In August 2019, we began work on a large extension to the side of our house in Bedfordshire taking the size to roughly 4,200sqft. At the time we had an oil fired combi boiler that was installed in 2014 when we purchased the house. The house was a converted barn/farm building with 70% of it being a new build.
The insulation was done to the standards of the time. It was a 4-bedroom house with three bathrooms and the combi boiler easily heated the house with underfloor heating downstairs and radiators upstairs. The hot water, however, was another story with it only being able to run one tap/shower at a time, and having 5-6 of us in the house was not suitable.
The extension was taking the house to 7/8 bedrooms and five bathrooms. We don’t have access to mains gas, so at the time I had priced a new oil fired system boiler and unvented cylinder into the extension. Early on in the build the company installing our wood burning stove visited to fit the ceiling brace and it happened to be the owner of the company doing the install – Artizan Heating.
The owner asked what we were doing for heating and hot water and suggested an air source heat pump (ASHP). I’d never really looked into them or knew much about them. He put together a quote for a Mitsubishi Ecodan 14kW heat pump with a 500 litre unvented cylinder.
They felt our house wouldn’t need any extra radiators, but I didn’t want to take any risks as our house is exposed to the wind and when it’s windy and cold the house really feels it, so I added two extra radiators to the largest bedrooms upstairs and this turned out to be the right decision.
At the time, the quote was roughly £13,000. Artizan Heating made me aware of the RHI scheme and helped get me set up very quickly and we will be paid back roughly £10,000-11,000 over seven years which takes the cost down to less than it would have been to fit a new high-powered oil fired system boiler and cylinder.
The installation took place over 3-4 visits due to issues with the main builder and the plumbers needing to do prep work for the heat pump installation. While all the work was going on, we used it as an opportunity to install a Stuart Turner mains boost system recommended by the plumber. For anyone wanting to run multiple taps, baths and showers at the same time, this system is a must. We also installed a secondary return as it’s quite a long pipe run from the hot water tank to the main house. All the pipes running in the trench under the drive were fully insulated.
Once all the building work was finished in January 2020 we were able to start learning to live with an ASHP. I think what most people will tell you after living with an ASHP is that it’s trial and error to start with. You have to invest time and effort into fully understanding how your system operates and what it likes and doesn’t like.
Downstairs all rooms have their own Heatmiser Neostat to control the underfloor heating. The upstairs radiators had a thermostat located on the landing. This was one of the first things I changed as I couldn’t get upstairs to a comfortable temperature in all rooms as the landing isn’t exposed to the wind, so it would stay warm while the bedrooms would be cold and the heating wouldn’t come on.
I switched to the Honeywell Evo system that transformed the heating upstairs allowing each individual radiator to call for heat when it needed it. Downstairs, I leave the Neostats all set to 24C 24/7. We keep our house warm, most people complain it’s too hot, but as its quite open plan I feel the thermostat readings aren’t comparable to traditional heating system thermostats, so I would say its closer to being 22C actual temperature inside the house.
Most people seem to suggest it’s more efficient to run the pump 24/7. While this makes sense on paper, in reality I leave the system to do what it needs to do to maintain the temperatures set in the house. On the coldest days it will run non-stop, but on days where it’s 6C outside with zero wind and loads of sun our house heats up quickly and the pump will go off for most of the day until the early evening when zones start to call for heat.
We learnt quite quickly that you have to think and plan 24 hours ahead for bad weather. You cannot simply arrive back from work in the evening on a cold windy day and assume you can crank the heating up. It will simply overwhelm the system. We have two log burners in the house that we light on the coldest days around 3-4pm to reduce the workload on the heat pump. If we didn’t run the log burners I would say the house would struggle if we had day-after-day of subzero temperatures with wind. It can cope with low temperatures and no wind, but you do feel a slight difference in certain rooms that struggle, but it’s not cold.
We have now lived with the system for over a year and I can give the following feedback. I often run the system on the weather compensation curve which automatically sets the flow temperature depending on the outside air temperature to save electricity.
If we have cold weather coming, I will manually set the flow temp to 45C to ensure it is ready, and to heat up the concrete slab of the house as well as possible. I’ve noticed that simply running the secondary return on the hot water system is responsible for a 2-3C an hour drop in the hot water cylinder. When it doesn’t run late at night the hot water tank hardly drops a degree. Sadly this is due to the length of the pipe runs and probably not insulating the pipe well enough within the ceiling, floors and walls of the house.
I have settled on our hot water setting being a target of 50C with a maximum drop of 5C to 45C before the system kicks in to heat up the water back to 50C. Once a week the system automatically heats the water to 65C for legionella protection.
Our running costs for a year are as follows. The system consumed 14,000kWh of electricity at a cost of £1,800 in electricity. Our electricity tariff was between 10-14p during this period. We could easily reduce this bill by turning down our room temps (which is not possible for us living with my nan who is always cold) and switching off the secondary return, although there would be an increase in water consumption waiting for the hot water to get to the taps.
I’m not sure how much oil we would have consumed as I have no before data for this, but I would assume it would be higher than the cost of electricity due to how hot we run our house and how much hot water we consume with up to 11 people living in the house at times.
We had the first service completed this year by a local certified Mitsubishi installer called Air Cool Engineering who were excellent. The cost was roughly £180 and they inspected the whole system including three manifolds, all radiators (and found two leaking valves I was unaware of) and a leak on the secondary return valve. The engineer wasn’t happy with the location of the pressure gauge as it was giving a false reading and he also wanted to fit a bypass valve to make the system more efficient.
We agreed to the work and it’s made the pressure gauge more accurate and the system seems to be flowing well with the bypass not allowing the freshly heated water to return straight back to the pump unless the pressure is high enough with the circuits closed.
Overall we are very pleased with our system and would happily install another one should we move or build another house. As long as it’s correctly specced to your requirements and installed well, it is a great alternative to gas or oil boilers. If you want a plug and play system that responds quickly to your needs, this is not for you.
There will be annoying days where you will feel it’s not powerful enough or working correctly but it helps you learn how to control your system better and prepare next time. There is very little work I need to do day-to-day except keeping an eye out for bad weather and checking the pressure gauge every few weeks.