Fire: Valve Orange OUT → T9 → Boiler SW-L + Pump live
Satisfied: Cyl Stat C+2 → T8 breaks HW demand when hot
⚠️ Isolate 230V before wiring. Orange = LIVE OUTPUT from valve — not an input.
📌 Resideo 95C-10820-06: Orange = boiler & pump live output.
S-PLAN · V4043H ZONE VALVES · HONEYWELL / RESIDEO
Component Info
Click any component for terminal connections
V4043H Wire Colours
Blue — Neutral → T1
Grn/Yel — Earth → T5
Grey — Permanent Live → T2
Brown — Motor live IN (zone demand)
Orange — End-switch OUTPUT ⚡ → T8 + T9
Brown energises the valve motor when its zone calls for heat. Once the valve is fully open, the internal end-switch closes and Orange becomes live — firing the boiler and pump.
Both valves' oranges wire in parallel to T8 (pump) and T9 (boiler SW-L).
CH demand: Prog CH ON → T3 → Room Stat pin 1 → (cold, 1+3 closed) → pin 3 → T4 → CH valve Brown (motor opens)
HW demand: Prog HW ON → T6 → Cyl Stat C → (cold) → 1 → T7 → HW valve Brown (motor opens)
Perm Live: T2 → Grey on both valves (powers motor coil & end-switch)
Fire: Valve fully open → end-switch closes → Orange OUT → T8 pump + T9 boiler SW-L
Satisfied: Stat opens → brown de-energises → spring closes valve → end-switch opens → boiler off
⚠️ Orange is a LIVE OUTPUT from the end-switch — never an input. Always isolate before working.
📌 Both zones fully independent. Either valve alone can fire boiler & pump via T8/T9.
⚠️ Safety First — Read Before Testing
Always use a properly calibrated multimeter rated CAT III 600V or higher. Never probe live terminals with the meter on resistance (Ω) mode.
Keep one hand clear of metalwork when probing. If in doubt, isolate and make safe. These tests are for qualified Gas Safe / competent persons only.
Y-Plan — V4073A
MID-POSITION VALVE
Multimeter Setup
Set meter to AC Volts — 600V range (or auto-range). Black probe to a known good earth or N (T1). Red probe to the terminal being tested.
All readings taken at the wiring centre top or bottom terminals with system calling for heat.
🔌 Wiring Centre — Voltage Tests (Y-Plan)
Terminal
Black probe
Condition
Expected
If wrong
T1 (N)
Earth / T5
Always
~0–5V AC
High V = N/E fault. Call electrician.
T2 (L)
T1 (N)
Spur ON
~230V AC
0V = spur off / blown fuse / no supply
T3 (HTG)
T1 (N)
Programmer CH ON, timed period active
~230V AC
0V = programmer fault / CH not timed on
T4 (CH OUT)
T1 (N)
CH demand + room stat calling
~230V AC
0V = room stat open circuit / stat wiring fault
T5 (E)
T1 (N)
Always
~0–5V AC
High V = earth fault. Isolate immediately.
T6 (HW OFF)
T1 (N)
Programmer HW OFF output active
~230V AC
0V = programmer HW OFF not wired / programmer fault
T7 (HW ON)
T1 (N)
Programmer HW ON, timed period active
~230V AC
0V = programmer HW output fault
T8 (CYL SAT)
T1 (N)
Cylinder cold — stat not satisfied
~230V AC
0V = cyl stat satisfied (normal when hot) or stat wiring open
T9 (SW-LIVE)
T1 (N)
Valve fully open (CH or HW demand)
~230V AC
0V = valve not fully open / orange end-switch fault
🌡️ Checking Heating is Working — Y-Plan
Set programmer to CH ON. Set room stat above room temperature. Confirm T3 reads ~230V (programmer output live).
Probe T4 — should read ~230V. If 0V, room stat contacts are open: check stat setting, replace stat contacts, or check T3→stat→T4 wiring.
