Wet Underfloor Heating Not Working? Common Faults and Repairs Explained

Wet Underfloor Heating Not Working? Common Faults and Repairs Explained

Wet underfloor heating is brilliant when it works.

The entire floor warms gently, the heat spreads evenly around the room and you don’t need radiators taking up half the wall.

Lovely.

When it stops working, however, you’re left staring at a complicated manifold covered in pipes, gauges and little plastic caps, wondering whether you should press something.

Probably don’t.

The good news is that the pipes buried beneath your floor are not automatically the problem. Most wet underfloor heating faults are connected to parts we can actually reach, such as the thermostat, actuator, pump, manifold, controls or boiler.

The important bit is diagnosing it properly.

Not randomly changing parts until something starts working. That’s not fault finding. That’s expensive bingo.

Quick answer: Why is my wet underfloor heating not working?

A wet underfloor heating system may stop working because of:

• A thermostat or control fault
• Dead thermostat batteries
• A failed or stuck actuator
• A seized pin valve
• Air trapped inside a heating loop
• A circulation pump fault
• Incorrect system pressure
• Closed manifold valves
• Poor system balancing
• A mixing valve fault
• A problem with the boiler or heat pump
• An electrical supply or wiring fault

If every room is cold, the problem is more likely to affect the main heat source, pump, controls or manifold.

If only one room is cold, the fault is more likely to be connected to that individual zone.

That distinction saves alot of pointless messing around.

What is a wet underfloor heating system?

A wet underfloor heating system uses a network of pipes installed beneath the floor.

Warm water travels through these pipes and heats the floor surface. The floor then releases that heat gradually into the room.

Instead of having one boiling hot radiator stuck beneath a window, you have a much larger surface warming the room more evenly.

Wet underfloor heating is commonly used in:

• Kitchens
• Bathrooms
• Extensions
• Open plan living spaces
• Dining rooms
• New build homes
• Rooms with large windows
• Spaces where radiators would be awkward or ugly

The warm water normally comes from a boiler or heat pump.

Because underfloor heating spreads heat over a large area, it can usually run at a lower water temperature than traditional radiators.

That does not mean the floor should feel like a frying pan.

It should feel comfortably warm.

If you’re expecting to cook an egg on it, your expectations may need adjusting.

How does wet underfloor heating work?

The main parts normally include:

ComponentWhat it doesBoiler or heat pumpProduces the heatThermostatTells the system when a room needs heatWiring centre or controlsSends signals between the thermostats and system componentsActuatorOpens or closes the water supply to an individual zoneManifoldDistributes water through the different heating loopsCirculation pumpPushes water around the underfloor heating pipesMixing valveControls the water temperature entering the floorFlow metersShow and help adjust flow through each loopPipe loopsCarry warm water beneath the floor

Your manifold is basically the system’s traffic controller.

It decides where the water goes, how much goes through each loop and which zones are allowed to heat.

If one part of that process fails, the floor may stay cold even though the boiler appears to be working.

Common wet underfloor heating faults

The whole underfloor heating system is cold

If the entire floor is cold, or several zones have stopped heating at once, it is unlikely that every loop has suddenly failed simultaneously.

That would be a spectacular level of bad luck.

The fault is more likely to involve something shared by the whole system.

This could include:

• The boiler or heat pump not producing heat
• The circulation pump not running
• A fault with the wiring centre
• A failed relay
• A closed isolation valve
• No electrical supply to the manifold
• A mixing valve fault
• The main thermostat or programmer not calling for heat
• Low system pressure

The first step is to confirm whether the system is actually receiving a demand for heat.

The next is to see whether the boiler responds.

Then the pump, manifold temperature, valves and flow can be checked.

There is a logical order to this.

Randomly turning everything on the manifold is not part of it.

One underfloor heating zone is not working

If most of the house is warm but one room remains stubbornly cold, the main heat source is probably doing its job.

The issue is likely to be local to that zone.

Common causes include:

• A faulty room thermostat
• Flat thermostat batteries
• A failed actuator
• A stuck pin valve beneath the actuator
• A closed flow meter
• Air trapped inside the loop
• Poor flow through that zone
• Incorrect balancing
• A wiring fault between the thermostat and actuator
• A blockage or restriction

An actuator is the small electrical component fitted to the manifold that opens a valve when a room asks for heat.

The actuator itself can fail.

The little valve pin underneath it can also become stuck.

Sometimes the actuator is doing exactly what it should, but the valve beneath it refuses to move. It’s like pressing a light switch when the bulb has already given up on life.

A proper test needs to establish:

1. Is the thermostat asking for heat?
2. Is the signal reaching the wiring centre?
3. Is the actuator receiving power?
4. Is the actuator opening?
5. Is the valve pin moving?
6. Is water actually flowing through the loop?

That is diagnosis.

Not just looking at the actuator and declaring it guilty.

One zone will not turn off

A zone that stays hot can be just as annoying as one that remains cold.

