Low-temperature heating, emitters and controls

Core idea

An emitter gives out less heat as the gap between its mean water temperature and the room temperature narrows. To meet the room’s needs you may have to cut the building’s heat loss, add emitter area or improve heat transfer, but the existing emitters may already be big enough. Compare their output at the proposed temperatures with each room’s calculated heat loss. A low-temperature system may run for longer, at steadier temperatures, than a high-temperature boiler system.

Approved Document L builds this in. A wet heating system that is newly installed, or fully replaced with its boiler, emitters and pipework, should be sized to meet the home's heating needs at a flow temperature of 55 °C or lower. Where that is not feasible, such as where there is no space for larger radiators, it is designed for the lowest flow temperature that still meets those needs.

What to recognise

• Calculate room heat loss before judging whether an existing radiator is large enough.
• Use emitter data at the proposed temperature difference, not only its high-temperature catalogue rating.
• Balance the circuit so design flow reaches each emitter.
• Weather compensation can reduce flow temperature as outdoor conditions become milder.
• Good interlock stops normal heat demand when rooms and stored water are satisfied.

Apply it

If a room needs 1.5 kW at design conditions, compare that load with the radiator output at the proposed mean water temperature. If the available output is short at the intended flow and temperatures, assess fabric improvements, emitter size or the design temperature. Check actual circulation too: increasing flow can help a starved radiator, but turning the pump up to maximum does not replace the output calculation.

Limits

Lower is not better if the system then cannot meet demand or keep hot water hygienic. The manufacturer’s minimum flow, defrost, backup heat and condensing-boiler limits still apply. Never disable protective controls to force continuous low-temperature running.

Automatic bypass valve

Some appliances need a minimum flow path when the TRVs or zone valves close. An automatic bypass gives one: it opens between flow and return as the pressure difference rises. Check the appliance’s approved layout, and whether it has an internal bypass, before specifying an external one. Set the valve by differential pressure, then check the required flow under the stated conditions, including pump overrun where it applies. Too much bypass flow takes water away from the emitters and raises the return temperature. A bypass gives a flow path; it does not give the active water volume a heat pump may also need.

Underfloor heating

Underfloor heating (UFH) is a large emitter that works at low water temperatures. Grant's UFH instructions, following BS EN 1264-2, limit the floor surface to 9 °C above room temperature: 29 °C in normally occupied areas with the room at 20 °C, which gives roughly 100 W/m². A water temperature control, usually a mixing valve at the manifold, blends return water into the flow to keep the floor below that limit. Where a boiler serves nothing but the UFH through such a control, a heat sink such as a towel rail may be needed before the mixing valve, so the boiler does not cycle and cut out on its high limit.

In screed floors Grant's standard pipe spacing is 200 mm with 16 mm pipe, closed up to as little as half that where heat loss is high, such as in a conservatory. The screed must dry and cure naturally before the heating is first used; the UFH must never be run to speed up curing. At commissioning, start from the lowest water temperature and expect each loop's thermal actuator to take 2 to 4 minutes to open fully. A screed floor stores a lot of heat, so it warms and cools slowly; a timber floor system responds faster.

Grant says the even warmth can allow the room thermostat to be set 1 to 2 °C below a radiator system's setting for comparable comfort. This is an air-temperature setting, distinct from the water and floor-surface limits.

Testing UFH before covering it

Grant's Uflex instructions test the filled loops at 4 to 6 bar for at least one hour. A small initial pressure drop can occur as the pipe expands, but the pressure should then remain steady. Reduce it to the specified working pressure of 1 bar and check for a further hour: it must not fall below working pressure. Keep the pipework at working pressure while the floor is laid so damage is noticed and repaired immediately, and lay the floor promptly after testing.

For semi-dry sand and cement screed, Grant quotes BS EN 1264-4's maximum area guide of 40 m² without expansion allowances. This area figure is not a complete joint design: follow the screed and system requirements. Anhydrite screeds have different allowances, to be checked with their supplier. Continue to let the screed cure naturally; do not use the UFH to accelerate drying.