The Heat Pump Guide

Imagine your home needs:

10,000 kWh of useful heat

over a period.

That difference is the basic reason heat pumps can work financially even though electricity is expensive.

You will see these terms frequently.

For a homeowner, the useful interpretation is simple.

If a system achieves a seasonal efficiency of around 3, it has delivered roughly three units of heat for every unit of electricity consumed over that period.

Higher is better.

But it is not simply a number printed on the side of the machine.

System temperatures, weather, controls and the home itself can all affect real-world performance.

This is one reason broad claims about heat-pump savings are not very useful.

Replacing:

direct electric heating

can produce a completely different financial result from replacing:

a modern gas boiler.

Likewise, homes currently using oil or LPG have a different starting point again.

The question should always be:

“What would this heat pump replace in my home?”

Lower running costs do not automatically make something a good investment.

Imagine a heat pump saves:

£500 per year

but costs:

£8,000 more after any available support

than the alternative you would otherwise install.

That extra capital cost matters.

Likewise, if grant support or the need to replace an ageing heating system substantially reduces the additional cost, the proposition changes.

So keep two questions separate:

1. Would it reduce my running costs?
2. Is the installation cost worth paying to achieve that?

Your home continually loses heat when it is warmer inside than outside.

Heat escapes through things such as:

• walls
• roof
• floors
• windows and doors
• ventilation and draughts

The colder it is outside, the harder the heating system needs to work to maintain the temperature inside.

A proper heat-loss calculation estimates how much heating power the property — and ideally each room — needs under cold design conditions.

That number is fundamental to sizing the system properly.

Better insulation generally means the home loses heat more slowly.

That reduces how much heating energy is required regardless of whether you use:

• gas
• oil
• electric heating
• a heat pump

But that does not mean a home must be perfectly insulated before a heat pump can work.

This is where the difference from a boiler becomes important.

A traditional boiler may supply very hot water to relatively small radiators.

A heat pump normally works more efficiently when the heating water does not need to be as hot.

Lower-temperature water means each radiator gives out less heat.

So some homes may need:

• larger radiators
• additional radiators
• other emitters such as underfloor heating

to deliver enough warmth at the lower temperature.

Flow temperature is the temperature of the water leaving the heating system and travelling towards your radiators or underfloor heating.

Generally:

lower required flow temperature → easier conditions for the heat pump → potentially better efficiency

This is why insulation, radiator sizing and system design all connect.

The aim is not simply:

“Can the heat pump make the radiators hot?”

It is:

“Can the house stay comfortably warm using sensible water temperatures?”

Many air-to-water heat-pump installations use a hot-water cylinder.

That means the available space and existing hot-water arrangement may matter.

Someone currently using a combi boiler with no cylinder should therefore understand what equipment the proposed system requires and where it will go.

An air-source heat pump also needs an appropriate outdoor location.

The installer needs to consider things such as:

• airflow
• access
• noise
• distance from the property
• where pipework will run

These are practical design questions and should be resolved before you commit.

Heat pumps generally become less efficient as you ask them to produce hotter water.

So there can be a big difference between a system able to heat the house comfortably with relatively low flow temperatures and one constantly being pushed to much hotter temperatures.

That is why Chapter 3 matters so much.

The radiators and heat loss determine what temperatures the system needs.

You do not necessarily need to choose between:

on all day

and:

off all day.

Modern controls can adjust how much heat the system supplies.

The important point is that repeatedly allowing the house to become very cold and then demanding rapid recovery may not play to a heat pump’s strengths.

A well-designed system should have controls configured around the property and household.

Weather compensation adjusts the heating-water temperature according to outdoor conditions.

If it is relatively mild outside, the system may not need particularly hot water to keep the house warm.

When it becomes colder, the required temperature can increase.

In simple terms:

1. milder weather
2. →gentler heating

1. colder weather
2. →more heating

without the homeowner constantly changing settings manually.

A heat pump does not simply stop being a heat pump because the temperature outside drops.

But its performance can change.

The building also loses heat faster in colder weather, meaning the system has more work to do at exactly the point when conditions are more demanding.

That is why the design calculation needs to consider cold weather rather than sizing the system around an average autumn afternoon.

Domestic hot water usually needs to be stored at a higher temperature than the water circulating through space-heating emitters.

The system therefore needs to manage:

• space heating
• hot water

appropriately.

You do not need to understand the control logic in detail.

But you should understand what hot-water arrangement has been proposed and whether it matches how your household uses hot water.

A heat pump can materially increase household electricity consumption because heating demand moves away from gas, oil or another fuel and onto electricity.

That means the tariff may matter more than it did before.

But do not choose a heating system purely around today’s specialist tariff.

The heating system is a long-term purchase.

Tariffs can change.

Think of a favourable tariff as something that can improve the economics — not something that rescues a badly designed system.

Solar or a home battery can change where some of the electricity comes from.

That may reduce the cost of some heat-pump electricity.

But heating demand is highest in winter, when UK solar generation is generally lower.

So avoid assuming:

“solar will run my heat pump.”

Some interaction can be useful.

The heating system still needs to make financial sense on realistic winter energy assumptions.

Before choosing the equipment, the installer should understand how much heat the property needs.

I would want the quotation to make clear:

• the design heat loss
• the outdoor design condition
• the proposed heat-pump output
• how the system meets the heat requirement room by room

When comparing quotes, look beyond the heat-pump model.

I would want to understand:

• heat-pump make and model
• proposed output
• design heat loss
• assumed flow temperature
• expected seasonal performance
• radiator or emitter changes
• hot-water cylinder
• controls
• electrical work
• outdoor-unit location
• installation work
• commissioning
• warranty
• total installed cost

Only then are you comparing complete systems.

Government support can make a large difference to the upfront economics.

But the schemes and eligibility differ across the UK and can change.

England and Wales currently use the Boiler Upgrade Scheme, while Scotland has separate support through the Home Energy Scotland Grant and Loan Scheme.

Treat support as a current discount to verify before buying rather than baking one UK-wide grant assumption permanently into your decision.

A useful approach is:

MCS has a formal heat-pump design standard that includes calculating the property’s heat requirement and designing the system around it.

MCS certification is therefore a useful check when choosing an installer.

It should not be the only check.

Also consider:

• experience
• clarity of the quotation
• how the heat loss was calculated
• warranty and aftercare
• whether the installer can explain the proposed system in plain English

There may be practical requirements around:

• outdoor-unit location
• noise
• planning
• electrical supply
• connection arrangements

The precise rules can vary by property and by UK nation.

You do not need to become an expert in them.

You do need to know that the installer has checked them.

The questions I would want answered are:

1. What is the calculated heat loss of my home?
2. What flow temperature is the system designed around?
3. Why have you chosen this size of heat pump?
4. Which radiators or emitters need changing, and why?
5. What seasonal performance have you assumed?
6. What is included in the complete installed price?
7. What support or grant has been assumed?
8. Who handles planning, electrical and certification requirements?

And perhaps most importantly:

“Show me what assumptions your running-cost estimate uses.”