Possible causes include:

• The thermostat continuing to call for heat
• Incorrect thermostat settings
• A faulty thermostat
• A permanently powered actuator
• An actuator stuck open
• A valve pin stuck in the open position
• Incorrect wiring
• Heat continuing to circulate after the demand has ended

Underfloor heating systems respond more slowly than radiators.

The floor may continue releasing heat after the thermostat has switched off because the floor itself stores heat.

That is normal.

A room remaining hot for hours while every other zone has stopped is not.

My underfloor heating is warm but not reaching temperature

This is one of the most misunderstood underfloor heating problems.

The system is not completely dead.

It is doing something.

Just not enough.

Possible causes include:

• Water temperature set too low
• A mixing valve problem
• Air trapped in one or more loops
• Insufficient flow
• Poorly balanced loops
• A weak or incorrectly set pump
• Low system pressure
• Incorrect thermostat settings
• Temperature sensor faults
• Heat loss from the room being greater than expected
• Unsuitable floor coverings
• A boiler or heat pump not providing enough heat

The manifold flow temperature should be checked and compared with the temperature being supplied by the heat source.

A large difference may indicate a problem with the mixing valve, circulation or temperature controls.

The flow rates through each loop should also be checked.

One loop may be taking more than its fair share while another gets virtually nothing.

Bit like a buffet, really.

My underfloor heating takes ages to warm up

Wet underfloor heating is not normally instant.

It heats a large floor surface rather than a relatively small metal radiator.

The floor construction also matters.

Pipes buried in a thick screed floor will usually respond more slowly than a system fitted closer to the finished surface.

That doesn’t automatically mean there is a fault.

However, if the system previously warmed properly and has suddenly become much slower, something may have changed.

Check for:

• Reduced water temperature
• Poor flow
• Air in the system
• A pump problem
• Incorrect controls
• Changes to thermostat schedules
• Thick rugs or new floor coverings
• Partially closed valves

Turning the thermostat up to 30 degrees does not make the system heat faster.

It simply tells it to keep heating for longer.

The thermostat is a target, not an accelerator pedal.

The water pressure is too high

The pressure shown on your boiler or heating system should remain within the range recommended for that system.

If the pressure is too high when the system is cold, possible causes include:

• The filling loop has been left open
• The filling valve is passing water
• Too much water has been added
• Another filling arrangement is allowing water into the system

If the pressure starts at a normal level but rises sharply as the system heats, the expansion vessel may need checking.

The expansion vessel allows heated water to expand safely.

Without enough expansion space, the pressure rises and the boiler’s pressure relief valve may discharge water.

Draining a little water might reduce the pressure temporarily.

It does not fix the reason the pressure rose.

Otherwise you’re just emptying it today so it can annoy you again tomorrow.

The water pressure is too low

Low pressure may stop the boiler or heating system operating properly.

Possible causes include:

• A leak from visible pipework
• A leaking manifold connection
• A boiler leak
• Water escaping through the pressure relief pipe
• Air being removed from the system
• A faulty expansion vessel
• A problem with another component
• A leak within the underfloor heating pipework

A leak beneath the floor is possible.

It is not the first conclusion we should jump to.

The manifold, boiler, valves and visible connections should be inspected before anybody starts discussing lifting your expensive floor.

Repeatedly topping the pressure up is not a repair.

If the pressure keeps dropping, the water is going somewhere.

The system needs investigating.

How can I tell if the underfloor heating pump is faulty?

The circulation pump moves warm water through the underfloor heating loops.

If it fails, the boiler may produce heat but that heat will not travel around the floor properly.

Possible signs of a pump problem include:

• The pump not running when heat is requested
• The pump running constantly
• A humming noise without circulation
• Unusual rattling or grinding sounds
• Hot pipework entering the manifold but little heat leaving it
• Flow meters showing no movement
• Several zones failing together
• The pump becoming excessively hot

The pump itself may not be the actual problem.

It could be receiving no electrical signal.

A thermostat, relay, wiring centre or control fault could prevent it from operating.

Likewise, a pump running constantly may be responding to a permanent demand created somewhere else.

Test the whole control sequence before replacing it.

Parts are expensive.

Guessing is more expensive.

Problems with the underfloor heating manifold

The manifold distributes warm water between the heating zones.

Common manifold related problems include:

• Closed isolation valves
• Stuck flow meters
• Incorrect flow settings
• Failed actuators
• Stuck valve pins
• Air trapped in individual loops
• Leaking connections
• A seized pump
• A failed mixing valve
• Incorrect balancing
• Temperature sensor faults

You may notice that some manifold pipes are hot while others remain cold.

That information can help diagnose where circulation is stopping.

It does not mean you should immediately start twisting every valve you can see.

Those settings may have been used to balance the system.

Once you change all of them, we now have the original fault and a completely unbalanced system.

Excellent. Two problems for the price of one.

Could it just be the thermostat?

Yes.

Sometimes the expensive looking heating failure is a couple of dead batteries.

