Do you need bigger radiators for a heat pump?

Last reviewed: 14 May 2026.

In short

Heat pumps run at lower water temperatures than gas boilers — typically 45-50°C flow instead of 70-80°C — and a radiator’s heat output drops sharply at lower temperatures. The arithmetic isn’t linear: a radiator delivers roughly half its boiler-era output at a 50°C heat-pump flow and around a third at 45°C. The fix is some combination of larger radiators in the rooms that need them, accepting a higher flow temperature with the SCOP penalty, or upgrading insulation so the room needs less heat. A typical Reading retrofit replaces 20-40% of radiators — usually 2 to 4 out of 7 — and retains the rest. Radiator upgrades are eligible plant under BUS, so they sit inside the £7,500 grant calculation rather than being a separate cost. Total radiator-works cost for a standard Reading retrofit lands between £1,000 and £3,500. The deal-breaker case is microbore (8mm) pipework — common in 1970s-1990s installs — where flow rates at low temperatures can’t be achieved without re-piping.

On this page

• Why heat pumps need bigger radiators
• Radiator types — K1, K2, K3
• How much heat at what flow temperature
• Which radiators need to change in a typical retrofit
• Designer radiators and constrained wall space
• Microbore pipework — the deal-breaker
• Cost — and how BUS absorbs radiator works
• What this means for homes in Reading
• Related guides

Why heat pumps need bigger radiators

The output of a radiator into a room depends on the temperature difference between the radiator’s surface and the room around it. A gas boiler running at 70-80°C flow creates a temperature difference of around 50-60°C, and a typical room radiator delivers a lot of heat. A heat pump running at 45-50°C flow creates a temperature difference of around 25-30°C — and the same radiator delivers far less heat.

The relationship is not linear. Halving the temperature difference doesn’t halve the output — it reduces it by roughly 60%. This is because of how convection works around the radiator’s fins: lower temperature difference means less air movement, less heat transfer per square metre, less output per unit time.

Worked example for a typical living-room radiator (a 700mm × 1000mm K2 Type 22):

• At gas-boiler flow temperatures (70-80°C): around 1,960 W output
• At 60°C flow (high heat-pump setting): around 1,480 W output (76% of boiler-era)
• At 50°C flow (typical heat-pump setting): around 1,010 W output (51% of boiler-era)
• At 45°C flow (efficient heat-pump setting): around 810 W output (41% of boiler-era)
• At 40°C flow (very efficient): around 600 W output (31% of boiler-era)

For a room that previously needed 1,000 W of heat from a single radiator at gas-boiler temperatures, the heat-pump version of the design needs roughly 2,200 W of nominal radiator capacity to deliver the same 1,000 W at 45°C flow.

The lower you push the flow temperature, the bigger the radiators have to be — but the higher the seasonal efficiency (SCOP) you get from the heat pump. Our SCOP explainer covers the efficiency-vs-emitter-size trade-off in detail.

Radiator types — K1, K2, K3

UK radiators are classified by panel count and fin count. The two naming conventions in common use are Type 11 / 22 / 33 (the BS EN 442 standard) and K1 / K2 / K3 (UK installer shorthand). They mean the same thing:

Type	Naming	Panels	Convection fins	Approximate depth	Output multiplier vs K1
Type 11	K1	1	1 set	~70mm	1.0× (baseline)
Type 22	K2	2	2 sets	~100mm	~1.8×
Type 33	K3	3	3 sets	~155mm	~2.5×

For the same height and width, a K2 puts out roughly 80% more heat than a K1, and a K3 puts out roughly 2.5× the heat of a K1. More surface area, more convection, same wall footprint.

The classic heat-pump retrofit move is to replace a K1 single-panel radiator with a K2 double-panel-double-convector in the same wall space. The K2 is 30-40mm deeper than the K1, but the height and width are the same — meaning the pipework runs don’t need to change, only the radiator itself. Output roughly doubles, install is a 1-2 hour swap per radiator.

Where a K2 is already in place and the room still needs more output, the upgrade route is K3 (which projects significantly into the room — 155mm depth) or wider/taller K2.

How much heat at what flow temperature

Here is the same 700mm × 1000mm K2 Type 22 across the practical heat-pump flow temperatures:

Heat pump flow temperature	Output (watts)
35°C flow	~400 W
40°C flow	~600 W
45°C flow	~810 W
50°C flow	~1,010 W
55°C flow	~1,250 W
60°C flow	~1,480 W

The pattern matters because the flow temperature is the install’s biggest single design decision — it determines both the radiator sizes you need and the seasonal efficiency (SCOP) the system delivers.

