Heat Pumps – the Good, the Bad and the Ugly
I have had a ground source heat pump for the last 15 years. In 2019 we had an extension built and added an air source heat pump to the system. This article is an objective view of the pros and cons of heat pumps from somebody with practical experience.
Heat pumps are supposed to be an efficient, renewable energy source. Is this true? They are expensive and take up much more space than, say, a gas combi boiler. Since 2021 new installations of heat pumps have been eligible for a grant of £5,000. Prior to this those who installed a heat pump in their home received a quarterly payment as they used the system; this payment was called the Renewable Heat incentive.
One of the most obvious disadvantages of a heat pump is the amount of space they take up. If you use a heat pump for room heating and hot water you need two tanks – one for the hot water and one for room heating. Add in the heat pump itself and it becomes impractical to house this inside most modern houses. The installation pictured is for a large four bedroomed house so would be smaller for a smaller house but still difficult to fit in. Most installations are therefore moved out to the garage. We now have two elements that combine to reduce the efficiency of the system.
Ground Source heat pumps are supposed to generate 4kW of output for every kW of energy input. For air source heat pumps this ratio is slightly less at 3:1. But as the water is stored it loses heat. As the water is transferred from the heat pump around the house it loses more heat. And since the heat pump is often situated away from the core of the house it is usually necessary to circulate the water around the house so that hot water arrives at the tap, but constantly circulating the water loses even more heat. I estimate that the true efficiency of the total system, including ground source heat pump, is around 1.75:1 and the air source heat pump around 1.5:1.
One of the features of a heat pump is that it does not heat the water as hot as a traditional gas boiler. For hot water heating a gas boiler is usually set to around 60°C. A heat pump is usually set to 50°C. The hotter you want the water the less efficient the heat pump becomes. The factory settings for my heat pump had a set temperature of 50° with a temperature drop of 10°. So, the heat pump heated the water to 50° then switched off. The water temperature would drop to 40° before the heat pump kicked in again. If you are unlucky when you run your bath the water is just above 40°. So, if you like a hot bath – tough. To help maintain a higher temperature I have changed the set points so that the set temp is now 51° and it drops to 43°before heating up again. These changes will have reduced the efficiency of the heat pump but make the water temperature a bit more acceptable.
The water for room heating is also lower. With underfloor heating the set temperature is usually around 35 – 40°. This usually varies with outside temperature. If it gets colder outside the set point increases so the water going through the underfloor heating is hotter. If the underfloor heating is running all the time and the room isn’t warm enough it is possible to increase the temperature of the water going to the underfloor heating. The trouble is, the higher the temperature, the less efficient it becomes. Underfloor heating is good for solid floors but not so good for suspended floors so the usual recommendation for upstairs rooms is low temperature radiators. These are larger than normal radiators and, like underfloor heating, are best left on a low temperature all the time. But this is very wasteful for bedrooms. Bedrooms are unoccupied most of the day and during the night you are snuggled up under a quilt so you only need heating for a short time in the evening and a short time in the morning. The best solution for this requirement is a traditional gas combi boiler. If you don’t have gas, a short burst from an electric heater is better than having your heat pump, underfloor heating or upstairs radiators running all day.
The good thing about underfloor heating is that (sorry for stating the obvious) you don’t have any radiators so this gives more freedom when designing your rooms. Unfortunately, underfloor heating isn’t reactive. It gives a low heat all the time but can’t give heat quickly. So, if you were out at work all day you couldn’t come home and switch on the underfloor heating and expect it to warm up quickly; you would reach temperature in time for the ten o’clock news. However, the underfloor heating/heat pump system is ideal for homes where the house is occupied most of the time, eg for elderly people.
If you think that a heat pump is the system for you, I would make the following recommendations:
Install in new builds only
Design so that the heat water is produced near the kitchen and bathrooms (to reduce heat losses in transfer)
Insulate all hot water pipes
The advice given by heat pump installers is to insulate the home properly before installation – make sure loft insulation is adequate, windows are double glazed and draught free, cavity wall is insulated. All this is true but it then makes it impossible to evaluate the impact of the heat pump. The suspicion is that most of the savings come from the improved insulation.
After you have done all that, you then have a problem with ventilation and condensation may become a problem. Extractor fans in kitchen and bathrooms are necessary but suck warm air out of the house so a heat recovery system should be considered. The problem is that these are very expensive and the payback time is about a hundred years. My solution to the excess moisture in the air is to run a dehumidifier, but then, of course, you are increasing your electricity usage and any savings are soon wiped out.