Home Heat Pump Guide

Heat Pump vs Combi Boiler: Key Differences

The combi boiler is the most popular heating system in the UK, installed in over half of British homes. It provides both central heating and instant hot water in a single compact unit, with no need for a hot water cylinder or cold water tank. It is simple, space-efficient, and well-understood by homeowners and plumbers alike.

A heat pump is a fundamentally different type of system. It does not burn fuel. It requires a hot water cylinder. It operates at lower temperatures. And it approaches heating in a completely different way. This guide explains every key difference so you can make an informed decision about which system is right for your home.

How Each System Works

Combi Boiler

A combi (combination) boiler heats water on demand. When you turn on a hot tap, the boiler fires up and heats water directly from the mains as it flows through a heat exchanger. For central heating, it heats water to 60-80°C and circulates it through your radiators.

Key characteristics:

  • Heats water instantly — no storage cylinder needed
  • Compact wall-mounted unit, typically in the kitchen or utility room
  • Burns mains gas (or LPG in off-grid properties)
  • Flow rate limits how much hot water you can use simultaneously
  • Runs at high flow temperatures (60-80°C for heating)

Heat Pump

A heat pump extracts heat from the outside air (or ground) and transfers it into your home. It operates like a refrigerator in reverse — compressing and expanding refrigerant to move heat from a cold source (outside) to a warm destination (inside).

Key characteristics:

  • Requires a hot water cylinder (typically 170-300 litres) — heat pumps cannot heat water fast enough for instant delivery
  • Outdoor unit (about the size of a washing machine) plus indoor cylinder and controls
  • Runs on electricity — no gas connection needed
  • Operates at lower flow temperatures (35-50°C for heating)
  • Runs more continuously at lower intensity rather than cycling on and off

For a deeper explanation, see our guide on how air source heat pumps work.

The Hot Water Cylinder Question

This is the single biggest practical difference between the two systems and the one that concerns most homeowners considering the switch.

Why a Heat Pump Needs a Cylinder

A combi boiler can heat water to 55°C in seconds. A heat pump heats water more slowly — it takes time to raise the temperature of the water in the cylinder to the required level (typically 50-55°C for domestic hot water). Storing hot water in a cylinder means you always have a ready supply available when you need it.

Where Does the Cylinder Go?

A typical hot water cylinder is 500mm to 600mm in diameter and 1,200mm to 1,800mm tall. Common locations include:

  • Airing cupboard: Many older homes had one before the combi boiler was installed — the same space can be reused
  • Utility room or garage: If you have one, this is often the most convenient location
  • Under stairs: A compact cylinder can fit in many under-stair cupboards
  • Bedroom cupboard: If space is limited, a slimline cylinder can be fitted in a built-in wardrobe

In some homes, finding space for a cylinder is genuinely challenging — particularly in small flats. This is worth discussing with your installer during the survey. Modern slimline cylinders are more compact than their predecessors, and creative solutions are usually available.

Installation Cost Comparison

Combi Boiler

  • Like-for-like combi boiler replacement: £2,500 to £3,800
  • Premium combi boiler: £3,500 to £4,500

Heat Pump (Replacing a Combi Boiler)

  • Before grant: £10,000 to £14,000 (includes heat pump, cylinder, controls, any radiator modifications)
  • After BUS grant (£7,500): £2,500 to £6,500

Replacing a combi boiler with a heat pump tends to cost more than replacing a system boiler, because you need to add a hot water cylinder and potentially reroute pipework. This adds £1,000 to £2,000 to the installation compared to a property that already has a cylinder.

Verdict on installation cost: A combi boiler is cheaper to install. After the BUS grant, the gap narrows significantly but the combi boiler still has an upfront advantage of £0 to £3,000 depending on the property. Use our cost guide for detailed breakdowns.

Running Costs

For a typical three-bedroom semi with 12,000 kWh annual heat demand:

Gas combi boiler (92% efficiency): Approximately £989 per year (gas plus standing charge)

Heat pump (COP 3.0): Approximately £980 per year (at standard electricity rate)

Running costs are effectively equal at current energy prices. With a heat pump electricity tariff, the heat pump becomes cheaper by £200 to £400 per year. See our running costs guide for detailed calculations.

Heating Performance

Combi Boiler Strengths

  • Fast warm-up: Heats radiators to high temperatures quickly — you feel the warmth within minutes
  • Responsive: Great for homes where people come and go at irregular times
  • Simple controls: Most homeowners are familiar with boiler thermostats and timers

Heat Pump Strengths

  • Consistent warmth: Runs continuously at low intensity, maintaining a steady temperature rather than cycling between hot and cold
  • Even heat distribution: Less temperature variation between rooms and over time
  • Summer cooling: Some heat pump models can reverse to provide cooling — increasingly valuable as UK summers get warmer
  • Quieter operation: No boiler ignition noise or pump cycling

Verdict on heating performance: A combi boiler offers faster response. A heat pump offers more consistent comfort. Many people switching from a combi boiler to a heat pump report that once they adjust to the different approach (setting a target temperature and letting the heat pump maintain it), they find the heat pump more comfortable overall.