Valve WHITE wire (CH demand) energises. Valve motor drives to mid-position. Allow 2–3 minutes for valve to fully open.
Probe T9 — should read ~230V when valve fully open (orange end-switch closed). If 0V, valve has not reached full open: check valve motor, check white wire from T4 reaching valve.
Boiler and pump should now fire. Confirm flow/return pipes getting hot within 5 minutes.
🚿 Checking Hot Water is Working — Y-Plan
Set programmer to HW ON. Confirm T7 reads ~230V (HW ON output).
Cylinder stat must be calling (cylinder cold). Probe T7 at cyl stat C terminal — should be ~230V. If stat is satisfied (cylinder hot), reading will be 0V at terminal 1 — this is normal.
If cylinder is cold and T7 reads 230V but T9 reads 0V: cyl stat wiring fault — check C → 1 path, check T7 loop through stat back to valve grey/white circuit.
Probe T9 when valve is driven to HW position (spring return, port B open). Should read ~230V from orange end-switch output.
Boiler fires. Cylinder heats. When stat satisfied (typically 60°C), T8 goes live breaking the HW demand — boiler off.
⚠️ Y-Plan Common Faults
Fault
Likely Cause
Test / Fix
🔥
No heating, no HW
No supply. Programmer off. Valve seized.
Check T2 = 230V. Check programmer timed on. Try manual override on valve.
🌡️
CH not working, HW OK
Room stat open. T3/T4 break. Valve white not energised.
Probe T3 then T4. Bridge room stat temporarily to confirm fault location.
🚿
HW not working, CH OK
Cyl stat satisfied / faulty. Programmer HW off. T7 break.
Probe T7. Check cyl stat set to 60°C. Bridge C→1 on stat to test.
⚡
Boiler won't fire — T9 = 0V
Valve not fully open. End-switch failed. Orange wire broken.
Listen for valve hum. Check orange wire continuity. Replace valve if motor failed.
🔄
Continuous HW — stat satisfied but boiler stays on
Cyl stat contacts welded. T6 (HW OFF) not wired. Grey wire missing from valve.
Probe T6 = 230V required. Check grey wire at valve. Replace cyl stat if welded.
💧
Valve jammed / motor hum but no movement
Valve head seized. Scale or corrosion on spindle.
Manually operate lever on valve head. Replace valve head or full valve body.
🌊
CH on when only HW selected
Valve stuck in mid-position. T4 continuously live.
Check T4 = 0V when only HW selected. Check room stat wiring. Check valve position.
S-Plan — V4043H
2-PORT ZONE VALVES
Multimeter Setup
Set meter to AC Volts — 600V range (or auto-range). Black probe to T1 (neutral) at wiring centre.
Red probe to the terminal being tested. System must be calling for heat with programmer and stats set correctly.
📘 V4043H Wire Roles — Per Resideo Issue 19BLUE = Neutral ·
G/YELLOW = Earth ·
GREY = Permanent Live ·
BROWN = Motor live IN (zone demand) ·
ORANGE = Switched live OUT (end-switch).
🔌 Wiring Centre — Voltage Tests (S-Plan)
Terminal
Black probe
Condition
Expected
If wrong
T1 (N)
Earth / T5
Always
~0–5V AC
High V = N/E reversal. Isolate immediately.
T2 (L)
T1 (N)
Spur ON
~230V AC
0V = spur tripped / blown fuse / no supply
T3 (CH PROG)
T1 (N)
Programmer CH ON, timed period active
~230V AC
0V = programmer CH not outputting / not timed on
T4 (CH MOTOR)
T1 (N)
CH demand + room stat calling (1+3 closed)
~230V AC
0V = room stat open / wiring break T3→stat→T4. Feeds CH valve BROWN.
T5 (E)
T1 (N)
Always
~0–5V AC
High V = earth fault. Stop work. Isolate.
