Bleed underfloor heating or is it not getting warm?

How underfloor heating works

With wet underfloor heating, warm water runs through plastic pipes in the screed. These pipes are divided into circuits: usually one or more loops per room. All circuits come together at the manifold, a box or rack with two horizontal pipes (flow and return) and a valve or flow meter for each circuit. Many installations also have a pump and a control that lowers the water temperature, because underfloor heating works with lower temperature water than radiators.

Underfloor heating responds slowly. A floor needs hours to reach temperature. That's normal and not a fault.

Signs that something is wrong

• One room or part of the floor stays cold, whilst the rest gets warm.
• You hear gurgling or rushing at the manifold.
• The electricity meter for a circuit is at zero or not moving.
• The floor no longer gets properly warm anywhere, or only after a very long time.
• The boiler pressure keeps dropping. This may indicate a leak; then read about underfloor heating leaks.

Bleeding underfloor heating

Air in the pipes usually collects at the manifold. There are bleed valves there: small nipples or automatic bleed valves at the top of the flow and return pipe. Broadly speaking, it works like this:

1. Switch off the underfloor heating pump and the heating and wait a moment, so the air can rise to the top.
2. Keep a cloth and a bowl ready under the bleed valve of the manifold.
3. Open the bleed valve carefully with a radiator key or screwdriver and let the air escape until water comes out without bubbles. Close it again.
4. Check the central heating pressure on the boiler and top up if necessary, just as you would after bleeding a radiator.
5. Turn everything back on and give the floor time to warm up.

If one loop stays cold, the air may be trapped in the loop itself. You won't always get it out via the bleed valve; the loop may need to be flushed through. Don't adjust the flow meters or balancing valves without good reason: they were set during installation so that each room receives the correct amount of water. Note the position if you do touch them.

Flushing underfloor heating: when is that necessary?

In older underfloor heating, especially with pipes without an oxygen-tight layer, sludge and sediment can form over the years. The water then flows less well through the loops and the floor becomes less warm. Air that won't come out can also be a reason to flush. When flushing, each group is flushed separately with water, and sometimes with air or a cleaning agent, until the water runs clear. This requires connections on the manifold and the right equipment. It's work for a tradesman, especially if a cleaning agent is involved.

Flushing isn't always the solution. If one room stays cold, it could also be the controls or the pump. So first have the cause established.

Underfloor heating not getting warm: other causes

• Central heating pressure too low. Check the pressure on the boiler.
• The pump isn't running. Feel whether the pump at the manifold is humming or vibrating slightly. If it's off, check the plug and the switch.
• The mixing control or thermostatic head is set too low, or is stuck.
• The room thermostat or the control per room is not controlling the zone, for example due to a flat battery or a faulty valve.
• The boiler isn't producing hot water. If the radiators aren't getting warm either, the problem is with the boiler. See boiler breakdown.
• The installation is not properly balanced or something has changed in the layout of the rooms.

What you'd better not do

• Opening the flow meters fully to make a cold room warmer. Then other rooms get too little.
• Drilling holes yourself in a floor with underfloor heating, for example for a door stop. A hit pipe causes a leak under the floor.
• Keep topping up when the pressure keeps dropping. Then there's a leak that needs to be traced.

Underfloor heating inspection or flushing: costs and approach

If one zone doesn't heat up whilst the rest do, there is often air or dirt in that zone, or a thermostatic valve or balancing valve is stuck. If an entire floor heats poorly, we first check the pump, the settings and the flow temperature. Only then is flushing considered.

When flushing, we push clean water through the pipes loop by loop, so that sludge and dirt come out. Afterwards we bleed, top up the system and rebalance the loops. If water is leaking somewhere, read more in underfloor heating leak.

Send a photo of the manifold and tell us how many circuits there are. Then we can give a good estimate in advance.