Home Heat Pump Guide

Can a Heat Pump Work with Single-Panel Radiators?

If your home has single-panel radiators — the slim, flat type without fins on the back — you might be wondering whether they can work with a heat pump or whether they all need replacing. The answer depends on your home, and it is not always the expensive outcome. Some single-panel radiators are fine. Others will leave rooms cold. Here is how to work out which applies to you.

Understanding Radiator Types

Before assessing compatibility, it helps to understand the different radiator types commonly found in UK homes:

Type 11 (Single Panel, Single Convector)

One flat panel with one set of convector fins behind it. This is the most basic type and delivers the least heat per unit of wall space. Common in bedrooms, hallways, and smaller rooms.

Type 21 (Double Panel, Single Convector)

Two panels with one set of convector fins between them. A step up from Type 11, these deliver roughly 40 to 50% more heat than a Type 11 of the same dimensions.

Type 22 (Double Panel, Double Convector)

Two panels with two sets of convector fins. The workhorse of UK central heating. A Type 22 delivers roughly 70 to 80% more heat than a Type 11 of the same size, making it the standard recommendation for heat pump systems.

Type 33 (Triple Panel, Triple Convector)

Three panels, three sets of fins. Maximum output but very deep (protrudes roughly 160mm from the wall). Rarely needed unless wall space is extremely limited.

The Problem with Single-Panel Radiators at Low Flow Temperatures

A heat pump operates at flow temperatures of 35 to 50°C, compared to 70 to 80°C for a gas boiler. At these lower temperatures, radiator output drops dramatically — and single-panel radiators, which have the lowest output to begin with, are hit hardest.

Here is a practical example. A Type 11 radiator measuring 600mm high by 1,000mm long has a rated output of roughly 890 watts at ΔT50 (standard boiler conditions). At the lower temperatures used by a heat pump:

  • At 50°C flow (ΔT25): Approximately 355 watts (40% of rated)
  • At 45°C flow (ΔT20): Approximately 267 watts (30% of rated)
  • At 40°C flow (ΔT15): Approximately 196 watts (22% of rated)

If that room needs 800 watts of heat (a typical figure for a small bedroom), the radiator at 45°C flow delivers only 267 watts — a shortfall of 533 watts. The room will not reach a comfortable temperature.

When Single-Panel Radiators Can Work

Despite the output reduction, single-panel radiators can work with a heat pump in certain situations:

1. The Radiator Is Already Oversized

In many older UK homes, radiators were fitted generously — a plumber might have installed a 1,600mm Type 11 in a bedroom that only needs 600 watts. If the radiator's output at 45°C still meets the room's heat loss, it can stay.

2. The Room Has Low Heat Loss

Small, well-insulated rooms (a box bedroom in a mid-terrace, for example) may only need 400 to 500 watts. A decent-sized Type 11 could deliver this even at lower flow temperatures.

3. The Home Has Been Insulated Since the Radiators Were Fitted

If you have added cavity wall insulation, loft insulation, or double glazing since the heating system was installed, the room's heat demand has dropped. Radiators that were correctly sized for the uninsulated home are now effectively oversized — which works in your favour with a heat pump.

4. You Accept a Slightly Higher Flow Temperature

Running the heat pump at 50°C instead of 45°C gives each radiator roughly 33% more output. This reduces efficiency slightly (COP drops from approximately 2.9 to 2.5), but it may allow you to keep most or all of your existing radiators. The running cost difference is around £100 to £200 per year for an average home.

When Single-Panel Radiators Need Replacing

In the following situations, single-panel radiators will almost certainly need upgrading:

  • Large rooms with high heat loss: Living rooms, kitchens, and dining rooms with big windows or external walls typically need 1,500 to 2,500 watts, which single-panel radiators cannot deliver at heat pump temperatures
  • Poorly insulated rooms: If the room has solid walls, single glazing, or poor loft insulation, the heat demand will be too high for a Type 11
  • Small radiators in medium-sized rooms: A 600 x 800mm Type 11 delivers very little heat at 45°C — if the room needs more than 250 watts, this will not suffice
  • Rooms where you want the lowest flow temperature: If you are targeting 40°C flow for maximum efficiency, the output reduction is too severe for single-panel radiators in most rooms

How to Assess Your Radiators: A Practical Process

Step 1: Get a Heat Loss Calculation

Your heat pump installer should perform a room-by-room heat loss calculation as part of the survey. This tells you exactly how many watts each room needs on the coldest design day (typically -3°C for most of England, colder in Scotland).

