Summary

"Intermittent hot water" is one of the most-reported and most-misdiagnosed plumbing complaints, because the same symptom — hot, then cold, then hot again — is produced by completely different faults depending on the system. The discipline that solves it quickly is localisation: is the problem at a single outlet or across the whole property? That one question splits the fault tree almost in half.

If only one outlet swings hot and cold, the fault is local — most often a thermostatic mixing valve (TMV) or shower cartridge whose wax/thermostatic element is failing or furred with scale, or a worn diverter inside a mixer shower. If every hot tap is affected, the fault is central — a combi boiler's diverter valve sticking between heating and hot water, limescale choking the heat exchanger, or a flow/pressure problem that stops a combi recognising hot-water demand.

The combi-specific trap is worth spelling out: a combi boiler only fires for hot water when it senses a flow rate above a threshold at the tap. Anything that drops flow — a scaled aerator, a partially closed valve, low or fluctuating mains pressure, a tired flow switch — can leave the boiler flickering on and off, giving short bursts of hot water between cold. So "the boiler keeps cutting out on hot water" is frequently a flow problem, not a boiler fault. Work the decision tree, prove the cause, and avoid the expensive mistake of condemning a boiler that's actually being starved of flow.

Key Facts

Quick Reference Table

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Pattern Scope Likely cause
One shower swings hot/cold Single outlet Thermostatic cartridge failing/scaled; shower diverter
One mixer tap lukewarm/varying Single outlet TMV cartridge; blend setting; supply imbalance
All taps: short bursts of hot then cold Whole house (combi) Diverter valve sticking; flow below firing threshold; scale
Hot water only when heating off (or on) Whole house (combi) Diverter valve fault
Gradual loss of hot-water performance Whole house (combi) Plate heat exchanger scaled (hard water)
Shower cools when another tap opens Pressure-linked Flow/pressure fluctuation; needs pressure-balanced/thermostatic valve
Cylinder: runs out / variable Stored system Immersion thermostat, coil scaling, motorised/blending valve

Detailed Guidance

The master decision tree

INTERMITTENT HOT WATER
======================
Q1: Is it ONE outlet or ALL hot outlets?

ONE OUTLET ----------------------------------------------------
  Thermostatic shower/mixer?
     -> Suspect THERMOSTATIC CARTRIDGE (scaled/failed element)
        - Temp swings, lukewarm, won't reach set temp
        - Service/descale or replace cartridge
  Manual mixer / bath-shower diverter?
     -> Worn DIVERTER or non-return/check valves letting cold
        cross over -> replace diverter/checks
  Does it cool when another tap opens?
     -> FLOW/PRESSURE imbalance at that outlet (see below)

ALL OUTLETS (COMBI) -------------------------------------------
  Hot comes in short bursts then cold?
     -> Is flow at the tap healthy?
          LOW  -> FLOW PROBLEM (scaled aerators/HEX, low mains,
                  failing flow switch) - boiler won't stay fired
          OK   -> DIVERTER VALVE sticking / DHW thermistor / control
  Hot water only when heating is OFF (or only when ON)?
     -> DIVERTER VALVE fault (stuck mid-position)
  Performance faded over months in hard water?
     -> PLATE HEAT EXCHANGER scaled -> descale/replace

ALL OUTLETS (STORED CYLINDER) ---------------------------------
  Runs out quickly / variable temperature?
     -> Immersion thermostat, scaled coil, or motorised/
        blending valve fault

Thermostatic cartridge faults (single outlet)

A thermostatic shower or TMV holds a set temperature using a wax/thermostatic element that moves a valve to blend hot and cold. When that element scales up or fails, you get temperature swings, a shower that won't reach temperature, or sudden cold. Symptoms localised to one thermostatic outlet point straight here. Fixes:

Combi diverter valve (all outlets)

The diverter valve sends the boiler's heat to either the heating circuit or the DHW heat exchanger. When it sticks:

Scale and flow (all outlets)

In hard water, scale builds in the plate heat exchanger and at outlets, progressively reducing hot-water flow and performance. Combined with the combi's flow-activated firing, this produces classic intermittent hot water:

WHY FLOW MATTERS ON A COMBI
---------------------------
Tap opens -> flow sensor sees >= threshold -> boiler fires for DHW
If flow dips below threshold (scaled HEX/aerator, low mains, valve
part-closed, another tap opens) -> boiler STOPS firing -> cold slug
-> flow recovers -> fires again -> short bursts of hot then cold.

Fix the FLOW, not the boiler:
  [ ] Clean/replace tap aerators and shower head
  [ ] Check inlet isolation valves fully open
  [ ] Measure mains flow & pressure; check for fluctuation
  [ ] Descale or replace the plate heat exchanger
  [ ] Check/replace the flow switch/turbine if worn

Pressure-linked temperature swings

A shower that goes hot or cold when another tap or the toilet is used is a pressure-balance problem, not a heating fault. The cure is a thermostatic or pressure-balanced shower valve that compensates for supply fluctuations, plus addressing any underlying low/unbalanced supply (e.g. gravity vs mains-fed mix, undersized pipework, or a pump that needs attention).

Frequently Asked Questions

My shower goes hot and cold but the taps are fine — why?

If only the shower swings and the hot taps are steady, the fault is local to the shower — most often a thermostatic cartridge whose element has scaled or failed, a worn diverter/non-return valve letting cold cross into the hot, or a pressure imbalance when another outlet is used. Service or replace the cartridge, check the inlet filters and non-return valves, and consider a thermostatic/pressure-balanced valve if it cools when other taps open.

Why does my combi only give hot water when the heating is off?

That is a textbook diverter valve fault. The diverter is meant to switch the boiler's heat between the heating circuit and the hot-water heat exchanger; when it sticks mid-position you get hot water tied to the heating state — only off, or only on. The diverter cartridge or actuator needs servicing or replacement. It is a common combi wear item, not a reason to replace the boiler.

My hot water comes in short bursts then goes cold — is the boiler failing?

Often not. A combi fires for hot water only when it senses enough flow at the tap. If flow keeps dipping below the threshold — because of scaled aerators, a fouled plate heat exchanger, partly closed valves, low or fluctuating mains, or a tired flow switch — the boiler keeps cutting out, giving short bursts of hot then cold. Restore the flow first; condemn the boiler only after proving flow is healthy.

Could limescale cause intermittent hot water?

Yes, especially in hard-water areas. Scale builds in the combi's plate heat exchanger and at outlets, gradually reducing hot-water flow and performance until the boiler struggles to maintain firing. It typically shows as a slow decline over months ending in intermittent or weak hot water. Descaling or replacing the heat exchanger restores it; fitting a scale-reduction device helps prevent recurrence.

How do I know if it's the cartridge or the boiler?

Localise it: run every hot outlet. If the problem appears at one outlet only, it's local — the mixer/cartridge/diverter at that point. If it appears at all hot outlets, it's central — the combi diverter, scale, or flow/pressure. This single test stops you replacing a boiler part when the fault is a £20 cartridge, or descaling a tap when the diverter valve is sticking.

Regulations & Standards