Home Heat Pump Guide

Air Source Heat Pump with Existing Radiators: Will They Work?

One of the biggest myths about switching to a heat pump is that you need to rip out every radiator in the house and replace them with larger ones. It is a myth that adds thousands of pounds to the perceived cost and puts many homeowners off making the switch.

The reality is more nuanced — and often more encouraging. Many existing radiators can work perfectly well with a heat pump. Some may need replacing. And in certain cases, your existing radiators might actually be an advantage.

This guide explains exactly how to assess your existing radiators, what determines whether they are adequate, and what to do if some of them are not.

Why Radiator Size Matters with a Heat Pump

To understand whether your existing radiators will work, you need to understand why size matters.

The Temperature Difference

A gas boiler typically runs at a flow temperature of 65-75°C. At these temperatures, even a relatively small radiator emits a lot of heat. An air source heat pump, for maximum efficiency, should run at 35-50°C. At these lower temperatures, each radiator emits less heat — roughly 30-50% less at 45°C compared to 70°C.

The question becomes: is each radiator big enough to emit the heat its room needs at the lower flow temperature the heat pump will provide?

The Maths

Every radiator has a rated heat output, typically stated at a standard condition called Delta T 50 (which corresponds to a flow temperature of 75°C with a room temperature of 20°C). At a heat pump flow temperature of 45°C, the radiator's actual output is roughly 44% of its Delta T 50 rating. At 40°C flow, it drops to about 35%.

So a radiator rated at 1,500 watts (Delta T 50) will only emit approximately 660 watts at a 45°C flow temperature. If the room needs 800 watts to stay warm, that radiator is too small. If it only needs 500 watts, the radiator is fine.

When Existing Radiators Work

There are several common scenarios where your existing radiators can work with a heat pump without any changes.

1. The Radiators Were Already Oversized

This is more common than you might think. Plumbers and heating engineers have historically tended to oversize radiators, partly as a safety margin and partly because larger radiators were not much more expensive. If your radiators were oversized for the original gas boiler system, they may be adequately sized for a heat pump at lower flow temperatures.

Homes fitted with radiators in the 1960s through 1990s are particularly likely to have oversized radiators, as sizing practices were less precise and plumbers tended to go bigger "just in case."

2. The House Has Been Insulated Since the Radiators Were Fitted

If your home has had insulation upgrades — cavity wall insulation, loft insulation, new windows — since the radiators were originally installed, each room's heat demand has decreased. The radiators were sized for the old, leaky building and are now oversized for the improved, better-insulated one. This works in your favour.

3. You Already Have Large Double-Panel Convector Radiators

Double-panel convector radiators (Type 22) have significantly higher heat output than single-panel radiators (Type 11 or Type 21) of the same dimensions. If your house already has Type 22 radiators, they have a good chance of being adequate for heat pump temperatures — especially if the house is reasonably well insulated.

4. The Room Has Multiple Radiators

Some rooms, particularly larger living rooms and kitchens, have two radiators. Even if each individual radiator would be undersized on its own, together they may provide enough heat at lower flow temperatures.

When Existing Radiators Do Not Work

1. Small Single-Panel Radiators in Poorly Insulated Rooms

The worst-case combination: a small Type 11 (single-panel, single-convector) radiator in a room with high heat loss — perhaps an external-facing room with single glazing or uninsulated solid walls. The radiator cannot emit enough heat at lower flow temperatures to keep the room comfortable.

2. Towel Rails in Bathrooms

Standard towel rails have very low heat output — often just 200-400 watts at Delta T 50, which drops to under 200 watts at heat pump temperatures. Bathrooms usually need more heat than a towel rail alone can provide at low flow temperatures. A larger radiator or underfloor heating may be needed alongside or instead of the towel rail.

3. Column Radiators and Decorative Radiators

Traditional column radiators and decorative designer radiators often have lower heat output per unit of wall space compared to modern double-panel convector radiators. They may look beautiful, but they were designed for high flow temperatures. At heat pump temperatures, they may not emit enough heat.

