Does your combi boiler run cold after a few minutes? Here we explain the most common causes and fixes.
Does your shower start lovely and hot, then turn freezing just as you’ve lathered up? Annoying.
The good news is there are some common culprits - and most can be sorted quickly by a qualified engineer.
Let’s begin!
PS - we fit new combi boilers with gas safe installation nationwide. Simply answer these questions, get your fixed price and you could get it installed as quick as tomorrow.
Why is your combi boiler running cold?
If your shower starts out piping hot but turns icy after a few minutes, you’re not alone. It’s one of the most common combi boiler complaints.
The good news? The causes are usually straightforward. Here’s what might be going on behind the scenes:
1. Faulty Diverter Valve
The diverter valve is the traffic controller of your combi boiler. Its job is to switch heat between your radiators and your hot water taps.
When you run a bath or jump in the shower, it should flick over to “hot water mode.”
If it sticks or starts to fail, the boiler gets confused and may divert heat back into your central heating instead.
Result? Your hot shower suddenly goes lukewarm, then cold - even though the heating might still be working perfectly.
Typical fix: Engineers often clean, repair, or replace the diverter valve. Costs usually fall between £90–£350 depending on whether it’s repair or replacement.
2. Clogged Plate Heat Exchanger
Inside your combi sits a clever little part called the plate heat exchanger. Its job is to transfer heat from the boiler’s system water to the fresh tap water coming through.
Over time, sludge (from your central heating) or limescale (especially if you’re in a hard water area) can clog the exchanger.
When this happens, it can’t transfer enough heat quickly, so your hot water temperature drops off a cliff after a few minutes.
Telltale signs: Water that runs hot, then cold, then maybe hot again… or showers that never stay stable.
Typical fix: Engineers can flush or chemically clean it. In bad cases, it needs replacing. Repairs tend to cost £80–£400 depending on the extent of the damage.
3. Flow Sensor or Thermistor Problems
Your combi boiler relies on a series of sensors to know what’s going on.
• Flow sensors measure how much water is moving through the system.
• Thermistors measure water temperature.
If either starts to play up, your boiler may “think” you’re not using much water - or that the water’s hotter than it really is.
The result? The boiler dials down the flame or switches off, leaving you with an unpleasant cold blast.
Typical fix: These sensors are relatively inexpensive parts (£60–£150 fitted) but need a pro to diagnose and replace.
4. Boiler Too Small for the Job
Combi boilers heat water on demand - they don’t store it in a cylinder.
That means if too many taps or showers are running at once, or if your home has high hot water demand, the boiler can’t keep up.
A small combi (e.g. 24kW) might cope with one bathroom but struggle with two.
Run a shower while the kitchen tap’s on full blast and you’ll likely notice the temperature yo-yoing.
Typical fix: Unfortunately, there’s no quick repair if your boiler’s undersized.
You’ll either need to upgrade to a larger combi (30–40kW+ for big families) or consider a system boiler with a hot water cylinder if you’ve got multiple bathrooms.
Can I Fix Hot Water Turning Cold DIY?
Unfortunately, there isn’t much you can do DIY if you find that hot water is running cold within minutes.
This is because the majority of potential fixes are only safe and suitable for a qualified heating engineer to undertake.
A professional plumber may also be qualified to fix the problem, depending on the nature of the issue.
If you suspect that there may be a problem with the thermostat or heating element, however, there are a few things worth trying DIY before calling out a professional.
For one, it’s possible that a power surge has limited the ability of the water heater tank to do its job.
Therefore, it’s worth resetting your fuse box by turning off the main switch, waiting several seconds and then turning it back on.
If you are fortunate enough, this will solve the problem and you’ll find that hot water continues to work as normal.
If you think there may be a problem with your thermostat specifically, it may also be worth replacing the batteries per the manufacturer’s instructions.
So, in summary, we suggest taking the following steps if you’d like to have a go at fixing this problem DIY:
Replace the batteries in your thermostat per the manufacturer’s guidance.
Reset your fuse box by turning the main switch off and on.
Then a short while later, check whether your hot water (e.g., via your shower) continues to flow out for longer than it was prior to your DIY efforts.
If the above strategy fails, however, you’ll need to hire a professional. Moreover, you may want to skip ahead to hiring an engineer or plumber either way since it’s not especially likely that the above steps will work (they are more a chance that may be worth taking before paying for the work of a professional - it’s really up to you).
Cost of Hiring a Professional
The average cost of hiring a professional to fix hot water running cold within minutes is about £30 to £70 per hour.
The average hourly rate in the UK for hiring a heating engineer and plumber is about the same.
However, a range of factors can still shape the total cost of having a combi boiler issue resolved such as hot water running cold within a matter of minutes.
Not only might supply costs also apply (e.g., if a dip tube needs to be replaced) but the cost of labour tends to differ across the UK.
Heating engineer and plumber rates are generally highest in the southeast of England (with London being especially notorious), while labour prices tend to be below the UK average in northern England, Scotland and Northern Ireland.
The average cost of hiring a professional to fix hot water turning cold:
Fixing a faulty dip tube - £30 to £120
Replacing a faulty tube - £35 to £220 *
Fixing a boiler heating element - £30 to £120
Replacing a boiler heating element - £50 to £170
Fixing a faulty thermostat - £30 to £120
Installing a brand-new thermostat – £130 to £320
Water heater replacement – £290 to £980
Brand new boiler installation - £800 to £2,800
*The actual supply price of a dip tube can vary widely hence the particularly wide price range.
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