How much is a heat pump – £15,000?       Actually, they start at £1.200

so let’s get going.

It’s easy to ruin a perfectly good house by doing a heat pump conversion. Thousands of people have already done so and yet the pressure is on to keep doing it. So, here’s how to do it logically and successfully. The measure of success here being better comfort with lower running costs and the satisfaction of lowering emissions too, but most of all, a system that actually works.

First of all, don’t rush in until you know how much power you have already and whether that is too much or too little. You might not know the answer to that until after you have done a big purge on insulation, and tested the result. All installers have to err on the side of caution so your boiler will certainly be bigger than it needs to be.

Stage 1/:

Insulate, insulate, insulate, floors, walls, loft windows, all of that. Start here because, compared to your old boiler a heat pump is relatively weedy and anyway very different. It’s a low temperature high flow device and it will need all the help you can give it.

Stage2/:

How much power?

After insulating take a look at how much energy you are currently using and hoping to replace. There are a couple of ways of doing this and it is best to do them both. First check the nominal power of your boiler then see how often it cycles on and off on a cold day. This might involve half an hour standing by the boiler with a stop watch. You might have a 30kW boiler but the chances are it will be short cycling, especially if it runs on oil. When you have the time to running ratio you can see the average power your boiler is delivering.

Secondly, check your total oil or gas bill and use this chart to calculate how many kilowatt hours of energy you have been buying.

There are about 200 days of winter so dividing the energy total by 200 will give you an idea of the average daily power requirement. There is no such thing as an average winter day so this is only going to give a rough idea. The peak requirement could be double the average. If you can get the data for shorter periods that would be even better.

Unfortunately, we are not there yet because there are transmission issues to consider. The pipe that runs from your boiler to the radiators is likely to be 22mm copper which with typical heat pump temperatures will transmit between 10 to 15 kW and more likely towards the lower end of that. On a COP of 3 that means the maximum heat pump power will be around 5kW (delivering 15kW), any more cannot be used. That almost certainly leaves a large hole in the requirement and is the reason why so many installations are a disaster. The solution is pretty obvious though; just add a mini-split air to air heat pump or even two. They will plug the gap and leave your existing pipework untouched. A mini-split has no connection to your existing system and all the pipes and radiators etc. It’s a stand-alone heat pump and that’s why it is so cheap to install.

Stage 3/:

Transmission

It’s not just the limitations of the pipework to consider, now the heat has to come out. Assuming there is no underfloor heating in place that leaves replacing existing radiators with really huge ones (not an inviting prospect) or fan coil units. The latter can be inexpensive and effective – see DIY fan-coil units. Again, the mini-splits make up for transmission shortfalls.

Hot water

Quite often you will be quoted for a new hot water cylinder to go with the heat pump. The reason being that the coil needs to be bigger to accommodate the low temperature heat pump. I’d ignore that suggestion and spend the money on solar panels and an energy diverter like the Eddi. Cheap night time electricity might be in your mix and that too can drive the immersion heater.

Looking back at that chart it appears that there are no savings to be made by running a heat pump. You can press on happily though because further tweaks will lead to the cheapest heating available. Heat pumps can run on renewable electricity too so they really will help you to save the planet.

If you have solar panels, they can make a free contribution and the low power mini-split(s) will often run for free on the first glimmer of sunlight.

In the long run you might have an electric car that allows the use of its battery to power the house. In one stroke that will easily halve your energy bill as you time-shift cheap night time electricity and amplify it with a heat pump.

For bigger houses with unusually high heating demands the answers are still the same although the transmission limitations can be bypassed with the aid of a heat bank with its multiple pumped outlets. That’s a longer discussion for another day.

So that’s it. Just sidestep the limitations and embrace the advantages. Easy really.

P.S. Some EVs can offer 240v vehicle to load with around 3kW available. They imagine you might plug in a toaster when you go camping. So you could plug in, for example, a Mitsubishi SRK60ZSX-WF Heat Pump System along with a changeover switch to pull in the mains when the car is not connected. This unit consumes 1.71kW (a lot less than a toaster) and delivers 6kW which is pretty punchy when it comes to making up any system shortfalls. Expect to pay £2,000 fully installed. This all gets you connected to super cheap heating without installing the expensive V2H charger and you can choose from a much bigger range of cars.

If I was building a house I’d be thinking about an extra ring main for EV powered devices.