If the hot water from your combi boiler is coming out lukewarm, the cause is usually one of a few simple things: water running too fast through the boiler, a temperature setting turned down, low system pressure, or limescale building up inside. It is often worse in winter, because the cold water coming into your home is colder and the boiler has more work to do to heat it.
This post explains how a combi heats your water, the checks you can safely make at home, and the signs that mean it is time for a Gas Safe registered engineer. You can look at dials, taps and the pressure gauge yourself, but never remove the boiler casing or touch any gas parts.
How a combi boiler heats your hot water
A combi (combination) boiler has no hot water tank. When you turn on a hot tap, cold mains water flows through a small heat exchanger inside the boiler, which is a set of thin plates or tubes that pass heat from the burner into the water. The water is heated as it passes through, on its way to the tap.
So a combi can only add so much heat to each litre. The faster the water flows, the less time it spends in the heat exchanger and the cooler it comes out. The colder the water going in, the cooler it comes out too.
Why it feels worse in winter
In summer, the water in the mains is fairly mild. In November and through the winter, it can be much colder. Your combi still raises the temperature by roughly the same amount, so the water reaching your tap or shower ends up cooler than it was a few months ago.
This is normal. Slowing the flow a little usually gives the boiler time to heat the water properly. If the water is barely warm even at a gentle flow, something else is going on.
Safe checks you can make at home
Work through these in order. None involve opening the boiler.
- Check the hot water temperature setting. Most combis have a dial or button for hot water, separate from the heating. It may have been turned down by accident. Turn it up a step and run a tap for a minute.
- Run one hot tap at a time. A combi can only supply a certain amount of hot water at once. If someone is running a bath while you shower, both will be cooler.
- Turn the tap down slightly. If the water gets hotter when the tap is only half open, the boiler is working but the flow is more than it can heat.
- Check the boiler pressure. Look at the gauge on the front. On most homes it should read around 1 to 1.5 bar when the system is cold. If it is low, our guide on how to top up boiler pressure safely talks you through it.
- Check every hot tap, not just one. If only the shower is lukewarm, the problem may be the shower's own mixing valve, which blends hot and cold water, rather than the boiler.
- Look for a fault code. If the display shows a code or a flashing light, write it down. It will help an engineer find the problem faster.
Limescale and hard water
Limescale is the chalky deposit left behind when hard water is heated. Hard water is water with a lot of dissolved calcium in it. SES Water, which supplies much of east Surrey from chalk and greensand sources, says many of its water sources are naturally hard.
Over time, limescale can coat the inside of a combi's hot water heat exchanger. This acts like a layer of insulation, so less heat reaches the water. Signs include hot water that slowly gets cooler over months, or a boiler that gets noisier when a hot tap is on.
You cannot clear limescale inside the boiler yourself. An engineer can check the heat exchanger and advise whether it needs cleaning or replacing, and whether a scale reducer would help.
Other causes an engineer will look for
If the simple checks do not help, the cause is usually inside the boiler. Common ones include:
- A sticking diverter valve. This valve switches the boiler between heating and hot water. If it sticks part way, some of the heat goes to your radiators instead of your tap.
- A faulty flow sensor. This tells the boiler a hot tap is on. If it misreads, the boiler may not fire properly.
- A burner or gas supply problem. If the boiler cannot burn enough gas, it cannot heat the water fully. This must only ever be checked by a Gas Safe registered engineer.
- Low mains water flow. Worcester Bosch points out that a combi needs an adequate flow from the incoming main, and that the flow depends on the size, type and condition of the pipe.
When to call an engineer
Call a Gas Safe registered engineer if the water stays lukewarm after the checks above, if it keeps swinging between hot and cold, if a fault code keeps coming back, or if the boiler is making new noises. Stop using the boiler and call straight away if you smell gas or your carbon monoxide alarm sounds. For a suspected gas leak, call the National Gas Emergency Service on 0800 111 999.
An annual boiler service is a good chance to catch problems like a scaled-up heat exchanger before they leave you with cold showers.
If your combi simply cannot keep up
Sometimes the boiler works fine but is the wrong fit for the home. A busy household with two bathrooms may need more hot water at once than a combi can give. In that case, a boiler with a stored hot water cylinder, such as an unvented cylinder, can supply several taps at full temperature. If your boiler is older and the repairs are adding up, it may be time to look at a new gas boiler that suits how your home uses hot water.
If your hot water has gone lukewarm and the simple checks have not helped, BCU Plumbing can take a look. Our Gas Safe registered engineers work across Godstone and nearby towns, and will find the cause and explain your options clearly. Book a boiler service or get in touch to arrange a visit.
