Most heating problems people call us about start with one sentence: "one of the radiators has gone cold." The useful part is that a radiator tells you roughly what is wrong by where it goes cold, and a fair few faults are ones you can sort yourself in ten minutes. Here is how to read your own system before anyone quotes you for work on it.
A radiator heats from the flow valve outwards and fills from the bottom up, so the pattern of cold gives the game away. Put your hand flat on it about twenty minutes after the heating fires, not two minutes, or you will misdiagnose a radiator that is simply last in the loop.
One warning: if several radiators upstairs are cold and the boiler pressure gauge is sitting under 1 bar, top up the system first. Low pressure mimics air locks and you will bleed for nothing.
South Lanarkshire is supplied with soft water, largely from the Daer and Loch Katrine catchments, so hard limescale furring is far less of a problem than in the south of England. What we do see is magnetite: black iron oxide sludge formed when air gets into a system and slowly eats the steel radiator panels from the inside.
It drops out in the coolest, slowest parts of the system, which is the bottom of your radiators and the base of the heat exchanger. A lot of the 1970s and 1980s housing around Fairhill, Hillhouse and Eddlewood runs on 8mm or 10mm microbore pipe, and microbore blocks with a fraction of the sludge a 15mm system would shrug off. If your radiators are cold along the bottom third, the water coming out at bleed points is black, and the boiler is kettling, that is the picture.
A power flush or a magnetic filter clean typically runs a few hundred pounds depending on the number of radiators and whether the pipework is microbore, and it should end with an inhibitor dose and a filter fitted if there is not one already. Inhibitor is the cheap part and the part most often skipped.
If the rooms nearest the boiler roast while the back bedroom never gets there, the system is unbalanced rather than broken. Balancing means throttling the lockshield valve, the one with the plastic cap, on the greedy radiators so more flow reaches the far ones.
Done properly it is measured, not guessed: you want roughly 11 to 12 degrees difference between the flow pipe and the return pipe at each radiator, checked with a clip-on or infrared thermometer while the system is up to temperature. Open the furthest radiator fully, then work back closing the nearest ones down a quarter turn at a time. It is fiddly and takes an hour or two on a full house, but it costs nothing in parts and it often removes the perceived need for a bigger boiler entirely.
Repair is usually right for stuck valves, weeping unions and single sludged panels. Replace when a panel is pitted or seeping from the body, when the radiator is visibly undersized for the room, or when the old single-panel type is struggling in a high-ceilinged Victorian sandstone flat of the sort you find around Burnbank and Low Waters.
Sizing matters more than brand. A room needs a calculated output in watts based on its volume, glazing, external walls and insulation, and a K2 double-panel double-convector puts out roughly 70 to 80 per cent more heat than a K1 of the same dimensions without taking any more wall space. A straight like-for-like swap, draining down and refitting on existing tails, is generally in the region of £150 to £300 per radiator including a new TRV, while moving a radiator to another wall means new pipework and costs more.
One forward-looking point: modern condensing boilers are most efficient running flow temperatures of 50 to 55 degrees rather than 75, and air source heat pumps lower still. Deliberately going one size up when you replace a radiator means you can drop the flow temperature later and keep the same warmth, which is worth doing now rather than replacing twice.
Once a year before the heating season is plenty for a healthy system. If you are bleeding the same radiator every few weeks, air is getting in somewhere, usually through a leak, a failed automatic air vent or a pump drawing air, and that needs looking at rather than bleeding repeatedly.
Yes, particularly on an older system with steel radiators, and most boiler manufacturers now require one to keep the warranty valid. It catches magnetite before it reaches the heat exchanger, though it only works if someone actually cleans it out at the annual service.
Usually, but it depends on the boiler output and the existing pipework. A combi with spare capacity can often take one more radiator, whereas a microbore system that is already struggling to heat the far end of the house will normally need balancing or a pipework upgrade first.
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