T6 (HW PROG)
T1 (N)
Programmer HW ON, timed period active
~230V AC
0V = programmer HW output fault / not timed on
T7 (HW MOTOR)
T1 (N)
HW demand + cyl stat calling (C+1 closed, cold)
~230V AC
0V = cyl stat satisfied (normal when hot) or stat wiring open. Feeds HW valve BROWN.
T8 (PUMP SW-L)
T1 (N)
Either zone valve fully open
~230V AC
0V = no valve fully open / no orange end-switch closed.
T9 (BOILER SW-L)
T1 (N)
Either zone valve fully open
~230V AC
0V = no orange end-switch closed — boiler will not fire.
🔬 At the Valve — V4043H Wire Tests
Wire
Black probe
Condition
Expected
Meaning
Blue
Earth / G-Y
Always
~0–5V AC
Neutral reference
Grey
Blue
Spur ON
~230V AC (always)
0V = no perm live at T2 / break in grey from T2
Brown
Blue
This zone calling for heat
~230V AC
Motor live IN — valve drives open. 0V = no demand reaching valve.
Orange
Blue
Valve fully open (after 2–3 min)
~230V AC
0V = end-switch not closed / valve not reached fully open
🌡️ Checking Heating is Working — S-Plan
Set programmer CH ON. Set room stat above room temperature. Probe T3 = ~230V confirms programmer CH output is live.
Probe T4 = ~230V confirms room stat contacts closed (terminals 1+3) and CH demand signal reaching CH zone valve brown wire.
Check CH valve grey = ~230V (perm live always present). Brown energised drives motor open — listen for motor running. Allow 2–3 minutes.
Probe CH valve orange = ~230V confirms end-switch has closed. Same voltage will now appear at T8 and T9.
Probe T8 = ~230V (pump running) and T9 = ~230V (boiler firing). Confirm flow/return pipes getting hot within 5 minutes.
🚿 Checking Hot Water is Working — S-Plan
Set programmer HW ON. Probe T6 = ~230V confirms programmer HW output is live.
Probe T7 = ~230V confirms cyl stat is calling (cylinder cold, C + terminal 1 closed). If 0V and cylinder is cold — cyl stat fault or wiring break.
HW zone valve brown energised. Valve drives open (2–3 minutes). Check grey = ~230V at valve (always present).
HW valve orange = ~230V when fully open — end-switch closed. Both valves' oranges wire in parallel — either alone fires boiler & pump.
Probe T8 = ~230V (pump runs). Probe T9 = ~230V (boiler fires). When cyl stat satisfied (~60°C), C+1 opens — T7 drops to 0V — brown de-energises — spring closes valve — orange drops to 0V — boiler off.
✅ Sanity Check — Back-feed via Orange
Per Resideo Issue 19 fault-finding table, when one zone is calling and the other is satisfied: the satisfied valve's orange wire will still show ~230V (back-fed from the active valve's orange via the parallel T8/T9 connection). This is normal and not a fault. The satisfied valve's brown wire will be 0V — that's how you tell which zone is actually driving.
⚠️ S-Plan Common Faults
Fault
Likely Cause
Test / Fix
🔥
No heating, no HW
Spur tripped. Programmer off. No perm live to grey on either valve.
Check T2 = 230V. Check programmer on. Check grey = 230V at each valve.
🌡️
No CH, HW OK
Room stat open. T3/T4 break. CH valve brown not energising.
Probe T3 then T4. Bridge room stat temporarily. Check brown at CH valve = 230V when calling.
🚿
No HW, CH OK
Cyl stat satisfied/faulty. T6/T7 break. HW valve brown not energising.
Probe T6 then T7. Check cyl stat set 60°C. Bridge C→1 to test stat. Check brown at HW valve.
Probe orange at each valve. If 0V at valve but brown is live — motor fault or end-switch failed; replace valve head.
💧
Pump runs, boiler won't fire
T9 break to boiler. Boiler CH terminal fault. Boiler lockout.
Probe T9 = 230V at wiring centre AND at boiler SW-L. Check boiler fault codes.
