Home Heat Pump Guide

What Happens During a Heat Pump Installation?

If you have never had a heat pump installed, the process can feel like a mystery. What exactly do the installers do? How messy is it? When does the old boiler come out? When does the heating come back on? This guide answers all of those questions with a detailed, day-by-day walkthrough of a typical air source heat pump installation in a UK home.

Every installation is slightly different depending on your property, the system being fitted, and the approach of your installer. But the core stages are consistent, and knowing what to expect makes the whole experience much less stressful.

Before the Installers Arrive

The work starts before anyone sets foot in your house. Your MCS-certified installer will have already completed a detailed heat loss survey, designed your system, and ordered all the equipment. Before installation day, you should:

  • Clear furniture away from any radiators being replaced
  • Clear the area outside where the heat pump unit will be placed
  • Make space for the hot water cylinder location (if a new one is being installed)
  • Move anything fragile or valuable away from work areas
  • Lay down dust sheets if you are particular about your floors (though good installers bring their own)
  • Make sure there is somewhere for the team to park close to the house

See our full installation checklist for a printable preparation guide.

Day 1: Stripping Out and Preparation

The installers typically arrive between 8:00 and 8:30 in the morning. Expect a team of two to three people — a lead installer (plumber/heating engineer) and one or two assistants. On larger jobs, an electrician may also be present from day one.

Morning: Draining and removing the old system

The first job is to drain the existing central heating system. The installers turn off the boiler, attach a hose to the lowest drain point, and empty the water from all the radiators and pipework. This takes 30 to 60 minutes depending on the system size.

Once drained, the old boiler is disconnected — gas supply capped, flue removed, electrical connection isolated — and physically taken out. If you have an older system with header tanks and a feed-and-expansion tank in the loft, those come out too. The old boiler and tanks are loaded into the van for disposal or recycling.

From this point, you have no central heating or hot water. If you have an electric shower, that will still work. Otherwise, plan ahead with an electric heater and a kettle.

Afternoon: Base preparation and wall penetration

While one member of the team works indoors preparing the pipework routes, another prepares the outdoor base for the heat pump unit. This typically involves either:

  • Laying a pre-cast concrete slab (the most common approach — quick and clean)
  • Pouring a small concrete pad (less common, takes longer to cure)
  • Installing anti-vibration mounts on an existing hard surface

The core drill comes out next. A 60 to 80mm hole is drilled through the external wall to route the flow and return pipes from the outdoor unit to the indoor system. This is the noisiest part of the entire installation — it takes about 15 to 30 minutes and produces brick dust. The installers will lay dust sheets, but some mess is unavoidable.

End of day 1

By the end of the first day, the old system is out, the base is prepared (or curing if concrete was poured), the wall penetration is done, and initial pipework routing has begun. The house may look a bit chaotic — don't worry, it comes together quickly from here.

Day 2: The Main Installation

Day two is the busiest day. This is when the new system physically takes shape.

Morning: Positioning the heat pump and cylinder

The outdoor heat pump unit is lifted onto its base and secured. Air source units weigh between 60 and 120 kg, so this is usually a two-person job. The unit is levelled and bolted down or secured with anti-vibration mounts.

Indoors, the hot water cylinder is positioned. If you are replacing an existing cylinder, this is straightforward — the new one goes in the same location. If you previously had a combi boiler and no cylinder, the installer will have identified a suitable location during the survey (typically an airing cupboard, utility room, or sometimes a bedroom cupboard).

Late morning to afternoon: Pipework

This is where the skilled work happens. The installers run copper pipework from the outdoor heat pump unit through the wall penetration to the indoor system. These pipes carry water (not refrigerant, in a monobloc system) and must be properly insulated to prevent heat loss and condensation.

The pipework connects to the hot water cylinder, the heating circuit, and any new components such as a buffer tank, expansion vessel, or system filter. The installers may also need to modify existing pipework — adding new connections, rerouting pipes, or replacing sections that are in poor condition.

Afternoon: Radiator work

If any radiators need upgrading — because heat pumps operate at lower flow temperatures than boilers, some radiators may need to be larger to deliver the same heat — this is when they are swapped. The old radiators come off, new ones go on, and the pipework is connected.

Not every radiator necessarily needs replacing. A good installer will have identified during the survey which ones are adequate and which need upsizing. Read more about whether you need new radiators.

End of day 2

By evening, the outdoor unit is in place, the cylinder is installed, most pipework is complete, and any new radiators are fitted. The system is physically taking shape but is not yet connected to the electrical supply or filled with water.

Day 3: Electrical, Filling, and Commissioning

Day three brings everything together.

Morning: Electrical connection

A qualified electrician (either part of the installation team or a separate contractor) installs a dedicated electrical circuit for the heat pump. This runs from your consumer unit (fuse board) to the outdoor unit and typically requires:

  • A dedicated MCB (miniature circuit breaker) in the consumer unit
  • An isolator switch near the outdoor unit
  • Appropriate cable routing (sometimes surface-mounted, sometimes buried in the wall)

If your consumer unit is old or does not have a spare way, it may need upgrading. Your installer should have identified this during the survey and included it in the quote.

