Why Is My Shower Going Cold

A shower that goes cold is nearly always one of four things, and how long it lasts tells you which. Cold in the first few seconds is just the water that was sitting in the pipe. Hot and then cold after a minute or two is usually the boiler, most often a sticking diverter valve or a heat exchanger furred up with scale. Fine for ten minutes and then cold means a stored cylinder that has run out. And one that swings hot and cold the whole time you are stood in it is a flow problem rather than a heating one.

The other thing worth settling before anything else is what kind of shower you actually have, because an electric shower heats its own water and has nothing to do with your boiler. We are a Gas Safe registered and OFTEC certified plumbing and heating business based in Bearsted, and this is one of the most common calls we take. What follows is how to work out which of the four you have, and which of them you can settle yourself before paying anybody to look.

Start with what kind of shower you have

An electric shower has a single cold water feed and heats the water itself as it passes through, using an element not unlike a kettle. If an electric shower has gone cold, your boiler is not involved and neither is your hot water cylinder. It is the shower's own element, its thermal cutout, or the flow going through it. That one distinction saves a lot of wasted call-outs.

A mixer shower is the other arrangement. It takes a hot feed and a cold feed and blends them, so the hot side comes from your boiler or your cylinder and anything wrong with either turns up in the shower. A manual mixer holds the position you set it to rather than the temperature, which is why it reacts to everything else happening in the house.

In our experience, a thermostatic mixer works better than a manual one on a combi, because a manual valve holds its position rather than its temperature, so every change in flow elsewhere in the house arrives at the shower as a change in heat.

A thermostatic mixer corrects for that, and it also shuts itself down if the cold supply fails, which is a real safety feature rather than a sales line. The trade-off is a cartridge that can seize, and in a hard water area it will eventually try to.

What the timing tells you

Before you call anyone, time it. Get in, note roughly how long the water stays hot, and note whether anything else in the house was running. That one piece of information narrows four possibilities down to one, and it is the first thing we ask on the phone.

Hot, then cold after a minute or two

This is the classic combi complaint, and there are two usual culprits. The first is the diverter valve, the component deciding whether the boiler is heating your radiators or your hot water. When it starts to stick you get hot water that gives up part way through, and often heating that comes on by itself when somebody runs a tap. We have found it is the most common mechanical fault on combis with a few years on them.

The second is the plate heat exchanger scaling up, and in this part of Kent that is the more likely of the two. The tell is that it got worse across a year or two rather than appearing overnight, and that the hot tap nearest the boiler is slower to come through than it used to be as well. A fault that arrived gradually is a scaling fault. A fault that arrived on a Tuesday is usually mechanical.

Descaling an exchanger is sometimes worth trying, but honestly not often. A chemical flush shifts what is loose and leaves the hard deposit bonded to the plates, so a shower that comes back tends to be back to lukewarm within a season. On a boiler otherwise worth keeping, replacing the exchanger is the repair that actually lasts.

It goes cold when someone runs another tap

A combi makes hot water on demand and it can only do it in one place at a time. Run the kitchen tap while somebody is in the shower and the boiler has to divide what it can heat, so the shower cools off. That is not a fault, it is how the design works, and it is the main practical drawback of a combi in a house with more than one bathroom.

Where it does become a fault is when it started happening and never used to. That normally means the flow has dropped somewhere, and less water arriving means less water the boiler has to work with, so the shower is the first thing in the house to notice.

It runs out rather than going cold

If your hot water is stored in a cylinder the failure looks quite different. It does not cool off gradually, it works perfectly and then simply runs out, usually at the least convenient moment. That is capacity rather than a fault, and the real question is whether the cylinder is the right size for the household or whether it is not reheating between uses.

Reheating takes time on an indirect cylinder heated by the boiler, so one drained by a bath will not be ready for a shower straight afterwards. If yours used to cope and now does not, the usual causes are an immersion heater that failed without anybody noticing because the boiler carried on doing the work, a coil scaled up on the outside, or a cylinder thermostat that has drifted out of calibration.

The burst of cold that is not a fault

One behaviour gets reported as a fault regularly and is not one. Run hot water, stop for a minute, then start again, and you get a short slug of cold before it comes back properly. The hot water left sitting in the pipe cools while the boiler stands down, and when you restart it the boiler has to relight and push that cooled water through ahead of the new hot.

It is more noticeable the further the shower sits from the boiler, which is why it turns up in a converted loft bathroom far more than in an en suite next to the airing cupboard. Nothing is broken. It is worth recognising before you pay somebody to hunt for a fault that was never there.

What we find on Kent jobs

• A brand new shower fitted to cure a problem that was in the boiler. The old shower was fine, and now there is a new one doing exactly the same thing.
• A thermostatic cartridge seized solid with scale, in a shower nobody has descaled since the day it went in.
• An immersion heater that failed months earlier and went unnoticed, because the boiler kept the cylinder up until somebody ran a bath.
• A diverter valve that had been sticking for most of a season, reported to us as the shower losing pressure rather than losing heat.

The first of those is worth dwelling on, because it is expensive and completely avoidable. From working with customers across Maidstone and the Medway towns, the honest check takes a minute. Run the hot tap at the basin nearest the boiler and watch whether it behaves the same way as the shower. If it does, the shower was never the problem, and replacing it will not fix anything.

What to check before you call

Three things are worth ruling out first. Whether the hot tap nearest the boiler does the same thing, which separates a shower fault from a hot water fault in about a minute. Whether it still happens with nothing else in the house running. And if it is an electric shower, whether it has simply cut out and needs resetting rather than anything more serious.

What to leave alone is the boiler itself. Anything behind the casing is gas work and belongs to a Gas Safe registered engineer, and a diverter valve or a heat exchanger is not a job to take on with a YouTube video and an afternoon free.

So it comes back to the timing. Seconds means the pipe. A minute or two means the boiler. Ten minutes means the cylinder. Swinging the whole way through means the flow. And if it is an electric shower, none of the above applies. We cover Maidstone and the rest of Kent from Bearsted, with free home visits and free quotes across Kent. If the hot tap nearest your boiler is doing the same thing as the shower, get in touch and we will come and look at the right end of it.