Heating & Hot Water System

Our hot water was provided by a gas boiler in the kitchen and our central heating by a vintage specimen gas boiler from 1985. Being a simple enough contraption it may well have carried on for years, but it would only be about 60% efficient- that is to say 40% of the chemical energy in the natural gas it consumes is lost as waste heat through the exhaust vent.

Gas boilers have improved vastly in the last couple of decades, so a modern condensing boiler that recovers heat from the exhaust fumes is now 90% efficient. You may be able to get Green Deal Cashback to help with the cost of upgrading to a condensing boiler.

We opted for a partly renewable heating system- a Heat Pump to provide our heating and hot water. A heat pump takes heat from outside at a low temperature and pumps it up to a higher temperature useful for the heating system. This isn’t some kind of weird black magic, but rather the same technology you already have happily in your fridge. A fridge takes heat at a low temperature from inside the fridge and moves it to the black grill on the back at a warm temperature, where it dissipates into the room. A heat pump is the same principle in reverse- moving heat from outside to the radiators in the house.


Air Source Heat Pump + Hot Water Tank

Product Air Source Heat Pump
Mitsubishi Ecodan air to water heat pump +
twin coil (solar ready) hot water tank
Rating 8.5kW
300 litre unvented tank
Cost £11,500
Running Cost expected £300 a year
approx £500 last year (complicated, see ‘Consumption’ below)
Saving £550 + 2.6 tonnes CO2 a year
36% saving on an old gas boiler
RHI Income £1000 a year for 7 years
Payback time  8 years
Supplier Pure Renewables


Renewable Heat Incentive (RHI)

The RHI is a government scheme that pays for renewable heat, such as generated by solar thermal or biomass boilers.  The domestic RHI finally launched in April 2014.

A Heat Pump supplies heat that comes partly from renewable energy (the outside air or ground) and partly from electricity.  The renewable portion of the heat is estimated and qualifies for the RHI.  The rate for an Air Source Heat pump is 7.51p per unit (kWh), as of Spring 2016.

Our Air Source Heat Pump will pay £1000 a year for 7 years.

How on earth does a Heat Pump work?

When you pump up tyres the pump gets hot, because when you compress a gas it heats up. This familiar effect is how a heat pump generates heat- by compressing a refrigerant until it’s hot enough to use in a heating system.

The cycle begins with a refrigerant as a gas at outside temperature (eg; 0˚C). The refrigerant gas is compressed until it’s a hot liquid (eg; 50˚C). That heat is transferred to the water that circulates through the central heating system, or into the hot water tank.

The refrigerant, which is still at high pressure, is then allowed to expand and quickly evaporates into a gas.

Another familiar principle- you know when you spray deodorant under your arms? “Oohh, ahh”, it’s cold, right? The deodorant is a room temperature liquid at high pressure in the can, but rapidly becomes a cold gas when it’s allowed to expand as it comes out. A drop in pressure means a drop in temperature- so the refrigerant in the heat pump is now really cold (perhaps -10˚C).

It’s now so cold it can absorb heat from outside. This absorption of heat from the ground or the air is where the renewable energy comes from.

The refrigerant warms up from it’s cold state, at say -10˚C, up to the outside temperature- 0˚C in our example.

It’s now in the same state as when we started, a gas at outside temperature, and ready to repeat the cycle.


Heating a home with electric heaters is a shockingly inefficient way to heat our homes. Over half the UK’s electricity is still provided by fossil fuels, and about half of that energy is lost as waste heat from the power station, for example all those clouds of steam billowing from rows of cooling towers either side of the M62.

Although a heat pump consumes quite a bit of electricity when it runs, for every unit of electricity it consumes it extracts several more units of renewable heat from outside. This ratio is called the Coefficient of Performance (COP)- the higher the COP, the better (more efficient).

An example of a Ground Source Heat Pump coil, for absorbing renewable heat from the ground. We didn't have room for one of these. Photo: Heatpump-Reviews.com
An example of a Ground Source Heat Pump coil, for absorbing renewable heat from the ground. We don’t have room for one of these. Photo: Heatpump-Reviews.com

Underground remains at a pretty constant temperature year round. Heat pumps can be connected to a pipe coiled at a shallow depth below the surface, or to a pipe in a bore hole, to use the ground as the source of heat. A Ground Source Heat Pump has a COP of around 2.8. ie; for every unit of electricity it consumes it delivers 2.8 units of heat. The manufacturers may claim COPs of 4 or 5, but the Energy Saving Trust measured real world installations and discovered the figures actually achieved aren’t as high as that. Report here.

Our heat pump collector unit is at the back of the garage.
Our heat pump collector unit is at the back of the garage.

