You step into the shower, it’s lovely and warm, then it runs cold, then hot again. Or the kitchen tap won’t settle on a steady temperature, even though the radiators are working fine. Combi hot water going hot and cold is frustrating, and the cause isn’t always inside the boiler. It can be the boiler, the water supply, or the shower or tap you’re using.
So where do you start? With the pattern. Whether it’s one outlet or every hot tap in the house, and what else is using water at the time, tells us a lot before anyone picks up a spanner.
- A combi heats water as it flows, so changes in flow can change the temperature.
- If only one shower or tap is affected, the problem may be local to that outlet.
- If every hot tap fluctuates, the boiler or the supply is more likely to be involved.
- Note what else was running, and report any fault code exactly as shown.
- If the water suddenly runs very hot, step away and turn it off.
How a combi heats your water
A combi boiler doesn’t store hot water in a cylinder. It heats water as it passes through, and only while a hot tap is open. To do that, it has to recognise that water is flowing and then fire up to match the demand.
That’s why flow matters as much as the temperature setting. If the flow through the boiler drops too low, it may not recognise the demand properly, and the burner can cut in and out. You feel that at the tap as a swing between hot and lukewarm.
Every combi also has a limit to how much hot water it can heat at once. Run a shower and a hot kitchen tap together and each gets a share of that output. If the temperature changes the moment someone else turns on a tap, that timing is a useful clue.
One tap or every tap?
This is the first thing we’ll ask. If the kitchen and bathroom hot taps stay steady but the shower keeps changing, the shower valve, the head, or the hose deserves a closer look before anyone blames the boiler.
If several hot taps are affected during normal use, that points more towards the boiler or the water supply to it. Tell us which outlets you’ve tried and whether the problem happens every time or only now and then.
It also helps to separate a short wait for hot water from repeated swings. Cold water sitting in the pipe has to clear before hot water arrives, especially at a tap a long way from the boiler. That’s normal. Hot water that arrives, then goes cold, then comes back is a different pattern.
Our plumbing services cover showers and taps as well as pipework, so a problem at one outlet doesn’t have to wait for a boiler engineer. Tell us where it happens and we’ll plan the right visit, rather than assuming every hot water complaint starts inside the boiler.
Other water use in the house
A toilet refilling, a washing machine taking water, or someone running a tap downstairs can all change the conditions at your shower. Appliances start and stop on their own, so you might not connect the two straight away.
Mira’s guidance on mixer showers fluctuating between hot and cold lists other outlets being used at the same time, and low flow, among the possible causes. That’s a reason to check the pattern, not a reason to put up with a shower that isn’t comfortable or safe to use.
Please don’t stand under the shower to test how far the temperature swings. If the water suddenly runs very hot, step away, turn it off, and mention that when you contact us.
Showers and combis
Not every shower suits a combi. If your home switched from a boiler and hot water cylinder to a combi at some point, the old shower may have been designed for a different type of supply. It can still be connected, but that doesn’t mean it’s the right match.
Worcester Bosch’s shower compatibility guidance says an existing shower should be checked for suitability when a regular boiler and cylinder are replaced with a combi. Tell us about any conversion you know of, even if it happened before you moved in.
A new showerhead, hose, or flow restrictor can change how the shower behaves too. If the swings started after something was changed, keep the product details. Clean the head only as the manufacturer’s instructions allow, and don’t take the shower valve apart to look inside.
When the boiler is the cause
If the fault affects all your hot taps, the boiler itself may need checking. A sticking diverter valve, a sensor that isn’t reading flow or temperature correctly, or a heat exchanger restricted by scale or debris are among the things an engineer may look at. The only way to know is to test it while the problem is happening.
It also helps to know the boiler’s make and model, roughly how old it is, and when it was last serviced. You’ll usually find the model on a label on the front or underneath, and the service date on your last paperwork. Please don’t remove any panels to look for it.
Anything inside the boiler casing is a job for a Gas Safe registered engineer. Our boiler repair service covers faults affecting hot water as well as heating, and the work needed depends on what we find.
Please don’t keep resetting the boiler to clear a recurring fault, and don’t top up the heating pressure because the shower feels weak. The pressure gauge on a combi shows the heating circuit, not the pressure at your taps.
What to note before you call
A few details make the first visit far more useful:
- Which outlets are affected. One shower, one tap, or every hot tap in the house.
- When it happens. Every time, at certain times of day, or only when another tap or appliance is running.
- What it feels like. A short wait for hot water, repeated swings, or water that never gets properly hot.
- Any fault code. Report it exactly as it appears, and take a photo of the display if you safely can.
- Recent changes. A new shower, showerhead, or boiler, or any settings you’ve already changed.
For a wider list of what helps an engineer before any boiler visit, our guide to boiler repair preparation covers the basics. For this problem, the outlet pattern and the timing are the details that matter most.
Hot and cold water questions
Does hot and cold water mean I need a new boiler?
Not on its own. The cause could be a shower or tap, the water supply, or a boiler fault that can be repaired. A new boiler should only come up after a proper look at the actual problem, not because of one symptom.
Why does my shower go cold when someone uses the kitchen tap?
Your combi has a limit to how much hot water it can heat at once, and opening a second tap changes the flow to the shower. Some shower valves also handle changes in supply better than others. If it happens every time, tell us, as it helps narrow down the cause.
Can an electric shower be affected by my combi?
An electric shower heats its own cold water supply, so it isn’t fed with hot water from the combi in the usual way. If you’re not sure which type you have, share the make and model rather than opening the unit to find out.
Should I turn the hot water temperature up to stop it going cold?
We’d advise against it. A higher setting doesn’t fix the cause, and it can make a sudden swing to hot more uncomfortable or risky. Leave the settings as they are and tell us what you’ve already changed.
Why is it worse first thing in the morning?
It might be that more water is being used at once, or that the problem shows up when the system has been sitting cold. Note what else is running at that time and whether the same tap behaves normally later in the day.
Could limescale be causing it?
It can play a part. Scale can build up in showerheads and, over time, inside parts of a boiler, which can affect flow and heating. Whether it’s the cause in your home needs checking, so mention any scale you can see on your showerhead or taps when you get in touch.
Will a boiler service fix it?
A service and a repair visit have different jobs, even though some checks overlap. When you book, tell us about the hot and cold problem so we can arrange the right visit rather than assuming a service covers every fault.
Book a combi check with us
If your hot water keeps swinging between hot and cold in Salford, Swinton, Worsley, Pendlebury, Seedley, Prestwich, or Kersal, contact our team with a note of which taps are affected, call 07960 042563, or email info@manchestergasandheating.co.uk.