Radiators for Heat Pumps: Do You Need to Upgrade?
TLDR: Heat pumps run at 35-50°C versus 60-75°C for gas boilers. This means some radiators may not provide enough heat at lower temperatures. Typically 20-50% of radiators need upsizing—not all. Modern double-panel radiators with convector fins often work fine. A proper room-by-room heat loss calculation identifies which radiators need attention. Budget £200-£400 per radiator that needs replacing, with most homes spending £500-£1,500 total on upgrades.
One of the most common concerns I hear from homeowners considering heat pumps is about their radiators. "Won't I need to replace every radiator in the house?" The short answer is: almost certainly not. But some radiators will likely need attention, and understanding which ones—and why—makes the transition much smoother.
The relationship between heat pumps and radiators is often misunderstood. Heat pumps aren't incompatible with radiators; they just work at lower temperatures than the gas or oil boilers most UK homes have been using for decades. This matters because radiator output is directly linked to water temperature. Lower water temperature means less heat output—unless you compensate with larger radiators or more efficient designs.
This guide explains the physics behind it all, helps you assess your own radiators, and shares real experiences from homeowners who've navigated this very decision. By the end, you'll understand what your home needs—and how much you're likely to spend.
Understanding Why Radiator Size Matters
The heat output of a radiator depends on two main factors: its physical size and the temperature of the water flowing through it. When you reduce the water temperature, output drops significantly—more than many people expect.
The Temperature-Output Relationship
Here's how a typical 600mm x 1000mm Type 22 radiator (double panel, double convector) performs at different flow temperatures:
| Flow Temperature | Heat Output | % of Max Output | Typical System |
|---|---|---|---|
| 75°C | 2,100W | 100% | Old gas/oil boiler at full blast |
| 65°C | 1,680W | 80% | Modern condensing boiler |
| 55°C | 1,260W | 60% | Heat pump (high temp mode) |
| 50°C | 1,050W | 50% | Heat pump (standard) |
| 45°C | 840W | 40% | Heat pump (efficient mode) |
| 40°C | 630W | 30% | Heat pump (highly efficient) |
At heat pump temperatures (typically 45-50°C), a radiator outputs roughly 40-50% of what it did at old-style boiler temperatures. If your room needed 1,500W and your radiator was sized to just meet that at 70°C, it will only deliver about 750W at 45°C—leaving you cold.
This is the crux of the radiator upgrade question: was your original radiator sized with excess capacity, or was it a tight fit?
Real Homeowner Experiences
Richard and Susan, Leicester (1980s detached): "When our installer did the heat loss survey, he checked every radiator against what the room needed. Of our eleven radiators, seven were fine—they were all Type 22s that the previous owner had oversized because they liked warm rooms. But four needed replacing: a tiny single-panel in the bathroom, two in bedrooms that were undersized, and the one in the conservatory that was fighting against all that glass. Total cost for the four new radiators was £1,100 including installation. We chose Stelrad Compact radiators—not the cheapest, but good quality. The rest of the system worked perfectly at 48°C flow temperature."
Emma, York (Victorian terrace): "Our house has beautiful cast iron radiators—the tall column style. I was dreading being told we'd have to rip them out. But actually, cast iron radiators are often huge compared to modern panel radiators, and ours were sized for the inefficient boiler that was here when the house was built in 1900. Our installer calculated that five of the seven cast iron rads were actually adequate, even at heat pump temperatures. We only needed to add one modern panel radiator in the bathroom and upsize the one in the kitchen, where we'd extended years ago. Total spend on radiators: about £650. The cast iron ones stayed, and they work beautifully with the heat pump—they're warm to touch rather than scalding hot like they used to be."
Michael, Glasgow (1960s semi): "We went through a Warmworks scheme (which handles ECO4 and BUS grants in Scotland), and they were very thorough. Every room was measured, every radiator assessed. Their recommendation was to replace six of our eight radiators. Most were undersized Type 11s (single panel, single convector) from when the house was built. The new Type 22 radiators were included in the installation package at no extra cost to us—the whole project was covered by the grant and ECO4 funding because we're on Pension Credit. The new radiators are definitely bigger, but the house is properly warm for the first time since we moved in twenty years ago."
Karen, Nottingham (new build 2018): "Our house was built with what the developer called 'efficient' radiators—which turned out to mean 'the smallest ones they could get away with'. When we switched from the original gas boiler to a heat pump, every single radiator needed replacing. The original installer had sized them for 70°C flow temperature with zero margin. At 50°C, they delivered about half what we needed. We replaced all ten radiators for £2,800 including installation. Expensive, but the house is now comfortable at 45°C flow temperature, which keeps our running costs down. If we'd known the developer was cutting corners on radiators, we'd have insisted on bigger ones from day one."
