Getting the best "figures" out of your heat pump, a cheats guide.

Our industry, heat pumps is not a place where marketeers live. You can tell this because we think talking about COP is interesting. Shock news, its not, but please keep reading.

COP is basically efficiency divided by 100, so a gas boiler is 90% efficient or it has a COP of 0.9

I often tell people COP is just miles per gallon. But there is a difference, the thing I like about miles per gallon is its quite hard to fudge and easy to test. You take the distance you've driven since you last filled up and divide it by the number of gallons or litres you put in the tank. Some new cars even tell you the figures for people who cant do division. Everyone knows that a figure of between 40 and 60 miles per gallon is realistic, anyone claiming 100mpg or higher is a dreamer, cant do maths or is a bull shitter.

In the heating world its a bit vaguer.

I'm boring, I measure the total electricity into my unit, its pumps and all the controls and the actual heat out using 3rd party RHI spec heat and electric meter. my figures are:

In 2022 I used 4750 kWhrs of electricity it delivered 13049 kWhrs of heat. That's an average cop of 2.74 over the whole year. Why so low?

Firstly I run my unit hot with room temperatures of 23C, the rads are very warm but weather compensated and we use a lot of hot water. The COP in hot water is never very high because you want the water hot no matter what the weather is like outside.

So how can I make my unit sound better without doing anything drastic?

simple, use creative maths.

Most heat pumps now tell you the cop you are getting on screen as an average over the last month or year. This figure is always much higher than the figures I measure, how?

Let me explain: your heat pump is made up of the following electrical components:

a compressor,

a fan motor,

some refrigerant valves,

a printed circuit board or two,

a water pump,

some controls thermostats etc

The compressor, fan motor and water pump are only on when the unit is making heat, they absorb the most energy and do the most work. My heat pump ran for 4296 hours of the 8760 hours in 2022.

All the other stuff is on all the time. I covered this in a previous blog https://www.dhirubhai.net/pulse/my-heat-pump-costing-me-fortune-while-sits-idle-summer-graham-hendra/

The conclusion of the blog is my unit uses a kWhr of electricity every 71 hours or 122 kWhrs a year. even if you never switch it on. this is just the power needed to keep the pcbs alive.

Now imagine if the inbuilt energy meter chose to ignore the power needed to power up the PCBs, in my case the power used would be 4750 - 122 = 4628 kWhrs. Divide 13049 kWhrs of heat by 4628 and the cop is now. That's an average cop is now up to 2.82, we are getting there.

my water pump ran for 4296 hours, it draws 75 Watts when running. That's 300kWhrs. If I "forgot " to measure the water pump energy use I would only use using 4328 units of electricity a year and producing 13049 kWhrs of heat that's a cop of 3.01 now we are motoring.

The fan motor in my unit draws another 75 Watts so that's another 300kWhrs a year. 4038 / 13049 = 3.23........

The truth is in nearly every heat pump I've ever worked on the onboard energy monitoring only measures the energy used by the compressor. All the other components are ignored.

So its not very hard to get a COP figure on the screen of the unit that gives a nice high figure.

Now before the boiler lads leap about and say, I told you heat pumps are crap, ask them how many kWhrs of heat they get for all that gas they pour in the front. No one ever measures it, everyone just accepts its 90% efficient. Man we are gullible fools.

So here is my guide to reasonable achievable real COPS.

If your heat pump is badly set up or you are using it incorrectly the COP will be low, anything less than 2.5 and you should be taking a look into the system. Call an engineer.

If you have radiators 3 to 3.5 is about right.

With a good underfloor heating system 4 is achievable.

Anything over 4 and you are a dreamer, or you run the house like a fridge or you are being lied too. Its possible to get insane COPs a number of ways, believe the onboard figures, never heat any hot water, turn your unit off at the mains when you are not using it and go on holiday for the Winter, then you can be the pub bore and claim a cop of 5.