Short answer: A sealed heating system with underfloor heating should hold steady at the heat source maker's cold pressure, often 1 to 1.5 bar on a boiler system. A small drop in the first days after filling is normal while air vents out. A drop that keeps happening means water is leaving the system, most often at a manifold joint, a weeping air vent, a pressure relief valve or a failed expansion vessel, and only rarely from pipe buried in the floor. To find out which, isolate the manifold with its ball valves and test that side on its own at 6 bar for 60 minutes.
Losing pressure is worrying with underfloor heating, because the pipe is hidden in the floor. The good news is that a pipe laid in one length, pressure tested before screeding and not drilled since is rarely the cause. Most losses are at parts you can see and reach.
This guide covers what pressure is normal, how to read the pattern of the drop, how to split the system to find the leak, and how to check the expansion vessel and relief valve. For testing a new floor before screeding, see our guide to pressure testing underfloor heating.
What pressure should underfloor heating run at?
The UFH loops share the pressure of the whole sealed system, so the target is the one your boiler or heat pump maker gives. One boiler maker's filling guide gives a normal range of 1 to 1.5 bar. Heat pump systems differ, so check the heat pump's instructions.
Pressure rises a little as the water heats and expands, then falls again as it cools. That swing is normal. What matters is whether the cold pressure, read at the same time of day, stays the same from week to week.
Two gauges can disagree. Water pressure changes by about 0.1 bar for every metre of height, so a manifold gauge 3m below a loft boiler reads about 0.3 bar more than the boiler gauge. Pick one gauge and always compare readings on that one.
Is a small pressure drop normal?
A drop soon after a fill, purge or drain-down is usually air leaving through the vents, and it should stop after a few heat-ups. A drop that carries on at the same rate is a leak. Read the pattern before you start taking things apart.
| Pattern | Likely cause |
|---|---|
| Falls for a few days after a fill | Air venting out |
| Falls steadily, week after week | Water leak |
| Very high hot, very low cold | Expansion vessel |
| Drips from outside pipe | Relief valve discharging |
| Rises when heating is off | Filling loop letting by |
If the pressure is rising instead, check that the filling loop valves are fully closed. On a combi boiler, a rise can also come from the mains side inside the boiler, which is a job for a Gas Safe registered engineer.
Where does an underfloor heating system lose water?
Look at the joints and valves first, because that is where most leaks are. Dry a suspect joint with tissue and check it again an hour later, with the system both hot and cold.
| Where | What to look for |
|---|---|
| Eurocone connections | Damp nut, wrong size core |
| Automatic air vent | Drips, crusty cap |
| End units, flow meters, plugs | Wet threads, white marks |
| Pump unions, ball valves | Drips under the pump set |
| Radiators, boiler pipework | Valves, joints under boards |
| Relief valve | Wet discharge pipe outside |
At the manifold, a core that doesn't match the pipe size, a pipe cut out of square or an overtightened nut can weep. Check the core matches the pipe, for example 16 x 2.0mm. A vent that drips is letting water out: isolate the manifold with its ball valves and replace it.
How do I find out if the leak is in the floor?
Split the system in two. Close the manifold ball valves and watch the manifold gauge and the heat source gauge separately. Whichever side loses pressure has the leak.
- Switch off the heating and let the system cool.
- Close both manifold ball valves so the manifold and loops are sealed off from the heat source.
- Note both gauge readings, then leave them for 24 hours.
- If only the heat source gauge falls, the leak is on the boiler or radiator side, not in the floor.
- If the manifold gauge falls, connect a hydraulic test pump to the manifold fill valve and raise it to 6 bar. Hold for 60 minutes. Check that every part on the manifold side is rated for that pressure first.
- If it drops, close all the loops at the manifold and retest. If the manifold now holds, the leak is in a loop.
- Open one loop at a time and retest each. The loop that loses pressure is the leaking one.
Change only one thing at a time, and let the water reach a steady temperature before you start timing. Water that warms or cools during a test changes the reading by itself.
Could the expansion vessel be the cause?
Yes. A failed expansion vessel is one of the most common causes of a sealed system that loses pressure for no visible reason. When the vessel's air charge is gone, the pressure climbs as the water heats until the relief valve opens and dumps water. When the system cools, the pressure is then lower than before.
Signs to look for:
- The gauge climbs much more than usual when the heating is on.
- The relief valve discharge pipe is wet or drips after the heating has run.
- Pressing the vessel's air valve gives little air, or water comes out.
