Combi hot water that runs hot, then cold, then hot again is known as the cold water sandwich. A short slug of pre-warmed water reaches the tap first, then the cold water standing in the pipe, then steady hot water once the burner settles. A brief sandwich at the start of a draw-off is normal on many combis. Water swinging hot and cold right through a long shower points to flow rate, settings or a fault inside the boiler. This guide covers both patterns, and where a Gas Safe registered engineer has to take over.
Why does combi hot water go hot, then cold, then hot again?
The cold water sandwich starts with water that was already warm inside the boiler. Many combi boilers hold the plate heat exchanger warm between draw-offs. Worcester Bosch calls this pre-heat: the exchanger is warmed in intervals so it is already hot when you open a tap. That first warm water reaches the tap quickly.
Behind it sits the cold water that has been standing in the pipework since the last time you ran the tap. That cold section arrives next, which is the middle of the sandwich. Once the boiler is firing on a proper hot water demand, steady hot water follows.
The pattern is most obvious on short draw-offs. A quick hand wash, or a shower switched off and back on within a minute, gives you all three stages. A sandwich that is brief and repeatable is how the appliance behaves, not a breakdown.
Low flow rate makes a combi boiler cycle and swing in temperature
Very low flow can make a combi switch on and off during a long draw-off, and that shows at the tap as temperature swings. Worcester Bosch puts it plainly in its technical bulletin: “Very low flow rates for long water draw offs may cause combi boilers to cycle on and off, which will result in temperature fluctuations at the outlet.” The boiler needs a certain volume of water moving through it to hold a steady burn. Below that volume it lights, overshoots, shuts down and starts again.
Flow is not the same as pressure. A combi's hot water performance depends on dynamic pressure and flow together. Flow depends on the size, type and condition of the incoming mains pipe. Fitting flow restrictors or restricted fittings to share out the available flow is work for an engineer, not a household adjustment.
Another tap running can pinch the water feeding your combi
Opening a cold tap or filling a washing machine can starve a combi of flow, so the shower turns cool while the other outlet runs. On any mains-fed combi, the incoming main has to be big enough to feed the boiler while other cold outlets draw water. If that pipe is undersized, the boiler gets pinched whenever more than one outlet is open. Piping the boiler as the first branch off the incoming main gives it priority, and that is installation work for an engineer.
Hot outlets share the load as well. A combi raises water temperature by an amount that depends on the volume passing through it. Open two hot taps and both the flow and the temperature are divided between them. Our guide on running two showers from one combi covers what that means for a second bathroom.
What the ECO button does to the wait for hot water
The ECO button on the front of many combi boilers switches pre-heat on or off, which changes how long you wait for hot water. On a Worcester Bosch combi, ECO lit green means economy mode: pre-heat is off, the boiler starts cold each time, and delivery to the tap takes slightly longer. ECO not lit means comfort mode, with pre-heat on and a faster response to a hot water demand. With pre-heat active it is normal for the boiler to fire during the day with no tap open.
Switching pre-heat off tends to lengthen the cold part of the sandwich rather than remove it. Some combis let you programme hot water times, so pre-heat only runs at the hours you actually use. The ECO button sits on the outside of the casing, so pressing it is not work on the appliance. Check your own manufacturer's user instructions, because the button behaves differently across models.
What temperature should a combi's hot water be set to?
Around 50 °C is the hot water setting Vaillant recommends for a combi boiler, with 50–60 °C as the usual range. Water above 45 °C at the outlet can scald, so it has to be blended down with cold at the tap. Where the boiler is set above 65 °C, Vaillant advises a thermostatic mixing valve at the outlet — a valve that blends hot and cold to a safer temperature. Below about 30 °C the water simply feels cold as it comes out.
Hot water temperature is set separately from the heating temperature on a combi. Both controls sit on the outside of the boiler, so adjusting them is not work inside the appliance. A low hot water setting makes every drop in temperature more noticeable. Open a shower fully and the extra flow can make lukewarm water feel like a cold spell.
A sticking diverter valve or limescale is work for a Gas Safe registered engineer
A sticking diverter valve or limescale in the heat exchanger can leave combi hot water lukewarm or erratic. The diverter valve sits inside the boiler and switches flow between hot water at the taps and the heating circuit. Vaillant lists the symptoms of a faulty one: lukewarm water, hot water working but no heating, or hot water only while the heating runs. Vaillant and Viessmann both say the same about the repair — have a stuck valve freed by a qualified engineer and do not attempt it yourself.
Limescale is calcium carbonate from hard water. It forms faster on appliances that heat water. Inside pipework and a heat exchanger it narrows the passage and worsens heat transfer, so the hot water output suffers.
In England, the Gas Safety (Installation and Use) Regulations 1998 as amended, regulation 3, prohibit anyone from carrying out work on a gas fitting unless they are competent to do so. HSE states that anyone working on gas appliances in a home must be Gas Safe registered and competent for that type of work. The competencies are listed on the back of the engineer's ID card, and HSE explains how to check the register.
What you can safely check yourself before calling an engineer
Four checks are safe for a householder when combi hot water runs hot and cold. None of them involves opening the boiler. If there is no hot water at all rather than a swing, our guide on no hot water but the heating still works takes a different route.
- Boiler pressure: the gauge should sit between 1 and 2 bar. Too low or too high and the system shuts down — see low boiler pressure for what the gauge means.
- Thermostat and programmer: confirm the hot water and heating settings have not been changed or knocked.
- Reset button: manufacturers list a reset as one of the first checks when hot water is missing.
- Bleeding radiators: releasing trapped air from a radiator that is cold at the bottom of the system is a normal household task.
Everything behind the casing is different. The diverter valve, the plate heat exchanger, the flue, seals and gas pipework are work for a Gas Safe registered engineer, never a household step.
Where a combi cannot keep up with two outlets at once, stored hot water is the other route, and you can arrange a free survey and a written fixed price for a Megaflo or other unvented cylinder before deciding.
Sources
- Worcester Bosch – Technical Bulletin TB 0123 A
- Worcester Bosch – How could mains water pressure affect my combi boiler?
- Worcester Bosch – Eco mode and Pre Heat mode
- Vaillant – What temperature should my combi boiler be set at?
- Vaillant – How to fix a boiler diverter valve
- Viessmann – How to prevent limescale in boilers
- Ideal Heating – Why can't I get hot water?
- legislation.gov.uk – Gas Safety (Installation and Use) Regulations 1998, regulation 3
- HSE – Gas safety: home owners