Why comparing a heat pump to an old boiler can distort the figures
When savings claims use an ageing gas boiler as the only baseline, and how to ask for a fair new-boiler comparison.
Worked example (illustrative, not a price quote)
Imagine a home needs about 12,000 kWh of useful heat a year.
- Old boiler at ~75% efficiency might burn ~16,000 kWh of gas.
- New boiler at ~90% might burn ~13,300 kWh for the same comfort.
- Heat pump at SCOP 3 might use ~4,000 kWh of electricity for the same heat.
Compare the heat pump only to the old boiler and electricity looks very attractive. Compare to the new boiler and the gap narrows. Add a higher electricity tariff or lower SCOP and the gap narrows again.
None of this uses real prices for your address. The point is structural: the baseline boiler changes the story.
Why installers use the old boiler
It is what you pay today. It is a fair question for “what happens if I do nothing different”. It is a weak question for “should I choose a heat pump over replacing the gas boiler?”
What to ask for
- Same heat demand figure for both options.
- New gas boiler annual kWh at realistic efficiency.
- Heat pump annual kWh at the quoted SCOP.
- Capital cost for each path (including radiators if needed).
- Heat pump cost on a standard electricity tariff, not only a special tariff.
You may still choose a heat pump
Lower carbon, future tariff options, or comfort reasons can matter. The quote should still show whether the money case relies on an old boiler comparison you would not actually face if you replaced the gas system anyway.
Use section 2 of the Heat Pump Quote Reality Checker for a new boiler baseline.
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