A radiator that never gets properly hot is usually not a faulty radiator. Either it is getting less water than the ones nearer the boiler, or it was never big enough for the room in the first place. Bleeding it again fixes neither of those.

Balancing: why the far end of the house is colder

Water takes the easiest route. Radiators closest to the pump take more than their share of the flow and the ones at the end of the run get what is left, which is how one room sits at temperature while another never quite arrives. Balancing corrects it by setting the lockshield valve on every radiator so the flow is shared out, judged on the temperature difference across each radiator rather than by hand. It is worth doing after a new boiler, after a radiator has been added or swapped, and any time the system has been drained and refilled.

Sizing: a hot radiator in a cold room

Output is worked out from the room, not from the wall. Room size, how many external walls and windows it has, the construction, and the flow temperature the boiler runs at all feed into it. Two things catch people out. Turning the boiler flow temperature down, which is sensible for running costs, lowers the output of every radiator in the house, so a panel that was already marginal becomes undersized. And vertical panels, designer radiators and towel rails give less heat for the space they occupy, so a towel rail on its own is rarely enough to heat a bathroom.

What a cold spot is telling you

• Cold across the top: air in the panel. If it comes back, something is drawing it in.

• Cold across the bottom: debris settled inside, restricting flow through the panel.

• One radiator cold, the rest fine: a seized thermostatic valve pin, or a lockshield closed years ago.

• A whole floor lukewarm: balance, or a pump not doing enough.

Repairs, replacements and new radiators

A weep at a valve nut or spindle can normally be re-made, or the valve changed. A leak from the body of the panel is corrosion working outwards and more pinholes will follow, so that one gets replaced rather than sealed. A like-for-like swap is straightforward. Adding a radiator or moving one to another wall means running pipework, usually under the floor, and rebalancing afterwards. Thermostatic valves go on as standard, except in the room where the main thermostat sits: that one stays on a manual valve so the thermostat can read the room properly.

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Common questions

What is balancing, and how do I know if my system needs it?

Balancing is setting how much flow each radiator gets, using the lockshield valve on the return side. The sign you need it is a house that heats unevenly for no obvious reason: rooms near the boiler roasting while the far bedroom stays cool, with no air in the panels and no stuck valves. It costs nothing in parts, and it is the first thing to look at before anybody buys a bigger radiator.

I turned the boiler flow temperature down and the house feels colder. Why?

Because radiator output depends on how hot the water inside it is. Drop the flow temperature and every panel in the house puts out less heat, which is fine in a well insulated room with a generous radiator and very noticeable in a cold room with a small one. The rooms that suffer are the ones that were already borderline. Either those panels get upsized, or the flow temperature comes back up a little.

Would a bigger radiator heat the room faster?

It will get the room warmer, which is a different thing from faster. A larger panel only helps if the pipework and the flow reaching it can feed it, and if the rest of the system is balanced, otherwise it simply takes heat from the radiators around it. We would rather check the flow and the balance first, because quite often the panel already on the wall is the right size and is just being starved.