Radiators still cold after a power flush? Here's why, and what to do
Don't open up the boiler, the pump or any filter. Everything in this guide is done from the outside. If the flush was recent, the company that did it should come back and put it right, often for free.
You paid for a power flush to sort out cold radiators, and some are still cold. Or radiators that were fine before are now cold. This is frustrating, but it's fairly common, and it doesn't always mean the flush was a waste of money. After a flush, the system usually needs bleeding and balancing (see step 3), and sometimes a stubborn radiator needs extra attention.
- Bleed every radiator and check the pressure.
- Check every radiator valve is open.
- Understand balancing, and whether it was done.
- Work out which kind of cold you have.
- Call the company back. Here's what to say.
Each step has important details below. Tap a step to jump to it.
Why it happens
A power flush pumps water and cleaning chemicals through your system at high speed to shift sludge (black, muddy deposits). Afterwards, radiators can stay cold for a few reasons:
- Air. The system is drained and refilled during a flush, so it's full of air at the end.
- Valves. Radiator valves are opened, closed and sometimes adjusted during the flush. If they're not reset, some radiators get too little hot water.
- Balancing. Once the system is clean, water flows more easily. The radiators nearest the boiler can then take most of the heat, leaving the furthest ones cold.
- Loosened sludge. Sometimes sludge breaks loose and settles in a radiator or valve, instead of being flushed out.
- A badly blocked radiator. Some radiators are too clogged for a flush to clear, and need taking off and cleaning separately, or replacing.
Step by step
1Bleed every radiator and check the pressure
With the heating off and cool, bleed every radiator, downstairs first, then upstairs. Close each bleed valve as soon as water appears. Then run the heating for half an hour and do it again.
If you have a combi or system boiler, check the pressure gauge after bleeding. It should usually be about 1 to 1.5 bar when cold. Our guide Boiler stopped working after bleeding the radiators shows how to top it up safely.
2Check every radiator valve is open
- Turn every thermostatic valve (the one with numbers) to its highest setting for now.
- On a cold radiator, check the TRV's pin isn't stuck. Unscrew the head and gently press the small metal pin. It should spring back up. Our guide Radiator cold after refitting shows how.
- The lockshield valve on the other end (under a plain cap) shouldn't be fully closed. Don't open it right up, though, as that can upset the balance. Note how many turns you change it by.
3Understand balancing, and whether it was done
Balancing means adjusting the lockshield valves so each radiator gets its fair share of hot water. The ones closest to the boiler are turned down a little, and the ones furthest away are opened more.
A clue that the system needs balancing: the radiators near the boiler get very hot quickly, while the ones furthest away (often upstairs or at the far end of the house) stay cool or take ages.
Many companies balance the system as part of a power flush, and they should. Ask whether it was done. You can balance radiators yourself with a lockshield key and some patience, but it's fiddly. If the flush was recent, it's reasonable to ask the company to come back and do it.
4Work out which kind of cold you have
| What you notice | Most likely cause |
|---|---|
| Cold at the top, warm at the bottom | Air. Bleed it again. |
| Cold at the bottom, warm at the top | Sludge still in that radiator |
| Completely cold, pipes going in are hot | A closed or stuck valve |
| Completely cold, pipes going in are cold too | A valve, a blockage in the pipe, or balancing |
| Far radiators cool, near ones very hot | Needs balancing |
| Was fine before, cold since the flush | Loosened sludge settled there, or a valve wasn't reopened |
5Call the company back
Most power flushing companies offer some kind of guarantee, and a radiator that's still cold straight afterwards is something they should look at. When you call, tell them:
- which radiators are still cold, and where they're cold (top, bottom or all over), using the table above;
- whether any radiators that were fine before are now cold;
- that you've bled them and checked the pressure;
- and ask: "Was the system balanced afterwards?" and "Was inhibitor added?" (Inhibitor is a chemical that protects the system from new sludge.)
A radiator that's still cold at the bottom after a flush may need taking off and flushing out on its own, or replacing if it's very old. That's normal, and a good engineer will tell you honestly which it is.
Anyone working on the boiler itself must be Gas Safe registered. You can check at gassaferegister.co.uk.
Questions people ask
Was the power flush a waste of money?
Not usually. If most of your radiators heat better and the boiler runs more quietly, it has done its job. One or two stubborn radiators are common and can usually be fixed. If nothing has improved at all, raise it with the company.
How long after a power flush should the radiators work properly?
Straight away, once they've been bled and balanced. If it takes more than a couple of days of bleeding, something else needs sorting.
Can I balance the radiators myself?
Yes, with a lockshield key and a bit of patience. You start with the radiator closest to the boiler, and turn lockshields down until each radiator heats up at a similar rate. It's easier and quicker for an engineer, and they should have done it after the flush anyway.