Viessmann describes a typical residential pre-charge of around 1 bar, with the exact figure in the boiler manual. A vessel maker's instructions say the air pressure should be slightly higher than the system's static pressure, and that it should be checked at least every six months. The charge must be checked with the water side of the vessel at zero pressure, or the reading is wrong. If the vessel is inside a boiler, have a Gas Safe registered engineer do it.
Is the pressure relief valve letting water out?
A relief valve opens to protect the system when the pressure gets too high. On most domestic boilers it opens at about 3 bar, but check the boiler's data. Find where its discharge pipe ends, usually outside near the boiler, and look for drips, staining or a wet patch after the heating has run.
- If it only drips when the system is hot, the expansion vessel is the likely cause.
- If it drips all the time, the valve may have dirt on its seat after opening once. It may need replacing.
- If someone has topped the system up too high, bring the pressure back to the normal range before judging the valve.
What if the leak is in a loop under the screed?
A leaking loop in a finished floor can be found and repaired. Before breaking anything out, be sure the loop really is the problem by testing it on its own, as above.
- Isolate the leaking loop at the manifold so the rest of the system can run.
- Find the leak. A thermal imaging camera with warm water in the loop often shows a warm spot or wet patch spreading from the leak. Specialist leak detection firms also use acoustic or gas tracing methods.
- Check the floor plan for where the loop runs, and think about recent work. Drilled fixings and new door thresholds are common causes.
- Break out a small area of screed, cut out the damaged section and join the pipe with a fitting the pipe maker approves for burying in a floor.
- Pressure test the repaired loop before you make good. MaxHeatUFH tests at 6 bar for 60 minutes. Keep it under pressure while the patch is screeded.
If the loop is beyond repair, it can stay isolated at the manifold. The room will then heat less, so check whether a neighbouring loop or another heat source can make up the difference.
How do I stop the pressure dropping again?
Fix the cause rather than topping up. Every top-up adds fresh, oxygen-rich water and more air, which leads to corrosion and more venting. Once the system holds:
- Set the cold pressure to the heat source maker's figure and note it down with the date.
- Check the gauge once a month during the heating season.
- Have the expansion vessel charge checked at each boiler service.
- Re-dose with inhibitor after any drain-down, following the inhibitor maker's instructions.
- Mark where loops run before anyone drills or fixes into the floor.
If you have just refilled, also purge the loops properly. Our guide to bleeding air from underfloor heating explains how.
Products for this job
Prices include VAT, October 2026.
- Hydraulic Pressure Test Pump, 0 to 50 bar: £114
- Manifold Pressure Gauge End Unit: £16
- Manifold Auto Air Vent End Unit: £10
- Manifold Ball Valve Set 1": from £22
- Eurocone Connector Cores 3/4": £1.42 each (16 x 2.0mm)
- Pressure / Temperature Gauge for UFH Mixing Unit: £10
- All manifolds and components
Related guides: Underfloor heating not getting warm and How to install water underfloor heating.
Adding a new floor to an existing system? Design it free with MaxHeatUFH ProLoop. Add your rooms and floor type, and get a loop design and full kit list with prices, including the manifold, valves and test kit. Start your design
Frequently asked questions
Why does my underfloor heating keep losing pressure?
Either water is leaving the system or air is still venting after a fill. A steady drop week after week is a leak, most often at a manifold joint, an air vent, the relief valve or because the expansion vessel has lost its charge.
Can underfloor heating pipes leak in the floor?
They can, but it is uncommon in a floor laid in one length and pressure tested before screeding. Damage from drilling or fixings is the usual cause. Rule out the manifold and the rest of the system first.
How much pressure drop is acceptable?
In normal running, the cold pressure should stay the same from week to week. During a 6 bar test, the gauge should hold for the full 60 minutes once the water temperature has settled.
Is it safe to keep topping up the pressure?
It's fine as a short-term measure, but it isn't a fix. Repeated top-ups bring in fresh water and air, which can cause corrosion and more venting. Find the cause.
Why is the pressure on my manifold different from my boiler?
Height. Pressure changes by about 0.1 bar per metre, so a gauge lower down reads higher. Compare readings on the same gauge each time.
Who should check the expansion vessel?
If it sits inside the boiler, a Gas Safe registered engineer. A separate vessel on a heat pump or system boiler can be checked by a heating engineer, with the water side at zero pressure.
Sources: Worcester Bosch system filling guide, Viessmann: what is an expansion vessel, Altecnic pressure vessel instructions, Wunda manifold instructions