Heat pump compatibility
Specifying liquid screed for heat pump-fed underfloor heating
Heat pumps run at lower flow temperatures than boilers, so screed depth, conductivity and commissioning need to be specified with that lower thermal driving force in mind.
Heat pumps typically deliver lower flow temperatures than a gas boiler, often in the region of 35 to 45 degrees Celsius rather than the higher figures a boiler-fed system might use, so the floor build-up has less thermal push available to reach a comfortable room temperature.
A liquid screed's high thermal conductivity relative to sand and cement is one of the reasons it pairs well with heat pumps, but the specification still needs to address depth over the pipe, pipe centres and the insulation beneath the slab if the lower flow temperature is to translate into an efficient, responsive floor.
Getting this wrong does not usually cause a failure of the screed itself, but it can leave the client with a heating system that struggles to reach setpoint on the coldest days, which is a performance and comfort issue rather than a defect in the floor.
Why flow temperature changes the specification
A boiler-fed screed floor has some tolerance for a slightly thicker or less conductive build-up because the flow temperature can be pushed higher to compensate. A heat pump has far less headroom, so decisions that would be marginal on a boiler system, such as pipe centres, cover depth or insulation thickness, become material to whether the room reaches its design temperature at all.
This means the screed specification cannot be finalised in isolation from the heat pump's design flow temperature and the heat loss calculation for the building. The two need to be read together, ideally before the screed depth and pipe layout are fixed on the drawings.
Depth, conductivity and responsiveness
Liquid screed's advantage over traditional sand and cement is largely down to conductivity and its ability to be laid thinner while still enclosing the pipe fully, which reduces the thermal mass the heat pump has to warm before the room feels the benefit.
Specifying the minimum practical cover over the pipe, consistent with the manufacturer's datasheet and any acoustic or structural requirements, keeps the response time as short as the system allows. Going thicker than necessary purely for a generous tolerance margin works against the heat pump's lower flow temperature.
Coordinating commissioning with the wider system
Because a heat pump often runs a slower, more gradual heat-up curve than a boiler, the commissioning heat-up procedure for the screed should reflect the actual equipment that will run the floor in service, not a generic ramp intended for a boiler. Screed manufacturers publish heat-up guidance that can be adapted, but the heating engineer should confirm the heat pump can deliver the flow temperatures the schedule assumes at the relevant time of year.
Screed for heat pump systems — the points that decide it
- Keep cover over the pipe as low as practically achievable within the manufacturer's minimum, since thinner cover responds faster to a lower flow temperature.
- Confirm insulation beneath the slab or floor meets the heat loss calculation used to size the heat pump, as under-insulation forces higher flow temperatures that erode efficiency.
- Check pipe centres against the emitter output required at the design flow temperature; wider centres sized for a boiler system may underperform with a heat pump.
- Specify a liquid screed with good thermal conductivity rather than assuming all screeds perform equally in this respect.
- Coordinate the commissioning and heat-up procedure with the heating engineer so the lower flow temperature is factored into the drying and initial heating sequence.
- Avoid floor coverings with high thermal resistance over heat pump-fed screed, as these can negate much of the benefit of the thinner, more conductive floor.
- Where zoning is used, confirm each zone's screed depth and pipe layout is consistent so the heat pump isn't forced to satisfy one poorly performing zone at the expense of the whole system's efficiency.
Screed for heat pump systems — your questions
Specifying a heat pump floor?
Send us the heat pump's design flow temperature and pipe layout drawings and we can advise on depth and cover.
Or call our landline on 01392 237700
In short
Heat pumps run at lower flow temperatures than boilers, so screed depth, conductivity and commissioning need to be specified with that lower thermal driving force in mind.
Specifying a heat pump floor?
Send us the heat pump's design flow temperature and pipe layout drawings and we can advise on depth and cover.