat the pool water.
Dependant on the size of your pool the heat pump will take between 3.5 to 4.5 days to get the pool water to your target temperature, this assumes a solar cover on the surface of the water.
​

This calculator suggests 43 hours

Volume: 9000 litres
Start Temp: 14 C
End Temp: 29 C
Energy input: 4000 watts
Efficiency: 90 % (this is probably way off?!)
Time taken: 43 hours 30 minutes
Energy used: 174.00 kWh
Cost: 33.06
​

This calculator (if I input the correct figures) suggests 36 hours

Hopefully tomorrow it will start to warm up where the outlet is ....

It is about 20 degrees out there tonight, and the pool has a bubble cover on the surface, and a tarp over to reduce airflow. It really should seem warmer after 12 hours ...

 

[conny]

The problem with these type of pools is that both the hot water coming in and the 'cold' water going out are located at the top.
So basically, as heat rises, what happens is the 'hot' water is pumped in at the top, swirls around on the surface and is then drawn into the skimmer compartment which naturally returns it to the heater. The heater is sensing the return flow as quite warm so doesn't heat up any more until it feels it is needed, but in actual fact the main body of water is quite cold so sits below the return outlet.
A pain in the backside but try going out each hour and giving the whole body of water a good stirring with a canoe paddle, if you have one, of a soft bristle sweeping brush. This will make the return flow much cooler so the heater will kick in and begin to warm it more often.

 

[JP_]

Well, the weekend was a success. Water temperature on thermometer in the pool was 31 degrees all the way down, but the temperature on the heater peaked at 21 degrees. But there is no way it was only 21 degrees. So heater works, but I suspect the heater's thermostat, or just the display, is not. Will speak to supplier today.

 

[MMarve]

As mentioned earlier in the thread but it may have passed you by so I'll try and make it clearer.

The Kw output of your heat pump is dependent upon the ambient temperature. So when the air temerature is 15 degrees, you will get nowhere near 4kw of power from it. When it is 30 degrees, you will closer but still not at the full 4Kw. Although it has been "hot" the last few days, it only really got much above 25 over the weekend and even then only for a few hours in the middle of the day.

Just something to bare in mind when you are trying to do your maths to work out how quickly you should be heating.

 

[JP_]

As mentioned just now, the pool heated up OK. But the heat pump reading suggests not.

The pool is very warm this morning, 30 degrees at the top, even though the heat pump was set to heat to 25 degrees. Warm water is still pumping out, so either it just did not cool down last night, or it is ignoring the thermostat

 

[JP_]

Time for a dip in the nice warm pool.

I've turned the thermostat down a bit now!

 

[conny]

Was this taken near the top of the water or at the bottom of the pool? As heat rises you may find the top layer of water is warm but the lower levels are quite cool. A good stir up first will give a truer reading.

 

[JP_]

no, this is fully heated. I put the thermometer at the bottom and just as warm. Was too warm really!
System is a bit annoying. The heat pump thermostat does not record correct temperature at normal pump flow, says it is cooler, so warms too much. I reduced setting to a lower temperature, but if the filter gets clogged and flowrate slows, the heat pump thermometer records a more accurate temperature, and cooling stops. Pool was a bit cooler today, still OK, but not warm.