Summary

"The manifold isn't calling for heat" is a specific and common complaint that's easy to misdiagnose because it sounds like a manifold fault, when in the large majority of cases the manifold itself is fine — it's just never receiving the instruction to open. Wet underfloor heating relies on a signal chain: a room thermostat or time programmer decides heat is needed, that call goes to the wiring centre, the wiring centre energises the relevant zone actuators and tells the boiler (via an interlock/volt-free contact) to fire, and only then does the manifold's pump or the primary heating pump circulate water through the loops.

A break anywhere in that chain — a dead thermostat battery, a tripped fuse on the wiring centre circuit, a broken interlock wire to the boiler, or a stuck relay inside the wiring centre — produces exactly the same symptom at the manifold: nothing happens. This is different from the more commonly documented UFH fault pattern (one zone cold while others work, or slow warm-up with cold patches), which is covered by the general wet UFH fault-finding guide and is almost always a zone-level actuator, balancing, or air/blockage issue.

This guide focuses specifically on total non-response — the manifold pump doesn't run and no actuators open, system-wide — and walks the demand chain from thermostat to boiler to pump so you don't waste time bleeding air or testing individual actuators when the actual fault is a dead RF thermostat battery or a tripped RCBO.

Key Facts

Quick Reference Table

Diagnosed the problem? Create a repair quote in minutes with squote.

Try squote free → 
SYMPTOM: Manifold pump/actuators never activate — no heat anywhere, ever
     |
     ├── Is a room thermostat or programmer actually calling for heat?
     │   (bridge the stat terminals, or raise setpoint 5°C above room temp to force a call)
     │   No → Check programmer time/date/holiday mode, thermostat batteries, RF signal/pairing
     │   Yes ↓
     ├── Does the wiring centre have mains power?
     │   (check indicator LED if fitted, or test terminals with a meter)
     │   No → Check fused spur, MCB/RCBO at consumer unit, isolator switch position
     │   Yes ↓
     ├── Does the wiring centre relay click / actuator output energise when a call is received?
     │   No → Wiring centre relay or PCB fault — replace wiring centre or relay module
     │   Yes ↓
     ├── Is the boiler receiving the demand/interlock signal from the wiring centre?
     │   (check the volt-free contact wiring between wiring centre and boiler)
     │   No → Trace and repair interlock wiring; check master/always-open zone hasn't failed closed
     │   Yes ↓
     ├── Does the boiler fire once it receives the demand signal?
     │   No → Boiler-side fault, not a UFH fault — see [boiler not firing](/knowledge/fault-finder/heating/boiler-not-firing)
     │   Yes ↓
     └── Does the manifold/system pump run once the boiler has fired?
         No → Pump fault, seized pump, or pump-overrun/interlock timer not triggering it
         Yes → Demand chain confirmed working — any remaining fault is zone-level, see
               [wet underfloor heating](/knowledge/fault-finder/plumbing/wet-underfloor-heating)

Detailed Guidance

Confirming a Genuine "No Call" Fault

Before assuming a wiring or control fault, force a definite call and watch the response. Raise a room thermostat setpoint at least 5°C above the current room temperature, or bridge the call terminals on the thermostat wiring at the wiring centre with a temporary jumper wire (only if you're competent to do this safely). If the wiring centre relay clicks and an actuator can be heard/felt moving within a few minutes, the demand chain from that specific stat downstream is working — the earlier fault-finding should focus on that particular thermostat, its battery, or its wiring back to the wiring centre.

If nothing happens even with a forced call at the wiring centre terminals themselves, the fault is downstream of the thermostat — in the wiring centre, its power supply, or beyond.