David, Oxford (1970s bungalow): "My radiators were a real mix—some original from the 1970s, some replaced piecemeal over the years. The survey identified three that needed attention. But rather than replace all three, we took a hybrid approach. One room got underfloor heating (we were redoing the floor anyway), one got an oversized designer radiator that my wife loved the look of, and the third got a low-temperature radiator with a built-in fan (Jaga brand). Different solutions for different rooms, based on what made sense. Total: about £3,500, but that included the UFH installation."
Which Radiators Usually Need Upgrading
Based on surveys of hundreds of heat pump installations, here's what typically needs attention—and what usually works fine.
Radiators That Often Need Upgrading
- Single panel radiators (Type 11, Type 10): These were common in 1960s-80s homes. With just one panel and minimal convector fins, they have low output for their size. Often the first candidates for replacement.
- Undersized radiators in large rooms: If a room feels cold even with the heating on full, the radiator was probably undersized from the start. This gets worse at heat pump temperatures.
- Radiators in high-heat-loss rooms: Conservatories, extensions with large windows, rooms over garages—anywhere with above-average heat loss needs above-average radiator capacity.
- Bathroom radiators and towel rails: Often decoratively sized rather than functionally sized. May work fine for keeping towels dry but won't heat the room adequately at low flow temperatures.
- Radiators sized for 75°C+ systems: Homes that still have original 1970s-80s boiler infrastructure were often sized for very high flow temperatures. These radiators struggle at modern heat pump temps.
Radiators That Often Work Fine
- Double panel, double convector radiators (Type 22): The most common modern type. If reasonably sized for the room, these usually have enough capacity for heat pump temperatures, especially if the home is well-insulated.
- Oversized radiators: Many 1990s-2010s installations used larger radiators than strictly necessary. This excess capacity becomes a benefit with heat pumps.
- Radiators in well-insulated rooms: If you've improved insulation, your heat loss is lower than the original installation assumed. The same radiator may now be adequate.
- Bedroom radiators: Bedrooms typically have lower heat requirements (we don't need 21°C while sleeping), so radiators that might be marginal elsewhere often work fine here.
- Cast iron radiators: Despite their vintage, cast iron column radiators are often physically large and have substantial output. Many work well at heat pump temperatures.
Upgrade Options and Costs
When a radiator needs upgrading, you have several options depending on budget, room constraints, and aesthetic preferences.
Option 1: Larger Standard Radiators
The simplest approach: replace the undersized radiator with a bigger one of the same type.
| Radiator Size | Radiator Cost | Installation | Total |
|---|---|---|---|
| 600 x 800mm Type 22 | £80-£120 | £100-£150 | £180-£270 |
| 600 x 1200mm Type 22 | £120-£180 | £100-£150 | £220-£330 |
| 600 x 1600mm Type 22 | £160-£220 | £120-£180 | £280-£400 |
| 700 x 1400mm Type 22 | £180-£250 | £120-£180 | £300-£430 |
Popular brands include Stelrad, Quinn, Kudox, and Brugman. Plumbers' merchants like Screwfix and Toolstation stock basic models; specialists like BestHeating offer wider selections.
Considerations: May need to relocate pipework if the new radiator is in a different position. Larger radiators take more wall space, which can be an issue in smaller rooms.
Option 2: Higher-Output Radiator Types
Replace with a more efficient radiator design that outputs more heat in the same footprint.
- Type 21 (double panel, single convector): About 50% more output than Type 11 in the same size.
- Type 22 (double panel, double convector): About 100% more output than Type 11.
- Type 33 (triple panel, triple convector): Maximum output but very deep (15cm+ from wall). Rarely needed but available for extreme cases.
This approach works well when wall space is limited. A Type 22 at 600 x 1000mm delivers similar output to a Type 11 at 600 x 2000mm—half the length for the same heat.
Option 3: Low-Temperature Radiators (Fan-Assisted)
Specialist radiators designed specifically for heat pump systems, often with built-in fans that boost convection.
- How they work: A small, quiet fan increases airflow across the radiator fins, dramatically improving heat transfer at low water temperatures.
- Output: Can deliver 2-3x the output of a standard radiator of the same size at heat pump temperatures.
- Cost: £400-£800 per unit plus installation.
- Brands: Jaga, Stelrad (LST range), Purmo (Narbonne).
Best for: Rooms where space is very limited, or where maximum output is needed without large radiators (conservatories, high-heat-loss spaces).
Option 4: Underfloor Heating (Partial)
Rather than upgrading individual radiators, install UFH in specific rooms—especially ground-floor living areas.
- Low-profile UFH: Adds 15-25mm to floor height. Can be installed room-by-room. Cost: £80-£120 per square metre.
- Ideal for: Kitchen, living room, extensions—anywhere you're renovating the floor anyway.
- Combine with radiators: UFH on ground floor, existing radiators upstairs. A common hybrid approach.
UFH operates at lower temperatures than radiators (35-45°C vs 45-55°C), which improves heat pump efficiency. It's worth considering if you're planning any floor renovation work.