🔄
CH on all the time
Room stat contacts welded. T4 permanently live. Brown never de-energises.
Disconnect room stat — if CH stops, stat is welded. Replace stat.
🌊
Valve won't close (spring return failed)
Spring return mechanism failed. Brown permanently live (wiring fault).
De-energise brown. If valve stays open mechanically — replace valve head.
🔊
Valve hums but won't open
Valve head seized. Scale on spindle. Motor capacitor failed.
Try manual lever. If seized — replace valve head. Check brown = 230V and grey = 230V at valve.
📋 Pro Tip — Voltage Drop Method
To confirm a wire is carrying current (not just showing voltage): probe each end of the wire. A good conductor shows the same voltage at both ends. A voltage drop of more than 5V across a wire indicates high resistance — check terminals, connections, and wire condition.
📡 Continuity Testing (Power OFF)
With system fully isolated, use Ω/continuity mode to check: room stat 1→3 (closes when cold, calling), cyl stat C→1 (closes when cold, calling), valve motor winding (typical 30–80Ω between brown and blue at valve). An open circuit reading means replace the component.
Test Cards — what it should read on the meter
On a job, the part's not working and you need to test it. Pick a component — each card shows how it's wired, the reading you should get on every lead, and what a wrong reading means. Live tests are AC volts, black probe to a neutral, unless the card says otherwise.
METHODUsing Your Multimeter — The Right Way
For fault-finding (is this terminal live?), a multimeter on AC volts is the right tool. For proving dead before you work on anything, GS38 says use a two-pole voltage indicator + proving unit — not a multimeter.
Do
Set the meter to AC volts (V~), correct range or auto, before you touch anything.
Black probe to a known neutral or earth; red to the test point.
Read it: ~230V = live present · 0V = no live.
Use CAT III 600V+ leads, GS38 probes (finger guards, ≤4mm tips). True-RMS is best.
One hand clear of metalwork. If in doubt, isolate and make safe.
Don't
Never probe a live circuit with the meter on resistance (Ω) or current (A) — it can draw fault current and damage the meter or hurt you.
Don't trust a multimeter to prove dead — wrong range or a flat battery gives a false zero.
Don't work the leads with bare fingers near the tips.
Watch for ghost voltages. A digital meter is high-impedance, so it can show a "phantom" reading (50–150V) on a wire that isn't really live — picked up from a parallel cable. If a reading looks odd or won't settle, confirm with a low-impedance / two-pole tester before you act on it.
Before you work on it. To replace a valve head, swap a stat, or open the wiring centre to work — isolate, lock off, and prove dead with a GS38 two-pole tester, proved on a known source before and after. Live testing is for fault-finding only.
Y-PLAN3-Port Mid-Position · V4073A
What it does: shares one boiler between heating and hot water. With no power it sits open to hot water (port B). A heating call drives it across to CH (port A); with both calling it holds mid-position, feeding both. Its end-switch sends the switched live that fires the boiler and pump.
TEST · black probe to neutral · AC volts · condition noted on each wire
If a reading's wrong
White 0V on a CH call → heating demand isn't arriving — check the room stat / T4.
Orange 0V with white live → the valve isn't driving open → change the valve head.
Grey never live → the HW-off hold isn't wired — the valve can't drop HW demand, so HW can stay on. (No HW-OFF terminal on the programmer = replace the programmer.)
Readings good but no heat? With CH calling, the pipe ~12″ from port A should get hot — if not it's plumbing / boiler, not the valve wiring.
Source: Honeywell V4073A 95C-10820 · Center CS0237 · 5-core lead (white, blue, grey, orange, earth — no brown). Spares: powerhead 40003916-003, motor 40002737-003.
How to test it
Live test — AC volts, black probe on a neutral.
Call for heating only: white should be live and orange should follow once the valve drives across.
Turn HW off: grey should go live (the hold). Turn HW on: grey drops.
Orange live but boiler dead → fault is past the valve (boiler / pump), not the valve.
