Combi Boiler Hot Water Goes Hot and Cold: Diagnostic Guide

Summary

"The shower keeps going cold and then scalding" is one of the most common combi callouts, and one of the most frequently misdiagnosed. The temptation is to condemn the boiler. In practice the boiler is usually doing exactly what it is designed to do — protecting itself from overheating — and the real fault is upstream (not enough water being drawn) or downstream (a scaled exchanger, a tired sensor, a mixer valve that cannot cope).

A combi makes hot water instantaneously: mains water passes through a plate heat exchanger heated by the primary circuit, and the burner modulates to hold the outlet at the set DHW temperature. That only works inside a band of flow rates. Draw too little water and even the boiler's minimum output heats it beyond the DHW limit, so the burner shuts off, the water cools, the burner relights, and you get the sawtooth the customer describes. Draw water through a scaled exchanger and the heat transfer is erratic, with the same result. If the flow turbine or diverter is marginal, the boiler loses "sight" of the hot water demand altogether, drops back to heating or idle, and then catches it again.

This guide gives a symptom-led route through the fault: what to measure, in what order, what each pattern means, and what to quote. It also covers the two things that mimic a boiler fault entirely — a thermostatic shower valve hunting on pressure changes, and a flow rate that is simply too low for the installed boiler — because fixing those costs the customer far less than a plate exchanger.

Key Facts

• Cycling pattern tells you the cause — slow regular hot/cold waves of 20–60 seconds point to the boiler cycling on its DHW limit; rapid fluttering points to a flow sensor or diverter; a sudden one-off cold slug when another tap opens is pressure-driven and points to the mixer valve
• Minimum DHW activation flow is typically 2.5–3 l/min on most domestic combis — below this the turbine will not reliably hold the hot water demand
• Minimum modulation is the key sizing figure: a 30 kW combi that modulates down to 7–9 kW still puts ~2.5–3 kW into every litre per minute at a trickle, so a 3 l/min basin draw will overshoot the DHW limit and cycle
• DHW high-limit on most combis is around 60–65 °C at the plate exchanger outlet; a setpoint of 45–55 °C leaves a margin and reduces cycling on low draws
• Plate heat exchanger scaling is the leading cause in hard-water areas (above ~200 mg/l CaCO₃, covering much of southern and eastern England); symptoms escalate from slow temperature response to oscillation to a weak, lukewarm flow
• Mains flow and pressure must be checked first — a combi cannot deliver more than the incoming main supplies, and low or fluctuating mains pressure produces temperature swings that are not the boiler's fault (see low mains water pressure)
• Thermostatic mixer valves on a mains-pressure system should be to BS EN 1111; a low-pressure valve (BS EN 1287) on a combi, or a tired thermostatic cartridge, hunts and produces exactly this symptom (see shower mixer valve temperature fluctuating)
• Flow restrictors fitted at the boiler DHW inlet or in the shower hose are a legitimate fix for a trickle draw-off that cycles the boiler — they raise the draw to a stable band, not lower it
• Water Supply (Water Fittings) Regulations 1999 govern fittings and backflow protection on anything you add to the cold main; Approved Document G3(4) requires a 48 °C limit at bath outlets in new dwellings
• Gas Safety (Installation and Use) Regulations 1998 — anything behind the boiler case (plate exchanger, diverter, flow sensor, PCB) is Gas Safe registered work
• Repair pricing — plate heat exchanger clean/descale £120–£250; replacement plate exchanger £180–£400 fitted; DHW flow sensor/turbine £90–£180 fitted; diverter valve or actuator £150–£350 fitted ****

Quick Reference Table

Detailed Guidance

Step 1 — establish the flow rate before anything else

Nothing else in this diagnosis means anything until you know how much water the customer is actually drawing. Take a 10-litre bucket and a watch to the problem outlet (usually the shower) and measure litres per minute at the setting the customer normally uses. Then repeat at the bath tap fully open.

