Hot water that runs lukewarm while the heating still works points at one of two things. A diverter valve or plate heat exchanger fault is likely if the change came on suddenly and the taps stay tepid whatever you set. Limescale is likely if you are in a hard water area, the hot flow has faded over months, and the boiler rumbles or kettles.
Diverter valve or plate heat exchanger against limescale: how the two patterns differ
Lukewarm hot water from a combi boiler has two common causes, and they behave very differently. A diverter valve or plate heat exchanger problem changes the hot water quickly, while limescale takes months or years to show.
| Criterion | Diverter valve or plate heat exchanger | Limescale from hard water |
|---|---|---|
| How it came on | Suddenly, or over a few days, with a clear before and after | Slowly over months or years, so the fade goes unnoticed |
| What the heating does | Heating may fire when you only asked for hot water, or radiators stay cold | Heating and hot water lose output together |
| Noise from the boiler | Usually quiet, though some valves click as they move | Kettling, a rumbling or boiling sound from trapped overheated water |
| Where it happens | Any area, because water hardness makes no difference to a failed valve or plate | Hard water areas, where calcium carbonate settles on hot surfaces |
| What helps afterwards | Annual servicing by a Gas Safe registered engineer | Scale protection on the cold mains, fitted to the maker's requirements |
A diverter valve is the part inside a combi that sends heated water either to the radiators or to the taps and shower. A plate heat exchanger is the stack of thin plates that passes that heat into fresh mains water on its way to the tap. Limescale is calcium carbonate left behind by hard water, and it settles on the hottest surfaces first. All three sit behind the boiler casing, so the diagnosis is work for a Gas Safe registered engineer.
What you can check yourself when combi hot water runs lukewarm
Four things are safely within reach of a householder, and the hot water setting is the one worth looking at first. A combi has separate controls for central heating and for hot water, and the hot water dial can be knocked down without anyone noticing.
- The hot water setting on the boiler. Around 50C is the usual manufacturer recommendation for a combi, within a 50 to 60C band.
- The pressure gauge on the front. The needle usually sits near 1 bar when the boiler is cold on a 0 to 4 bar scale, and roughly 1 to 2 bar when the heating is running.
- Programmer and room thermostat settings. A timer that has slipped after a power cut can leave the hot water behaving oddly.
- The reset button. If a fault code is showing, your manual says where the reset is and how to use it, holding it for no longer than ten seconds.
- Bleeding radiators. Trapped air explains a cold radiator rather than a tepid tap, and the system pressure drops slightly afterwards.
Everything beyond those settings is work for a Gas Safe registered engineer. The isolation valve and filter on the cold feed, the diverter valve and the plate heat exchanger all sit inside or behind the casing. If the hot taps run properly cold rather than tepid, the guide on no hot water but the heating still works covers that different pattern.
Why combi hot water feels lukewarm in winter with nothing actually broken
Incoming mains water is colder in winter, so the boiler has to lift it further in the same few seconds. A combi delivers a temperature rise that depends on how fast the water passes through it. Open the tap wide and the rise falls, which is why the same shower can feel fine in August and tepid in January. Worcester Bosch user instructions advise reducing the flow at the tap or shower when the mains is very cold, so the water has longer in the heat exchanger.
Running two outlets at once splits the flow and the temperature between them. The kilowatt rating of the boiler sets how many litres a minute it can supply at a stated temperature rise, which is why an undersized combi feels weak in a busy house. Our guide on whether a combi boiler can run two showers at once works through the same flow limit. A tap that runs hot, then cold, then hot again is a different symptom, set out in combi boiler hot water goes hot then cold.
When the diverter valve or plate heat exchanger is the likely cause of lukewarm water
Suspect the diverter valve or the plate heat exchanger when lukewarm hot water arrived suddenly and the temperature barely responds to the controls. Viessmann names tepid water at the taps with the heating still working as one of the clearest signs of a failing diverter valve. The valve sticks part way and lets some of the heated water carry on round the heating circuit instead of reaching the tap.
