Heat pumps do not need special radiators. They need bigger ones. A heat pump runs water at roughly 35°C to 55°C where a gas boiler runs it at 70°C to 80°C, and a radiator gives out far less heat at the lower temperature. To deliver the same warmth, the radiator has to be larger.
The good news is that most homes do not need every radiator replaced. A proper room-by-room survey usually finds that some rooms are already adequately served and only a subset need upgrading, most often by swapping a single panel for a double rather than covering more wall.
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- Radiator outputs are published at delta-T 50, a boiler-era baseline - that catalogue figure assumes water far hotter than a heat pump will ever supply
- At heat pump flow temperatures a radiator delivers roughly 50% of its rated output - around 35% if the system runs cooler still, which is why sizing has to be recalculated
- You typically need about 2 to 2.5 times the rated output - usually achieved by going single panel to double, or double to triple, not by hanging a much wider radiator
- Not every radiator needs replacing - a room-by-room heat loss survey identifies which rooms fall short, and in many homes several radiators are already big enough
- Microbore pipework is the hidden risk - 8mm and 10mm pipe common in 1970s and 80s homes can restrict the higher flow rates a heat pump needs
Why Heat Pumps Need Bigger Radiators
A radiator’s heat output depends on how much hotter its surface is than the room around it. Drop the water temperature and the output falls sharply. That is the entire reason heat pump systems need larger emitters, and it is physics rather than anything specific to heat pump technology.
UK radiators are rated under BS EN 442 at what the industry calls delta-T 50: a mean water temperature 50°C above room temperature. In practice that means water at roughly 70°C to 75°C heating a 20°C room. Every output figure in every manufacturer’s catalogue assumes those conditions, because they are the conditions a gas boiler produces.
A heat pump does not produce those conditions, and deliberately so. Efficiency falls as flow temperature rises, so a well-designed heat pump system runs as cool as the building allows. That is what makes it cheap to run, and it is also what makes the old radiator sizing invalid.
The Output Correction, in Numbers
Radiator output at a lower temperature is calculated by correcting the rated figure:
Actual output = rated output × (actual delta-T ÷ 50)n, where n is approximately 1.3 for a standard panel radiator.
Worked through, that gives the following approximate results. Treat these as working figures rather than exact values, because the exponent varies slightly between panel, column and fan-assisted radiators.
| System | Approx. delta-T | Radiator delivers | A 1,500W rated radiator gives |
|---|---|---|---|
| Gas boiler (catalogue baseline) | 50 | 100% of rated output | 1,500W |
| Heat pump, higher flow temp | 30 | Roughly 50% | About 770W |
| Heat pump, low flow temp | 20 | Roughly 30% | About 455W |
This is why the rule of thumb is that heat pump radiators need roughly two to two and a half times the rated output of the ones they replace. A radiator that comfortably heated a room on a boiler will leave the same room cold on a heat pump, not because the heat pump is underpowered, but because the emitter is now undersized for the water temperature reaching it.
Doubling output usually means changing radiator type rather than dimensions. A single-panel radiator swapped for a double-panel of identical height and width can roughly double output, and a triple panel more again. Most homeowners expecting enormous radiators are pleasantly surprised - the radiators get deeper, not wider.
Do You Need to Replace All Your Radiators?
Usually not. This is the most persistent myth about heat pumps, and it puts people off before they get a survey. In most homes a subset of radiators needs upgrading, not the whole system.
The reason is that radiators are rarely sized precisely. Many were installed oversized, or the room has since been insulated, double glazed or had its loft topped up, all of which cut the heat the room needs. A radiator that was generous for a 2005 heat loss may be adequate at heat pump temperatures for a 2026 heat loss.
Under MCS standard MIS 3005-D, an installer must design the system to meet the dwelling’s calculated heat loss at the design outdoor temperature, at flow temperatures the heat pump can actually deliver. That obliges a room-by-room heat loss calculation, and it is that calculation, not a rule of thumb, that determines which radiators change.
If an installer quotes for a heat pump without measuring rooms and calculating heat loss individually, they cannot know which radiators need changing. Either the system will underperform in the coldest rooms, or you will be sold replacements you did not need. A room-by-room calculation is a requirement of the MCS standard that underpins the £7,500 Boiler Upgrade Scheme grant, so any MCS-certified installer must do one.
What Radiator Upgrades Cost
Where a substantial radiator upgrade is required, we record a range of £3,600 to £9,000 in our air source heat pump cost guide. That covers a full or near-full replacement across a typical house, and it sits alongside the heat pump installation rather than inside it.
We do not publish a per-radiator price, because it varies too much with size, type and how accessible the pipework is to be useful. What matters more for budgeting is which of these applies to your property:
- A handful of radiators upgraded. The common outcome in a reasonably insulated home with generously sized existing radiators
- Most radiators upgraded. More likely in a poorly insulated property, or one where radiators were sized tightly to begin with
- Radiators plus pipework. The expensive case, driven by microbore pipe rather than by the radiators themselves
Insulating first changes the answer. Reducing a room’s heat loss reduces the radiator output it needs, which can take a room from “needs replacing” to “adequate”. Where budget is tight, insulation often buys more than radiators do.
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The Microbore Pipework Problem
Microbore is small-diameter pipework, typically 8mm or 10mm, widely installed in UK homes during the 1970s and 1980s. It works acceptably with a boiler, which moves a modest volume of very hot water. It is a genuine constraint with a heat pump, which does the opposite: it moves a larger volume of cooler water.
Push more water through narrow pipe and you get higher resistance, more pump energy, and potentially not enough flow to deliver the design output at the far end of the circuit. Where that happens, pipework has to be upsized, and because pipework runs under floors and inside walls, replacing it is disruptive in a way that swapping a radiator is not.
There is no reliable national figure for what this costs, because it depends entirely on your floor construction and how much pipe has to be reached. What you can do is find out early: ask any installer surveying your property to confirm the pipework diameter before you commit, because it is the single largest source of unexpected cost in a heat pump retrofit.
Alternatives to Bigger Radiators
Larger panel radiators are the default answer, but they are not the only one. Three alternatives are worth raising with your installer, particularly where wall space is genuinely limited.
- Fan-assisted radiators. A small fan moves air across the radiator, greatly increasing output from the same physical size. Useful where a wall simply cannot take a bigger panel, at the cost of a small electrical draw and some noise
- Underfloor heating. The natural partner for a heat pump, because the huge emitting surface works at very low flow temperatures. Cost and disruption make it realistic mainly in extensions, renovations and new floors
- Accepting a slightly higher flow temperature. Running the heat pump warmer lets existing radiators work, but efficiency falls and running costs rise, so it is a trade-off rather than a free fix
That last option deserves care. Every degree of extra flow temperature costs efficiency, and efficiency is the whole economic case for a heat pump. Our running costs guide shows how the numbers move, and the running cost calculator lets you test flow temperature assumptions against your own tariff. If you are still deciding on system type at all, heat pump sizing is the place to start.
How We Researched This Guide
Radiator output figures follow the BS EN 442 rating standard and its standard delta-T correction, which is the method UK manufacturers and installers use. Design obligations are taken from MCS standard MIS 3005-D, the heat pump design standard that MCS-certified installers work to and which underpins Boiler Upgrade Scheme eligibility. Cost ranges come from our own heat pump cost research, last verified August 2026.
The correction figures here are approximate by nature: the exponent varies between radiator types, and your actual requirement comes from a room-by-room heat loss calculation for your specific property. We give the method rather than a universal multiplier because no universal multiplier is honest.
Last updated: August 2026.
