Heated Towel Rail Not Getting Hot

What's happening?

A heated towel rail that is plumbed into the central heating works like a radiator, with a few differences. It has no bleed valve on some designs, so air can sit at the top. It is often on a small-bore pipe that clogs more easily. And which circuit it is on decides when it is hot. A rail on the heating circuit is hot only when the heating runs, so it is cold in summer, which is normal. A rail on the hot water circuit is hot whenever the cylinder is being heated, and cold the rest of the time. If it is cold when it should be hot, the usual causes are air at the top, a closed valve, or a blockage. A rail that heats from an electric element is a different appliance with its own fault list.

What usually causes it?

• Air trapped at the top of the rail, so only the lower part heats
• One or both valves closed or part-closed
• A rail on the heating circuit during summer, when nothing calls for heat
• A rail on the hot water circuit that only heats when the cylinder does
• Sludge or a blockage in the narrow rail tubes or pipework
• A thermostatic valve on the rail set low or stuck
• On a dual-fuel rail, an electric element switched off or failed

Is it dangerous?

No. A cold towel rail is a comfort fault. Hot rails can burn, so keep small children clear of one that is running.

Can I fix it myself?

With the heating off, bleed the rail using the bleed valve at the top or a manual vent. Check both valves are fully open and the thermostatic head, if there is one, is set high. Work out which circuit it is on by whether it heats with the heating or the hot water. In summer, a heating-only rail will be cold until the heating is switched on, which is normal.

When should you call a heating engineer?

Call a heating engineer if bleeding and valves do not help, if the rail is cold in winter when the heating is on, or if it was only ever half warm. Call an electrician for an electric-only rail, and see the electric radiator page for that.

What will the heating engineer actually do?

• Establish which circuit the rail is on and what should heat it
• Bleed the rail and fit a bleed valve where the design lacks one
• Check valves, thermostatic head and pipe sizing
• Flush the rail and pipework if blocked
• Rebalance or alter the circuit so the rail heats when needed

Who fixes this

• Heating not working
• Power flush

Questions people ask

Sources & References

1. Vaillant: heating controls
2. Vaillant: what is a magnetic boiler filter?
3. Vaillant: most common boiler problems