Turning on a tap and getting only cold water is one of the most frustrating boiler problems. A few simple checks can sometimes sort it, and if not, they help narrow down the fault.
Gas Safe registered 25 years in the trade 12 years running the business
Written by Chris Gill, C Gill & Son Plumbing & Heating ·
Combi or cylinder: why it matters
Before checking anything, work out which kind of system you have, because the checks differ. A combi heats water the moment you open a tap, so there is no hot water tank. A system or regular boiler heats water in advance and stores it in a cylinder, usually in an airing cupboard.
With a combi, no hot water points to the boiler itself. With a cylinder, the problem could be the boiler, the controls telling it to heat the water, or the cylinder. If you are unsure which you have, our guide comparing a combi and system boiler explains how to tell them apart.
Safe checks you can do yourself
These checks involve nothing more than looking at gauges, buttons and settings. None of them involve opening the boiler.
Pressure: on a combi or system boiler, check the gauge. If it has dropped low, the boiler may refuse to fire. Our boiler losing pressure guide shows how to top it up safely.
Settings and programmer: check the hot water is switched on, the timer has not been reset by a power cut and the hot water temperature has not been turned right down.
Other taps: try a different hot tap. If one works and another does not, the fault may be the tap or a mixer valve rather than the boiler.
Frozen condensate pipe: in a cold snap, the plastic pipe running outside can freeze and block, and the boiler locks out. Pouring warm, not boiling, water along the outside section often thaws it.
Immersion heater: if you have a cylinder, it may have an electric immersion heater you can switch on as a backup while you wait for a repair.
The faults usually behind it
If the simple checks do not help, the cause is normally a part that needs an engineer. On a combi, a diverter valve switches the boiler between heating and hot water; when it sticks, you can end up with one but not the other. A flow sensor tells the boiler that a tap is open, and if it fails the boiler simply does not respond.
On a cylinder system, the cylinder thermostat or a motorised zone valve can fail, so the boiler is never asked to heat the stored water. These parts sit inside the boiler or are wired into the controls, so they are not DIY jobs. Anything inside a gas appliance has to be dealt with by a Gas Safe registered engineer.
Hot water but no heating, or the other way round
When one works and the other does not, it narrows things down. On a combi, heating without hot water often points to the diverter valve or flow sensor. Hot water without heating can be the same diverter valve stuck the other way, a room thermostat issue or a pump problem.
On a cylinder system, a zone valve that has failed in one position can give exactly this split. Note which works and which does not before you call, because it saves time on the visit.
Repair, or a sign the boiler is tiring?
A single failed part is usually a fair repair, even on a boiler that is a few years old. It is different if the hot water has been unreliable for a while, the boiler has needed several repairs lately or parts are getting harder to find.
Chris will give you an honest view either way. If a new boiler does make more sense, our combi boiler installation page explains what that involves, though plenty of hot water faults are put right with one repair.
Booking a repair
Call or WhatsApp 07429 632804 and tell us what type of boiler you have, any code on the display and whether the heating still works. Chris is Gas Safe registered with 25 years in the trade. Our boiler repairs page covers the wider work we do, and if you have no heating or hot water at all, see emergency boiler repair; Chris will tell you honestly whether and when he can attend.
Common questions
No hot water FAQs
Clear answers before you arrange a visit.
Why do I have heating but no hot water on my combi?+
The most common causes are a sticking diverter valve or a faulty flow sensor. Both need a Gas Safe registered engineer to replace.
Can a frozen pipe stop hot water?+
Yes. A frozen condensate pipe makes the boiler lock out. Thawing it with warm water and then resetting the boiler often gets it going again.
Should I reset the boiler?+
One reset is fine once you have checked the pressure and settings. If it locks out again, stop resetting it and get the fault looked at.
How do I use the immersion heater as a backup?+
If your cylinder has one, there is usually a switch nearby. Turn it on and give it time to heat the water, then switch it off when you no longer need it.
Is no hot water classed as an emergency?+
It can be, for example with young children or vulnerable people in the house. Call 07429 632804 and Chris will tell you honestly whether and when he can attend.
