Radiators Hot When Only the Hot Water Is On? Here Is Why

First, make sure the heating really is off

Before looking for a fault, rule out the controls. Set the programmer so the hot water channel is on and the heating channel is off, and turn the room thermostat right down as well. Some programmers have a setting that runs the heating and hot water together, and on older systems the heating channel can be set so that it cannot be switched off while the hot water is on. A smart thermostat or a wireless room stat may also be calling for heat on its own schedule, so check the app or the wall unit too.

Then give it half an hour and feel the radiators. Note which ones warm up and how hot they get. All of them properly hot tells a different story from a warm towel rail and a lukewarm radiator on the landing, and that difference is what an engineer will want to know.

• Every radiator in the house heats up fully: suspect the heating valve or the wiring
• Mainly upstairs radiators and towel rails, and only warm rather than hot: suspect gravity circulation
• Only a towel rail, and only while the cylinder is heating: it may be plumbed on the hot water circuit by design
• Radiators warm up whenever someone runs a hot tap, and you have a combi: suspect the diverter valve

On a system with a cylinder: the motorised valve

A boiler that heats a cylinder shares its hot water between the radiators and a coil inside the cylinder, and motorised valves decide which gets it. If you are not sure which layout you have, our guide to Y plan and S plan heating systems shows how to tell from the pipework in the airing cupboard.

On an S plan there are two separate two port valves, one for the heating and one for the cylinder. The heating valve should be closed whenever the heating is not calling. If its lever has been pushed across and latched in the manual position, it stays open. People sometimes do that to get heat during an earlier fault and forget about it. If the lever is in auto, the valve can still be stuck open because the brass spindle has seized with scale or sludge, or it can close but let water past a worn seat. Any of these lets boiler water into the radiators every time the hot water runs.

On a Y plan there is one three port valve. With only the hot water calling, it should sit fully across to the cylinder port. If the spindle has seized part way, or the valve is worn and no longer seals, some of the flow leaks through to the heating side. You get warm radiators and hot water that takes longer than it used to. The lever on the head also has a manual setting that holds the valve in its middle position, which sends water both ways, so check that first.

The valve head runs on mains voltage. Moving the lever back to auto is fine, but do not take the cover off, and do not force a seized lever. A snapped spindle turns a head swap into a drain down.

Gravity circulation: why upstairs radiators warm up by themselves

Hot water rises. When the boiler is heating the cylinder, the water in the flow pipe is hot, and if the heating pipework branches off below the radiators without anything to stop it, some of that hot water will rise into the radiator circuit on its own, with no pump involved. It is called gravity circulation or thermosiphoning. It shows up most on radiators and towel rails upstairs, because they sit highest above the boiler, and they usually get warm rather than properly hot.

It is common on older houses where the pipework has been altered over the years, and on systems converted from gravity hot water to fully pumped without all the pipework being changed. The usual fix is a small anti gravity valve, a spring loaded non return valve, on the heating flow. It stays shut until the pump pushes water through it, so natural circulation cannot open it. Fitting one means partly draining the heating, so it is a job for an engineer rather than a quick adjustment.

A towel rail that is meant to be warm

In a lot of homes with a cylinder, the heated towel rail is plumbed onto the pipework that feeds the cylinder coil, not onto the heating circuit. That is done on purpose so the bathroom has a warm rail in summer when the heating is off. If the only thing warming up is a towel rail, and it only gets warm when the cylinder is being heated, it is probably working as it was designed.

The clue is in the pattern. A towel rail on the hot water circuit warms and cools with the cylinder heating times, not with the heating timer. If it bothers you, it can usually be re-plumbed or given its own valve, but it is not a fault.

On a combi boiler: the diverter valve

A combi has no cylinder and no hot water channel. It heats water as it passes through to the tap, and a diverter valve inside the case decides whether the boiler heat goes to the radiators or to the hot water. When you open a hot tap, the valve should swing fully over to the hot water side and shut the heating off.

If the valve is worn or sticking, it does not seal on the heating side, so part of the heat goes to the radiators while you run the tap. You notice radiators getting warm in summer after a bath or a shower, and hot water that is not as hot as it was. Left alone, it tends to get worse until the valve sticks and you have no hot water but the heating works. Our page on the diverter valve replacement cost explains what is involved, and the page on no hot water from a combi boiler covers the related faults.

The diverter valve is inside the case of a gas appliance, so any work on it is for a Gas Safe registered engineer.

Wiring and control faults

Less often, the valves are fine and something is telling the heating side to open. A programmer with a failing relay, a room thermostat that is stuck calling, or a wiring fault in the wiring centre next to the boiler can all leave the heating valve powered when it should not be. The give away is that the lever on the heating valve moves across by itself when the hot water comes on. If the heating valve is being driven open with the heating switched off, the fault is in the controls, not the valve, and replacing the valve would not fix it.

The same controls are behind the opposite problem, where the hot water only works when the heating is on. If that sounds more like your system, that guide goes through the checks from the other direction.

Why it is worth fixing

Heat that goes into radiators nobody asked for is heat that is not going into your cylinder. The hot water takes longer to come up to temperature, the boiler runs longer than it needs to, and in summer you are paying to warm rooms that are already warm. On a Y plan, a valve that has started letting by is also a valve that is likely to stick completely at some point, and that usually happens on the first cold evening of the autumn.

Most of these faults are one part and one visit. A motorised valve head can often be changed without draining the system. A seized valve body, an anti gravity valve or re-plumbing a towel rail all involve draining part of the heating and refilling it, which takes longer. A combi diverter valve is a boiler repair.

Getting it looked at

Call us on 020 4652 5111 or send an enquiry. It helps to tell us whether you have a combi or a cylinder, how many motorised valves you can see near the cylinder, where the levers are sitting, and which radiators warm up with only the hot water on. A photo of the airing cupboard pipework saves time.

Hot water systems are our core work and we have 20 engineers across London, so the valve, the controls or the pipework is usually found and put right in one visit. If the fault turns out to be inside the boiler, a boiler diagnosis tests the parts in turn rather than guessing. Our hourly rate and call-out are on the pricing page and agreed before we attend. The rate does not change on the day; the total depends on which part has failed and how long the job takes.