4. Very Old or Corroded Radiators

If your radiators are heavily corroded internally (sludge build-up, pinhole leaks, cold spots), they are not operating at their rated output regardless of size. A power flush might restore them, but if they are beyond saving, replacement is the better option — and an opportunity to upsize.

How to Check Your Existing Radiators

Here is a practical approach to assessing whether your radiators are up to the job.

Step 1: Get a Room-by-Room Heat Loss Calculation

This is essential and should be carried out by your heat pump installer as part of the design process. A heat loss calculation determines exactly how many watts of heat each room needs to maintain a comfortable temperature (usually 21°C in living areas, 18°C in bedrooms) on the coldest expected day.

Step 2: Identify Your Existing Radiator Sizes and Types

Measure each radiator (height, width, and depth) and identify the type (single panel, double panel, single convector, double convector). You can then look up the manufacturer's rated heat output or use online radiator output calculators.

Step 3: Compare Output at Heat Pump Flow Temperature

Calculate each radiator's heat output at the target flow temperature (typically 40-50°C for a heat pump) using the Delta T correction factors. Then compare this to the room's heat loss from Step 1.

  • Radiator output at HP temp >= room heat loss: The radiator is fine. No change needed.
  • Radiator output at HP temp is within 10-20% of room heat loss: Marginal. It may work, especially if you run the flow temperature slightly higher (e.g., 50°C instead of 45°C). Consider upgrading if budget allows.
  • Radiator output at HP temp is significantly less than room heat loss: Replace with a larger radiator or add a second radiator to the room.

Step 4: Prioritise Replacements

You may find that most of your radiators are adequate and only two or three need upgrading. This is very common. Replacing three radiators at £300-£500 each is far cheaper and less disruptive than replacing all twelve.

For a comprehensive guide on this topic, see our detailed page on whether you need new radiators for a heat pump.

Oversized Radiators: Your Secret Advantage

Here is something that surprises many homeowners: having oversized radiators is actually a good thing when switching to a heat pump.

With a gas boiler, oversized radiators waste energy — the room heats up too quickly, the thermostat clicks off, the boiler cycles on and off inefficiently. It is one of the reasons gas boilers in real homes are often less efficient than their rated figures suggest.

With a heat pump at low flow temperatures, those same oversized radiators are an advantage. They provide enough surface area to heat the room at 35-40°C flow temperature, allowing the heat pump to run at maximum efficiency. What was a flaw with a gas boiler becomes a feature with a heat pump.

If you are renovating or replacing radiators before installing a heat pump, deliberately oversizing them by 50-100% is a smart move. It costs a little more upfront but pays back in lower running costs for the life of the system.

What About Cast-Iron Radiators?

Cast-iron column radiators are common in Victorian and Edwardian homes. They are often very large — which is good for heat pump compatibility. However, there are some considerations:

  • Heat output: Cast-iron radiators have lower output per square metre of surface than modern steel radiators, but their large size often compensates. Check the output figures for your specific radiators.
  • Thermal mass: Cast iron is heavy and stores a lot of heat. This means they take longer to warm up and cool down — similar to underfloor heating. With a heat pump running continuously, this high thermal mass is actually beneficial, providing stable, even heat.
  • Condition: Check for corrosion, leaks, and sludge. Old cast-iron radiators can have significant internal build-up. A power flush is advisable before connecting to a new heat pump.
  • Connections: The pipework connections on old cast-iron radiators may need updating. Modern TRVs (thermostatic radiator valves) should be fitted to all radiators for proper heat pump control.

The Role of Flow Temperature

Flow temperature is the bridge between radiator size and heat pump efficiency. If your radiators are slightly undersized for 40°C flow temperature, running the heat pump at 45-50°C may solve the problem — at the expense of some efficiency.

