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Here's the good news first: not every home needs new radiators for a heat pump. Many UK homes already have radiators that will work perfectly well — and a proper heat loss survey will tell you exactly which rooms, if any, need an upgrade before anything goes in.
The longer answer is that it depends. Heat pumps run at lower temperatures than gas boilers, which means your existing heat pump radiators need to be a certain size to keep rooms warm. Whether yours make the cut comes down to your radiators, your insulation, and your home's heat loss. This guide explains the difference and tells you what to expect if some radiators do need upgrading.
Why Heat Pumps Need Different Radiators
A gas boiler pushes water around your system at 55–75°C. A heat pump works differently — it heats water more slowly and runs most efficiently at 35–45°C. The lower the flow temperature a heat pump can run at, the less electricity it uses. Every 5°C increase in flow temperature costs roughly 10–12% more to run, so keeping it low matters.
The trade-off is that cooler water means radiators need to be large enough to still release enough heat into the room.
How Does Lower Temperature Affect Radiator Output?
A radiator's heat output drops significantly when the water running through it is cooler. Here's how that looks in practice:
| Flow temperature | Approx. output vs rated | Typical use |
|---|---|---|
| 75°C (old gas boiler) | 100% | Traditional boiler system |
| 55°C (modern combi) | ~70% | Modern boiler on lower setting |
| 45°C (heat pump) | ~42% | Heat pump — typical retrofit |
| 35°C (heat pump, ideal) | ~27% | Heat pump — optimal efficiency |
Based on EN 442 radiator output standards. Figures are indicative for a standard single-panel radiator.
So a radiator rated at 1,000W under boiler conditions only delivers around 420W at 45°C. That's why sizing matters — and why a room-by-room calculation is the only reliable way to know what's needed.
Do You Need New Radiators to Install a Heat Pump?
Many homes don't need to replace all their radiators, and some don't need to replace any.
When Existing Radiators Might Work Fine
Your current radiators are likely to work without upgrades if:
- They're already double-panel (Type 22) or larger. These have roughly twice the output of single-panel radiators at the same size — they were probably oversized for your boiler, which is actually ideal for a heat pump.
- Your home is reasonably insulated. Good loft and wall insulation reduces how much heat each room needs, which means smaller radiators can keep up.
- Rooms reach temperature comfortably with the boiler currently turned down. If your boiler thermostat runs at 60°C or below and rooms are still comfortable, the radiators are already working at relatively low temperatures.
- The radiators feel warm rather than hot when the heating is running. That's a sign your system is already running at a modest temperature differential — closer to heat pump territory than you might think.
When You Do Need to Upgrade
Radiator replacement is more likely if:
- You have old single-panel (Type 11) radiators, particularly in living rooms or larger bedrooms.
- Your home was built before 1980 with original insulation — or very little of it. Higher heat loss means radiators need to work harder.
- Some rooms struggle to reach temperature now, even with the boiler running. That's a sign the radiators are already undersized.
- The heat loss calculation confirms the existing output isn't sufficient at 45°C flow.
An MCS-certified installer is required to complete a room-by-room heat loss calculation as part of any heat pump installation — this tells you exactly which radiators (if any) need changing.
One more thing worth knowing: improving insulation before or alongside a heat pump install often reduces room heat loss enough that fewer radiator upgrades are needed. If your loft insulation is thin or your walls are uninsulated, addressing that first can save you money on radiator replacements.
How Much Bigger Do Heat Pump Radiators Need to Be?
A heat pump sends warm water round your radiators, rather than the very hot water a gas boiler uses. Cooler water means each radiator gives off less heat, so to keep the room just as cosy you need a radiator that can put out more heat – usually around one and a half to two times as much. That doesn't always mean a bigger radiator. Often it's simply a double-panel radiator the same size as the one you have now, so it fits in the same spot on the wall.
Room-by-Room Heat Loss
For accuracy, a room-by-room heat loss calculation is a reliable method. This uses:
- Room dimensions
- Window area and glazing type
- Wall construction and insulation levels
- Desired indoor temperature vs design outdoor temperature
The calculation gives the exact wattage each room needs to maintain temperature on the coldest days. Your MCS-certified installer is required to complete this as part of the installation design. Switch Together's home survey includes this calculation — the results specify exactly which radiators need changing, so there's nothing left to chance.
Types of Radiators That Work Best with Heat Pumps
Type 21/22 double-panel convectors are generally what you'll find as the default fit in most UK living rooms and bedrooms today. If upgrades are needed, here's how the main options compare:
| Radiator type | Output vs Type 11 | Best for | Supply cost | Heat pump suitability |
|---|---|---|---|---|
| Type 22 double panel | ~2× | Most rooms — standard upgrade | £80–£200 | Excellent |
| Type 33 triple panel | ~2.5–3× | Large rooms, high heat loss | £150–£350 | Best output |
| Fan convector | ~3–4× at low flow temp | Problem rooms, bathrooms | £300–£700 | Outstanding |
| Aluminium panel | Similar to Type 22 | Fast heat-up, low water volume | £100–£300 | Very good |
| Cast iron | Lower at low temps | Period properties, aesthetics | £300–£800 | Works, not ideal |
| Underfloor heating (wet) | Area-based | Living rooms, kitchens, bathrooms | £50–£100/m² | Ideal emitter |
Double and Triple Panel Convectors (Type 22 / Type 33)
Standard steel convectors are the most common upgrade. Type 22 (two panels, two rows of fins) is the default recommendation for most rooms — maximum output with minimum disruption, as they fit the same wall brackets as the radiator they're replacing. Type 33 adds a third panel for the highest output in the smallest wall space, useful for living rooms or rooms with large windows.
