What to sort out before a heat pump

There is a natural order to this and most people do it backwards. They get quotes first and find out about their house afterwards, which means the quotes were guesses and the decision was made on guesses.

Five things are worth doing first, and only one of them — the insulation — costs any money.

Find out what your house burns

This is the free one that changes everything, and it takes twenty minutes.

Two figures matter, and one of them almost nobody has to hand.

• Gas or oil used last year. Gas is given in kWh on the bill; oil comes in litres from your delivery notes. Your supplier's annual statement has the whole twelve months in one figure, which saves adding up bills.
• Gas or oil used in one summer month — July or August. In summer the heating is off, so what you burn is hot water and cooking. That single month separates the two halves of your bill exactly, and without it the split between heating and hot water is the biggest guess in the whole sizing calculation.

A year of your own bills describes your house better than any survey of it, better than any assumption about its age, and much better than an estimate made in your kitchen. Everything downstream gets sharper the moment you have real numbers: how big a heat pump, what it will cost to run, whether the quote in front of you is sensible.

It also arms you. When somebody says your house needs a 14 kW heat pump, you can check that against what the house burnt last winter, and the heat pump size calculator will do that arithmetic from the figures above.

Deal with the fabric before the machine

The order matters here and it costs real money to get wrong.

Insulation makes a house need less heat. Needing less heat means a smaller heat pump, which is cheaper to buy. It also means the radiators you already have are relatively more generous, which means a lower water temperature, which means a lower bill for as long as you own the place. Do it the other way round and you have paid for a machine sized for the house you used to have.

In rough order of how much they are worth for what they cost:

• Loft insulation. Cheap, easy, and if yours is thin or was laid decades ago it is the best money you will spend. 270 mm is the modern depth; a lot of lofts have 100 mm or less.
• Draught-proofing. Letterboxes, floorboards, loft hatches and unused chimneys are cheap to deal with and make more difference than people expect.
• Cavity wall insulation, if you have cavities and they are empty. A house built between roughly 1930 and 1980 probably has cavities. Ask; do not assume, because an assumed cavity fill that was never installed is the single commonest reason a heat loss calculation comes out far too low.
• Floor insulation, if you have suspended timber floors with a draughty void under them.
• Glazing. Expensive per unit of heat saved, so best done when the windows need replacing anyway and not for the heating benefit alone.
• Solid wall insulation. The big one for older houses, and the disruptive and expensive one. Worth serious thought, but it is a project and not a job.

You do not have to do all of it. You do want to know which of it applies to you before somebody sizes a system around the house as it stands today.

Go and look at your radiators

A heat pump is cheap to run when the water going round the house is cool, and how cool it can be is decided by the radiators. Bigger radiators give out the same heat at a lower temperature, so this is the part of the house that sets your running cost for the next fifteen years.

Take a note of what is there, so that the conversation about emitters starts from your actual radiators.

Walk round with a tape measure and note the height and width of every radiator, and whether each one is a single panel or a double. Note which rooms feel cold now, on the coldest days, because those are the ones most likely to need a bigger emitter. Note whether the pipework you can see is microbore, which is the very thin sort, since that sometimes has to be replaced and it is much better to know early.

A design that keeps your existing radiators and a design that replaces six of them are different prices for the same heat pump. Both can be right, and a quotation that says nothing either way has not considered the question.

Work out where the unit would stand

The outdoor part is roughly the size of a large chest freezer on its end, and it needs air moving through it, so it cannot go in a sealed cupboard or a tight passage between two walls.

Look for a spot on an outside wall with clear space in front, ideally not directly under a bedroom window and not tight against a boundary. Where it goes affects how far the pipes have to run, which affects the price, and how audible it is to you and to a neighbour.

Most installations need no planning permission, but the exemption comes with conditions attached, including one about noise at the neighbour's wall. The article on noise and permitted development covers what those are.

Somewhere indoors also has to take a hot water cylinder, about the size of a tall fridge, which often ends up where the airing cupboard used to be. A house on a combi boiler has no cylinder at present, so this is the point at which space has to be found for one.

Check the electricity supply

A heat pump is a substantial new electrical load on a supply that may have been sized decades ago. The limit is the main fuse where the supply enters the house, and on older properties it can be 60 A or 80 A, which a heat pump, an electric shower and a car charger together can exceed.

The installer has to satisfy themselves about this and to notify the company that owns the wires in your street. It is worth raising early, because a supply upgrade runs on that company's timescale and can take months. There is more in telling the network operator.

This is also the moment to think about metering, since the tariff you can get is a large part of what a heat pump costs to run, and the cheaper tariffs need a working smart meter. That question is covered in do you need a smart meter for a heat pump?

Get your expectations straight

Nothing to do here, but this causes more disappointment than anything else on the list.

A heat pump is not a boiler that runs on electricity. It runs gently and continuously, the radiators feel lukewarm to the hand, and turning it off when you go out costs money instead of saving it. Houses where somebody tried to run a heat pump like a boiler are a large share of the unhappy ones.

It is not a money-making scheme. A well designed system in a reasonably insulated house can cut a heating bill substantially and can pay for itself over a decade or so, helped by the Boiler Upgrade Scheme grant. Anybody presenting it as an investment with a two-year payback is selling something.

Then get quotes

With a year of bills in hand, a summer month, a rough idea of your fabric and a list of your radiators, you are in a much better position than most people who ring three installers on a Tuesday.

Put your figures through the heat pump size calculator first. You are not trying to design anything — you are trying to know roughly what the answer should be, so that a quotation which is nothing like it prompts a question and not a signature.

Then read what a good quote looks like.