Heat Pump Installation: What to Expect (From an Engineer)

Most guides about heat pump installation are written by people who've never actually done one. This guide is different — it's written from the perspective of a heating engineer who's been hands-on with these systems. Here's what the process genuinely looks like, what takes longer than you'd expect, and what to look out for.

Stage 1: The survey

Before anything else, an MCS-certified installer will survey your property. This isn't just a quick look around — a proper survey takes 1–2 hours and covers:

• Your current heating system and how it's plumbed
• Room-by-room heat loss calculations
• Radiator sizes and whether any need upgrading
• Insulation levels — loft, walls, floors
• Hot water demand (number of bathrooms, baths vs showers)
• Where the outdoor unit will go — clearances, noise considerations, planning
• Electrical supply — whether your consumer unit can handle the load

If an installer quotes you a price without doing a proper survey, that's a red flag. Heat pump sizing is critical — too small and it won't heat your home; too large and it'll short-cycle, waste energy, and wear out faster.

Stage 2: Design and quotation

After the survey, the installer designs the system — selecting the right heat pump model, cylinder size, any new radiators or pipework changes, and the control strategy. You'll get a detailed quote showing:

• The heat pump unit and model
• Hot water cylinder specification
• Any radiator upgrades needed
• Pipework modifications
• Electrical work
• The BUS grant deducted (if eligible)
• Total cost to you

Get at least two or three quotes. Compare not just the price but the system design — the cheapest quote isn't always the best if the installer has under-sized the system or skipped necessary radiator upgrades.

Stage 3: Pre-installation prep

Before the installers arrive, a few things might need doing:

• Radiator upgrades — heat pumps run at lower flow temperatures than boilers (typically 35–45°C vs 60–80°C), so some radiators may need upsizing
• Pipework — the main flow and return pipes may need increasing from 22mm to 28mm
• Base for the outdoor unit — a concrete pad or anti-vibration mounts
• Electrical upgrade — if your consumer unit is old or at capacity

Some of this work can happen at the same time as the main installation, but it depends on the installer's schedule and the scope of work.

Stage 4: Installation day(s)

A straightforward air source heat pump installation typically takes 2–3 days. More complex jobs — ground source, extensive pipework changes, or full radiator upgrades — can take 4–5 days. Here's what happens:

Day 1

• Old boiler removed and disposed of
• Outdoor unit positioned and secured
• Refrigerant lines run between outdoor and indoor units
• Hot water cylinder installed

Day 2

• Pipework connections to the heating circuit
• Electrical connections and controls wiring
• System filled, pressurised, and leak-tested

Day 3

• Commissioning — the system is tested at various conditions
• Flow temperatures and weather compensation set up
• MCS paperwork and building regs sign-off
• Handover — how to use the controls, what to expect in the first few weeks

What to expect in the first few weeks

Heat pumps work differently to boilers. The biggest adjustment is that they deliver heat more gently — lower flow temperatures, running for longer periods rather than short blasts. This is normal and by design. A few things to be aware of:

• Radiators feel warm, not hot — this is correct at 35–45°C flow temp
• The system runs more hours per day — heat pumps work best running steadily, not in short bursts
• Weather compensation takes time to dial in — the system adjusts output based on outdoor temperature, and it can take a few weeks to fine-tune
• Hot water may take longer to heat — especially if you've come from a combi boiler

Give it a full heating season before judging performance. Most complaints about heat pumps come from the first few weeks when the system hasn't been optimised and the homeowner is still adjusting to the different heating pattern.

Red flags to watch for

• Installer quotes without surveying your property
• No MCS certification
• Pressure to make a quick decision
• Unusually low price compared to other quotes
• No mention of heat loss calculations
• The quote doesn't include a hot water cylinder when you clearly need one

A good installer will take their time, explain their recommendations, and not rush you into signing. The BUS grant isn't going away tomorrow — take the time to choose well.