Air Source Heat Pumps

[Anton]

I am a relatively new user with air source heat pumps. I need to understand the electric consumption of these pumps, however my access to this data is limited. I have the flow rate and temperature difference of the return and flow which gives the heat power generated. The nominal electric power supply is at 240V and 5A for the heat pump. Is there a way to calculate/if there is formualic expression with the actual electric consumption of the pump from the compressor frequency?  The heat capacity of the heat pump is 8.5KW at 2.93 KW input at normal operating conditions.

Edited January 30, 2020 by Anton
Edit

[ProDave]

Electrical power is simply volts multiplied by amps *

 

So in your example, 240V multiplied by 5 amps = 1.200 Watts.

 

* I won't complicate it my including power factor

[SteamyTea]

I am not 100% sure what you are asking here.

The power of the pump is the voltage times the amperage, so 240 [V] x 5 [A] = 1200 W

But you say that it it a 2.93 kW input which is 2.93 [kW] / 240 [V] = 12.2 A

 

To get the consumption, regardless of the actual number, you multiply the power by time.

So if it is really 2.93 [kW] and it runs for 10 hours, that is 2.93 [kW] x 10 [h] = 29.3 kWh

 

Now if you have a inverter type ASHP, then the speed of the fan, pumps and compressor may all vary, this will change the amount of power used.

[Jeremy Harris]

The only way to accurately know the electrical input is to measure it.  As the power factor (PF) may well not be unity for a heat pump, measuring just voltage and current independently may not give an accurate indication of input power.  Fitting an simple energy meter, one that has a power display as well as energy, to the supply to the heat pump will give an accurate measure of input power.  Small, DIN rail mount energy meters like this are fairly cheap and seem to work well.  I have one fitted in a small enclosure on a test lead that I made up:

 

 

I have a similar one connected in the supply to our air source heat pump, and find it pretty useful.  It not only gives the energy (in kWh) that the heat pump uses, but also displays voltage, current, power factor and instantaneous power.  I think the one above cost about £15 or so.

 

[Anton]

Thank you all for the information. I understand that I would need the power factor to obtain electric consumption of the heat pump on a daily basis.

Thank you for Jeremy for that idea. I was just wondering if I can do anything with the heat compressor frequency to understand its electric consumption without any access to the other data mentioned like the power factor.

 

[SteamyTea]

23 minutes ago, Anton said:

was just wondering if I can do anything with the heat compressor frequency to understand its electric consumption without any access to the other data mentioned like the power factor

Do you mean the actual rpm of the compressor, or how frequently it is on?

Basically no is the answer.

Have you thought of getting an energy meter. There are some optical ones that read the pulses on your main meter.

[Jeremy Harris]

26 minutes ago, Anton said:

Thank you all for the information. I understand that I would need the power factor to obtain electric consumption of the heat pump on a daily basis.

Thank you for Jeremy for that idea. I was just wondering if I can do anything with the heat compressor frequency to understand its electric consumption without any access to the other data mentioned like the power factor.

 

 

 

The main problem is that many heat pumps will modulate their input power over quite a wide range, plus the compressor and fan are often not running at the same speed, or even at the same time.   For example, our ~7 kW output heat pump draws between about 400 W to over 2,000 W, and that input power fluctuates a fair bit.  On average it seems to sit at around 600 W or so, which is around 2,000 to 2,400 W output.