Can an R290 Heat Pump Replace a Gas Boiler Without Replacing the Radiators?

Homeowners and installers planning a boiler replacement often ask this first. The short answer is **often yes, but not automatically.** Many homes can keep most or all of their existing radiators. Whether yours can depends on three things: how much heat each room loses, how much heat each radiator can deliver at a lower water temperature, and what flow temperature the heat pump is asked to run at.

R290 heat pumps widen the range of homes where keeping the radiators is realistic, because many are designed for higher flow temperatures than earlier heat pumps. Being able to reach a temperature is not the same as running efficiently at it, though. This guide explains how to tell the difference, and how to check your own radiators.

Why this question matters more for a heat pump than a boiler

Gas boiler systems were traditionally designed around high flow temperatures, often 70–80 °C, with radiators sized to match. Modern condensing boilers also benefit from lower return temperatures, but heat pumps are generally more sensitive to water temperature, because the temperature lift the compressor has to provide directly affects efficiency.

A heat pump moves heat from outdoor air into the heating water, and the smaller the gap between those two temperatures, the less work the compressor has to do. Lower flow temperatures therefore generally mean higher efficiency. The practical question is not "can the heat pump make hot water?" It is "what is the lowest flow temperature at which each room still reaches its target temperature on a cold day?"

For a deeper look at how flow temperature affects efficiency, see [What Is the Best Flow Temperature for Underfloor Heating with a Heat Pump?](https://nordthermglobal.com/blog/best-flow-temperature-underfloor-heating-heat-pump). The same principle applies to radiators.

What R290 changes, and what it doesn't

Many modern R290 air-to-water heat pumps are designed to deliver higher leaving-water temperatures than earlier heat pump generations. For radiator retrofits, this can give designers more flexibility when a few rooms need warmer water.

Three caveats apply:

- **Capacity may reduce at high flow temperatures and low outdoor temperatures.** A unit's output at 35 °C flow and at 65 °C flow, on a −7 °C day, can differ significantly. Check the manufacturer's capacity tables for your design conditions, not just the maximum flow temperature.

- **Efficiency falls as flow temperature rises.** A system designed around 45–50 °C will generally run more efficiently across the season than one designed around 65 °C.

- **High-temperature capability provides design flexibility, but it should not automatically become the default operating target.** Where one or two rooms require unusually high water temperatures, upgrading those emitters may allow the whole system to operate more efficiently at a lower temperature.

How radiator output changes at lower flow temperatures

Radiators are usually rated in Europe under EN 442 at a mean water-to-room temperature difference of 50 K, for example 75/65 °C water in a 20 °C room. Output does not fall in straight proportion as water temperature drops. It falls somewhat faster, following an exponent typically around 1.3 for panel radiators.

The table below gives indicative output as a share of the ΔT50 rating, assuming a 20 °C room:

| Flow / return (°C) | Mean water-to-room ΔT | Approx. output vs ΔT50 rating |

|---|---|---|

| 75 / 65 | 50 K | 100% |

| 65 / 55 | 40 K | ~75% |

| 55 / 50 | 32.5 K | ~57% |

| 50 / 45 | 27.5 K | ~46% |

| 45 / 40 | 22.5 K | ~35% |

*Illustrative only. Actual output depends on radiator type, the manufacturer's declared exponent, and installation.*

At first glance this looks like a serious problem, since a radiator at 45 °C gives about a third of its rated output. In practice many homes still work, for the reasons below.

Why many existing radiators may still be adequate

**Radiators are frequently oversized.** Boiler-era systems were often sized with generous margins, rounded up to the next panel size, or selected by rule of thumb rather than calculation.

**Heat loss may have fallen since installation.** Loft insulation, cavity wall insulation, new glazing and draught-proofing can reduce a room's heat demand considerably, while the radiator stays the same size.

**The design condition applies only on the coldest days.** With weather compensation, the heat pump runs at its highest flow temperature only when it is coldest outside. For most of the heating season it can run cooler, where both radiator output needs and efficiency work in the system's favour.

The only reliable way to know is to check each room, as described below.

