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Updated 01 November 2020   Boiler Services Ltd 2009-2020

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Radiator Balancing — Balancing a Central Heating System

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Radiator balancing is normally simple when we use this method. It takes a bit of time but it’s not likely to be difficult. People also talk about balancing the central heating system. Unless it's referring to a warm air central heating system or underfloor heating it means the same thing, ensuring that all the radiators get equally hot. Ideally they will all get hot together, in about the same time. If all your radiators get hot pretty evenly you don't need to balance the heating system any further. Adjusting radiator valves can sometimes make them leak so have some old towels ready.

Before you try balancing it's important to make sure that the radiators have been bled (that is, all the air trapped at the top of radiators has been released). If some of your radiators are hot at the bottom but cold at the top (even the very top) you need to bleed the radiators before balancing the heating system. If you bleed a lot of air out of a radiator with the system warm, the water entering the radiator to replace the air may make the radiator hot. You'll only really know whether the radiators need balancing if you start from cold.

Also, before attempting to balance radiators, make sure the pump is not set to too low a speed. Try turning it up; it shouldn't pose problems. If it causes your system to pump water over into the feed and expansion tank (known as the F+E tank or the header tank) or causes air to be pulled down the open vent pipe into the system you may have to slow the pump down again. This usually indicates a system design fault or a blockage somewhere.

Balancing radiators takes a little time but it’s not difficult. There are more complicated methods but the following method works fine for us. We don't use thermometers and we judge radiator temperature by touch.

There are some basic things you need to know to balance radiators but they're simple too. If you're not sure of any of the terms, follow the blue links for an explanation then come back here.

Almost all radiators have 2 radiator valves, generally at the bottom on opposite ends. (Some very old systems may have one at the top and one at the bottom on opposite ends; some newer systems have both valves at the bottom in the middle of the radiator.)

Water enters the radiator through one valve (the flow end or flow valve) and leaves the radiator through the other valve (the return end or valve).

Usually we are restricting a lockshield valve. Different manufacturers' valves turn a different number of times from fully open to fully closed. Some turn only 2 turns and some as much as 5 or 6 turns. If your valve turns 2 turns from open to closed, closing it by 80% means 2 full turns closed (or only a turn open). If it turns 5 turns open to closed 80% closed means 4 full turns closed (or only 1 turn open).

If you close a valve on the flow end by 80% or more, the gap left open in the valve is so small that even a small air bubble coming in with the flow can become trapped in the valve and stop the flow. It may sound daft but it happens. If the flow valve is fully open and you restrict on the return valve, any air bubbles come through the flow valve and rise to the top of the radiator, safely out of the way. They don't enter the return valve which is substantially closed so they don't block it and stop the water flowing through the radiator.

The type of valve fitted is not a guarantee of which is the flow and which the return end. To be sure, start with the heating system cold and go round checking which end of the radiator (which pipe or valve on each radiator) gets hot first. Mark the end in some temporary way; a bit of tape will be fine. Sometimes slightly colder water coming along the flow pipe from a different part of the house can deceive you, making the flow end temporarily cooler than the return end. Check again as they warm up to be sure.

You may need to be quick moving round the radiators because a very small radiator connected to a powerful boiler like a combi may get hot too quickly to be sure which end is which. If necessary, check the smaller radiators first. You can check them in any order and if you've marked with tape you'll know you've checked them all.

If a central heating system needs balancing we start by opening all radiator valves fully, both flow and return. Adjusting radiator valves is not without its consequences, it can make the valves leak. If you're not sure how to deal with leaking valves, read that section first.

Different parts of the central heating circuit have different resistance to water flow but generally, the nearer a radiator is to the boiler, the quicker it will start to get hot. In a well designed and installed heating system little balancing is needed.

With all the radiator valves now fully open, if all the radiators become similarly hot in a similar time there is not much more to do and your radiator balancing is completed. If not, and "not" is usually the case, we go to the radiators which get hottest quickest and restrict the flow through them. We set no fixed order in which to restrict radiator valves except to do the hotter radiators first. This pushes more flow through the remaining, slower, radiators. Where possible we always restrict the water flow on the valve on the return end (the cooler end) and there's a good reason for this. To balance a poorly designed system it may be necessary to close a valve more than 80%, perhaps even 90%. Let me explain what I mean.

Having restricted the return valves on the hottest radiators by 50% or 60% to start with, we wait to see what happens. Cooler radiators will start to get hotter. Some previously cool radiators may get fully hot. If some are still cool we go round again, restricting all the hotter radiators, some which we restricted before get closed down even more (always, if possible, on the return end) and some which weren't restricted first time round are restricted this time because they are now hot. Again we wait to see what effect we've had and again, if necessary, we restrict the hotter radiators. We'll do it yet again if necessary.

Hopefully this process brings the radiator system into balance; you'll only know for sure when you heat the system up again from cold. If you close a valve down too far, that radiator will cool down and may go cold. Just open the restricted valve a little (10% of its total travel between closed and open) and see if that sorts it. Open it progressively more if you need to.

If balancing radiators doesn't resolve the problems, and no matter how you try you still have some cold or very slow radiators, there are two likely causes. First is a failing pump (but make sure someone hasn't partially closed one of the pump isolation valves). Check to see whether you can turn up the pump speed. Second likely cause is sludge build up and partial blockage of pipework or of an air separator. We would always opt to try changing the pump before trying a process like power flushing to clear possibly blocked pipework.

You can find another article we wrote about radiator balancing at

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