Mid-morning: Controls and sensors

The thermostat, weather compensation sensor, and any zone controls are installed and wired. The weather compensation sensor is a small device fitted on an external north-facing wall — it tells the heat pump what the outside temperature is so it can adjust its output accordingly.

Late morning: System filling and testing

The system is filled with water (and inhibitor to prevent corrosion) through the filling loop. The installers pressurise the system to the correct level — typically 1 to 1.5 bar — and then carefully check every joint, connection, and fitting for leaks. Any leaks are fixed and retested.

The system is then bled — air is released from each radiator and from the heat pump circuit — until water flows freely through the entire system without air locks.

Afternoon: Commissioning

Commissioning is the formal process of verifying the system works correctly. The heat pump is powered on and the installer checks:

  • Flow and return temperatures at the heat pump and at radiators
  • Hot water heating performance (time to reach target temperature)
  • System pressures under operating conditions
  • Defrost cycle operation
  • Noise levels from the outdoor unit
  • Weather compensation response (adjusting output to match outdoor temperature)
  • All safety controls and fault indicators

The installer programmes your heating schedule and sets up the weather compensation curve — a setting that determines how the heat pump responds to changes in outdoor temperature. This is crucial for efficiency and comfort.

The Handover

Once commissioning is complete, the installer walks you through your new system. This handover should cover:

  • How to adjust the heating temperature and schedule
  • How hot water heating works (it is usually on a separate timer)
  • What the weather compensation setting does (and why you should not override it)
  • What is normal: defrost cycles, fan noise changes, warm (not hot) radiators
  • How to check system pressure and top it up if needed
  • When to book your first annual service
  • Emergency contact details

Documentation you should receive

  • MCS installation certificate
  • Manufacturer's warranty registration
  • Building regulations compliance certificate
  • Commissioning record with all measured values
  • Heat loss calculation and system design documents

If your installer does not provide all of these, ask for them. They are requirements under MCS standards.

What About Mess and Disruption?

Let's be honest about the disruption:

  • Noise: The core drilling is loud (30 minutes). General installation noise (sawing pipe, drilling brackets) is moderate — comparable to any plumbing work
  • Dust: Core drilling produces brick dust. Good installers contain this with dust sheets and a vacuum, but some dust will escape
  • No heating/hot water: Typically 2 to 3 days between old boiler removal and new system commissioning
  • Garden: The outdoor unit takes up a space roughly 1 metre by 0.5 metres, plus clearance around it. The base preparation is quick and relatively clean
  • Indoor mess: Comparable to having a bathroom refitted — some disruption to daily routines, but you can still live in the house

Most homeowners are surprised by how manageable it is. It is significantly less disruptive than a kitchen or bathroom renovation.

What If Something Goes Wrong?

Delays and complications do happen, but they are manageable:

  • Hidden pipework problems: Sometimes existing pipework is in worse condition than expected and needs replacing. This adds time but is better to fix now than later
  • Electrical supply issues: If the consumer unit cannot accommodate the heat pump circuit, an upgrade is needed. A competent installer will have identified this at the survey stage
  • Weather delays: Heavy rain can slow outdoor work. This is more of an issue for ground source installations than air source
  • Equipment issues: Very occasionally, equipment arrives damaged or incorrect. Good installers have contingency plans and will reschedule quickly

Frequently Asked Questions

Do I need to be at home during the installation?

You do not need to be present the entire time, but someone should be available to let the installers in and be contactable for questions or decisions. Being home on the first morning and for the handover is advisable.

Can I use my bathroom during installation?

Yes. Your toilets and cold water taps work throughout. If you have an electric shower, that works too. You will lose stored hot water when the old system is drained, so no baths or mixer showers until the new system is commissioned.

Will the installers make good any mess?

Reputable installers clean up after themselves and make good any damage — filling core drill holes with fire-rated sealant, touching up paint where brackets have been moved, and removing all packaging and old equipment. Clarify this in your contract before work begins.

What if I have pets?

Keep pets away from work areas, especially during core drilling and when doors are propped open. A bedroom with the door closed works well. Noise-sensitive animals may be stressed by drilling — consider whether a day with a friend or family member would help.

Do the installers need access to all rooms?

They need access to every room with a radiator (to bleed them and check heat output), plus the room where the cylinder is located and wherever the consumer unit is. Clear access paths between rooms help the job go smoothly.

How soon after installation can I use the heating?

Immediately after commissioning is complete — usually late on day 3 or on day 4. The installer will have the system running and tested before they leave.

Ready to get started? Get free quotes from MCS-certified installers in your area. Our complete installation guide covers every stage from survey to handover, and the cost calculator will show you what to expect financially. Check our guide on choosing the right installer to make sure you get the best team for the job.