Both are compatible with all heat pump systems, widely available, and straightforward to install.
Fan Convectors (Fan-Assisted Radiators)
A fan convector has a built-in low-wattage fan that actively blows warm air through the unit. The result: high heat output even at very low flow temperatures (as low as 35°C), and rooms that warm up quickly. They're the best option for problem rooms that are hard to heat, or for bathrooms where fast warm-up matters more than passive heat. More expensive than panel radiators, but worth it where a standard upgrade would need a very large radiator to achieve the same output.
Aluminium Radiators
Aluminium is worth highlighting because it's often overlooked — and it suits heat pump systems particularly well.
Aluminium has much lower thermal mass than steel. This means it heats up and cools down much faster when the flow temperature changes. In a heat pump system that modulates temperature based on demand (which most modern heat pumps do), an aluminium radiator responds quickly and doesn't waste energy maintaining temperature in a room that's already warm. The lower water volume inside the radiator also slightly reduces the total water the heat pump needs to heat — a small but real efficiency gain.
If you're replacing radiators as part of a heat pump retrofit, aluminium is an increasingly popular choice and well worth including in your installer's quotation.
A Note on Cast Iron Radiators
Cast iron has high thermal mass — it's slow to heat up and slow to cool down. In a heat pump system, this means the pump has to run longer to bring the radiator up to temperature before the room benefits, which can reduce efficiency. Cast iron radiators work, and many period properties keep them, but they're not the optimal choice for a new heat pump installation. If you're replacing radiators anyway, switching to steel or aluminium will serve you better.
How Much Does Radiator Upgrading Cost?
When upgrades are needed, here's what to expect:
| Item | Approximate cost | Notes |
|---|---|---|
| Single radiator replacement (supply + fit) | £200–£400 | Type 22 double panel, standard size |
| Fan convector radiator (supply + fit) | £400–£800 | Premium option; best for problem rooms |
| 3-bed house (4–6 radiators needing upgrade) | £800–£2,400 | Rarely all rooms need it |
| 4-bed house (6–8 radiators needing upgrade) | £1,200–£3,200 | Upper end if all rooms upgraded |
Cost estimates based on HeatPumpChecker.co.uk data. Actual costs vary by radiator size, location, and local labour rates.
With Switch Together, radiator upgrades are assessed separately during the home survey. This means you won't be offered upgrades you don't actually need. Additionally, the cost for standard radiator replacements is capped at £200 per radiator.
My Heat Pump Radiators Aren't Getting Hot — Why?
If you already have a heat pump and your radiators feel lukewarm, the most likely explanation is also the least alarming one.
| Cause | What to check |
|---|---|
| Heat pump running at low flow temperature | This is normal. Heat pumps run cooler than boilers. If rooms are reaching your target temperature, the system is working correctly — radiators that feel warm, not hot, are doing their job. |
| Radiators need balancing | If some radiators are warm and others are cold, the system needs balancing — adjusting lockshield valves to distribute flow evenly. This is standard commissioning and should be done at installation. |
| Air in the system | Bleed the radiators using a radiator key at the bleed valve. Air pockets prevent hot water circulating and cause cold spots, usually at the top of the radiator. |
| Flow temperature set too low | If the heat pump is set to a very low flow temperature and radiators are undersized, they may struggle on cold days. An installer can adjust the weather compensation curve. |
| Undersized radiators | If rooms genuinely can't reach temperature after checking the above, the radiators may need upsizing. A service visit will confirm this. |
If issues persist after checking these, it's worth booking an annual ASHP service — a trained engineer can diagnose flow, pressure, and commissioning issues in one visit.
Frequently Asked Questions
Do heat pumps work with normal radiators?
Yes — heat pumps work with standard radiators. Because they run at lower flow temperatures (35–45°C vs 55–75°C for gas boilers), existing radiators may need to be larger to deliver the same heat output. Many UK homes already have radiators that are oversized for the room, which means they'll work as-is. A heat loss survey confirms exactly what's needed for your home.
What is the best radiator for an air source heat pump?
Type 22 double-panel convectors are the most cost-effective upgrade for most rooms — they offer roughly twice the output of single-panel radiators in the same wall space. Fan convectors are best for problem rooms or very low flow temperatures. Aluminium radiators offer fast response and low water volume, making them well-matched to the way heat pumps modulate temperature.
How do I calculate radiator size for a heat pump?
The accurate method is a room-by-room heat loss calculation, which an MCS-certified installer is required to complete as part of the system design. As a rough guide, radiators need approximately twice the output of standard boiler radiators when a heat pump runs at 45°C flow temperature. A Type 11 to Type 22 swap of the same dimensions usually achieves this in most rooms.
Do I need to bleed or balance radiators with a heat pump?
Yes to both, as needed. Bleed radiators if you notice cold spots at the top — this indicates air in the system, and the process is the same as with a gas boiler. Balancing (adjusting lockshield valves) ensures flow is distributed evenly across all radiators. This should be done at commissioning and checked if uneven heating develops over time.
Can I mix radiators and underfloor heating with a heat pump?
Yes — underfloor heating in ground-floor living areas and the kitchen, with radiators in bedrooms and hallways, is a popular and practical combination. Both systems run at the same low flow temperature from the same heat pump. This setup is common in new build properties and works well in retrofits where ground floors are being renovated.
Next Steps
Most homes need fewer radiator upgrades than people expect — especially if there's already a mix of double-panel radiators and reasonable insulation. The heat loss survey is the definitive answer, and it's included as standard in every Switch Together installation.
If you're curious about whether your home is a good candidate for a heat pump, start there. We'll survey your home, tell you exactly what (if anything) needs changing, and quote for the full system — including any radiator upgrades.