The Assessment Process: How Installers Decide
Any MCS-certified heat pump installer will conduct a detailed assessment before recommending radiator changes. Here's what they calculate:
Step 1: Room-by-Room Heat Loss
Using MCS-standard software (like Heatloss.co.uk or MasterSpec), the installer calculates how much heat each room loses based on:
- Room dimensions
- Wall construction and insulation
- Window size, type, and orientation
- Floor and ceiling construction
- Ventilation and air changes
- Target room temperature (usually 21°C living areas, 18°C bedrooms)
Step 2: Current Radiator Output at Design Temperature
Each existing radiator is identified (type, dimensions) and its output calculated at the planned flow temperature—typically 45-50°C for efficient heat pump operation.
Step 3: Gap Analysis
The installer compares what the room needs versus what the radiator can deliver. If there's a shortfall, they calculate how much.
- Small shortfall (10-20%): May be acceptable—the heat pump can run slightly hotter occasionally.
- Moderate shortfall (20-40%): Upgrade recommended for comfort and efficiency.
- Large shortfall (40%+): Definite upgrade required, or consider UFH/fan-assisted options.
Step 4: Recommendations
The installer proposes specific solutions for each undersized radiator, considering:
- Available wall space
- Pipework requirements
- Budget
- Aesthetic preferences
- Any planned renovations that might affect choices
A good installer explains the trade-offs clearly and gives you options at different price points.
The Running-Hotter Question
Can't you just run the heat pump at higher temperatures to make existing radiators work?
Yes—but there's a significant cost. Heat pump efficiency drops as flow temperature rises:
| Flow Temperature | Typical COP | Electricity to Heat £100 Worth |
|---|---|---|
| 35°C | 4.5 | About £22 of electricity |
| 45°C | 3.5 | About £29 of electricity |
| 50°C | 3.0 | About £33 of electricity |
| 55°C | 2.6 | About £38 of electricity |
| 60°C | 2.2 | About £45 of electricity |
Running at 55°C instead of 45°C might avoid radiator upgrades, but you'll pay 30% more in running costs—potentially £200-£400 extra per year for a typical home. Over 10 years, that's £2,000-£4,000, far more than most radiator upgrades would cost.
There's a middle ground: run at a slightly higher temperature (48-50°C) and accept modest efficiency loss, while upgrading only the worst-performing radiators. Many homeowners find this balance works well.
Q&A: Radiators and Heat Pumps
Q: Do I have to replace all radiators?
A: Almost never. Typically 20-50% of radiators need attention, and sometimes fewer. Many existing radiators, especially double-panel types installed since the 1990s, work fine at heat pump temperatures. A proper survey tells you exactly which ones need upgrading.
Q: What about underfloor heating instead?
A: UFH is ideal for heat pumps because it operates at lower temperatures by design. Adding UFH to ground floors while keeping radiators upstairs is a common hybrid approach. It's worth considering if you're renovating anyway, but installing UFH throughout an occupied home is disruptive and expensive.
Q: Can I just run the heat pump hotter to keep my radiators?
A: Yes, but efficiency drops significantly. Running at 55°C instead of 45°C increases electricity consumption by about 25-35%. Over the system's lifetime, upgrading radiators is usually cheaper than accepting permanently higher running costs.
Q: My radiators are already quite big. Will they definitely be okay?
A: Probably, but get them checked. Visual inspection isn't enough—you need the actual heat loss calculation. An oversized Type 22 might be fine; an undersized Type 22 might still need upgrading. The numbers tell the story.
Q: How long does radiator replacement take?
A: A straightforward swap takes 1-2 hours per radiator. If pipework needs extending or relocating, allow 2-4 hours. A typical home needing 3-4 radiators replaced can usually be done in a day.
Q: Should radiator upgrades be included in my heat pump quote?
A: Often yes—many installers include necessary radiator upgrades in their overall quote, particularly if claiming the BUS grant. Ask for a breakdown so you understand what's included. If radiators are quoted separately, expect £200-£400 per radiator installed.
The Bottom Line
Radiator upgrades are a normal part of transitioning to a heat pump—not a deal-breaker or a sign of incompatibility. Most homes need between zero and five radiators replaced, at a typical total cost of £500-£2,000. This is usually included in installer quotes or factored into the overall project budget.
The key is getting a proper room-by-room assessment from your MCS installer. Don't assume all radiators must go, but equally don't assume they'll all work. The truth is usually somewhere in between, and a good installer will give you clear recommendations with costs.
The radiators you have may well be fine with a few strategic upgrades. And even if more work is needed, the running cost savings from an efficient, well-designed heat pump system—combined with the £7,500 BUS grant—make the investment worthwhile.
Think of radiator upgrades not as an obstacle, but as an opportunity to optimise your home's heating for the next 20 years. Done right, you'll have a warm, comfortable home at lower running costs than you've ever achieved before.