Interpretation:

• Below about 3 l/min — you are at or below the activation flow of most combis. The turbine may be dropping out, and even if it holds, the boiler cannot modulate low enough to avoid overshooting. This is a flow problem, not a boiler fault. Look for eco shower heads, aerators, flow regulators in the hose, kinked flexible hoses, part-closed service valves, scaled shower head.
• 3–6 l/min — marginal on many 28–35 kW combis. Expect cycling on the DHW limit when the incoming mains is warm (summer) because the burner cannot go low enough. A flow restrictor at the boiler's DHW inlet (many manufacturers fit one as standard — check it is present and the right size) or a lower DHW setpoint usually settles it.
• Above 8 l/min with the fault still present — the boiler is well inside its working band. Now it is legitimately a heat exchanger, sensor or valve problem.

Also check the mains itself: static pressure with a gauge on an outside tap (1–3 bar is typical; see water pressure flow rate) and dynamic pressure with a second outlet open. A main that drops hard when a second outlet opens produces pressure-driven cold slugs that no boiler component will cure.

Step 2 — measure temperature at the boiler, not the tap

Run the hot tap nearest the boiler at a steady, generous flow (bath tap, 8–10 l/min) and put a probe thermometer on the water, or read the boiler's own DHW outlet temperature on the display or service menu.

• Boiler holds a steady temperature within ±2–3 °C of setpoint at the bath tap, but the shower swings — the boiler is fine; the fault is the shower valve or the shower's flow rate. Go to Step 5.
• Boiler outlet itself swings in slow waves — it is cycling. Check whether the burner is cutting out (listen, or watch the flame indicator). If the burner shuts off at the top of each wave, it is hitting the DHW high-limit: either flow is too low (Step 1) or the plate exchanger is scaled so the thermistor sees a spike (Step 3).
• Boiler outlet is sluggish — takes a long time to come up, never quite reaches setpoint, then overshoots — classic scaled plate exchanger.
• Boiler drops the DHW demand entirely (display flicks back to standby or heating mid-draw) — flow sensor or diverter (Step 4).

Step 3 — the plate heat exchanger

The DHW plate heat exchanger is a stack of thin stainless plates with primary water on one side and mains water on the other. In a hard-water area it scales on the mains side. Scale is an insulator, so the primary side runs hotter for the same DHW output, the thermistor sees erratic temperatures, and the control system over- and under-shoots. End-stage symptoms are a hot water flow that is both weak and lukewarm, sometimes with a kettling noise on demand.

Confirming it:

• Hard-water postcode (most of the Thames basin, East Anglia, Lincolnshire, Yorkshire Wolds, the Midlands chalk/limestone belts).
• Boiler age over 5–7 years with no scale reducer on the cold inlet.
• Temperature response slow and oscillating at every flow rate, including a full bath tap.
• Flow rate from the hot tap noticeably lower than from the cold at the same outlet (scale narrowing the DHW passages).

Fixes: on many boilers the plate exchanger can be removed and descaled in a proprietary descaler, or back-flushed in situ with a descaling pump. If plates are heavily blocked or the exchanger is leaking between circuits (symptom: system pressure creeping up on its own — see boiler loses pressure repeatedly for the opposite), replace it. Fit an inline scale reducer (electrolytic or polyphosphate) on the cold inlet to the boiler afterwards; Approved Document L (England) expects scale-prevention measures where hardness exceeds 200 mg/l on new combi installations, and it is good practice on any repair. All of this is behind the case and is Gas Safe registered work.

Step 4 — flow sensor and diverter valve

The DHW flow sensor (turbine or paddle switch, depending on the make) tells the PCB a hot tap is open. A scaled or sticking turbine gives an intermittent signal — the boiler sees demand, fires, loses the signal, drops out, sees it again. That gives a rapid flutter rather than slow waves, and the boiler's own DHW demand indicator will be flicking on and off while the tap is running steadily. Cleaning the turbine sometimes works; replacement is usually the honest repair and is a low-cost part.

