Heat pumps not only have to heat your home they also have to provide hot water which presents several new challenges.
Traditional oil and gas boilers work at high flow temperatures, usually greater than 60C, which are perfect for heating hot water to a sensible temperature for household use. Well designed systems can heat water and house radiators at the same time. There’s sufficient power available that many households have ‘combi’ boilers that heat water as required, doing away with the need for a hot water cylinder.
Heat pumps can’t work this way. The flow temperatures used for heating are usually 30 – 40C, not enough to heat water. They can’t heat water instantly so a hot water cylinder is a must. A separate operation mode is needed with a higher flow temperature, typically 50C, for water heating. You can’t heat your house and hot water at the same time. In order to heat water quickly a heat pump specific cylinder with a larger heat exchanger coil is required. Heat pump cylinders tend to be higher capacity so that enough hot water to meet the house demand is readily available – we moved from a 140L cylinder to a 250L cylinder when our heat pump was installed.
It all sounds a bit daunting but if you understand the fundamentals it works well. The graph shows our hot water tank temperature over almost two days. At this time of year the heat pump is set to heat water to 50C at 01:45 am using cheap overnight electricity – it usually takes less than an hour. The water in the tank stays hot enough for a decent shower or two until the next day – the benefit of a large cylinder capacity. 40C is enough for a hot shower.
And then there’s legionella…
…which is a type of bacteria that can cause serious illness in humans. It can multiply in warm water between 25 and 45C but is killed by higher temperatures – the higher the temp the quicker it dies, 2 hours at 50C, 2mins at 60C – source Wikipedia. With a heat pump we are not usually storing water above 50C and for some of the time the water temperature in the cylinder will allow any legionella bacteria to multiply. So probably a good idea to take steps to mitigate the risk.
Incredibly it is really hard in the UK to find any official advice that refers to domestic heat pump hot water heating systems – at least we couldn’t find anything. There’s plenty of ‘information’ out there from all sorts of sources but it is a struggle to know what to believe. Even our heat pump manufacturer dodges the issue in their video on the subject advising you to talk to your installer. We don’t think it is good enough, the UK government are currently offering a £5,000 grant towards a heat pump installation but there’s no easy to find legionella guidelines.
Anyway our installer recommended heating the cylinder to 60C once a week to reduce the risk. We have a timer on the immersion heater set to run once a week and the immersion thermostat is set to just over 60C. It takes a little over and hour to raise the water from 50C to over 60C.
Using cheap overnight electricity the cost is 3 kWh x 8.25 p/kWh – around 25p a week or £13 a year. Given the hot water tank is above 50C for a couple of hours every day we are not completely convinced this additional heating is necessary but for £13 a year aren’t going to worry. If we were paying 34p a unit that would be £53 a year and we wouldn’t be so relaxed. Come on UK government step up and provide clear sensible guidelines for heat pump owners.