A manifold looks like a lot of components on one bracket, and that is roughly what it is. But three of those components do three quite different jobs that get run together in conversation all the time — one indicates, one regulates, one switches — and separating them properly makes the whole assembly legible.
This article covers Module 4 of the PlumbMate underfloor heating course: the three system categories, taking the primary from the heat source, what is on a manifold and what each part does, following the water through a blended assembly, and how room control and compensation work. There is a 10-question mock test at the end.
Three categories of system
Underfloor installations divide into three, and the category decides how much control gear you need:
- Full house. Underfloor throughout, room by room control, one or more manifolds.
- Extension. Underfloor in the new work, radiators in the existing house. This is a mixed temperature system, and blending is not optional — the radiators want 70 to 80 °C and the floor wants 40 to 45.
- Single zone. One room or one area. Typically controlled by a return temperature limiter valve and sensor, with no room-by-room control at all.
Taking the primary
Every manifold is served from the primary flow and return of the central heating source. Not from a cylinder draw-off, and not from anywhere downstream of a blending valve serving something else. The primary runs at 70 to 80 °C; the floor wants 40 to 45; the blending arrangement on the manifold is what bridges that gap.
Where you are adding underfloor to an existing system, three checks belong at survey and not later:
- Boiler size. Is there capacity for the added load on top of what the house already takes?
- Primary pipe size. Can the existing pipework carry the extra flow to where the manifold is going?
- Primary pump size. Can the pump move it?
Miss those three and you get the classic extension fault: the manifold is warm rather than hot, the extension never reaches temperature, and the radiators in the rest of the house have got slower since the work. Nothing on the underfloor side is wrong at all.
Two further points. Some boilers require a bypass valve between flow and return, and cooker-boilers often carry extra requirements of their own — read the instructions. And insulate the primaries: their heat cannot be switched off by any room thermostat, so anything they give up on the way is uncontrolled and unwanted.
What is on a manifold
The core features you will find on almost any manifold:
- Air vent and drain cock ports
- Full bore end caps
- Union ball valves
- Isolator valves on both the flow and return bars
- Flow meters
- Actuator heads
And the secondary features, which vary by system:
- A pump with a 3-port mixer, or a pump with a 2-port injector
- An automatic bypass
- A high temperature cut-off
Four component names come up by name again and again: the lockshield, the differential valve, the injector valve and the regulating valve.
Now the three jobs, which is the part worth getting straight:
- Flow meters indicate. They tell you the rate in litres per minute. They do not set it.
- Lockshields regulate. On the return bar, they are what actually sets each circuit's flow rate.
- Actuators switch. On or off, from a room thermostat. Nothing in between.
And the differential valve regulates pressure across the manifold — which is what makes it possible to set the flow meters accurately in the first place. Without it, adjusting one circuit moves all the others.
Follow the water
The clearest way to hold a manifold in your head is to trace one drop of water through it:
- Hot primary water enters at the injector valve
- The circulating pump drives it on
- It passes the temperature sensor
- Into the flow bar
- Out through the floor circuits
- Back into the return bar
- And is recirculated, rather than replaced
That last step is the one that surprises people. A large proportion of the water going round the floor has been round already. If the sensor reads above the thermostatic head setting, the injector closes and the same water keeps circulating until it has cooled enough to need topping up with hot.
There is a useful mnemonic for the order: mix it, pump it, underfloor flow.
Safety sits on top of that. A thermostat with an immersion sensor shuts the pump down if the thermostatic head fails, so that primary water at 80 °C bypasses the circuits rather than entering a screed that is only allowed 55 °C around the pipe. It is the last line of defence, and on a manifold serving a buried floor it earns its place.
Blending: two ways to do it
A 3-port mixing valve blends hot primary flow with cooled return water from the floor to arrive at the blend temperature.
A 2-port injector valve gets to the same place differently: the floor circuit is already recirculating, and the injector meters hot water into it as required.
Alongside either, the automatic bypass gives the pump somewhere to go when every actuator has closed. On a system with room-by-room control that state is not a possibility — it is guaranteed to happen, several times a day. The bypass is not optional.
Even a factory-assembled blending package still needs its blend temperature set on site and recorded. Assembled is not commissioned.
Room control and compensation
Room control comes in two forms: hard wired, at 230 V or 24 V, and wireless radio.
The electrothermic actuator is an on/off device fitted to the return circuit isolator valve. The single most useful thing to know about it is that it takes two to four minutes to open fully. That delay is behind a good half of the “it isn't working” calls in the first week after handover, and it is worth explaining at the door before anyone else has to.
A four-wire actuator has an auxiliary contact, which can switch a pump, a zone valve or the boiler itself.
All of it is electrical work to BS 7671, and commissioning waits until that work is complete and has been checked.
Two control strategies for the water temperature:
- Fixed temperature control holds one blend temperature and lets the room thermostats switch circuits on and off underneath it.
- Weather compensation moves the flow temperature along a heat curve against outdoor air temperature. Colder outside, hotter flow.
Compensation suits underfloor particularly well, and the reason is the thermal lag from Module 1: when the emitter takes hours to respond, switching is a blunt instrument, and modulating the water temperature is a much better fit. Set the curve to the lowest flow temperature that still holds comfort, and then go back and review it in genuinely cold weather rather than in October.
One last thing about the primary. Hot water for the cylinder is taken from the primary at full temperature, upstream of the blending. Blended water at 45 °C will not heat a cylinder properly, and it will not satisfy the legionella requirement either.
📝 10-Question Mock Test
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The radiators want 70 to 80 °C and the floor wants 40 to 45. Blending is what bridges that, and on this kind of job it is mandatory rather than a refinement.
Always the primary of the heat source. And hot water for the cylinder is taken upstream of the blending, at full temperature — blended water at 45 °C will not heat a cylinder or satisfy the legionella requirement.
Miss those three and you get the classic extension fault: a manifold that is warm rather than hot, an extension that never reaches temperature, and radiators elsewhere that have got slower — with nothing wrong on the underfloor side at all.
Flow meters indicate, lockshields regulate, actuators switch. Three jobs, three components — and they are run together in conversation constantly.
Without it, adjusting one circuit's lockshield moves the reading on all the others, and accurate balancing becomes impossible.
Mix it, pump it, underfloor flow. And note the last step: a large proportion of the water is recirculated rather than replaced, with the injector closing when the sensor reads above the thermostatic head setting.
Same destination, different route. The 3-port blends hot flow with cooled return; the 2-port injects hot water into a recirculating circuit as required.
With room control, every actuator being shut at once is not a remote possibility — it happens several times a day, and the pump needs somewhere to circulate.
Two to four minutes, and that delay is behind a good half of the “it isn't working” calls in the first week. Worth explaining at handover before anyone else has to.
When the emitter takes hours to respond, moving the flow temperature along a curve is a far better fit than switching. Set the curve to the lowest flow temperature that still holds comfort, then review it in genuinely cold weather.
Underfloor is what lets a condensing boiler condense nearly all the time, because the return stays low all day. It is also what keeps a heat pump's flow temperature down and its efficiency up — and on ground source jobs you will often see 150 or even 100 mm pipe centres, buying the required output at a lower flow temperature than 200 mm could manage. The manifold is where all of that is actually delivered.