• 35°C flow typically means underfloor heating or massively oversized radiators. Excellent SCOP (4.0+), expensive to retrofit.
• 45°C flow is the design sweet spot for many Reading retrofits — moderate radiator upgrades, SCOP in the 3.7-4.0 range, achievable in most cavity-walled or partially-insulated solid-wall properties.
• 50°C flow keeps more existing radiators in place — fewer upgrades, marginally lower SCOP (3.5-3.8).
• 55°C flow is the “minimal radiator changes” target — fewer upgrades, but a noticeable SCOP penalty for the life of the system (typically 0.5-0.8 SCOP below the 45°C case).

The right answer for your property depends on your heat-loss figures, your existing radiator sizes, and your budget for upgrades. The heat loss survey is where this gets decided.

Which radiators need to change in a typical retrofit

The heat-loss survey produces a room-by-room heat-loss figure in watts. The radiator schedule cross-checks each existing radiator’s output at the chosen design flow temperature against the room’s heat-loss requirement. The result is a list flagged into three categories:

Keep as-is. The existing radiator delivers the required output at the design flow temperature with margin. Typically 40-60% of radiators in a Reading retrofit, more in modern estate stock with already-oversized emitters. Bathroom towel radiators are often in this category — they were sized generously for gas-boiler-era specifications.

Upsize in the same footprint. Replace K1 with K2 (or K2 with K3) in the same wall space. Output roughly doubles, install cost is the new radiator unit plus 1-2 hours of plumbing. Typically 20-30% of radiators in a Reading retrofit.

Replace with a larger unit or split the load. The room needs more emitter area than the existing wall space allows in same-footprint K-class. Options: a wider or taller standard radiator, a designer/column radiator that goes vertical, splitting the load across two radiators on different walls, or moving to underfloor heating where viable. Typically 10-20% of radiators in a Reading retrofit. Costs vary — designer radiators run £400-£1,000+ per unit.

For a typical Reading 3-bed retrofit with seven radiators, this distribution produces a radiator schedule with 2 to 4 radiators changed. The aggregate radiator-works cost lands at £1,400-£3,500 including labour — and sits inside the BUS-eligible plant scope, absorbed within the install quote rather than billed separately.

Where the radiator-replacement count goes above 50% of the system, it’s worth pushing back on the design. Either the flow temperature target is too low for the property’s emitter footprint (push to 50°C and re-run the calculation), or there’s a fabric-upgrade route — better insulation in the worst-performing rooms — that brings the heat-loss figures down enough to keep more radiators in place.

Designer radiators and constrained wall space

Standard panel radiators (K1, K2, K3) are the dominant retrofit choice — cheapest per kilowatt of output, easiest to fit, widest availability. But they only work where there is enough horizontal wall space.

Designer radiators — tall narrow vertical units, column radiators, finned-tube designs — solve the constrained-wall-space problem at a price. The use cases:

• A central Reading Victorian terrace with deep skirting, fitted bookcases, panel walls, or architraves — limited horizontal wall space, but enough vertical wall space for a tall narrow unit
• A bay window or alcove configuration where a standard radiator can’t fit
• A room where the homeowner has aesthetic preferences against a standard panel radiator

The cost premium runs at roughly 30-100% above a same-output standard panel radiator. For a Reading retrofit where 1-2 designer radiators are needed (typical conservation-area case), the radiator-works budget rises by £200-£800 depending on the units chosen.

Designer radiators are BUS-eligible plant — they sit inside the install quote on the same terms as standard panel radiators.

Microbore pipework — the deal-breaker

The radiator-output side of the picture is one half of the retrofit. The other half is the pipework feeding the radiators.

A heat pump delivers heat at lower temperatures than a gas boiler, which means it needs to push larger water flow rates through the system to carry the same total heat to the radiators. The same Reading 3-bed using 18 litres per minute at 70°C flow needs roughly 35 litres per minute at 45°C flow — roughly double.

Standard 15mm copper pipework handles this without problem. 8mm microbore pipework — common in UK installations from the late 1970s through the 1990s — often cannot. The pressure drop through 8mm tubing at the higher flow rate exceeds what the heat pump’s circulator can deliver. The result: the system under-delivers heat at the radiator end, the design SCOP isn’t achieved, and the only fixes are:

• Re-pipe radiator runs in 15mm copper. Material cost is £15-30/m for copper and fittings; the disruption is significant — lifting floorboards, chasing walls, redecoration. A typical 3-bed re-pipe runs £3,000-£6,000, on top of the radiator-replacement cost.
• Accept a higher flow temperature. Running at 55-60°C uses smaller flow rates that microbore can handle, but tanks the SCOP for the install’s life — typically a 0.5-0.8 SCOP penalty.
• Decline the install. Rare, but the case where the design just doesn’t make economic sense. A reputable installer flags this at survey, not at commissioning.

Your survey should explicitly check pipework material and gauge. A microbore-heavy property finds out at survey, not at commissioning — and the installer’s report should call this out and price the re-pipe transparently if it’s recommended.

