Wet Underfloor Heating Fault Finding: No Heat, Uneven Zones & Pressure Loss Diagnosis

Summary

Wet underfloor heating (UFH) is generally very reliable when correctly installed — the pipe loops themselves rarely fail if correctly installed and pressure-tested. When problems arise, they are almost always in the controls (actuators, thermostats, programmers) or in the hydraulics (pump, manifold, air, sludge). Pipe leaks do occur but are far less common than control or hydraulic failures.

Fault diagnosis in UFH requires understanding the system architecture: the boiler/heat pump supplies hot water to a manifold (or multiple manifolds). The manifold has ports for each loop (zone), each with its own actuator (electronically operated valve). When a zone thermostat calls for heat, the actuator opens, allowing hot water to flow through that loop. A pump (either built into the manifold or separate) circulates the water.

Most UFH systems also have a blending valve (thermostatic mixing valve) that mixes primary flow with return water to deliver the correct flow temperature to the UFH circuits — typically 35–45°C for gas boiler systems, or as low as 30°C for heat pump systems.

Key Facts

• Flow temperature for UFH — Typically 35–45°C (gas boiler). 30–35°C for heat pump. Primary boiler temperature is higher (60–70°C) and mixed down to UFH temperature at the blending valve
• Actuators — Normally closed (NC) electrothermal actuators. Energised to open. If no power = valve closed = no heat in that zone. They click audibly when opening
• System pressure — Sealed system: 1.0–1.5 bar cold. Below 0.8 bar: risk of pump cavitation and air draw-in. Above 2.0 bar: PRV may activate
• Air in system — Air collects at high points. UFH manifolds typically have air vent valves. System must be filled and bled correctly. Air causes cold patches in loops
• Sludge/magnetite — Black ferrous sludge from corrosion. Blocks loop ports in manifold and reduces flow. Diagnose by checking flow rates at manifold balancing valves
• Pump failure — If the pump fails, no zones will heat even if actuators are open. Check pump runs when system calls for heat
• Zone balancing — At commissioning, each loop is balanced to achieve equal flow. Over time, partial blockages can upset this balance. Check manifold flow meters
• Polypipe/Hep2O fittings — If the floor has been modified or fittings replaced, check for correct insert use in push-fit connections — see push fit fittings
• Manifold types — Kliemann, Emmeti, JG Underfloor, Polyplumb — all have similar architectures but different actuator specifications

Quick Reference Table

SYMPTOM: No heat from any zone
     |
     ├── Boiler/heat pump running? 
     │   No → Diagnose boiler fault first
     │   Yes ↓
     ├── Pump running? (touch pump body — should vibrate)
     │   No → Check power to pump, check pump is not seized
     │   Yes ↓
     ├── Blending valve set correctly?
     │   No → Adjust flow temperature (clockwise = hotter)
     │   Yes ↓
     └── All actuators powered? (listen for click, check LED if fitted)
         No → Check wiring centre power/fuse
         Yes → Check manifold isolation valves are open

SYMPTOM: One zone cold, others warm
     |
     ├── Is the zone thermostat calling for heat?
     │   (raise setpoint 5°C above room temp to test)
     │   No → Thermostat fault — check battery, wiring, setpoint
     │   Yes ↓
     ├── Is the actuator receiving power? (multimeter on actuator terminals)
     │   No → Trace wiring back to wiring centre
     │   Yes ↓
     ├── Is the actuator moving? (listen for click; remove and press pin manually)
     │   No → Replace actuator
     │   Yes ↓
     ├── Is flow showing at manifold flow meter for that zone?
     │   No → Zone blocked — check for debris, balance valve closed accidentally
     │   Yes ↓
     └── Flow rate correct? (compare to adjacent balanced zones)
         Low → Partial blockage or loop too long — powerflush or reroute

Detailed Guidance

Pressure Loss Diagnosis

A sealed UFH system that loses pressure has a leak. The leak can be in:

• The UFH pipe loops within the screed (rare, but occurs at fittings embedded in screed)
• The manifold connections
• The primary circuit (boiler side)
• The expansion vessel (waterlogged vessel causes apparent pressure loss)

Step 1: Isolate the UFH system from the primary circuit at the manifold isolating valves. Pressurise the UFH loops to 1.5 bar. Monitor pressure over 24 hours. If pressure holds, the leak is on the primary/boiler side.

Step 2: If UFH pressure drops with the primary isolated: the leak is within the UFH loops. Pressurise to 3 bar (within manufacturer specification for the pipe). Use a thermal imaging camera to inspect the floor surface — wet patches will show as temperature anomalies. Trace dye test can also be used.

