Underfloor heating control: a practical guide for installers

Underfloor heating works with low-temperature water and a lot of thermal mass. Those are its strengths: even comfort, no draughts, excellent efficiency with heat pumps. They are also why underfloor heating control needs proper thought. A floor that responds in hours rather than minutes does not forgive control mistakes.

This guide is for installers: which parts you need, which choices to make, and how it all comes together in the control panel. In an earlier article we looked at how much zone control can save. Here we look at how to do it.

The parts of the system

Let’s start with the water and end at the panel.

The heat source. A condensing boiler or a heat pump. Both work best with a low flow temperature: underfloor heating usually runs at 30–35 °C, rarely above 40 °C.

The three-way mixing valve. It blends hot water from the heat source with water returning from the floor, so the loops get the right temperature. You need it when the heat source produces water hotter than the floor needs, for example a boiler that also feeds bathroom radiators or domestic hot water. It is driven by an actuator, either 3-point or modulating 0–10 V.

The underfloor circuit pump. It circulates the mixed water. These days it is almost always an electronic, variable-speed pump.

The manifold. It distributes water to the loops, one or more per room. Each return port carries a thermoelectric actuator that opens or closes the loop. They are slow, taking a few minutes to open, which suits a floor perfectly.

The sensors. One room sensor per zone, wired or wireless. A flow sensor, just downstream of the mixing valve. An outdoor sensor, if you use weather compensation. Systems that also cool in summer need a humidity sensor.

The safety device. A manual-reset high-limit thermostat on the flow, which stops the pump if the water exceeds the maximum temperature allowed for the screed. It is hard-wired and does not depend on the controller.

Fixed or weather-compensated flow

The first choice is how to decide the temperature of the water going into the floor.

Fixed flow temperature. The mixing valve always holds the same temperature, say 35 °C, and the actuators open and close the zones. It is simple, but on mild winter days the floor gets hotter water than it needs and the zones cycle on and off a lot.

Weather compensation. The flow temperature follows the outdoor temperature along a curve: the colder it is outside, the warmer the water. It is the right choice for almost every system, and essential with a heat pump, since every degree off the flow temperature improves its efficiency. We will cover the heating curve in detail in a dedicated article.

The best setup combines both: weather compensation on the flow, actuators fine-tuning room by room. The controller can also adjust the curve when it sees that every zone has been calling for heat for hours, or that none has.

Zones and screed response times

A 5–7 cm screed takes hours to warm up and hours to cool down. That changes how you think about zones.

• Zones are for comfort and weekly schedules more than short set-backs. Switching a room off for two hours in the middle of the day changes little.
• Pre-heating matters. If bedrooms must be at 20 °C by 7 am, the zone has to start well before. A good controller learns how early, rather than using a fixed time.
• Night set-back should be moderate. Two or three degrees, no more: a cold floor in the morning takes too long to recover.
• Loops in the same room belong to the same zone. If the living room has three loops, the three actuators are switched together.

Before deciding on zones, check the manifold layout. Loops are often laid room by room but labelled wrongly. An hour spent checking them at the start saves many during testing.

Wiring the control panel

Heating control lives in the electrical panel. A good heating control panel has a few recurring features.

• Separate protection for the controller, the actuators and the pumps, so a fault on one load does not shut everything down.
• An actuator supply that matches the model chosen. Actuators come in 230 V and 24 V versions: 24 V ones need a transformer sized for the combined inrush current, which at switch-on is much higher than the running current.
• Numbered terminals for every sensor and output, matching the diagram. Whoever comes after you will be grateful.
• Sensor cables run apart from power cables, to avoid noisy readings.
• The high-limit thermostat in series with the pump, outside the controller’s logic.

Wireless sensors, such as Zigbee, make upgrades much easier: no chasing walls, one sensor per room, freestanding or wall-mounted. In new builds, wired sensors remain the most robust choice.

Brick configurations compared

Brick’s ready-made configurations cover the most common cases and share the same principle: the controller manages the heat source, mixing valve, pumps and zones, and logs all the data.

• A configurations. Underfloor heating with up to 4 zones (A0) or 8 zones (A1), with zone thermostats. They are designed for upgrading existing systems without changing what is in the rooms.
• B configurations. Underfloor heating with up to 4 or 8 zones, wireless Zigbee sensors and actuators, controlled from the app. B4 adds a controller in each room. B3 and B5 are for fan coils.
• Configuration C0. Mixed systems, underfloor and fan coils, with Modbus room controllers.
• Configuration D0. Mixed systems with KNX controllers, terminals in sequence and multi-stage heat sources.

Every configuration also handles domestic hot water, distribution pumps and mechanical ventilation, where present. Each one comes with an architecture diagram and a wiring diagram in DWG.

Before you switch on

Well-designed controls can still get off to a bad start. Three checks before commissioning:

1. Each actuator matches its zone. Open one zone at a time and check on the manifold which loop warms up.
2. The flow sensor really reads the flow, downstream of the mixing valve, in good contact with the pipe and insulated.
3. The high-limit thermostat trips. Test it by lowering the set point, not by waiting for a fault.

We will devote an article to commissioning, with the full checklist. For now, one piece of advice: update the wiring diagram with the changes made on site, and leave a copy in the panel.