Fault guide
Radiators cold at the bottom, or not heating at all
Where the cold patch is tells you what is wrong. Read the pattern first and you will know whether it is a five minute job or a system problem.
Read the pattern
Run the heating for twenty minutes, then feel each radiator top, middle and bottom.
- Cold at the top, warm at the bottom: air in the radiator. Bleed it and top the pressure back up.
- Cold at the bottom, warm at the top: sludge and corrosion debris settled inside. Cleaning, not bleeding.
- One radiator stone cold, the rest fine: seized valve, closed lockshield, or that radiator is airlocked.
- All radiators lukewarm: boiler firing low, poor circulation, or a failing pump.
- Radiators cold with hot water working: diverter or zone valve, or the thermostat is not calling for heat.
What causes the sludge
System water corrodes steel radiators from the inside over the years, especially where inhibitor has never been topped up after work on the system. The debris settles in the bottom of each radiator and in the heat exchanger, which is why kettling and cold bottoms often turn up together.
A magnetic filter fitted on the return catches most of it, and inhibitor keeps new corrosion down. Both are inexpensive next to a heat exchanger.
What we do
We check pump operation, valve travel, flow and return temperatures and the state of the system water, then balance the radiators properly across the house. Where cleaning is needed we say whether a chemical clean or a full power flush is justified, and we fit a filter and inhibitor so it does not come straight back.
Questions we get asked
- Cold at the bottom or cold at the top, does it matter?
- It matters a lot. Cold at the top is air, and bleeding usually sorts it. Cold at the bottom is sludge settling in the radiator and needs the system cleaning rather than bleeding.
- Only the upstairs radiators are cold, why?
- Usually a circulation problem: pump speed, a balancing issue, or air sitting in the highest part of the system. On older gravity-fed systems it can also be the feed and expansion tank.
- Do I need a power flush?
- Sometimes. We test the system first, because a chemical clean plus a magnetic filter is enough on plenty of systems, and flushing will not fix a seized valve or a failing pump.