Before assuming something catastrophic has happened, check:

• Is the thermostat switched on?
• Is it displaying the correct temperature?
• Is the set temperature higher than the current room temperature?
• Is the schedule correct?
• Is it in holiday or frost protection mode?
• Are the batteries working?
• Is the wireless thermostat connected to its receiver?
• Is the correct zone being controlled?

A thermostat that lights up is not proof that the entire control system is working.

But it is a sensible place to begin.

Air trapped in underfloor heating pipes

Air inside an underfloor heating loop can reduce or completely stop circulation.

You may notice:

• One zone staying cold
• Uneven floor temperatures
• Gurgling noises
• Flow meters showing little movement
• A zone warming briefly and then cooling
• The pump sounding noisy

Removing air from an underfloor heating system is not always the same as bleeding a radiator.

Individual loops may need to be isolated and flushed properly.

Opening a random valve and hoping the air feels intimidated enough to leave generally doesn’t work.

Common homeowner mistakes

Turning every valve on the manifold

Please don’t.

Those valves and flow meters may have been adjusted to balance the system.

Take photographs before changing anything.

Better still, don’t change anything unless you understand what it controls.

Increasing the thermostat again and again

If the room is not heating at 22 degrees, setting the thermostat to 30 degrees rarely solves the fault.

You’ve just asked the broken system to try harder.

Constantly topping up the pressure

If pressure keeps falling, there is a reason.

Repeated topping up can introduce fresh oxygen into the heating system, which may contribute to corrosion.

Find the fault.

Assuming the pipes beneath the floor have failed

They might have.

But thermostats, actuators, pumps and valves fail far more conveniently, because apparently even heating systems understand that lifting tiled floors is a nightmare.

Test the accessible components first.

Replacing parts without testing them

An actuator may not open because it is faulty.

It may also not open because it is receiving no power.

Replacing the actuator will not repair a wiring fault.

Expecting underfloor heating to behave like radiators

Underfloor heating is slower and more gradual.

It may need different scheduling.

Turning it off completely and expecting immediate heat later can leave you disappointed and wearing two jumpers.

Wet underfloor heating fault guide

What is happening?What may need checking?Every zone is coldBoiler, heat pump, controls, pump, power supply, pressure and manifold valvesOne zone is coldThermostat, actuator, valve pin, air, flow and balancingOne zone stays onThermostat, wiring, actuator and valve pinFloor is only slightly warmWater temperature, mixing valve, flow rate, pump and controlsSystem takes much longer than beforeAir, reduced flow, pump, settings or floor covering changesPressure rises when heatingExpansion vessel and pressure controlPressure keeps fallingLeaks, pressure relief discharge, expansion vessel or pipeworkPump runs constantlyPermanent demand, wiring, thermostat, actuator or relayPump makes noise but no heat circulatesAir, stuck pump, blockage or pump failure

This table is a starting point.

It is not a diagnosis.

Two completely different faults can create very similar symptoms, which is why testing matters.

Practical checks you can do safely

Before arranging an underfloor heating repair, you can check:

• Whether the boiler or heat pump is working
• Whether other heating zones are working
• Whether the thermostat is asking for heat
• Whether the thermostat batteries need replacing
• Whether the system pressure is unusually high or low
• Whether any obvious leaks are visible
• Whether the manifold has electrical power
• Whether any fault codes are showing
• Whether the issue affects one zone or the whole system
• Whether any recent settings or schedules have changed

Write down what you find.

Photos of the thermostat, manifold, pressure gauge and any fault code can be helpful.

Do not remove electrical covers.

Do not dismantle the boiler.

Do not start opening manifold connections.

Water and electrics remain a fairly poor combination.

Our professional recommendation

When diagnosing wet underfloor heating faults, work through the system in order.

Start with the demand.

Then check the heat source.

Then the electrical controls.

Then the pump.

Then the manifold.

Then the individual zone.

Finally, look at flow, air and balancing.

That approach gives you information.

Information stops you replacing perfectly good components.

A proper repair should involve finding the cause of the fault, explaining it clearly and then repairing the right thing.

Not arriving with six spare parts and hoping one of them earns its keep.

When should you call a heating engineer?

Arrange a professional inspection if:

• The boiler pressure keeps rising or falling
• The whole system has stopped heating
• Several zones have failed
• The pump is noisy, hot or not running
• Water is leaking from the manifold
• Electrical components are not operating
• One zone remains cold after basic thermostat checks
• The system needs flushing or purging
• Faults keep returning
• You do not understand the manifold controls

A wet underfloor heating system is relatively simple once you understand how each part works.

Until then, it looks like somebody has connected thirty pipes to a wardrobe and hoped for the best.

Wet underfloor heating repairs across North Somerset and Bristol

Kesselmann Heating & Plumbing can inspect wet underfloor heating faults in Weston-super-Mare, Clevedon, Portishead, Nailsea, Bristol and surrounding parts of North Somerset.

We assess the controls, circulation, manifold, pressure and heating source to work out what is actually failing.

Then we explain it in plain English.

No vague nonsense.

No pretending every fault requires the floor to come up.

And no changing expensive components because they looked a bit suspicious.