The diverter valve switches the primary circuit between radiators and the plate exchanger. A diverter that moves to DHW but does not hold there (worn diaphragm, weak actuator, debris) lets primary flow leak back to the heating side, so the plate exchanger is starved and the hot water fades. Tell-tales: radiators warming up during a long hot water draw in summer; hot water fine for the first minute then fading; the opposite fault (no hot water at all) covered in no hot water but heating works. On some boilers the diaphragm is a service item; on others the whole valve is replaced.

Step 5 — the shower valve and pressure balance

If the boiler holds steady at a bath tap but the shower swings, you are looking at the valve. Three common causes:

1. Wrong valve type. A low-pressure thermostatic valve (BS EN 1287) fitted to a mains-pressure combi system hunts because it cannot control the higher, more variable pressure. The correct valve is to BS EN 1111 (high pressure). Check the valve's marking or data sheet.
2. Tired thermostatic cartridge. Wax-element cartridges lose their response over 5–10 years, especially in hard water. The valve overshoots in both directions. A cartridge swap is a 30–60 minute job.
3. Pressure imbalance. When a WC refills or a washing machine fills, cold pressure at the valve drops, the thermostat tries to compensate, and by the time the cold recovers the user has a hot slug. A good thermostatic valve copes with modest swings; a marginal main or an undersized cold feed does not. Separate dedicated 15 mm feeds to the shower from the first available point help; a pressure-reducing valve on the incoming main stabilises a high, fluctuating supply.

A non-thermostatic manual mixer on a combi will always do this to some degree, because a combi's outlet temperature moves with flow. The fix there is a thermostatic valve, not boiler work. For new-build or where G3(4) applies, a TMV limiting bath outlets to 48 °C is required in any case.

Step 6 — when it really is the boiler

Patterns that do point inside the boiler:

• Fluctuation accompanied by a fault code (DHW thermistor open/short, flow sensor fault) — read the code history and follow the brand diagnosis (see boiler error codes vokera and the brand guides linked below).
• DHW thermistor drifting — the boiler reads a wrong temperature and controls to it. Resistance-check the NTC against the manufacturer's table.
• Pump running slow or primary circuit partly airlocked, so the plate exchanger is not getting enough primary flow — radiators also under-performing, boiler kettling (see boiler kettling).
• PCB fault — intermittent control of the gas valve or fan; usually other symptoms present too.

Diagnostic decision tree

Hot water goes hot / cold / hot on a combi
                 |
                 v
   Bucket-test flow at problem outlet
                 |
      +----------+----------+
      |                     |
  < 3 l/min             >= 3 l/min
      |                     |
 Flow too low for      Measure temperature at bath tap
 boiler minimum.       (steady 8-10 l/min) and boiler
 Remove restrictions,  display DHW outlet
 lower DHW setpoint,            |
 fit correct inlet     +--------+---------+
 restrictor            |                  |
                  Steady at bath,    Boiler outlet itself
                  swings at shower   swings or sluggish
                       |                  |
                  Shower valve:     +-----+------+
                  type (EN 1111?),  |            |
                  cartridge,     Slow waves,  Demand drops
                  pressure       burner cuts  in/out, flutter
                  balance        at top of        |
                                 wave         Flow sensor /
                                    |         turbine; then
                               Scaled plate   diverter not
                               exchanger /    holding DHW
                               DHW limit --
                               descale or
                               replace, fit
                               scale reducer

Frequently Asked Questions

Why does the shower go cold when someone flushes the toilet?

Because the cold supply pressure at the shower valve drops while the cistern refills, so the mix goes hot, and then overshoots cold when it recovers. A thermostatic valve to BS EN 1111 should absorb a modest drop; if it does not, the valve is tired or the wrong type, or the cold main is marginal. Dedicated 15 mm feeds to the shower and a pressure-reducing valve on a high, fluctuating main both help. This is not a boiler fault and you should not be quoting boiler parts for it.

Will a bigger combi stop the hot water fluctuating?

Usually the opposite. A bigger boiler has a higher minimum output, so on a trickle draw it overshoots the DHW limit even sooner. The figure that matters for low-flow stability is the minimum modulation, not the maximum output. A customer who mainly uses an eco shower head and a basin tap is better served by a boiler with a low minimum output (some modulate down to 3–4 kW) than by a 40 kW unit. For sizing arithmetic see combi boiler flow rate explained.