How to check whether your existing radiators are large enough

Step 1: Calculate room-by-room heat loss

A whole-house estimate is not enough. One under-radiated room can force the whole system to run hotter. In the UK, MCS-compliant heat-pump design uses room-by-room heat-load calculations. Requirements vary elsewhere, so installers should check the applicable local standard.

Step 2: Survey the radiators

Record each radiator's type (e.g. Type 11, 21, 22, 33), height and length, then find its rated output at ΔT50 from the manufacturer's data or a reference table.

Step 3: Compare output with heat loss at the proposed design condition

Apply the correction factor for the proposed flow temperature to each radiator's rated output, then compare the result with the room's heat loss.

> **Illustrative example**

>

> - Existing radiator, EN 442 output at ΔT50: **2,000 W**

> - Proposed design condition: **50/45 °C**, 20 °C room

> - Approximate correction factor: **~46%**

> - Estimated radiator output: 2,000 × 0.46 ≈ **920 W**

>

> If the calculated room heat loss is **800 W**, the existing radiator may be adequate at this design condition.

>

> If the room heat loss were **1,200 W**, the same radiator would fall short. The designer could then consider a larger radiator, reducing the room's heat loss, or a higher design flow temperature.

Step 4: Work it the other way — what flow temperature does each room need?

Often the more useful question is: *if this radiator stays, how warm does the water need to be?*

1. Divide the room heat loss by the radiator's ΔT50 output to get the correction factor required.

2. Use the radiator manufacturer's correction table, or its declared exponent, to find the mean water-to-room ΔT that gives that factor.

3. Add the room temperature to get the mean water temperature needed.

> **Illustrative example**

>

> - Radiator ΔT50 output: **2,000 W**; room heat loss: **1,000 W**

> - Required correction factor: 1,000 ÷ 2,000 = **0.50**

> - With an exponent of 1.3, this corresponds to a mean water-to-room ΔT of roughly **29 K**

> - Mean water temperature: 20 + 29 ≈ **49 °C**

> - Assuming a **5 K flow-to-return temperature difference**, this corresponds to approximately **52/47 °C flow/return**

For preliminary estimation, the approximate relationship is: required ΔT ≈ 50 K × (room heat loss ÷ ΔT50 output)^(1/n), where n is the radiator's declared exponent (often around 1.3). Use the radiator manufacturer's correction data for final selection where it is available.

**In a system where the radiators share a common supply temperature, the room requiring the highest radiator water temperature will typically determine the system's design flow temperature.** Repeating Step 4 for every room shows which one that is. It often turns out that one or two rooms are holding the whole house at a higher flow temperature.

> **Practical preliminary check:** During a suitably cold period, and where the boiler and controls allow it, a homeowner may be able to reduce the boiler flow temperature and observe whether the radiators can maintain the target room temperatures during steady operation. The appropriate settings and procedure should be confirmed by a competent heating professional. This test can provide useful preliminary evidence, but it does not replace a room-by-room heat-loss calculation or demonstrate heat-pump performance under design conditions.

Other things to check before replacing the boiler

**Pipework and flow rate.** Heat-pump systems are commonly designed for a smaller flow/return temperature difference than many legacy boiler systems, which means a higher water flow rate is needed to transfer the same heat output. Small-diameter pipework, especially long microbore runs, may limit flow and should be assessed during design.

**System condition.** Sludge, magnetite and air in older systems reduce radiator output and can affect heat pump components. A system clean or flush and a suitable filter are commonly recommended before connecting a heat pump.

**Controls, hydraulics and hot water.**

- Existing thermostatic valves, bypasses and zoning should be reviewed so the heat pump sees stable, adequate flow.

- Whether a buffer or other hydraulic separation is needed depends on the system's active water volume, zoning, required flow rate and the heat-pump manufacturer's requirements. It should not be added by default.

- Homes replacing a **combi boiler** usually need space for a domestic hot water cylinder, because heat pumps typically heat stored water rather than delivering instantaneous hot water. This is often a bigger practical constraint than the radiators.

What to do if some rooms fall short

Replacing every radiator is rarely the only option, and often not the best one. Because the most demanding room sets the flow temperature, upgrading just one or two radiators can sometimes allow the whole system to run cooler. Common approaches include:

- **Upgrade only the rooms that fall short**, for example by replacing a Type 11 with a Type 22 or 33 of similar size.

