The first cold snap of the season is when most people find out a radiator is not pulling its weight. The good news is that where a radiator is cold usually tells you why.
Here are the common patterns, what usually causes each one, what you can try yourself and when it is worth getting an engineer in.
Cold at the top, warm at the bottom
This is the classic sign of trapped air. Air rises to the top of the radiator and stops hot water filling it. You might also hear gurgling or trickling in the pipes.
The fix is to bleed the radiator:
- Turn the heating off and let the radiators cool, so you do not get sprayed with hot water
- Fit a radiator key to the bleed valve at the top corner and hold a cloth under it
- Turn the key slowly anticlockwise until you hear air hissing out
- When water starts to come out steadily, close the valve again
- Check the boiler pressure gauge and top up if it has dropped
If one radiator needs bleeding every few weeks, air is getting into the system from somewhere, or gas is being produced inside it by corrosion. That is worth looking into rather than living with.
Warm at the top, cold at the bottom
When the top of a radiator is hot but the bottom or middle stays cold, sludge is the usual suspect. Over the years, rust and debris from inside the radiators and pipes settle at the bottom of the panels and block the flow.
Bleeding will not help here. If you do bleed it and the water that comes out is dark or black, that is a good sign the system is dirty. Depending on how bad it is, the fix might be taking a single radiator off and flushing it through, or cleaning the whole system and adding a filter to keep it clean afterwards.
One radiator stone cold
If a single radiator stays cold while the rest of the house is warm, check the valves before anything else:
- Make sure the thermostatic radiator valve (TRV) is turned up
- If it is, the pin underneath the TRV head may have stuck down over the summer. Radiators that have been off for months are prone to this
- The lockshield valve on the other end may have been closed, for example after decorating
A stuck pin can sometimes be freed by taking the TRV head off and gently working the pin, but if you are not comfortable doing that, it is a quick job for an engineer.
Radiators furthest from the boiler are cold
When the radiators near the boiler get hot quickly and the ones at the far end of the house barely get warm, the system usually needs balancing. Hot water takes the easiest route, so the nearest radiators take most of it unless the lockshield valves are set to share the flow out evenly.
Balancing means adjusting each lockshield valve and checking temperatures across the system. It takes a bit of patience. A weak or failing pump can cause similar symptoms, so if balancing does not help, the pump is the next thing to check.
All the radiators are cold
If every radiator is cold, the problem is not the radiators. Check the obvious things first: is the boiler on and showing no fault, is the pressure in the normal range, is the timer calling for heat and is the room thermostat turned up?
On a combi boiler, if you have hot water at the taps but no heating, the fault is often in the part that switches the boiler between heating and hot water, or in the pump or controls. On a system with a hot water cylinder, a stuck motorised valve is a common cause. These are jobs for a heating engineer.
Upstairs cold, downstairs warm
If the downstairs radiators are fine but the upstairs ones are cold, it can be air trapped high in the system, a sticking zone valve on homes with separate upstairs and downstairs heating, or a pump that is struggling. Bleeding the upstairs radiators is worth a try first. If that does not help, it needs proper fault finding.
When to call Gary
Bleeding a radiator and checking the valves are fair DIY jobs. It is time to call an engineer when:
- Radiators keep needing bleeding
- Bleeding brings out black water or the radiators are cold at the bottom
- Several radiators are cold, or the whole system is slow to warm up
- The boiler keeps losing pressure after bleeding. Our guide to why a boiler keeps losing pressure explains why
Gary traces heating faults back to the cause rather than guessing, and carries out radiator and heating system work including new radiators and system cleaning. For harder-to-find problems, see heating fault finding and repair.
He covers homes across Cheltenham, Churchdown, Gloucester and nearby. There is no call-out charge. Call 07454 117678 or get in touch.