Underfloor Heating Manifold Installation

A manifold consists of two horizontal bars, one above the other. Warm water from the heat source enters one of them (the flow), and cooled water from the floor returns to the other (the return). Each loop connects to both bars, one pipe end on each.

• Flow meters show how much water runs through each loop and let you adjust it
• Actuator-ready valves allow room thermostats to be added later, so each room can be controlled separately
• Air vents and drain valves are used for filling, flushing and bleeding air out
• Isolating ball valves separate the manifold from the rest of the heating so repairs can be done without draining everything

Why is there sometimes a pump and a mixing valve as well? Underfloor heating needs much cooler water than radiators. If the heat source delivers high temperatures, for example an older boiler that also feeds radiators, a mixing unit is fitted in front of the manifold. With a heat pump or a condensing boiler running at low temperatures it is often not needed. The heating engineer should decide this based on the system design.

First, a manifold with the right number of ports is chosen. This depends on how many loops there are, how long they are and the output of the heat source. It makes sense to leave one spare port if you might add another room in future.

1. Prepare a manifold cabinet, either recessed into the wall or surface-mounted, or a technical niche.
2. Fix the manifold on its brackets, level and higher than the loops in the floor, which makes bleeding the air easier.
3. Connect the flow and return from the heat source, including isolating valves and any mixing unit.
4. Connect each loop using compression fittings made for that type of pipe.
5. Label every port with the room name and the loop length.
6. Fill and flush the loops one at a time until the water runs without air.
7. Carry out a pressure test and record it in a test report.
8. Set the flow rates and check the temperatures when the heating first runs.

Where should the manifold go? Ideally as close to the centre of the heated area as possible, so the loops are of similar length and the supply pipes do not have to run through other rooms. It also has to stay accessible: the cabinet should not end up behind kitchen units or a fitted wardrobe.

Pressure testing and screeding over the pipes

The pressure test is always done before the screed is poured, while the pipes are still visible. The test pressure and duration are set by the system manufacturer, and the procedure is based on EN 1264. The pipes stay under pressure while the screed goes down, so any damage shows up straight away. The screed holds the pipes in place and spreads the heat evenly into the floor. The heating is switched on for the first time only once the screed has cured, usually after several weeks for a cement screed and sooner for an anhydrite screed. The temperature is then raised gradually, following the manufacturer's instructions.

Fitting the manifold is only half the job. Water always takes the easiest path, so without adjustment a short bathroom loop would "steal" heat from a long living-room loop. That is why the system is hydraulically balanced: the heating engineer sets each flow meter so the loop gets exactly as much water as it needs for its length and the room's heat loss. The values should be in the system design. If there is no design, they come from a calculation or the manufacturer's tables.

A well-balanced system heats evenly, runs more quietly and the pump does not have to work harder than necessary. It is also ready for room thermostats. Actuators on the valves then open and close the loops on their own according to the room temperature. The wiring of the actuators and the control unit must be done by a qualified electrician.

The simplest case is a new build where the supply pipes are ready, the cabinet is set at the right height and the loops have not been screeded yet. More work is involved with:

• A large number of loops or very long loops
• The need for a mixing unit with a pump
• Replacing an old manifold whose dimensions do not match, so fittings or supply pipes have to be adapted
• Unlabelled old loops, where you first have to work out which one goes where
• Tight spaces, or a cabinet that has to be cut into the wall afterwards
• Old loops full of sludge that need a thorough flush

• The manifold is hidden so well that nobody can reach it for servicing
• The loops are not labelled, and years later no one knows which belongs to which room
• All flow meters are left fully open, so no balancing was actually done
• All loops are filled at once and air stays trapped in them
• Screed is poured without a pressure test, or without the pipes being under pressure
• The underfloor heating is connected straight to high-temperature water from the boiler with no mixing

It is quite common for an owner to suspect the boiler when the real fix is simply setting the flow rates correctly and bleeding the loops.

Old manifolds often have no flow meters and no way to add actuators. A renovation is therefore a good moment to fit a new one, especially if you are changing the heat source or the room layout. If the floor loops are watertight and not clogged, a new manifold can usually be connected without breaking up the floor.

If the boiler is being changed at the same time, a gas boiler may only be disconnected and reconnected by a qualified, registered gas engineer. If you smell gas, leave the property and call the local emergency number or 112. For larger changes to the heating system, check with your local building authority whether any approval or notification is needed under local building regulations.

In a block of flats things are different. If your flat is connected to a shared central heating system, discuss any work with the building management or owners' association beforehand. Connecting underfloor heating to the building's system without approval can upset the heating for your neighbours too.

• The manifold is firmly fixed, level and easy to reach
• Every port is clearly labelled with its room
• All joints are dry and nothing is dripping
• You have the pressure test report
• The flow meters are set according to the design and you have the values in writing
• The loops are bled and all of them get warm when the heating runs
• The isolating valves close and open easily
• If screed was laid, you have a record of the gradual heat-up procedure
• The heating engineer has shown you how to bleed a loop and isolate the manifold