How much do heat pumps cost to run? An update.... part 3

Weather compensation

This is the 3rd of 3 posts updating our thoughts on the cost effectiveness of heat pumps.

Weather compensation in a general sense is just – it gets colder so you put on warmer clothes. In the context of heat pumps, weather compensation is a control function within the heat pump that adjusts the flow temperature in relation to the outside air temperature. 

Referring back to our previous posts, heat pumps use less energy to heat the water if the flow temperature is lower. The Samsung R290 produces 4.74kWh of usable heat for one kWh of electricity used at a flow temperature of 35°C and 2.84kWh of usable heat at a flow temperature of 65°C.

The rate of heat transfer is affected by many factors, including the temperature difference between the inside of the house and the outside. The larger the temperature difference, the more heat is lost. The house needs more heating when it's -4°C outside than when it's 10°C. 

So, if the house loses less heat when it's not as cold outside, it needs less heat put into it. If it needs less heat input, the heat pump can work at a lower flow temperature to provide that heat. If it's working at a lower flow temperature, it uses less electricity, and the cost goes down.

Heat pump manufacturers use different approaches to setting up weather compensation. This graph is from Vaillant:

With Vaillant, you specify the compensation curve. A curve of 1.2 has the following characteristics:

At 20°C outside air temperature, the heat pump flow temperature is 20°C = no heating. At -4°C outside air temperature, the heat pump will deliver a heating flow at around 55°C, which would be appropriate for a property with moderately oversized radiators.

Setting the weather compensation incorrectly can have a significant effect on cost. We recently serviced a Mitsubishi heat pump that provides heating via underfloor heating throughout the property. The control unit was set for a constant output flow temperature of 48°C with no weather compensation. We changed this to 20°C flow at 20°C outside air temperature and 45°C for an outside air temperature of -3°C. On a subsequent visit we asked the customer how they were getting on. They said they hadn't noticed any difference in the warmth of the house, but the electricity costs had significantly reduced.

Setting the weather compensation curve can be a little tricky and may need to be revisited a few times. We are currently fine-tuning the new heat pump system at Artimus Towers, an old Victorian house with solid walls and radiators sized for an oil boiler. We've had to adjust the heat pump flow temperature for low outside air temperature a couple of times. We now have a warm house with a flow temp of 58°C, which corresponds to an outside air temperature of -1°C. 

There is a long-standing belief that heat pumps do not heat old houses. This is totally wrong.

As we renovate the house, we are adding insulation to the walls and oversizing the radiators to further reduce our heating bills.

In summary, weather compensation saves money, so you should make sure that the heat pump being installed has this function and that it's set up correctly.