Your Home Climate

Understanding · Heating

Why can't I just swap my boiler for a heat pump?

Because the boiler was making heat and a heat pump moves it, and the two machines want completely different things from the rest of the house. This page follows the mechanism through one 1930s semi as it climbs the Heat Loss Ladder, one rung at a time.

January · gas combi at 75°C · fabric leaking

January, evening. A gas combi is quite happy shovelling 75°C water through undersized radiators; every mistake it papers over is a mistake a heat pump would inherit.

The boiler makes heat; the pump moves it.

A gas boiler burns fuel and pushes the resulting heat into the water at whatever temperature you ask for; 75°C is easy for it and hides a great deal. A heat pump captures low-grade heat from outside air and upgrades it by compression. The wider the gap between the outside air and the flow temperature, the harder the compressor has to work and the lower its seasonal efficiency. Every 5°C higher on the flow costs roughly 8 to 12% of seasonal COP; this is why a well-designed pump runs at 45°C rather than the 75°C the boiler was throwing about.

The moment the flow temperature comes down, everything downstream of it has to change. Radiators sized for 70°C water only deliver about 45% of their nameplate output at 45°C; MIS 3005 requires every existing radiator to be checked, and roughly two-thirds of UK radiators fail that check and need upsizing.

The pump isn't the intervention; the flow temperature is.

Smaller loss, smaller system.

A house that loses 8 kW on the coldest day of the year needs an 8 kW heat pump. The same house with the loft brought up to 300 mm and the worst-exposed window replaced often drops to 5 kW; that is a smaller pump, quieter outside, cheaper to buy, and running at a lower flow temperature for a higher COP. Every decision above the fabric rung inherits the number the fabric sets.

This is why the Heat Loss Ladder is stubbornly ordered. Skip fabric and the pump is oversized; skip the emitter check and the pump runs hot to compensate; skip controls and the pump never quite lands on the room condition. Each step compounds the next. The physics does not care which order feels most convenient.

Same January · loft topped, worst window replaced

Same January, after fabric. The heat-loss calculation has dropped by roughly three kilowatts before any plant is chosen.

Nothing about this is an argument against heat pumps. It is the honest price of a system that delivers three units of heat for one of electricity when it is designed properly.

Same house · radiators upsized to K3, flow at 45°C

Radiators grown to K3, some pipework upgraded to 22 mm. The house can now be kept at 21°C on 45°C water; the pump has a job it can do well.

Verdict

What the Ladder actually tells you to do.

Settle the fabric before the plant. The pump is the last rung, not the first.

The honest answer to 'can I just swap the boiler' is almost always 'yes, once the rung below it is doing its job'. That is not a delaying tactic; it is the difference between a pump that quietly hits its rated COP for the next fifteen winters and a pump that behaves like an expensive electric heater. Every well-run heat-pump installer starts with a room-by-room heat-loss survey for the same reason; the survey is the Ladder, written up.

Why we think thisOpen

Reasoning

The relationship between flow temperature and coefficient of performance used here (8 to 12% loss per 5°C rise) is drawn from monitored UK installations reported by the Energy Systems Catapult's Electrification of Heat programme and reflected in the CIBSE CP1 heat networks code. The 45°C design flow is the current MCS assumption for well-designed retrofit systems.

The claim that roughly two-thirds of existing radiators fail an MIS 3005 check comes from installer-reported survey data across the Boiler Upgrade Scheme cohort; the exact share varies by property age and previous refurbishment history.

Assumptions

  • 1930s three-bed semi with cavity walls, existing gas combi, radiators sized for a 65 to 70°C flow.
  • Loft insulation currently between 100 and 200 mm; scope to top up cheaply to 300 mm.
  • One or two south-facing rooms with the oldest single or early double glazing on the property.
  • Householder open to a room-by-room survey rather than a like-for-like swap quote.

Sources

  • Electrification of Heat: Final Report — Energy Systems Catapult, 2023
    Monitored SPF data across 742 UK heat-pump installations; the empirical basis for the flow-temperature / COP relationship.
  • MIS 3005: Heat Pump Systems — MCS, current edition
    Requires per-radiator sizing checks at design flow temperature; underpins the emitter-resizing rung of the Ladder.
  • CIBSE CP1: Heat Networks Code of Practice — CIBSE
    Reference for low-flow-temperature design principles applied here at dwelling scale.

Next January · heat pump running · weather comp on

Next winter. The compressor hums quietly outside; weather compensation trims the flow every hour. Same house, different physics.

If this were our house

If this were our house, we would settle the fabric before we chose a plant, and we would refuse to accept a like-for-like swap quote from any installer who offered one.

  1. 1
    Top the loft to 300 mm and price cavity fill where the walls allow. A single-day job that shifts the whole heat-loss calculation and every downstream decision.
  2. 2
    Replace single glazing on the worst-exposed elevation. Not every window; the one or two that dominate the room heat-loss figures. Sizing every downstream decision to a glazing spec you intend to change is expensive.
  3. 3
    Commission a room-by-room heat-loss survey. Any MCS installer worth using will do this before offering a system size. Refuse quotes that skip it.
  4. 4
    Turn weather compensation on and leave it on. It is fitted to almost every modern boiler and every heat pump; on most it is never activated. Six to fifteen per cent, quietly, all winter.

We would still say the same thing if we were installing this ourselves; the Ladder is the physics, not a preference.

Related questions this page hasn't answered.

What if I can't do the fabric first?
You can still install a heat pump; it will run at a higher flow temperature and its seasonal COP will sit lower, closer to 2.5 than 3.5. It will heat the house; it will cost more to run than a well-designed system would. Every rung of the Ladder can be added later, at proportionately smaller benefit.
Does hybrid gas and heat pump solve this?
It defers the problem rather than resolving it. A hybrid runs the heat pump on mild days and hands over to the boiler in the cold snap; on the coldest weeks of the year the pump is doing very little. Useful as a transitional step in a large property with high peak loads; rarely the right answer for a small UK semi.
Is any of this changing under the Clean Heat Market Mechanism?
The pricing and the grant landscape shift regularly; the physics does not. Every question this page addresses about flow temperature, emitter sizing and controls will be true of every heat pump sold in 2027 as it is of every heat pump sold today.
Evidence apparatus
Last reviewed
4 July 2026
Evidence quality
High· Rests on monitored UK trial data and the current MCS installer standard.
Primary sources
Assumptions
  • 1930s three-bed semi with cavity walls, existing gas combi, radiators sized for a 65 to 70°C flow.
  • Loft insulation currently between 100 and 200 mm; scope to top up cheaply to 300 mm.
  • Householder open to a room-by-room survey rather than a like-for-like swap quote.
Related evidence
Changelog
  1. 4 July 2026First publication with full evidence apparatus.

Reviewed on a rolling six-month cadence; the physics is stable, the grant landscape is not.