Home Heat Pump Guide

Can You Mix Radiators and Underfloor Heating?

Yes — mixing radiators and underfloor heating with a heat pump is not only possible, it is one of the most common system configurations in UK homes. Underfloor heating on the ground floor, radiators upstairs. It is a practical, cost-effective approach that gives you the best of both systems.

This guide explains how mixed systems work, the equipment involved, and the design considerations that ensure everything runs smoothly from a single heat pump.

Why Mix Radiators and Underfloor Heating?

The most practical reason is renovation constraints. Installing wet underfloor heating on a ground floor during a kitchen or extension project is relatively straightforward. Running UFH upstairs in an existing home is more difficult — you need to lift floorboards, fit between-joist panels, and manage floor height changes at the stairs.

Keeping existing upstairs radiators (or fitting new, correctly sized radiators) while installing UFH downstairs gives you the comfort advantages of underfloor heating where you spend most of your time, without the disruption and cost of a whole-house UFH retrofit.

Other Reasons for Mixed Systems

  • Extensions — new extensions with screeded floors are ideal for UFH, while the existing house keeps its radiators
  • Bathrooms — UFH beneath bathroom tiles is a common upgrade, even when the rest of the house has radiators
  • Different heating needs — UFH provides gentle, even warmth in living spaces; radiators respond faster in bedrooms where you might want a quick boost
  • Budget — whole-house UFH is expensive in a retrofit; mixing systems lets you prioritise UFH where it matters most

How Mixed Systems Work

The fundamental challenge is temperature. Radiators and underfloor heating need different flow temperatures:

  • Underfloor heating: 30°C to 40°C flow temperature
  • Radiators (with heat pump): 40°C to 50°C flow temperature

If you send 50°C water through your underfloor heating, the floor will overheat — it will be uncomfortably warm underfoot and could damage certain floor finishes. If you send 35°C water to your radiators, they may not deliver enough heat (unless they are very generously sized).

The solution is to run two separate circuits at different temperatures, both fed by the same heat pump.

The Blending Valve (Mixing Valve)

A thermostatic blending valve (also called a mixing valve) is the key component in a mixed system. It sits on the underfloor heating circuit and mixes the hot flow from the heat pump with cooler return water to reduce the temperature entering the UFH pipes.

How It Works

The heat pump sends water at its full flow temperature (say 45°C) to both the radiator circuit and the blending valve. The blending valve mixes some of this hot water with cooler return water from the UFH circuit (say 30°C) to create a blended temperature of around 35°C for the underfloor pipes. The valve adjusts automatically to maintain the set temperature.

Types of Blending Valve

  • Thermostatic mixing valve — the simplest option; a self-regulating valve that maintains a preset output temperature. No electrical connection needed. Costs £40 to £100.
  • Motorised mixing valve with actuator — a more sophisticated option controlled by the UFH controller or heat pump. Can adjust the blended temperature based on room demand or outdoor conditions. Costs £80 to £200.
  • UFH pump station — a complete unit containing the blending valve, circulation pump, temperature gauge, and isolation valves in one compact package. Costs £150 to £350 and simplifies installation considerably.

Do You Always Need a Blending Valve?

With a heat pump, the answer is sometimes no. If your heat pump runs at very low flow temperatures (35°C or below), both the UFH and the radiators can potentially operate at the same temperature — provided the radiators are large enough to deliver sufficient heat at 35°C. In this scenario, both circuits connect directly to the heat pump without a blending valve.

However, this only works in very well-insulated homes with generously sized radiators. Most mixed systems in existing UK homes benefit from a blending valve to optimise both circuits independently.

System Design for Mixed Heating

The Low-Loss Header Approach

The cleanest way to design a mixed system is with a low-loss header (also called a hydraulic separator). This is a simple vessel that hydraulically separates the heat pump circuit from the heating circuits. The heat pump circulates water through the header at its own flow rate, and separate pumps draw water from the header to the radiator circuit and UFH circuit at their respective flow rates.

This approach ensures the heat pump always has sufficient flow, regardless of how many zones are open or closed. It also makes it easy to add circuits — you could have a radiator circuit, a UFH circuit, and a hot water circuit all fed from the same header.

The Direct Connection Approach

Simpler systems connect the radiator circuit directly to the heat pump flow and return, with the UFH circuit branching off via a blending valve and its own pump. This works well for smaller systems but can cause flow issues if multiple zone valves close simultaneously.

Buffer Vessel

In some installations, a small buffer vessel (20 to 50 litres) is added to prevent the heat pump from short-cycling when demand drops — for example, when the UFH is up to temperature and only one or two radiators are calling for heat. Not all systems need one, but your installer will assess whether it is necessary based on your heat pump model and system design.