Two further patterns point the same way. Hot water that only appears once the radiators have fired is one, and radiators that stay cold while the taps work is the other, covered in hot water works but the radiators stay cold. Vaillant describes diverter valve faults as common, particularly on boilers of around ten years and older. A blocked or scaled plate heat exchanger produces a quieter version of the same complaint, because less heat crosses into the mains water. The engineer tests the valve and the plate heat exchanger against the manufacturer's service instructions before anything is replaced.
When limescale in a hard water area is the likely cause of lukewarm water
Suspect limescale when you live in a hard water area and the hot water has weakened gradually rather than overnight. Hard water carries calcium and magnesium, and calcium carbonate drops out onto the hottest surfaces first because hot water evaporates faster. Inside a heat exchanger that deposit narrows the waterways and insulates the metal, so less heat reaches the water going to your tap. Viessmann cites a figure from British Water of up to 12 per cent lost boiler efficiency from scale.
Kettling is the giveaway noise. Scale traps small pockets of water that overheat and flash to steam, which you hear as banging, rumbling or boiling from the boiler. Protection is fitted on the cold mains where it enters the house, using a scale reducer or a softener chosen to the boiler manufacturer's requirements. Descaling or replacing a scaled exchanger means opening the appliance, which is engineer's work and never a householder task.
What temperature should combi hot water be set to, and when is it too hot?
Around 50C is the usual recommendation for the hot water setting on a combi, inside a 50 to 60C range. Vaillant warns that anything above 45C at the tap itself can scald, so the outlet should be mixed with cold. The heating side is a separate setting, normally 60 to 70C on a condensing boiler, and pushing it higher takes the appliance out of condensing operation.
HSE puts the scald risk at outlet temperatures above 44C. At 55C about twelve seconds is enough for a second degree burn, while at 48C it takes around ten minutes. HSE guidance is written for employers and care settings rather than private homes, but it also expects bath and bidet outlets not to exceed 48C, and a type 3 thermostatic mixing valve limited to 44C where vulnerable people have access and the risk is significant. Our guide on a thermostatic mixing valve on a bath and the 48C limit explains where that applies.
Who may work on a combi diverter valve or heat exchanger in England
In England the Gas Safety (Installation and Use) Regulations 1998, as amended, allow work on a gas fitting only by a person competent to do it. HSE states that in homes and workplaces this means a Gas Safe registered engineer who is qualified for that particular work, and that it is illegal for anyone else. The duty covers the appliance, its flue and the pipework, not just the burner.
HSE also advises having every gas appliance, flue and pipe checked at least once a year by a Gas Safe registered engineer. Manufacturer warranties usually carry the same condition, requiring an annual service to keep the cover in place. A service is the point at which weakening hot water, early scale and a sticking valve tend to be picked up.
Diverter valve, plate heat exchanger or scale: which to act on
Treat it as a diverter valve or plate heat exchanger problem if the hot water went tepid suddenly, the heating fires when you only asked for hot water, and the boiler is around ten years old or more. Treat it as limescale if you are in a hard water area, the hot flow has faded over months and the boiler kettles. Treat it as flow and seasonality if the hot water is only weak in cold weather, with the tap wide open or a second outlet running.
Check the hot water setting, the pressure gauge, the programmer and thermostat, the reset button and your radiators, then note any fault code on the display for the engineer. The parts behind the casing are not a householder job in any of the three cases. Where a boiler is beyond sensible repair, a replacement starts with a free home survey and a written fixed price, so nothing is agreed on a guess over the phone. If the property is let, the annual gas safety check is a separate duty from any repair, and our landlord gas safety certificate (CP12) page sets out how that is arranged.
Sources
- Viessmann — How to fix a diverter valve
- Vaillant — How to fix a boiler diverter valve
- Viessmann — How to prevent limescale in boilers
- Vaillant — What temperature should my combi boiler be set at?
- HSE — Scalding and burning
- The Gas Safety (Installation and Use) Regulations 1998
- HSE — Gas safety: home owners
- Worcester Bosch — Users Instructions, Greenstar HE combi