Hot Water Delivery

Combi Boiler

Unlimited hot water on demand, but flow rate is limited. A typical 28 kW combi boiler delivers 10-12 litres per minute. This is fine for one tap or one shower at a time, but running two showers simultaneously will reduce flow rate and temperature. Larger families in homes with multiple bathrooms often find combi boilers limiting.

Heat Pump

Hot water is stored in the cylinder and delivered at mains pressure (with an unvented cylinder). A 200-litre cylinder provides enough hot water for two to three consecutive showers. The flow rate is not limited by the heat pump's output — it is limited by your mains water pressure, which is typically higher than a combi boiler can deliver.

The trade-off is that once the cylinder is depleted, it takes time to reheat — typically one to two hours for a full reheat. However, with good scheduling (heating water overnight and during the afternoon), most families never run out.

Verdict on hot water: For households with multiple bathrooms, the heat pump with a cylinder often delivers better hot water performance than a combi boiler. For single-bathroom homes with low hot water demand, the combi boiler's instant delivery is convenient and perfectly adequate.

Space Requirements

Combi Boiler

Extremely compact. A wall-mounted combi boiler takes up minimal space — typically 700mm × 400mm × 300mm. No cylinder, no tank, no additional equipment.

Heat Pump

Requires:

  • Outdoor unit: Approximately 1,000mm × 400mm × 750mm, mounted on a concrete base with clearance around it for airflow
  • Hot water cylinder: Approximately 500-600mm diameter × 1,200-1,800mm tall, located indoors
  • Controls: Wall-mounted controller, similar in size to a boiler thermostat

Verdict on space: A combi boiler is the clear winner for space efficiency. If you live in a small flat or a home with absolutely no room for a cylinder, this is a genuine constraint. However, most houses — even terraced homes — can accommodate a cylinder with creative placement. Your installer can advise on options during the survey.

Radiators

A combi boiler runs at high flow temperatures (60-80°C), which means standard radiators work efficiently. A heat pump runs at lower flow temperatures (35-50°C), which means radiators need to be correctly sized to deliver the same heat output.

In many homes, existing radiators are already oversized for the room (a common occurrence in UK properties) and will work perfectly well at lower flow temperatures. In some cases, a few radiators may need upgrading to larger models. Your installer will calculate this during the survey.

For a detailed guide, see do I need new radiators for a heat pump?

Lifespan and Reliability

  • Combi boiler: 12-15 years typical lifespan, annual servicing required, multiple components that can fail (gas valve, diverter valve, heat exchanger, PCB)
  • Heat pump: 20-25 years typical lifespan, service every 1-2 years, fewer mechanical components, no combustion wear

Over a 25-year period, you would likely need two combi boilers (£5,000-£7,600 total) versus one heat pump (£2,500-£6,500 after grant). The total cost of ownership over this period increasingly favours the heat pump.

Our Verdict

If you currently have a combi boiler and it is working well, there is no urgent need to switch — the running cost savings at current energy prices are minimal. However, when your combi boiler reaches end of life (typically 12-15 years), a heat pump is worth serious consideration.

The main barrier for combi boiler homes is finding space for the hot water cylinder. If you can accommodate one — and most homes can — the heat pump offers a longer-lasting, lower-maintenance, lower-carbon system with running costs that are equal today and expected to improve.

For homes with multiple bathrooms, the heat pump with a cylinder actually provides better hot water performance than a combi boiler — higher pressure and the ability to run multiple taps simultaneously.

Use our suitability checker to assess your property, or get quotes from MCS-certified installers to see what the switch would involve for your specific home.

Frequently Asked Questions

Can a heat pump replace a combi boiler directly?

Not directly — a heat pump requires a hot water cylinder, which a combi boiler does not use. Your installer will need to find a suitable location for the cylinder and route pipework to connect it. This adds some cost and complexity but is standard practice.

Will I lose cupboard space for the hot water cylinder?

You will need to allocate space for a cylinder, typically in an airing cupboard, utility room, under stairs, or a built-in wardrobe. Modern slimline cylinders are more compact than old-style tanks and include useful drying space on top for towels and clothes.

Is the hot water from a heat pump as hot as from a combi boiler?

Yes. Hot water in the cylinder is stored at 50-55°C — the same temperature as a combi boiler delivers. The difference is that it is stored rather than heated on demand, but the temperature at the tap is identical.

Can I run multiple showers with a heat pump?

Yes — and often better than with a combi boiler. An unvented hot water cylinder delivers water at mains pressure, so flow rate is not limited by the heat pump. You can run multiple showers simultaneously, provided the cylinder has sufficient stored hot water.

How long does the switch from combi boiler to heat pump take?

A typical conversion takes three to five days, including installing the outdoor unit, hot water cylinder, connecting pipework, and commissioning. This is longer than a combi boiler swap (usually one day) but the work is straightforward for experienced installers.

Should I wait until my combi boiler fails before switching?

There is no financial penalty for switching before your boiler fails, but the case is strongest when your boiler needs replacing anyway. If your combi boiler is over 10 years old, it is worth getting heat pump quotes now so you are prepared when the time comes. The BUS grant is available until 2028 — waiting too long risks missing out.