Wiring Centre Power and Relay Testing

Most wiring centres are mains-powered (230V) with either 24V or 230V outputs to actuators depending on the system design. Start with the obvious:

  1. Check the wiring centre's indicator light (if fitted) shows power present
  2. Check the fused spur or dedicated isolator switch feeding the wiring centre is switched on and the fuse is intact
  3. Check the relevant MCB/RCBO at the consumer unit hasn't tripped — an RCBO nuisance trip on this specific circuit can go unnoticed for weeks if the UFH isn't needed daily
  4. With power confirmed present, force a call (see above) and listen/feel for the internal relay clicking
  5. If power is present but no relay click occurs on a forced call, the wiring centre's internal relay or PCB has likely failed and the unit needs replacing or its relay module servicing

The Boiler Interlock — the Step Most Often Missed

Many UFH installations are wired so that the boiler will only fire once it receives confirmation that at least one zone (an actuator, a zone valve, or a dedicated interlock output) has actually opened — this is a standard no-volt/interlock requirement intended to stop the boiler firing into a system with no open path for water to flow, which would cause it to overheat and cut out on its own high-limit stat.

If this interlock wire is broken, disconnected, or the confirming zone valve/actuator has itself failed shut, the boiler will never receive a valid demand signal — even though every room thermostat, the programmer, and the wiring centre relay are all functioning correctly. This fault presents identically to a wiring centre power fault from the homeowner's perspective (nothing happens, ever) but the wiring centre itself will show a call being registered; the missing link is between the wiring centre and the boiler.

Trace this wiring with the boiler's own wiring diagram and the UFH wiring centre's terminal labelling — they are rarely self-explanatory and vary between manufacturers, so keep or request the original installation documentation for the specific wiring centre model fitted.

Frost Protection and Programmer Faults

A frost protection thermostat, where fitted, is designed to force heat on regardless of room stat settings if the building temperature falls below a set threshold (typically 5°C) — its purpose is to prevent pipe freezing, not to interfere with normal operation, but a miswired or failed frost stat can either block normal calls or force the system on permanently.

Time programmers are a frequent, low-cost cause of "no call at all" faults, especially after a power cut has reset the clock or if the unit has silently entered holiday/away mode. Always check the programmer's current time, day, and active heating periods before moving on to electrical fault-finding — this single check resolves a meaningful proportion of "the UFH has stopped working" callouts.

Pump Behaviour and Overrun

Don't be misled by a pump that is running — many systems run a pump-overrun cycle for several minutes after the last genuine call ends, to circulate residual heat and protect the boiler from overheating on shutdown. A pump running during overrun does not confirm the demand chain is currently working for a new call; wait for the overrun period to finish (check the boiler/wiring centre documentation for the typical duration) before testing a fresh call.

Frequently Asked Questions

The room thermostat display shows "calling" or its heat icon is lit, but nothing happens at the manifold. What's wrong?

This points to a break between the thermostat and the manifold's actual response — most commonly a wiring centre power fault, a failed relay in the wiring centre, or a broken interlock signal to the boiler. Confirm the wiring centre has power first, then force a call directly at the wiring centre terminals to isolate whether the fault is in the thermostat's wiring run or downstream of the wiring centre.

Can a single faulty thermostat stop the whole underfloor heating system?

Yes, but only if that thermostat is wired as a "master" or interlock-triggering zone in the system design — some UFH control layouts use one zone's call (or a dedicated always-open valve) as the confirmation the boiler needs before it will fire. If that specific stat or valve fails, the whole system can stop even though every other room stat is functioning normally. Check the original wiring centre documentation to see if a master-zone arrangement was used.

Do I need an electrician for this kind of fault-finding?

Diagnosing and repairing existing wiring on a UFH wiring centre is not itself notifiable work, but it should be carried out by a competent person given it involves mains electrics, often in a cupboard or utility space that may be a special location under BS 7671. If the fix requires installing a new circuit (rather than repairing an existing one), that new circuit is notifiable under Part P.

How do I tell a pump-overrun cycle from a genuine working call?

Overrun typically runs for a fixed, short period (commonly a few minutes) immediately after a call ends, and won't respond to a fresh thermostat call raised during that window in the normal way. If in doubt, wait for the pump to stop completely, then force a new call from a room thermostat and confirm the relay clicks and the pump restarts in direct response — that confirms a genuine working demand chain rather than residual overrun.

Regulations & Standards