Step 2: Identify Your Existing Radiators

For each radiator, note the type (check the depth — Type 11 is typically 50 to 60mm deep, Type 22 is about 100mm), height, and length. Your installer can look up the rated output from manufacturer tables.

Step 3: Calculate Output at Your Target Flow Temperature

Apply the correction factor for your chosen flow temperature. At 45°C flow, multiply the rated output by 0.30. At 50°C, multiply by 0.40. Compare this to the room's heat loss requirement.

Step 4: Decide Room by Room

Some radiators will pass; others will not. You may find that bedrooms and hallways are fine while the living room and kitchen need upgrades. This targeted approach keeps costs down.

Upgrade Options When Replacement Is Needed

Replace with Type 22 Double-Panel Convectors

The most common upgrade. A Type 22 of the same height and length as your existing Type 11 delivers roughly 80% more heat. In many cases, a like-for-like swap of Type 11 to Type 22 radiators is sufficient. The pipe connections are usually in the same position, so the swap is straightforward.

Cost: £150 to £400 per radiator plus £100 to £200 labour per radiator.

Replace with Larger Radiators

If a Type 22 of the same dimensions is still not enough, you may need a longer or taller radiator. This can mean moving pipework, which adds cost.

Cost: £200 to £500 per radiator plus £200 to £400 labour if pipework needs extending.

Switch to Aluminium Radiators

Aluminium radiators deliver more heat per kilogram of metal because aluminium conducts heat better than steel. They heat up faster and are more responsive at low flow temperatures. However, they cost more than standard steel panels.

Cost: £300 to £800 per radiator plus installation.

Add Fan-Assisted Convectors

Where wall space is very limited, a fan convector can deliver two to three times the output of a standard radiator of the same size. The built-in fan pushes air over the heat exchanger, dramatically boosting output at low flow temperatures.

Cost: £400 to £1,200 per unit plus installation.

What About Adding a Second Radiator?

Rather than replacing your single-panel radiator with a huge Type 22, another option is to keep the existing radiator and add a second one on a different wall. This doubles the room's total radiator area, which may provide enough output at low flow temperatures without needing to remove the existing unit.

This approach works well in rooms with two or more external walls, where heat loss is high and a single radiator — however large — creates uneven warmth.

The Cost of Doing Nothing

If you keep undersized single-panel radiators and do not replace them, the heat pump installer will need to raise the flow temperature to compensate. Running at 55°C instead of 45°C typically costs £200 to £350 extra per year in electricity. Over 10 years, that is £2,000 to £3,500 — often more than the cost of upgrading the radiators.

In short, investing in appropriately sized radiators pays for itself relatively quickly through lower running costs.

What Your Installer Should Tell You

A good heat pump installer will include a radiator assessment as part of their survey. They should:

  • Perform a room-by-room heat loss calculation
  • Survey every existing radiator (type, size, and condition)
  • Calculate each radiator's output at the proposed flow temperature
  • Clearly list which radiators need replacing and which can stay
  • Provide a quote that includes radiator upgrades where needed

If an installer tells you all your radiators need replacing without doing this assessment, or conversely tells you everything is fine without checking, consider getting a second opinion. The Boiler Upgrade Scheme requires MCS-certified installers, who should follow a proper design process.

Frequently Asked Questions

Do all single-panel radiators need replacing for a heat pump?

No. If a single-panel radiator is large enough relative to the room's heat demand, it can work at heat pump flow temperatures. A heat loss calculation is the only way to confirm this — do not guess.

Is it cheaper to upgrade radiators or raise the flow temperature?

Upgrading radiators has a higher upfront cost (typically £1,500 to £4,000 for a whole house) but saves £200 to £350 per year in running costs. The upgrade usually pays for itself within 5 to 10 years and then saves money every year thereafter.

Can I replace radiators myself?

Swapping a radiator on existing pipework is a common DIY task, but you need to drain the system, remove the old unit, fit the new one, refill, and bleed the system. If valve positions differ or new pipework is needed, it is best left to a plumber. Your heat pump installer can often include radiator replacements in the overall installation quote.

Will the Boiler Upgrade Scheme grant cover new radiators?

The BUS grant covers the heat pump installation, and many installers include necessary radiator upgrades as part of the package. Check with your installer whether radiator replacements are included in or additional to the quoted price.

How do I know what type my radiators are?

Measure the depth from the wall to the front of the radiator (excluding the bracket gap). Type 11 is roughly 50 to 60mm deep. Type 21 is about 65 to 75mm. Type 22 is approximately 100mm. Type 33 is around 160mm. You can also look at the top — single panel has one visible panel edge; double has two.