- **Consider fan-assisted radiators** where wall space is limited.

- **Improve the room's fabric** (insulation, glazing, draught-proofing) to reduce its heat loss.

- **Accept a modestly higher design flow temperature**, weighing the efficiency cost against the cost and disruption of emitter changes.

In practice, "keep most radiators, upgrade a few" is a common retrofit outcome and can avoid unnecessary emitter replacement.

The trade-off of running at higher flow temperatures

Designing for a higher flow temperature to avoid emitter changes is a legitimate choice, but it has a running cost. Seasonal efficiency generally decreases as design flow temperature increases. How much depends on the unit, the climate and the control strategy.

One useful approach is to compare two or three scenarios, such as "all existing radiators at 55 °C" versus "three radiators upgraded, system at 45 °C". Weigh the one-off upgrade cost against the expected difference in annual running cost.

Quick guide

| Situation | Likely outcome |

|---|---|

| Radiators meet room heat loss at 45–50 °C | Existing radiators may be retained; favourable for lower-temperature operation |

| One to three rooms fall short | Upgrade selected radiators or improve those rooms |

| Most rooms fall short at 55 °C | Review fabric improvements and emitter upgrades together, rather than raising flow temperature further |

| Microbore pipework throughout | Assess flow capacity before finalising the design |

| Existing combi boiler, no cylinder | Plan space for a hot water cylinder |

FAQ

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

Not necessarily. Many homes keep most or all of their radiators. A room-by-room comparison of heat loss against radiator output at the design flow temperature shows which rooms, if any, need attention.

**How do I know what flow temperature my radiators need?**

Divide each room's heat loss by its radiator's rated ΔT50 output, then use the manufacturer's correction data to find the water temperature that delivers enough heat. Where radiators share a common supply temperature, the room requiring the highest radiator water temperature will typically determine the system's design flow temperature.

**Will my radiators feel less hot with a heat pump?**

Usually yes. They run cooler for longer periods rather than hot in short bursts. Rooms can still reach the same temperature if the system is designed correctly. A radiator that is warm rather than hot is expected behaviour.

**Can a heat pump work with microbore pipes?**

Sometimes. Heat-pump systems are usually designed with higher flow rates than legacy boiler systems, and long microbore runs may restrict flow. This should be checked during design rather than assumed either way.

**Is R290 better than other refrigerants for existing radiators?**

Many R290 heat pumps are designed for higher flow temperatures, which may help where some rooms need warmer water. Efficiency at any given flow temperature still depends on the unit's design, so compare capacity and efficiency data at your design conditions.

**What about hot water if I currently have a combi boiler?**

Heat pumps typically heat a stored hot water cylinder, so space for one usually needs to be planned. Cylinder sizing depends on household demand and usage patterns.

Continue Exploring

📖 **Related guides**

- [What Is the Best Flow Temperature for Underfloor Heating with a Heat Pump?](https://nordthermglobal.com/blog/best-flow-temperature-underfloor-heating-heat-pump)

- [Can Underfloor Heating Be Installed in an Existing House?](https://nordthermglobal.com/blog/can-underfloor-heating-be-installed-in-an-existing-house-a-retrofit-guide)

- [Why Is Air-to-Water Heat Pump Installation So Expensive?](https://nordthermglobal.com/blog/why-is-air-to-water-heat-pump-installation-so-expensive)

📸 **Real projects**

- [UK house retrofit with underfloor heating and fan-coil cooling](https://nordthermglobal.com/case/air-to-water-heat-pump-retrofit-for-an-existing-uk-house-with-underfloor-heating-and-fan-coil-cooling)

🏠 **Related products**

- [R290 air-to-water heat pump for heating, cooling and hot water](https://nordthermglobal.com/products/r290-air-to-water-split-heat-pump-for-heating-cooling-and-hot-water)

**Planning a boiler replacement?**

Share any available room-by-room heat-loss results, together with radiator types and sizes. For an initial review, room dimensions, floor areas, insulation details and current heating-system information can also be useful. Final system design should still be based on a proper room-by-room heat-loss calculation. [Contact Nordtherm:[email protected] to discuss your project with our engineering team.