S-PLAN2-Port Zone Valve · V4043H
What it does: a simple on/off gate for one circuit (a heating zone or the hot water). The motor drives it open when that zone calls; a spring snaps it shut when the call stops. Once fully open, an internal end-switch makes and sends a switched live on to fire the boiler and pump.
TEST · zone calling for heat · black probe to neutral · AC volts
If a reading's wrong
Grey 0V → no permanent live reaching the valve — break in the grey feed. Orange can never go live without it.
Brown 0V on a call → the demand isn't arriving — fault is back up the stat / programmer, not the valve.
Brown live but orange 0V → the valve isn't reaching fully open, or the end-switch has failed → change the valve head.
Readings good but no heat? With the zone calling, the pipe ~12″ from the valve should get hot — if not it's plumbing / boiler, not the valve wiring.
28mm / 1″ valve (V4043H1106 / 1080) has a 6th white wire — it's the SPDT changeover model; the white must be isolated (made safe) in S-Plan. The 22mm is SPST (no white).
Leave it live — these are live voltage tests. Meter on AC volts, black probe on a neutral.
With the zone calling, read each coloured lead against neutral.
Work down: grey (should always be live) → brown (the demand) → orange (the result).
Any reading that doesn't match → use the guide above to find the fault.
CENTREWiring Centre · 10-Way · Honeywell 42002116
What it is: the junction box where the programmer, stats, valve and boiler all meet. Every wire lands on a numbered terminal — so it's the best place to test. Readings below are to neutral, on a heat call (Y-Plan shown).
Terminal
What's on it
Expect (to N)
If it's wrong
T1 · N
Neutral bus
0V
reference point
T2 · L
Permanent live
230V always
spur off / fuse / MCB tripped
T3 · HTG
Programmer CH output
230V
CH not timed on / programmer
T4 · CH out
Room stat out → valve white
230V
room stat open / wiring
T5 · E
Earth bus
0V
reference point
T6 · HW OFF
Prog HW-off → valve grey
230V when HW off
not wired / programmer
T7 · HW ON
Prog HW-on → cyl stat C
230V when HW on
programmer HW output
T8 · CYL SAT
Cyl stat satisfied feed
230V when hot
cyl stat / wiring
T9 · SW-L
Valve orange → boiler + pump
230V on a heat call
valve not open / end-switch
T10
Spare
—
—
S-Plan version: same box, same method — T3/T4 feed the CH valve brown, T6/T7 feed the HW valve brown, and T8/T9 are the paralleled pump and boiler switched lives. I'd build a Y and an S version.
ROOMRoom Thermostat · T6360B
What it does: switches a live out of terminal 3 when the room is below the dial setting; opens (satisfied) when it's warm enough. SPDT, 230V, 10(3)A. Double-insulated — earth is a parking terminal only.
TEST · black probe to neutral · AC volts · wind the dial up to force a call
Terminals 4 = cooling (changeover), 5 / 6 = indicator lamp — not used on a heating-only stat.
If a reading's wrong
1 dead → no permanent live reaching the stat — spur / fuse / wiring back to it.
1 live but 3 never lives on a call → stat not switching (wind dial well above room temp first) → faulty stat.
3 stuck live even when satisfied → contacts welded or dial wound to max → heating won't turn off.
What it does: SPDT changeover on the hot-water cylinder. Cold → makes C–1 (calls for HW). Hot → makes C–2 (satisfied — used on Y-Plan). ~10°C differential, normal set 60°C. Class II, no earth.
TEST · black probe to neutral · AC volts · HW timed ON · wind dial up = call, down = satisfied
Changeover: only one of 1 or 2 is live at a time. Cold = C–1, hot = C–2.
If a reading's wrong
C dead → no HW live arriving — programmer HW output / T7, not the stat.
C live but 1 never lives when cold → stat not calling (wind dial above cylinder temp) → faulty stat.
1 always live, never satisfies → dial wound too high or stat stuck → HW overheats / never drops.