Is turning the hot water temperature down a real fix or a bodge?

A real fix in the right case. Lowering the DHW setpoint from 60 °C to 45–50 °C widens the gap to the high-limit and stops the boiler cycling on low draws, and on a combi there is no stored water so the Legionella argument for 60 °C storage does not apply. The customer mixes in less cold at the tap, so the usable flow of hot water actually improves. It does not fix a scaled exchanger or a failing sensor, so confirm the pattern first.

How do I tell a scaled plate heat exchanger from a faulty flow sensor without opening the boiler?

Pattern and speed. A scaled exchanger gives slow, smooth temperature waves and a hot flow that is weaker than the cold from the same outlet, at every flow rate, and the boiler stays "in DHW" throughout. A flow sensor fault gives a rapid flutter and the boiler visibly loses and regains the hot water demand (display changes, burner cuts in and out abruptly) while the tap runs steadily. Hard-water postcode and boiler age push you towards scale. Only a Gas Safe engineer should confirm by opening the case.

How much should I quote?

Diagnosis visit £60–£120. Then by outcome: shower cartridge £90–£180 fitted; new thermostatic bar valve to BS EN 1111 £150–£300 fitted; plate exchanger descale £120–£250; plate exchanger replacement £180–£400 fitted; flow turbine £90–£180 fitted; diverter valve/actuator £150–£350 fitted; inline scale reducer £80–£180 fitted. **** — quote the diagnosis separately and explain that the boiler may not be at fault, because the customer has often already been told it is.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — all work behind the boiler case (plate exchanger, diverter, sensors, PCB) must be carried out by a Gas Safe registered engineer

• Water Supply (Water Fittings) Regulations 1999 — fittings, backflow protection and materials for anything added to the cold main (scale reducers, PRVs, check valves)

• Building Regulations Approved Document G (England), Part G3 — hot water safety; G3(4) requires bath outlet temperature limited to 48 °C in new dwellings

• Building Regulations Approved Document L (England) — scale-prevention measures expected where water hardness exceeds 200 mg/l on new combi installations; Domestic Building Services Compliance Guide

• BS EN 1111:2017 — thermostatic mixing valves for high-pressure systems (combis and unvented cylinders)

• BS EN 1287:2017 — thermostatic mixing valves for low-pressure (gravity) systems — not suitable on a combi

• BS EN 806 (Parts 1–5) and BS 8558:2015 — design and installation of water supply installations inside buildings

• BS 7593:2019 — treatment of water in domestic hot water central heating systems (primary-side cleaning and inhibition when a plate exchanger is changed)

• Benchmark Commissioning Checklist — DHW flow rate and temperature at commissioning should be recorded; a missing or blank Benchmark is a useful clue to a boiler that was never set up for the property's flow

• Gas Safe Register — statutory register for gas engineers in Great Britain

• HSE — Gas Safety — statutory guidance on gas appliance safety

• GOV.UK — Approved Document G: Sanitation, hot water safety and water efficiency — G3 hot water safety including the 48 °C bath outlet limit

• WRAS — Water Regulations Approval Scheme — approved fittings and Water Fittings Regulations guidance

• Heating and Hotwater Industry Council (HHIC) — Benchmark scheme and combi commissioning guidance

• BEAMA — TMV2 scheme — thermostatic mixing valve approval scheme referencing BS EN 1111 and BS EN 1287

• no hot water but heating works — the diverter valve fault in its complete form

• shower mixer valve temperature fluctuating — thermostatic valve diagnosis in detail

• shower runs cold — shower-specific cold-water faults

• low mains water pressure — when the incoming main is the problem

• combi boiler flow rate explained — flow rate arithmetic, ΔT and sizing for hot water

• water pressure flow rate — measuring pressure and flow properly

• boiler kettling — primary-side scale and circulation faults

• system cleansing inhibitors — cleaning and inhibiting after component replacement