Is it a fair COP?

When consumers are comparing heat pumps the no 1 thing, they search out is the COP. COP or coefficient of Performance is just a measure of percentage efficiency. Take power out divided by power in and you get COP. In all heat pumps this figure is always above 1, if it was expressed in percentages that would give a figure above 100% which although true is hard to believe so we use COP instead.

COP is usually measured at 35-degree flow temperature at an ambient of 7 degrees C, its known as the bullshit COP. It is a totally unrealistic measurement.  I have never seen a unit run in this condition, at 7 degrees ambient no one wants their radiators running at 35 degrees C.  It would be far more realistic to publish the COP at -2C and 50 degrees flow but I do not make the rules.

I liken COP to miles per gallon MPG in cars. The better the MPG the cheaper it is to run. Everyone knows the advert says the car will do 50 MPG but only if you drive everywhere at 56 mph. COP is the same, as long as the temperature outside is a constant 7 degrees C you will achieve the bullshit COP figure.

 A few years ago, a measure called SCOP or seasonal COP was devised, this gives an average or Seasonal COP across the whole year. Its not a bad measure and gives a pretty good estimation of the efficiencies you are going to achieve. So, my advice is ask for the SCOP and ignore the COP figures.

Now everyone who is a master of Heat Pumpery knows that ALL heat pumps are more efficient if the weather is warmer and the water you are supplying is colder. I've plotted an example below. You can see the COP or efficiency falls as the water temperature rises and the COP falls with the ambient temperature. You must pay for “the lift” or lifting the temperature from the outdoor air to the water temperature.

OK that is clear, most people believe heat pumps are more efficient in Summer and less efficient in winter, but…………………. my cop is lowest in August and September, Why?

The answer is a simple one. Every heat pump consumes power just by being switched on, it is not much power but like a boiler if the power is on the controls and the PCB are all lit up just waiting to go. Let us assume that my unit uses 1 kW hr of electricity per day just being switched on. That is the equivalent of 40 Watts. This is wasted energy it is in the garden warming up the air.

In August, my unit does no heating and only heats the tank once a day, it only produces 3.1 kWhrs of heat a day. So that kW hr of just being switched on represents 33% of the heat produced.  This ruins the cop. In February and March my unit produces 60-70 kW hrs of heat a day (its winter so the heating is on) so the 1 kW hr standby power is less than 2% of the heat produced.

So, watching your COP every day is a mugs game, the figures are skewed in summer. If you never turned your heating or hot water on the heat pumps would still use 365 kWhrs of electricity a year and produce no heat, that is an SCOP of 0.

Top hint, your boiler is doing the same, but no one is measure int eh amount of electricity it uses. They only watch the gas.

So don’t watch the COP every day, look at the actual run cost. In the end its the bit you are paying for.