Test the output type first. A modern stat is one of two kinds — and you test them differently. Get this wrong and you'll chase a fault that isn't there.
SWITCHED LIVE
Passes a live through
Takes a permanent live and switches it out on a call — same as the old stats. Test: output reads 230V calling, 0V satisfied. Many digital stats (e.g. wired Honeywell T4, ESi, Drayton Digistat) do this.
VOLT-FREE
Dry contact only
A relay closes between COM and a call terminal — the stat has no voltage of its own. 0V there is normal. Test: isolate, check continuity COM→call closes on a call; or if you've linked a live into COM, that live appears at the output when calling.
Wireless & smart (Hive, Nest, Tado, Drayton Wiser, Honeywell T-series): test at the receiver / heat-link, not the wall unit. Most receivers are volt-free — e.g. Hive (1 L, 2 N, 3 COM, 4 NO), Nest Heat Link (COM / NO / NC). Check the wall unit has battery and signal/pairing, then test the receiver relay as volt-free above.
Always go by the maker's terminal labels — COM / NO / NC, or A / B, or 1 / 2 / 3 — they vary by brand. When in doubt, the datasheet for that exact model wins.
PROGRAMMERProgrammer · ST9400 / ST6400
What it does: a time clock with two changeover switches — one for CH, one for HW. Each flips a live between its ON and OFF terminal as the program (or the override button) demands. 230V, 2 × SPDT, 3(3)A.
TEST · black probe to neutral · AC volts · use the override buttons to force each state
Numbers change with the make — go by function. ST6400 / ST9400 = 1–4 (above). ST699 / ST799 = 3–8. Drayton Lifestyle = 1–4 in its own order. Single-channel timeswitches (e.g. Lifestyle LP111) have a volt-free output — link L→terminal 1 to get a switched live, so 0V there without the link is normal.
If a reading's wrong
L dead → no permanent live — spur / fuse / isolator. Nothing works, clock blank.
CH set on but terminal 4 never lives (and 3 stays live) → programmer relay not switching → faulty programmer.
Both CH terminals dead → no perm live reaching the switch, or unit faulty.
Source: Honeywell ST9400 50022736 · Drayton Lifestyle 06490033001. Each channel is a changeover — only ON or OFF is live at once.
How to test it
Confirm L is live (perm). Black probe to N, AC volts.
Override CH ON → terminal 4 lives, 3 dies. Override CH OFF → it swaps.
Same for HW: ON → 2 lives, OFF → 1 lives.
Right output live but nothing downstream → fault is past the programmer (stat / valve / wiring centre).
PUMPCirculating Pump
What it does: the simplest part of the system — it runs whenever it's given a switched live (from the valve orange / boiler). Speed is set on the head (1–2–3 or auto). No live = it sits still.
TEST · black probe to neutral · AC volts · with a heat demand on
Pump overrun. Many boilers keep the pump running after they stop, to shed residual heat. Those take a separate permanent live + neutral at the pump (or via the boiler) as well as the switched live — so the pump can run when the boiler isn't calling. Wiring is boiler-specific: check the boiler manual.
If a reading's wrong
No live on a demand → the switched live isn't arriving — test back at the valve orange / wiring-centre T9, not the pump.
Live present but pump silent → seized rotor, failed motor or capacitor → free it off / replace the pump.
Runs but no flow → airlock or speed set too low — bleed it, check the speed selector.
What it does: fires the burner and runs its own pump when it gets a heat demand. The demand reaches it as either a switched live or a volt-free contact — many modern boilers ship with a link across two terminals that you remove to wire in a stat or programmer.
TEST · black probe to neutral · AC volts · with a heat demand on
Volt-free type. Instead of an SL terminal, many boilers have two terminals with a factory link. Remove the link and wire a volt-free stat across them — there's no 230V here, the boiler does the sensing, so test that the contact closes on a call (continuity), not for voltage. OpenTherm boilers use a 2-wire digital link instead of a simple on/off — different again.
