Heating 21 August 2026

Hot Water but No Heating: What Has Actually Failed

If you have plenty of hot water at the taps but the radiators stay stone cold, the boiler itself is usually working perfectly well — something on the heating side of the system has stopped asking it for heat. On a system or regular boiler that is nearly always a motorised valve, the programmer or room thermostat, or the pump; on a combi it is almost always the diverter valve stuck in the hot water position.

That distinction decides whether this is a five-minute fix you can do yourself or a job for a Gas Safe registered engineer. Below is the order our engineers work through it on Kent call-outs, so you can rule out the cheap causes before you pick up the phone.

Why is my central heating not working but hot water is?

Hot water and heating are two separate demands. Both are served by the same boiler, but each has its own controls. When one works and the other does not, the fault is almost never the burner — if the burner had failed you would have neither.

So the question is really: what stands between your thermostat and the boiler on the heating side only? On a stored-hot-water system that chain is the programmer, the room thermostat, the motorised valve for the heating circuit, and the pump. On a combi it is the diverter valve inside the boiler. Work along that chain in order and you will find it.

First, work out which system you have

Go and look at your airing cupboard.

• A hot water cylinder in the cupboard means a system or regular boiler (regular if there is also a small tank in the loft). These have external controls — one or two brass-bodied motorised valves on the pipework near the cylinder, each with a plastic head and a small manual lever.
• No cylinder anywhere, hot water instantly at the tap means a combi boiler. Everything is inside the case.

This one check changes the whole diagnosis, and it is the first thing we ask on the phone.

Combi boilers: the diverter valve

A combi heats your radiators until you open a hot tap, at which point a diverter valve swings over and sends all the heat to the tap instead. When you close the tap it should swing back.

When that valve sticks in the hot water position, you get endless hot water and cold radiators. Often the boiler fires briefly when the heating calls, runs for a minute, then cuts out — the heat has nowhere to go.

There is no homeowner fix for this. The valve sits inside a sealed gas appliance, so freeing or replacing it is Gas Safe work — never open a boiler case yourself. It is a common repair and usually a same-visit one.

System and regular boilers: the motorised valve

On a stored-hot-water system, a motorised valve physically opens to let hot water into the radiator circuit. If the heating-side valve has failed — the motor, the microswitch inside the head, or the valve body seizing after a summer of sitting still — the boiler never learns that the heating wants anything, so it carries on serving the cylinder and nothing else.

We see far more of these in September and October than at any other point in the year, and the reason is mundane: the valve has not moved since March.

Telling an S-plan from a Y-plan

• S-plan has two separate two-port valves, one for heating and one for hot water. Only the heating one is faulty here, which is why the hot water is unaffected — and it is the more straightforward of the two to diagnose.
• Y-plan has one three-port mid-position valve doing both jobs. When it fails it often parks itself in the hot water position, giving the same symptom.

If you can see two valve heads, it is S-plan. One head with three pipes coming off the body, it is Y-plan.

Central heating coming on with the hot water

The reverse complaint points at the same component. If your radiators get warm whenever the hot water is on — even with the heating switched off at the programmer — that is usually a Y-plan mid-position valve failing to return to its hot-water-only position, or circulation through a valve that is no longer closing fully. It wastes gas all winter and is worth putting right rather than living with.

What you can safely check yourself

Before you book anything, run through these. They cost nothing and they resolve a fair share of the calls we take.

1. The programmer or timer. Check heating is set to ON or to a time period that is currently active, not just hot water. After a power cut a programmer can reset to a default schedule, and it is easily missed.
2. The room thermostat. Turn it well above the current room temperature and listen for a click. No click, no call for heat. If it is battery powered, change the batteries — flat batteries in a wireless stat are one of the most common causes we attend, and the display often looks fine right up to the point it stops transmitting.
3. Boiler pressure. Look at the gauge. If it has dropped below the green band, the boiler may lock out the heating while still managing hot water. Our guide to what pressure your boiler should be at walks through topping it up. If it keeps dropping, you have a leak somewhere and that needs looking at.
4. Thermostatic radiator valves. TRVs seize shut over the summer. If only some radiators are cold, pull the plastic head off and check the pin underneath moves freely — a gentle tap and a wiggle with pliers frees most of them.
5. The manual lever on the motorised valve. Every motorised valve head has a small lever for manual opening. If sliding it across makes the radiators warm up, you have confirmed the valve head is the fault rather than the boiler.

If none of that changes anything, stop there. Anything further involves the boiler, the gas or mains wiring, and needs an engineer.

Why is my radiator not heating up when the others are?

That is a different fault entirely, and a much better one to have. A single cold radiator, or one cold at the bottom and warm at the top, is a circulation problem in that radiator — air at the top, sludge at the bottom, or a seized TRV. It is not the boiler. Bleed it first, and if the bottom stays cold while the top is hot, the system is carrying sludge and wants a proper clean rather than another bleed.

When every radiator is cold and the water is hot, you are back to the valve, controls and pump list above.

When it becomes Gas Safe work

Anything inside the boiler case — diverter valves, pumps built into a combi, gas valves, PCBs — is work for a Gas Safe registered engineer, without exception. Plumbing Angels is Gas Safe registered under number 951167, and we cover the whole of Kent from our Tonbridge base.

Repair work is charged by the hour: £95+VAT for the first hour, which is always billable, and £47.50+VAT for each hour after that. Any parts needed are confirmed and agreed with you before we fit or order them. Our workmanship carries a 90-day guarantee.

If the system struggled through the last two winters, book a boiler service at the same time — a flat £95+VAT, and the sensible way to catch a tired pump or a sticking valve before January rather than after.

Get it sorted before the cold sets in

Cold radiators in August are an inconvenience. The same fault in December, when every heating firm in Kent is booked solid, is a genuinely miserable week. If you have hot water but no heating, call us on 01732 792 454 — the answers to two or three questions will usually tell us whether it is a valve head, a thermostat or the boiler.

For a diagnosis and repair, see boiler breakdowns and repairs. If you have no heating and no hot water and there is someone vulnerable in the house, our emergency plumbers page explains how to reach us out of hours.