We don’t have enough space for a ground source pipe under our lawn, and aren’t about to spend a couple of grand having a bore hole drilled. Recently though Air Source Heat Pumps have become feasible for domestic applications, and qualify for the RHI. They have a big fan that circulates outside air to extract heat from it, and achieve a COP of 2.5. They look like a large air conditioning unit.

A gas boiler is at its most efficient producing really hot water- so radiators are hot to the touch, and only running when needed- on a timer to heat the house for when you return from work.

A heat pump runs more efficiently producing lower temperatures. The harder it has to work, the lower the COP. So it’s better to have it run gently all the time, rather than let the house get cold and then heat it up when you need it. Heat pumps therefore work best with low temperature heating systems, such as underfloor heating, or radiators that are warm to the touch rather than hot. In fact the controller rather cleverly throttles the heat pump so it never works any harder than necessary, based on the outdoor and indoor temperatures. So the temperature of the radiators varies, according to the weather and room temperature.

What we did

The supplier should carry out a detailed analysis of the property, measuring the size of each room, windows, radiators & level of insulation, to calculate the heat demand and accurately specify the size of heat pump required. Be wary of a supplier that doesn’t bother to do this. All but one of the quotes we got specified a heat pump bigger than necessary (wildly wrong).

The control unit and hot water tank are the other side of the wall, inside the garage. The overhead pipes are the central heating and water connections to the house.
The control unit and hot water tank are the other side of the wall, inside the garage. The overhead pipes are the central heating and water connections to and from the house.

The collector unit is just behind the garage on the patio, across which it blows icy cold air when working hard to run the heating in winter. Fortunately we’re not in the habit of taking breakfast al fresco at that time of year, so it’s no great loss. The fern on the far side of the patio seems to have suffered though.

The fan whirs on & off when it’s running, but you can barely hear it inside the house.

We decided to save space inside the house and had the control unit, itself the size of a gas boiler, and the hot water tank installed in the garage, where access was easier than the roof space. Locating it in an unheated space means increased heat loss, but I mitigated that as well as I could by wrapping hot exposed pipes in insulation.

What we didn’t

Thermodynamic Panels

There is now a heat pump available that connects to black panels, wall or roof mounted, to capture heat from the ambient air and the sun. A sort of cold radiator that absorbs heat instead of emitting it. It’s an interesting idea. They’re called thermodynamic panels and the manufacturers claim COPs as high as 9, marketing ‘solar panels that work at night’, which sounds amazing. However despite repeated requests by suppliers we were unable to get real world figures for UK performance. And I can’t help noticing that the manufacturers are based in Portugal, which I can personally attest from visiting my wife’s family, is ridiculously hot and sunny.

So perhaps a product to watch, but we assigned it to the experimental category.

Underfloor Heating

As heat pumps work better with low temperature heating systems, underfloor heating was considered. We are exposing the floorboards throughout the main house, and you can get underfloor heating systems that fit between the joists under the floorboards (diffuser plates). However, as well as costing another £1000 or so, we were advised that it works best with newly engineered boards, whereas we were keeping the original floorboards which might be more likely to creak or warp. The extension is solid concrete floor, which would have been ideal for underfloor heating had they installed it when it was built. But they didn’t. The only way to do it now would be to drill out the concrete and lay insulation and a fresh floor with a heating pipe in it. So stuff that. We stuck with the radiators we already had.


It’s difficult to accurately calculate how much our heat pump actually consumes.

In fact, without a smart meter we don’t even know how much the whole house uses, because some of our needs are met by the solar PV. All that our 2 meters measure is the total generated, and the total imported. How much we export (or consume) isn’t measured.

It was predicted the heat pump would consume £977 worth of electricity a year, based on heating the whole house to 22˚C, 24 hours a day and without solar thermal or solar PV.

Our total electricity cost last year was ~£500, mostly for the heat pump. Although a little of the heat pump’s electricity is met by the solar PV, the heating is only on in winter when the PV doesn’t produce much.

The single biggest energy saving measure, the EWI, hasn’t yet been done. When it is, the heat pump will be able to heat the radiators to a lower temperature and therefore run even more efficiently.

Legionella Prevention in the Hot Water Tank

Our controller is set to heat the hot water tank to 47˚C. This is plenty hot enough for a household’s general needs, and the heat pump becomes less efficient at higher temperatures- but it’s not hot enough to kill the legionella bug. So, if the solar thermal hasn’t achieved it already, every couple of weeks the controller heats the tank to 65˚C with help from an immersion heater.


Last winter just half an hour or so after the heating turned off the house was so cold I’d have to wear my fleece inside. The heat pump was only specified to be able to heat the house after all the insulation had been done, but the timing of works meant the heating system was installed in February 2013 before the insulation. It was a particularly cold spell, so we didn’t know whether it would be able to cope.

It coped absolutely fine, and the house is now a comfortable temperature all the time. The difference when the insulation is complete will be lower running costs and improved efficiency.