If a reading's wrong
SL live but boiler won't fire → it's a boiler fault (lockout, ignition, gas, flow/return) — read the fault code and use the fault-code library, not a wiring fault.
No SL on a call → demand isn't arriving — test back at the valve orange / wiring-centre T9.
Boiler has its own internal frost protection — it can fire to protect itself regardless of the controls.
Terminal layouts vary a lot by make/model — always check that boiler's own manual (R2 library). Source: Drayton LWC1 (boiler = switched live).
FROSTFrost / Pipe Thermostat
What it does: a stat set low (~5°C) that overrides the time and temperature controls to fire the heating when it's cold enough to risk freezing — for pipework or boilers in exposed spots (garages, lofts, outhouses). Best as two-stage: a frost stat (senses air) in series with a pipe stat (senses the pipe) — both must be cold before it fires, so it doesn't run needlessly.
TEST · black probe to neutral · AC volts · chill below ~5°C, or wind the dial up to force it
If a reading's wrong
Won't go live when cold → faulty or set too low → no frost protection (freeze risk).
Stuck live → heating runs constantly regardless of the programmer → stat jammed or set too high.
Two-stage: if either the frost stat (air) or the pipe stat is open, there's no output — they're in series, so check both.
Source: Drayton LWC1 / Honeywell T6360 frost-protect (RTS3 + PTS1 two-stage). Wired to override time and temperature controls.
WEATHER COMPWeather Compensation
What it does: adjusts the boiler's flow temperature to the outside temperature on a heating curve — colder outside = hotter flow, milder = cooler flow. Running cooler when it can lets the boiler condense more and burn less. Uses an outside sensor (an NTC thermistor) on a north wall, wired to the boiler by a thin 2-core.
TEST IS DIFFERENT · it's a SENSOR · meter on OHMS (Ω) · never apply 230V
Never apply 230V to the sensor. It's a low-voltage sensor circuit. Set the meter to ohms, measure across the two sensor cores (or at the PCB plug, to include the leads), and compare to the boiler maker's resistance/temperature table. Base value varies — commonly ~10kΩ at 25°C, but some are 1kΩ or 12kΩ. Check that boiler's manual.
If a reading's wrong
Boiler reports a fixed / silly outside temp whatever the weather → corroded or failed sensor, or a broken lead → flow temp ends up wrong.
∞ (open) or 0Ω (short) → sensor or wiring fault → boiler logs a sensor fault and usually defaults to a fixed flow temp.
Resistance is correct but comfort's poor → that's the curve / slope setting, not a fault — adjust it in the boiler, don't change the sensor.
NTC = resistance rises as it gets colder. Test in Ω against the maker's table. Curve set in the boiler's installer menu.
WIRELESS RXWireless Receiver · EPH & ESi
What it does: receives an RF signal from a battery-powered wall thermostat and closes a volt-free relay (COM / NO / NC) to call for heat. You wire L & N to power the receiver, then link L into COM so the NO terminal gives a switched 230V output to the boiler. Terminal numbers are DIFFERENT between brands — get the link wrong and the boiler never fires.
TEST · black probe to neutral · AC volts · turn thermostat above room temp to force a call
Source: ESi ESRTERFW v6.10.2 / ESRTP4RF+ installation instructions. Same receiver for all ESi wireless models.
⚠️ Don't mix up the COM terminal: EPH = link L to terminal 2 (COM). ESi = link L to terminal 1 (COM). Same numbering, different function. Get it wrong and the boiler never fires — the NO terminal stays at 0V because it has no source voltage.
Scenario 1 · Combi Boiler (standalone receiver)
Receiver on its own fused spur. You supply L and N to the receiver yourself, so you must link L to COM to give the relay a source voltage. NO output goes to the boiler's 230V RT terminal.
Isolate. Mount receiver near the boiler. Wire L and N from the fused spur (3A fuse).
Link L to COM on the receiver (EPH: L→terminal 2, ESi: L→terminal 1).