This is a trade-off your installer should discuss with you:

  • Lower flow temperature (35-40°C): Highest heat pump efficiency, but requires large radiators or UFH
  • Moderate flow temperature (40-50°C): Good efficiency, works with many existing radiators
  • Higher flow temperature (50-55°C): Lower efficiency, but works with most existing radiators including smaller ones

A well-designed system uses the lowest flow temperature that keeps every room comfortable. Weather compensation adjusts this automatically based on outdoor conditions — lower flow temperature on mild days, higher on cold days.

Practical Tips for Making Existing Radiators Work

1. Power Flush the System

Before connecting a heat pump to an existing radiator system, a power flush removes sludge, rust, and debris from the pipes and radiators. This can restore up to 20% of lost heat output in a dirty system and protects the heat pump's heat exchanger from damage.

2. Fit Thermostatic Radiator Valves (TRVs)

TRVs on every radiator (except the one in the room with the main thermostat) allow individual room temperature control. This is important for heat pump efficiency — rooms that are warm enough have their radiators throttled back, allowing the heat pump to run at a lower output.

3. Bleed Radiators Regularly

Trapped air in radiators creates cold spots and reduces output. With a heat pump, where every watt of output matters, keeping radiators properly bled is more important than ever.

4. Ensure Good System Water Quality

Treat the heating system water with corrosion inhibitor and, if appropriate, a magnetic filter on the return pipe. This protects both the radiators and the heat pump's plate heat exchanger.

5. Consider Adding a Second Radiator

If a room's existing radiator is undersized but there is wall space for a second one, adding a radiator can be cheaper and less disruptive than replacing the existing one with a larger model — especially if the existing radiator is in good condition.

Cost of Radiator Upgrades

If some radiators do need replacing, here is what to budget:

  • Standard Type 22 double-panel convector radiator: £80-£250 per radiator (depending on size), plus £100-£200 for fitting
  • Larger or designer radiator: £150-£500+ per radiator, plus fitting
  • Power flush (whole system): £300-£600
  • TRVs (if not already fitted): £15-£30 each, plus fitting

For a typical house where three to four radiators need upgrading, budget £600-£1,500 for the radiator work on top of the heat pump installation cost.

Frequently Asked Questions

Do I need to replace all my radiators for a heat pump?

Almost certainly not. A room-by-room heat loss calculation and radiator assessment will identify exactly which radiators (if any) need upgrading. Most homeowners find that the majority of their existing radiators are adequate, with only two to four needing replacement. See our full guide on radiators and heat pumps for more detail.

Will my radiators feel cold with a heat pump?

They will feel cooler to the touch than with a gas boiler — warm rather than hot. At 45°C flow temperature, a radiator surface will be around 35-40°C, compared to 55-65°C with a gas boiler. The room should still be warm; the radiator just does not feel as hot when you touch it. This is normal and expected.

Can I keep my old cast-iron radiators?

Often, yes. Cast-iron radiators are typically large and have good thermal mass, both of which suit heat pump operation. Have them assessed for output at heat pump flow temperatures and check their condition. A power flush is advisable to clear any internal sludge build-up.

What if I want the most efficient system possible?

For maximum efficiency, you want the lowest possible flow temperature, which means the largest possible radiators (or underfloor heating). If budget allows, replacing radiators with models oversized by 50-100% allows the heat pump to run at 35-40°C and achieve the highest COP. The extra cost of larger radiators pays back through lower electricity bills over the system's lifetime.

Should I replace radiators before or during the heat pump installation?

Ideally during — your heat pump installer can supply and fit appropriately sized radiators as part of the overall installation. This ensures the whole system is designed as a coherent unit. If you want to do it in advance (perhaps as part of a renovation), get your heat pump installer to specify the sizes first.

Can a high-temperature heat pump avoid the need for new radiators?

Yes. High-temperature heat pumps can run at 60-80°C, similar to a gas boiler, making almost all existing radiators adequate. The trade-off is lower efficiency — a high-temp unit running at 65°C achieves a COP of roughly 2.0-2.5 instead of 3.0-3.5 at lower temperatures. It is a valid approach, especially for older houses where radiator replacement would be particularly disruptive or expensive.