By property era:

• Pre-1970s — Victorian, Edwardian, inter-war, immediate post-war. Microbore is uncommon; 15mm copper was the standard.
• Late 1970s to 1990s — microbore is a real possibility that needs verification at survey.
• 2000s onwards — generally 15mm copper or pex/plastic of equivalent capacity. Microbore unusual.

Cost — and how BUS absorbs radiator works

UK radiator replacement costs in May 2026:

Item	Cost range
K1 single-panel — unit cost	£40-£150
K2 double-panel — unit cost	£100-£400
K3 triple-panel — unit cost	£200-£700
Designer / column radiator — unit cost	£400-£1,000+
Labour (straightforward swap)	£150-£300 per radiator
Labour (with chasing pipework, new TRV, balancing)	£200-£400 per radiator
Microbore re-pipe (per radiator run)	£200-£500

A typical Reading 3-bed retrofit with 3 radiator changes:

• 3× K2 units at £250 average = £750
• 3× labour at £250 average = £750
• Sub-total: £1,500

A more involved retrofit (5 radiator changes, partial microbore re-pipe, one designer radiator):

• 4× K2/K3 units = £1,200
• 1× designer radiator = £600
• 5× labour = £1,400
• Partial microbore re-pipe = £1,500
• Sub-total: £4,700

How BUS handles this. Radiator upgrades, pipework changes, and the associated labour are all eligible plant under BUS per the Ofgem installer guidance. They sit inside the install quote. The £7,500 BUS grant deducts from the total install quote (heat pump + cylinder + emitters + pipework + electrical + commissioning) — the homeowner doesn’t pay the radiator works as a separate line outside the BUS calculation. See our BUS eligibility guide for the full scope of what BUS covers within the install.

For most Reading retrofits where the total install cost lands in the £10,000-£14,000 range — see our UK heat pump cost guide for the detailed cost picture — the radiator works are economically absorbed by the BUS grant. For more expensive retrofits (extensive microbore re-pipe, multiple designer radiators), the radiator works add to the post-grant balance the homeowner pays.

What this means for homes in Reading

Reading’s housing-stock distribution shapes the radiator-upgrade picture meaningfully:

Central Reading and lower Caversham — Victorian and Edwardian terraces. Original pipework is typically 15mm copper (predates microbore), but existing radiators are often 1970s-1990s K1 single-panel units sized for high-temperature gas-boiler operation. Heat-loss figures are high (typically 7-12 kW for a 3-bed terrace before fabric upgrades), pushing required radiator outputs up. Conservation-area properties face wall-space constraints from period features. Typical conservation-area retrofit: 3-5 radiators changed, 1-2 of them designer units in constrained rooms. Total radiator-works cost commonly £2,000-£4,500.

Tilehurst, Earley, Whitley and the eastern inter-war/post-war wedge — semis with cavity walls. Original pipework is typically 15mm copper. Some 1980s extensions used microbore, so the survey should check explicitly. Heat-loss figures are moderate (5-8 kW typical). Standard retrofit: 2-3 radiators changed, total radiator-works £1,000-£2,500.

Lower Earley, Woodley, modern southern and western estates — 1980s+ construction. Microbore likelihood is higher in 1980s-1990s phases; standard 15mm in 2000s+. Existing radiators are often appropriately sized for the lower heat-loss figures. Some 2000s+ properties need no radiator changes at all. Standard retrofit: 1-2 radiators changed, total radiator-works £500-£1,500.

Caversham Heights, Caversham Park, parts of central Reading — listed and conservation-area concentrations. Highest design complexity. Listed-building consent may constrain radiator choice. Cost commonly £3,000-£6,000 for the radiator-and-pipework portion on a fully retrofitted listed-property install.

The pattern: standard Reading suburban retrofits handle radiator works comfortably inside the BUS-grant envelope. Listed and conservation-area properties run higher total costs, but the BUS still pays £7,500 of the larger total — and on these properties the radiator design is the most consequential single decision the survey makes.

Related guides

• Heat loss surveys for heat pumps — what they are, what to expect — the survey that produces your room-by-room heat-loss figures and radiator schedule
• SCOP, COP and HSPF — heat pump efficiency metrics explained — the efficiency-vs-flow-temperature trade-off that radiator sizing sits inside
• BUS grant eligibility — does your property qualify? — what BUS covers within the install, including radiator and pipework works
• Heat pump installation cost in the UK 2026 — where the radiator-works budget lands within the total install cost

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Want to know which of your existing radiators stay and which need upgrading? Our MCS-certified team produces a room-by-room radiator schedule as part of every heat-loss survey — with the design flow temperature, the existing radiator outputs at that temperature, and the upgrades flagged before you commit.

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