Run a wire from NO (EPH: terminal 1, ESi: terminal 2) to the boiler's 230V RT terminal.
Power up. Pair thermostat to receiver. Set thermostat above room temp. Meter NO — should read 230V.
Scenario 2 · System Boiler / S-Plan (receiver in existing wiring)
Receiver replaces the old wired room stat. The switched live is already coming from the programmer via the 5-core cable — so the receiver's COM and NO just take two cores from the existing wiring. No L-to-COM link needed because the programmer is already supplying the live through the circuit.
Isolate. Remove the old wired room stat. Identify the two switching cores in the cable (typically grey and black in a 5-core).
Wire L and N to the receiver from the cable to power it.
Connect the switched live in (from the programmer) to COM on the receiver (EPH: terminal 2, ESi: terminal 1).
Connect NO (EPH: terminal 1, ESi: terminal 2) to the wire going back to the zone valve / wiring centre — this is the call-for-heat output.
Power up. Pair. Set thermostat above room temp. The programmer supplies the live, and when the relay closes, 230V passes through to fire the valve and boiler.
Why no link? On a combi the receiver is standalone — it has no source of 230V on its relay side unless you bridge it yourself. On an S-Plan, the programmer's switched live IS the source — it arrives at COM from the existing wiring, so linking L to COM would bypass the programmer's time control.
⚠️ Glowworm / Vaillant eBUS Boilers
Glowworm boilers default to eBUS (24V) stats — their own Climapro / MiGo thermostats use the eBUS terminals, NOT the 230V RT terminals. If you're fitting a third-party 230V wireless receiver (EPH, ESi, Honeywell, etc.), you must use the 230V RT terminal on the boiler PCB, not the eBUS connection. Connecting a 230V stat to the eBUS terminals will not work and can damage the PCB.
Glowworm PCB terminal block typically has: L, N, E (mains supply), RT (230V room thermostat — use this one), eBUS (24V for Glowworm's own stats — don't use with 3rd party 230V), and sometimes OFF (burner lockout, for UFH or condensate pump). On some models the RT terminal has a factory link fitted — remove it when wiring a thermostat. Vaillant boilers have the same eBUS system.
Check the boiler manual for the RT terminal location on the PCB connector block.
If a Glowworm eBUS stat (Climapro/MiGo) was previously fitted, disconnect its eBUS wiring completely.
Remove any factory link on the RT terminal.
Wire the receiver's NO output to the boiler's RT terminal. On most Glowworm combis, L and N for the receiver can come from the boiler's own fused spur.
Set the boiler's internal CH control to ON so it responds to the external thermostat demand.
Glowworm Climapro1 manual: "Do not connect the room thermostat to a 230V terminal" — that warning is about eBUS stats, not 3rd party. 3rd party 230V stats use the RT terminal specifically.
If a reading's wrong
NO terminal always 0V (combi) → #1 cause: no L-to-COM link. Volt-free contacts have no voltage of their own — you must bridge live in.
NO terminal always 0V (S-Plan) → check the programmer is set to CH ON and is outputting a switched live. Meter at COM — if COM is 0V, the issue is upstream (programmer or wiring), not the receiver.
L and N live but boiler won't fire → check thermostat is paired and calling (flame symbol on ESi LCD, or M/A LED red on EPH). If stat is calling but NO is 0V, relay or pairing fault.
Heating won't turn off → check manual override isn't engaged (green MAN/MANUAL LED solid). Press M/A then MANUAL to exit override.
Glowworm: wired to eBUS but using a 230V stat → wrong terminals. Move wires from eBUS to the 230V RT terminal. Remove eBUS wiring completely.
Thermostat calling but no RF signal → re-pair. EPH: connect button on receiver + connect on stat. ESi: hold M/A 10 secs until green flashes, then pair.
Intermittent signal → receiver too close to metal, microwave, or Wi-Fi router. Must be ≥1m clear. Range ~20–30m open.