Controls for Mixed Systems

Each circuit needs independent control:

Underfloor Heating Controls

  • Room thermostats for each UFH zone (typically one per room)
  • Manifold actuators that open/close each circuit based on room demand
  • UFH controller or wiring centre connecting thermostats to actuators
  • Blending valve setting or actuator to manage UFH flow temperature

Radiator Controls

  • Thermostatic radiator valves (TRVs) on each radiator — ideally smart TRVs for individual room control
  • Zone valves if the radiator circuit is split into zones (e.g., upstairs bedrooms vs bathroom)
  • Room thermostat for each radiator zone

Integration with the Heat Pump

Modern heat pump controllers can manage multiple circuits, adjusting the flow temperature and scheduling for each. Some heat pumps (notably Vaillant, Daikin, and Mitsubishi) have built-in support for mixed systems and can control the blending valve directly. Ask your installer about your heat pump's specific capabilities.

Costs of a Mixed System

A mixed system costs slightly more than a radiator-only or UFH-only setup due to the additional components (blending valve, separate pump, controls). Here are typical additional costs over a single-circuit system:

  • UFH pump station (blending valve + pump): £150 to £350
  • Low-loss header (if used): £100 to £250
  • Additional zone controls: £100 to £300
  • Labour for mixed system setup: £200 to £500
  • Total additional cost over single circuit: £400 to £1,000

The UFH installation itself follows standard wet underfloor heating costs. The radiator circuit may need radiator upgrades to work at the lower heat pump flow temperatures.

The overall cost of the heat pump and system combined is covered in our heat pump cost guide, with the BUS grant of £7,500 helping to offset the investment.

Design Tips for Mixed Systems

Size Radiators for Heat Pump Temperatures

Do not assume your existing radiators are adequate. They were sized for boiler temperatures. Your installer must calculate whether each radiator delivers enough heat at the heat pump's flow temperature. Oversizing radiators slightly is always better than undersizing.

Balance Each Circuit Independently

Balance the radiator circuit separately from the UFH circuit. The UFH manifold has its own flow meters for balancing individual loops. Treat them as two separate systems that happen to share the same heat source.

Consider Response Times

UFH with screed takes 2 to 4 hours to respond to changes; radiators respond in 30 to 60 minutes. Set up schedules accordingly — the UFH should run on constant or near-constant mode, while radiators can tolerate a small setback if needed.

Floor Finishes Matter

In rooms with UFH, choose floor finishes that conduct heat well — tiles, stone, or engineered wood. In rooms with radiators, floor finish is less critical for heating performance. See our UFH guide for detailed floor finish advice.

Common Mixed System Layouts

Ground Floor UFH + First Floor Radiators

The most popular configuration. UFH in the kitchen, living room, dining room, and hallway. Radiators in bedrooms, bathroom, and landing. This suits most renovation projects and new extensions.

Kitchen/Bathroom UFH + Radiators Everywhere Else

A more targeted approach — UFH only in rooms with tiled floors where warm feet matter most. Radiators handle the rest. Lower cost than full ground-floor UFH.

Extension UFH + Existing House Radiators

When adding an extension with a screeded floor, UFH is the obvious choice for the new space. The existing house keeps its radiator system. The heat pump serves both.

Skirting Board Heating + UFH

In rooms where wall space is limited and radiators are impractical, skirting board heating can replace radiators and work alongside UFH circuits from the same heat pump.

Frequently Asked Questions

Can one heat pump run both radiators and underfloor heating?

Yes. A single heat pump provides the heat source for both circuits. The circuits are separated by a blending valve or low-loss header to manage the different temperature requirements.

Do I need a bigger heat pump for a mixed system?

The heat pump is sized based on the total heat demand of your home, not the type of emitters. Whether you use radiators, UFH, or both, the heat pump size is determined by your property's heat loss calculation. A mixed system does not require a larger heat pump.

Is a mixed system less efficient than UFH only?

Slightly, because the radiator circuit requires a higher flow temperature than UFH. However, the difference is modest — perhaps 5% to 10% lower seasonal efficiency compared to a full UFH system. The practical and cost advantages of a mixed system usually outweigh this small efficiency difference.

Can I add UFH to one room later?

Yes. If you have a radiator-only system, you can add a UFH circuit to a single room (e.g., a bathroom or kitchen renovation) with a small dedicated manifold and blending valve. Discuss this with your installer to ensure the heat pump has sufficient capacity.

Do I need separate thermostats for radiators and UFH?

Yes. Each circuit (or zone) needs its own thermostat because the response times and temperature requirements differ. A single thermostat cannot effectively control both UFH and radiators.

What if my radiators are already big enough for heat pump temperatures?

If your existing radiators are oversized for the room (common in older properties that have been insulated since the radiators were fitted), they may work at the same flow temperature as the UFH. In this case, you may not need a blending valve — your installer will calculate whether this is the case.