UFH pipe leak repair: If a pipe is identified under screed:

• Mark the location precisely
• Cut the screed carefully — an SDS drill or disc cutter only to confirmed depth
• Identify the damaged section and repair with approved fittings (push-fit for PE-X or PB — ensure insert stiffener sleeves are used)
• Pressure test the repair before re-screeding
• Use a proprietary screed repair compound (not cement mortar) to reinstate

Actuator Testing

A UFH actuator is an electrothermal device — a heating element that expands wax to push a plunger, opening the valve. When de-energised, the wax cools and contracts, allowing the spring to close the valve.

To test an actuator:

1. Verify wiring to the actuator (typically two wires: live and neutral from the wiring centre)
2. Apply 230V AC to the actuator terminals (with insulated probes and appropriate care)
3. After 3–5 minutes, the actuator pin should extend and the valve open
4. Touch the actuator body — it should feel slightly warm
5. If no extension after 5 minutes, actuator is faulty — replace

Actuators are standardised to M30×1.5 thread in the UK (Danfoss use M28×1.5 — a different standard). Most actuators are interchangeable within the same thread standard.

Manifold Balancing

Each port on the UFH manifold has a flow meter (a float in a tube) and a balancing valve (lockshield). At commissioning, the balancing valves are set to achieve equal or appropriate flow rates in each loop.

Checking balance:

1. Open all actuators (energise all zones or manually open each valve)
2. Run the pump
3. Check flow meter reading on each port
4. Adjust lockshield valves (clockwise = reduce flow) until all loops have equal flow per metre (longer loops need slightly more flow to achieve the same heat transfer)

If one flow meter shows zero or very low flow, that loop has a blockage or the balancing valve has been accidentally closed.

Air Purging Procedure

Air in UFH loops causes cold patches and noise. UFH systems must be fully purged at commissioning:

1. Connect a hose to the manifold drain/fill valve
2. Energise all actuators (all zones open)
3. Fill system from the manifold drain valve one port at a time — flush each loop individually until no air bubbles are visible in the drain hose
4. Close each port and move to the next
5. After all loops, pressurise to 1.5 bar and check all air vents
6. Run the pump for 30 minutes and re-check pressure — some air will de-gas
7. Use automatic air vent valves at the manifold to allow residual air to escape over time

Blending Valve Adjustment

The thermostatic blending valve (mixing valve) controls the UFH flow temperature by mixing primary (boiler) water with return water. It has a temperature adjustment knob (typically 20–60°C range).

If the floor doesn't feel warm enough:

• Turn the blending valve knob clockwise (typically increases flow temperature)
• Allow 30–60 minutes for the screed to warm and re-check
• Do not exceed 45°C for standard screed UFH, or 55°C for dry system UFH

If the floor is too hot:

• Turn anti-clockwise
• Check thermostat setpoints in each room — if correctly set, the blending valve may be over-temperature

Frequently Asked Questions

My UFH was working last winter but doesn't heat now. Where do I start?

Start with the boiler/heat pump — is it calling and running? Then check the wiring centre (typically a small box near the manifold) — is it powered, are any fuses blown? Then check each actuator — are they clicking and opening? Finally, check system pressure and manifold flow rates. A seasonal fault that was not there last winter is most often a control failure (thermostat battery, actuator, or wiring centre).

Can I work on UFH pipes under screed?

Screed repairs above UFH pipes are high-risk and should only be attempted after careful thought. Use a thermal camera to pinpoint the leak precisely before cutting. Use a depth gauge to establish the screed depth before cutting. Keep cut depth to minimum. Have the right materials ready before starting — you cannot leave an open trench in a screed overnight.

How often should a UFH system be serviced?

UFH pipe loops: never, if correctly installed with good-quality inhibitor. Controls and manifold: annual check by a competent plumber. Inhibitor concentration: test annually with test strips. If the water appears dark (magnetite), arrange a powerflush before inhibitor levels cause further corrosion.

Regulations & Standards

• BS EN 1264 — Water-based surface embedded heating and cooling systems (installation standard)

• BS 7593 — Treatment of water in heating systems (inhibitor requirements apply equally to UFH)

• Approved Document L — Energy efficiency: UFH thermostat control requirements

• Polypipe UFH Technical Support — Installation and fault-finding guides for wet UFH

• JG Underfloor Heating Technical Guide — Manifold and controls specification

• BPEC UFH Module Guidance — Training and assessment guidance

• underfloor heating — UFH installation specification and pipe spacing

• zoning systems — Zone control wiring for UFH systems

• low pressure — System pressure diagnosis

• expansion vessels — Expansion vessel in sealed UFH systems