Why one room is always cold, what sludge actually is and when flushing is worth it, what a magnetic filter does and does not catch, and how radiators should be sized.
Three causes, most of the complaints
Central heating problems present in a lot of different ways and resolve to a short list of causes.
- Dirty system water. Iron oxide – sludge – forms wherever oxygen reaches steel, and it settles at the bottom of radiators, blocks heat exchangers and wrecks pumps. Radiators cold at the bottom is the classic symptom.
- No balancing. Water takes the easiest route, so radiators near the pump get more flow than they need and distant ones get too little. The result is the room that is always freezing while another is stifling. Balancing a heating system covers it.
- Emitters that were never sized. A radiator either can or cannot deliver the heat a room loses, and a great many were fitted to fill the space under a window rather than to a calculation. Sizing radiators properly sets out the actual method.
Cold at the bottom is sludge – the radiator is full of sediment. Cold at the top is air – it needs venting. Warm but never hot, when everything else is fine, is usually balancing. Those three symptoms point at three completely different jobs and they are constantly confused.
Start here, depending on the symptom
| Symptom | Start with |
|---|---|
| Radiator cold at the bottom | Radiators cold at the bottom |
| One room never warms up | Balancing a heating system |
| Told you need a power flush | Power flush, chemical flush, or neither |
| Black water when you bleed | Magnetic system filters |
| Rooms too hot and too cold at once | Heating controls and TRVs |
| New radiator not making a difference | Sizing radiators properly |
| Considering underfloor heating | Underfloor heating vs radiators |
Flushing, and when it is genuinely needed
Power flushing is one of the more oversold services in the trade, and also one that is genuinely necessary in the right circumstances. Both of those are true at once.
Power flush, chemical flush, or neither separates them. The short version:
- A chemical clean – dosing the system, running it, then draining and refilling – resolves a great deal at modest cost, and is frequently all that is needed.
- A power flush uses a pump to force water through at high velocity in alternating directions. It shifts material a chemical clean will not, and it is the right answer on a genuinely heavily fouled system.
- Neither is the answer on a system that is simply unbalanced, or where one radiator has a problem the other twelve do not.
- On a very old system, flushing can reveal leaks that the sludge was effectively sealing. That is not a reason to avoid it, but it is a reason to know it before starting.
What matters more than the flush is what happens afterwards: the correct inhibitor dose, and a magnetic filter to catch what forms next. A system flushed and then left without inhibitor will be back where it started.
Controls, which is where the easy savings are
A heating system with good emitters and poor controls wastes a great deal. It is also the cheapest thing on this page to put right.
- Heating controls, TRVs and smart thermostats – what each control actually does, and the combinations that work.
- The single most common fault we find: a TRV in the same room as the main thermostat, which makes the two fight each other.
- The second most common: TRVs that have seized, which happens to any valve left in one position for years, and which is usually recoverable rather than a replacement.
- Weather compensation and load compensation are genuinely worthwhile on a modern condensing boiler and are routinely omitted from installations.
And the point that underlies all of it: a condensing boiler only condenses – only achieves the efficiency it is sold on – when the return water temperature is low enough. Controls, balancing and correctly sized emitters are what make that happen. A modern boiler on an unbalanced system with oversized flow temperatures is being run as an ordinary boiler.
See also power flushing, system balancing, system filters and radiator installation.
One room that is always cold? That is usually balancing, and it is usually an afternoon rather than a new radiator.
Questions we get asked about this
Why is one radiator always cold?
Usually balancing – water takes the easiest route, and the distant radiator gets too little flow. Cold at the bottom instead means sludge; cold at the top means air.
Do I need a power flush?
Sometimes. A chemical clean resolves a great deal at lower cost, and neither is the answer if the real problem is balancing. The guide separates them honestly.
What does a magnetic filter actually do?
Catches iron oxide circulating in the system before it reaches the boiler’s heat exchanger. It catches magnetic debris; it is not a substitute for inhibitor and it does not catch everything.
Should I have a TRV in the same room as the thermostat?
No, and it is the commonest control fault we find. The two end up fighting each other, and the room with the thermostat should generally have a lockshield pair.
My new boiler does not feel more efficient.
A condensing boiler only condenses when return temperatures are low enough. On an unbalanced system with high flow temperatures it runs as an ordinary boiler regardless of its rating.
How do I know what size radiator a room needs?
From a heat loss calculation for that room, not from the size of the gap under the window. It is the reason a replacement radiator sometimes makes no difference.
What we cover
Central heating
- Balancing a heating system, and why one room is always freezing
- Heating controls, TRVs and smart thermostats
- heating guide 1
- heating guide 2
- heating guide 3
- heating guide 4
- heating guide 5
- heating guide 6
- heating guide 7
- Magnetic system filters: what they catch and what they do not
- Power flush, chemical flush, or neither
- Radiators cold at the bottom: sludge, and what it really means
- Sizing radiators properly: heat loss, not guesswork
- Underfloor heating vs radiators: an honest comparison
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.