Does your combi boiler run cold after a few minutes? Here we explain the most common causes and fixes.
Do you find that while your hot water works initially, it suddenly turns cold after a few minutes?
If so, you’ve come to the right place and with the correct steps taken, you can stop the frustration of your shower or hot water taps turning cold.
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?
There are a few likely reasons why (if you have a combi boiler) your hot water may run cold within a matter of minutes.
The potential explanations generally differ from those of why your home has no hot water at all (we’ll touch on that later).
But without further ado, let’s explore the most common causes of combi boiler hot water running cold within minutes.
Faulty Dip Tubes
One way that the water heater (which sits combined inside your combi boiler unit along with the boiler itself - hence where the term comes from) may experience an issue is with a faulty dip tube.
This can, in turn, result in water running cold after a few minutes.
A water heater dip tube plays a key part in keeping cold and hot water separated. Should it break, it can result in hot water turning cold quickly.
Usually, a water heater will contain hot water toward the top of the tank.
Cold water will be supplied at the bottom via the dip tube so that it can be heated up before ascending to the top of the tank to eventually be distributed via pipework (as would be the case for the initially hot water toward the top of the tank).
Some of the hot water being distributed will be sent to radiators to heat rooms, whereas some of it will be used for direct use through hot water outlets (e.g., showers and hot water taps).
If your water heater has a broken dip tube, the water heater will not be able to properly separate the cold and hot water.
Ultimately, this will likely lead to the hot water being generated by your combi boiler not staying for particularly long.
Alternatively, your water heater may suddenly be incapable of producing any hot water at all and may only generate lukewarm water at best.
Thermostat or Heating Element Are Acting Up
Electric water heaters tend to feature a thermostat allowing you to set the water temperature as you wish.
If so, a relevant point to consider about the thermostat is that it controls two distinct heating elements, those being the upper and lower elementary.
As water enters the water heater tank the bottom heating element will warm the water till it reaches the temperature you’ve set on the thermostat.
As the hot water reaches the top of the tank, the upper heating element will act on the hot water to keep it hot.
However, should either (or both) of these heating elements fail, it can result in hot water turning cold quickly or there may be a lack of hot water entirely.
In the case of the upper heating element experiencing a fault, this would mean that the hot water will not stay at the temperature you’ve set on the thermostat.
Alternatively, it may become cold.
However, if the lower heating element is acting up, hot water will run out sooner than expected. You may also notice leaks and water discolouration.
Boiler Not Large Enough
Last but not least, it’s entirely possible that your water heater is simply not big enough.
On average, approximately three-quarters of a hot water tank should serve the purpose of providing hot water.
If your hot water heater is too small to adequately meet your property’s hot water needs, you may end up in a scenario where hot water runs out too fast.
It may not be the case that your hot water is always running out too quickly (it may also be running out a little faster than you’d like as opposed to within just a few minutes), however, it may still run out quickly when your home has a particularly high demand for hot water.
Whatever the case, if your hot water supply is depleting faster than you’d like and it’s at an unacceptable level, assuming that no other issues are at play, you’d need to look at having your water heater replaced.
It goes without saying, that you should have all other possible causes checked by a professional first before splashing out on a replacement.
Of course, given that we are talking about combi boilers in this article, replacing a water heater inside a combi boiler with a larger one may not be feasible.
However, you should consult a qualified heating engineer to determine whether or not it could work in your case.
Otherwise, you’d, unfortunately, require a brand-new combi boiler with a larger water heater built in.
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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