Case Study: 18 Months in a Leeds 3-Bed Semi on a Heat Pump

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Exterior of a 1937 red-brick semi-detached home in north Leeds with an air source heat pump unit on the side wall, mature front garden, autumn light

The Wilkinson family — names changed at their request — switched their 3-bed semi in north Leeds from a 12-year-old gas combi to an 8.5kW Mitsubishi Ecodan air source heat pump in October 2023. They’ve kept careful records ever since. With their permission, here are the numbers.

This isn’t a sponsored case study. We met the Wilkinsons via an MCS installer in our directory, asked if they’d share their data, and they agreed. We pay them nothing. No installer or manufacturer reviewed this article.

The property

3-bed semi-detached, built 1937
Solid brick walls (no cavity), externally re-rendered 2019
Loft insulation 300mm, top-up 2022
Modern double-glazed UPVC windows throughout
11 radiators across two floors
Previous heating: 24kW gas combi, installed 2011, working but inefficient

The install

8.5kW Mitsubishi Ecodan air source heat pump
210L Mixergy hot water cylinder (replacing combi)
4 radiators upgraded (living room, master bedroom, dining room, study)
Microbore pipework retained — installer judged it adequate
Outdoor unit on north wall, 800mm from boundary
Quoted with the £7,500 BUS grant deducted

Quote breakdown:

Heat pump unit + indoor controller: £4,800
Cylinder + ancillaries: £2,100
Radiators (4 upgraded): £1,400
Installation labour: £3,200
Commissioning: £400
Gross: £11,900
BUS grant: –£7,500
Net to homeowner: £4,400

Pre-install baseline

The Wilkinsons have detailed bills going back to 2021. Their last full year on gas:

Gas: 12,400 kWh @ blended 8.7p/kWh = £1,079/year
Electricity: 3,200 kWh @ standard variable = £864/year
Total energy cost: £1,943/year

Boiler estimated efficiency: ~82% (12 years old, no service in last 4 years).

Year 1 (Nov 2023 – Oct 2024)

They moved to Octopus Cosy tariff on day one. Smart cylinder programmed to heat water during off-peak windows.

Electricity total: 7,800 kWh
- Off-peak (Cosy): 5,400 kWh @ 7.2p = £388
- Peak: 2,400 kWh @ 28.5p = £684
Annual standing charge: £216
Total electricity cost: £1,288/year
No gas (gas account closed, saving £85/year standing charge)

Total Year 1 energy cost: £1,288 Saving vs gas baseline: £655/year

That’s a 34% reduction. Not the £800/year the installer projected, but well within the modelled range.

The miss was partly behavioural: the Wilkinsons run the heat pump warmer than the installer modelled. They’re used to a gas boiler “blast” heating pattern. Over time, this evened out.

Year 2 partial (Nov 2024 – Apr 2025)

Six months in. Heating-season data only — the meaningful months.

Electricity used Nov–Apr: 5,100 kWh (vs 5,300 kWh same period Year 1)
Cost on Cosy: £703 (Year 1 same period: £728)

Slight improvement on Year 1, attributed to:

Lower flow temperature setting (45°C → 42°C after installer follow-up)
Less manual override behaviour
Slightly milder winter (Nov–Feb mean 5.2°C vs 4.7°C Year 1)

Projected full Year 2 total: ~£1,200. Saving vs gas: ~£743/year.

What happened in cold spells

January 2024 had a 9-day cold snap with overnight lows of -6°C. The Wilkinsons reported:

Heat pump ran near-continuously through coldest days
No backup immersion engagement (cylinder kept up)
Indoor temp held at 20.5°C set point
Defrost cycles 6–10× per day, each ~5 minutes
Brief drop in flow temp during defrost noticed but not uncomfortable
Total electricity for the 9-day cold spell: 285 kWh (vs ~140 kWh on milder week earlier in month)

Cold weather increased electricity usage by ~100% short-term, but the system held the house. No emergency immersion heating. No collapse in performance.

What broke / what they fixed

Issue 1 (week 6): First-floor bathroom radiator not warming up. Diagnosis: balancing valve needed adjustment after install. Resolved on installer’s first 6-week check.

Issue 2 (month 4): Persistent humming from outdoor unit at certain wind angles. Diagnosis: vibration through wall mount. Installer added anti-vibration pads. Resolved.

Issue 3 (month 11): Cylinder pressure relief valve dripping. Diagnosis: filling loop valve left slightly open during a service top-up. Resolved on annual service.

None of these were catastrophic. All were dealt with under warranty. The installer’s response time was 2–4 days for each.

The Wilkinsons’ verdict

“We knew it wouldn’t be as fast at heating the house as the boiler. We expected lukewarm radiators. We didn’t expect how steady the warmth would feel — it’s actually more comfortable than the old system, which would over-heat then cool down. The first three months were us learning to leave it alone. After that, it just runs.”

“Bills are lower than gas, but not as much as we hoped. £600 a year is real money over 15 years, though. And we like that we’d be on the right side of any future gas price spike.”

Total cost of ownership picture

Install net cost: £4,400 (after BUS)
Annual saving Year 1: £655
Annual saving projected Year 2: ~£743
Simple payback (Year 1 saving rate): 6.7 years
Simple payback (Year 2 projected rate): 5.9 years

These figures are at the better end of the typical 8–14-year range. Three reasons:

They were replacing an old, inefficient boiler
They’re on a heat-pump-optimised tariff
Their solid walls were already externally insulated

A typical UK retrofit with a newer gas boiler and standard variable tariff would pay back slower.

What I’d do differently (if I were them)

Asked this directly, Mr Wilkinson said:

“I’d specify a slightly larger cylinder. 210L is fine but tight when we have visitors. I’d also have asked the installer harder about microbore — we got lucky that ours works, but I see online that other people have problems. Otherwise, no regrets.”

For homeowners considering it

The Wilkinsons’ experience is broadly representative of what a well-installed heat pump in a reasonably insulated UK home looks like over the first 18 months. Not transformational savings, but consistent ones. Some teething issues, all resolvable. Comfort change took a few weeks to adjust to but ended up preferred to the gas system.

The £7,500 BUS grant was the decisive financial factor. Without it, the £11,900 gross install would have been hard to justify on running-cost savings alone. With it, the maths gets a lot easier.

Run your own numbers in the cost calculator. If you’re in the LS postcode area, the Leeds installer page lists MCS-certified businesses serving the city.