Underfloor heating not heating evenly: common causes

Air trapped in a loop is the most common culprit

Underfloor heating works as a set of loops of pipe run through the screed, each fed from a manifold. If air gets into one loop, water cannot circulate through it properly and that section of floor stays cold even though the rest of the room is warm. This happens most often after the system has been drained and refilled, after work elsewhere in the building, or simply over time as small amounts of air work their way in.

Most manifolds have a bleed valve on each loop, usually a small screw or valve at the top. Turning it slightly with a flat screwdriver, with a cloth ready to catch the water, lets trapped air escape until water runs through steadily. It is worth doing this on a system that has been recently drained or on a loop that has gone from working fine to being noticeably colder than the others.

If bleeding a loop makes no difference, or the same loop keeps needing it every few weeks, the problem may not be air at all but a slow leak somewhere letting air back in, or a balancing issue at the manifold. That is a reasonable point to stop and call a plumber rather than keep bleeding the same valve.

A stuck or badly set manifold valve

Each loop at the manifold has its own valve, and these are set during installation to balance flow between loops of different lengths. Over time a valve can seize slightly, get knocked out of position, or simply be set wrong from the start, so one loop gets far less water than the others and that room never quite reaches temperature.

You can usually see the manifold in a utility cupboard or under a floor hatch, with a row of small dials or handles, one per loop. If one dial is turned right down compared to the others while its room is cold, that is worth noting, but changing the balance yourself without knowing the original settings can throw off the whole system rather than fix one room.

This is one to describe rather than adjust: note which loop looks different, which room it feeds, and mention it when you call for help. A plumber can rebalance the manifold properly, and if a valve is actually stuck rather than just misadjusted, it may need freeing or replacing.

Thermostat placement often misleads you about the whole room

A thermostat measures the air, or in some systems the floor, at one specific spot. If it sits on a wall that gets afternoon sun, near a draught from a door, or above a warm appliance, it reads a temperature that has nothing to do with the rest of the room and calls for less heat than the room actually needs.

Cold patches caused by thermostat placement usually show up as a whole room running cool rather than one section of floor, and the pattern often matches the time of day or which doors have been open. It is worth checking whether the thermostat sits somewhere unusual before assuming there is a fault in the loops themselves.

Relocating a thermostat means running a new cable or a wireless sensor to a better spot, and because it is wired into the heating control, this is electrical work best left to a qualified electrician rather than something to rewire yourself.

Furniture, rugs and thick flooring block the heat

Underfloor heating relies on the floor surface radiating warmth into the room, so anything that insulates the floor from the air above it reduces output in that spot. A large rug, a low sofa with no gap underneath, or a wardrobe sitting flush to the floor can all create a noticeably cooler patch nearby, even though the loop underneath is working exactly as it should.

This is the easiest cause to rule out and often the actual answer. It is worth checking simply by feeling the floor with a bare foot in the cold spot compared with elsewhere, and by noting what sits on top of it.

Flooring material matters too: thick carpet with a dense underlay, or engineered wood laid over an uneven subfloor with air gaps, both reduce how well heat gets through. If you are planning new flooring over underfloor heating, it is worth checking the manufacturer's suitability for it before laying it — our flooring service, from 15 € per job, can advise on this as part of the work.

• Move rugs and thick mats off suspect cold patches and see if the floor warms within a day
• Check for furniture sitting flush to the floor with no airflow gap underneath
• Feel the floor surface directly rather than judging by room air temperature alone
• Note whether the cold area matches a specific loop or the whole room
• Check flooring material and underlay thickness if the whole floor runs cooler than expected

What is worth worrying about and what can wait

A cold patch on its own is a comfort issue, not a safety one, and there is no urgency to fix it. It is reasonable to live with it for a while, note the pattern, and deal with it when convenient.

What does deserve prompt attention is any sign of water where there shouldn't be any: a damp patch on the floor, a manifold cupboard that smells musty, or a pressure gauge on the system dropping steadily over days. Underfloor pipework is embedded in the screed, so a leak there is harder to trace and more disruptive to fix the longer it runs.

A system that used to heat evenly and has recently developed several cold spots at once, rather than one that has always run slightly uneven, is also worth investigating sooner rather than later, since that pattern points more towards a manifold or pump problem than simple trapped air.

When to call a plumber and roughly what it costs

If bleeding the obvious loops and checking for furniture and rugs hasn't solved it, the next step is having someone look at the manifold and check the balance and pressure properly. Our plumbing service starts at 45 € per hour, with a call-out of 25 € within Tallinn that is deducted from the bill if the work goes ahead.

A straightforward check and rebalance of a manifold usually takes an hour or so; tracing an actual leak in the loops takes longer and is normally quoted as a fixed price after an inspection, valid for 30 days. If the fix also involves moving a thermostat or checking wiring, that part is handled by a qualified electrician.

For anything unrelated to the heating loops themselves, such as adjusting furniture placement or checking flooring fit, our odd jobs service from 39 € per hour or the flooring service from 15 € per job can help without needing a plumber at all.