Heat pump vs gas boiler: which one works best with cast iron radiators?
Estimated reading time: 6 minutes
A gas boiler suits homeowners who want a familiar, lower-cost installation and don’t mind a slightly higher long-term running cost. A heat pump suits homeowners planning to stay in their home for years, who can claim government grant support and want lower-carbon heating. The real difference between the two isn’t about whether cast iron radiators work: they work with both. It comes down to flow temperature, and that changes how many sections you need.
Which system suits cast iron radiators better: heat pump or gas boiler?
Gas boiler: strengths and impact
- Familiar technology, fitted by any qualified gas engineer, with a typical installation cost of £2,000 to £4,500
- Runs at a higher flow temperature, commonly a Delta T of 50°C to 60°C, so fewer radiator sections reach your target BTU output
- No grant paperwork or MCS-certified installer required, useful if your existing boiler fails and you need heat back on quickly
- A modern condensing boiler runs at around 90% to 92% efficiency, per Energy Saving Trust figures
Heat pump: strengths and impact
- Cast iron’s thermal mass genuinely suits a heat pump – the radiators hold their heat and return warmer water to the system, which helps the heat pump run more efficiently
- Qualifies for the government’s Boiler Upgrade Scheme: £7,500 towards an air source or ground source heat pump, rising to £9,000 for eligible off-gas-grid oil and LPG homes between 21 July 2026 and 31 March 2027, according to Ofgem
- Installations currently attract 0% VAT on the heat pump and its fitting, a government incentive running until 31 March 2027
- Typically delivers a seasonal COP of 3 to 4, meaning 3 to 4 kWh of heat for every 1 kWh of electricity used, against under 1 kWh of heat per kWh of gas from a boiler
Whichever system you choose, cast iron holds its heat longer than steel or aluminium. Browse the cast iron radiator range to find the right output for your home.
How do heat pumps and gas boilers compare for cast iron radiator owners?
| Feature | Gas Boiler | Heat Pump |
|---|---|---|
| Typical flow temperature | 50°C to 60°C (ΔT50/60) | 30°C to 40°C (ΔT30) |
| Typical installation cost | £2,000 to £4,500 | £8,000 to £14,000 (before grant) |
| Government grant available | ✘ | ✓ (Boiler Upgrade Scheme, up to £9,000) |
| VAT on installation | Standard rate | 0% until 31 March 2027 |
| Typical efficiency | Around 90% to 92% | SCOP of 3 to 4 |
| Typical system lifespan | 10 to 15 years | 15 to 20+ years |
| Works with cast iron radiators | ✓ | ✓ |
| Usually needs more sections for the same output | ✘ | ✓ |
| Required in new-build homes from 2027 (Future Homes Standard) | ✘ | ✓ |
Why does comparing heat pumps and gas boilers matter before you choose your radiators?
Getting this right before you order saves you from resizing your radiators later. A heat pump’s lower flow temperature means each section puts out less heat than the same section would on a gas boiler, so you may need extra sections or a deeper column profile to hit the same room temperature. The government’s Warm Homes Plan, published in January 2026, confirmed there’s no fixed deadline forcing existing gas boilers out of UK homes, so sticking with a boiler remains a perfectly reasonable choice. But if you’re planning ahead, whether for running costs, for the Boiler Upgrade Scheme grant, or for new-build compliance from 2027, sizing your cast iron radiators around a heat pump’s lower running temperature from the outset avoids an expensive retrofit further down the line.
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