High temperature heat pumps: do they really cost a fortune?

In his latest column, Graham Hendra addresses a really hot topic – high temperature heat pumps

The heat pump industry is currently split into two camps, separated by temperature. The low temperature heat pump police hate a high temperature heat pump. Meanwhile, those of us who love a high temperature heat pump can’t see what the fuss is about. Most of the low temperature arguments come from manufacturers still peddling old low-temperature technologies who have not upgraded their ranges.

All the good manufacturers now offer heat pups which can reach 80 deg C, thanks to R290 the new wonder refrigerant. But just because it can get to 80 deg C doesn’t mean you have to run it at that temperature. My car can do 150 mph, but I don’t drive it that fast, officer.

All the smart money tells you run the heat pump super cold and save a fortune. But is it true? Let’s take a look.

Running the numbers

The average UK house is a 3-bed semi, with a floor area of 120 square metres, built in the 1930s, but now has loft insulation and double glazing.

This house uses 12,000 kWh of gas to deliver 11,000 kWh of heat and hot water per year. The rest goes up the flue.

It is assumed by all the people ‘in the know’ that running a heat pump at high temperature is massively expensive.

Au contraire. Here are the figures based on a top-ten performing unit from a mass manufacturer:

If you buy a heat pump and run it at 65 deg C, in this house it will consume 3,666 kWh of electricity per year.

If you run it at 60 deg C, it will use 3,313 kWh;

If you run it at 50 deg C, it will use 2,835 kWh;

If you run it at 45 deg C it will use 2,650 kWh;

And at 40 deg C, it will use 2,494 kWh.

If we assume a cost of £0.30  per kWh, then at 65 deg C, it will cost £1,099 a year or £91 a month

At 60 deg C it will cost £994 a year;

At 50 deg C it will cost £945 a year;

At 45 deg C it will cost £795 a year;

At 40 deg C it will cost £748 a year.

A sensible idea?

“Ok,” I hear some people saying, “So if you buy a heat pump and run it at 40 deg C, it will save 50% against a unit running at 65 deg C? Then surely it’s case closed: high temperature heat pumps are a bad idea?”

Not so fast: Most people who can do the maths – or who have actually measured it – have worked out that while a gas boiler may be set at 70 deg C, the boiler cycles and is time-clocked, so its average outlet temperature is much lower than 70 deg C.

You can get the same warmth in the house by running the boiler (or a heat pump) for longer periods at lower constant temperatures. The radiators, when installed with a gas boiler, are not sized to run at 70 deg C all day. If you measure the radiator temperature over a 24-hour period on a very cold day it is likely that it averages 50-55 deg C.

So, setting the heat pump to 60 deg C means you can keep the old rads and pipes and run the heating system for longer periods of time and you will be nice and warm. If you don’t believe me, try it with your boiler: next time it’s cold, set the boiler to 60 deg C and run it 24/7, I’m willing to bet you will be warm. Then turn it down to 55 deg C for a day, then 50 deg C the following day  until the house no longer gets to temperature. This way you can find out how hot you actually need the water to be. It’s a way of trying a heat pump without buying anything.

So based on this illustration you can see why using run costs for a system set to 65 deg C is not a sensible comparison – no one runs their heat pump at 65 deg C.

Now let’s look at the cold end of the scale. In theory you can run your radiators at 40 deg C on the coldest day: all you have to do is make the radiators bigger. A radiator running at 40 deg C needs to be twice the size of a radiator running at 60 deg C to spit out the same amount of heat. If you are happy to install new radiators which are much bigger than the old radiators, that’s great. But this is the reason why most 40 deg C systems are done using underfloor heating: because the size of the floor is huge compared to a radiator, so they can run at much lower temperatures.

A better comparison

Back to our average house: we can normally dismiss the 40 deg C run temperatures, as they are very hard to achieve. And, as described above, the 65 deg C run temperature is never going to happen. So, in practice, we need to be comparing a cold system running at 45 deg C, with all new radiators, to an existing system running at 60 deg C. That gives a saving of £195 a year.

If each new radiator costs you £200-300 to replace, that’s quite a long payback.

But most importantly, if you want super-efficient low-cost go for low temperature. If you want quick, cheap, simple, no fuss installation go for a higher temperature. Any decent installer will give you the option, costs and run cost to make your own decision. That’s what we do at Genous.Earth: you put some data into our website and we provide you with all the figures.

At the end of the day, it’s down to the homeowner. It’s their house and they can do whatever they want. All we can do is try to persuade them.