Balancing a CH system

24 May 2005
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United Kingdom
Dear all,
I have a few questions about balancing a CH system. I ahve looked at:

1) It says to start on the rad nearest rad to boiler, does this mean the closest in feet and inches to the boiler or the first rad on the loop from the boiler?

2) It says:
Fit the radiator thermometers to the INLET and OUTLET pipes of the nearest radiator to the boiler. DO NOT fit them to the main flow and return pipes.
Switch on the central heating system.
Close the lockshield valve on the first radiator to almost closed, as the temperature of the systems comes up, gradually open up the valve until the temperature difference between the two thermometers is about 20°F (12°F).

I know the difference should be about 20°F, but what should the inlet pipe temp be?

3) I have TRV fitted on some of my rads, does this affect balancing of the system, as som of the lockshield valves have gone?

Thanks in advance

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if the lockshield vavles don't function you can't balance the system.

The flow temerature depends on the type of boiler and wether or not the thermostat is adjustable. Some old boilers have fully adjustable with a knob or a high/low switch.

The flow temp should be set on the boiler to correspond to the temperature at which the radiator manufacturer calculated the heat output of your radiators I guess.
Interesting post, this. When it says use "radiator thermometers", where can I buy these? I've looked at thermistor types, but to try and stick the little thermistor beads to the rad without them coming off as the rad heats up, seems to be a bit of a challenge. Or do you just hold it up against each end of the rad (or the pipes) for a minute or so? What do "real plumbers" use?
I've bought an infrared thermometre, seems very accurate and doesn't require me burning my fingers when taking the temperature of anything :).

<--- btw DIYer :)
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Sorry chaps I have to have this straight in my head before I attempt.

Am I right in thinking that balancing a systems involves adjusting the flow of water through the FLOW tail into the radiator?

If so, some of my rads do not have lockshields on the flow. Some have TRVs on the flow and handwheel types on the return, how do I balance these as obviosly somepeople will adjust the TRV thus adjusting the balance of the system.

Or have I got this arse about face?

Thanks for you patiance

You adjust the lockshield valve - it doesn't make any difference whether it's on the flow or return.
Sorry mate, one of the rads has a TRV on one end and a normal rad valve the other, what do I need to do for this on, should I replace the rad valve with a lockshield?

Stolen from another forum. Author unknown.
Really it is irrelevant which rad is nearest the boiler. What you need to find is which rad gets hot first when all rad valves and all lockshields are FULLY OPEN.

btw, if you have trv's it's best to remove the trv head during balancing otherwise they may throttle back automatically which will give false readings elsewhere.

So start with a cold system, all valves fully open, all trv heads removed, room stat on maximum, programmer to ON.

Switch on boiler and run around like a maniac feeling which rad seems to be warming up first. After about half an hour see which rads are hottest, warm, cool and write this down.

Now restrict the lockshield by about 80% of its possible amount (i.e. lockshield valve only about 20% open) on the rad that got hot quickest and wait about ten minutes. Now feel the rads that were cool. They should have improved a bit but you will then need to restrict the second quickest rad to get hot and so on and so on.........

It's a tedious and laborious job, especially on a massive three storey house with 38 radiators ...........

The general idea is that the pressure needed in the system is equal (ish) throughout to stop the water simply taking the route of least resistance. So a distant radiator may be harder for the water to get to so you have to make it harder for the water to get to the nearer (easier) rads.

It can take days of fine tuning to get it perfect but you rarely need it to be perfect, so long as all rads heat up it doesn't really matter if some take 5 minutes longer than others.

Now next time you remove a rad to decorate (or whatever) you should note down the number of turns you had to turn the lockshield valve to close it off. Now when you refit that rad you can open the lockshield valve by the same number of turns and "hey presto" you haven't buggered up the balancing!

Decorators please note the above...............
Excellent, Moss5. Best explanation I've seen for this balancing job. I've been fiddling with a friend's house for ages, now I will go and do it properly.

But what's with all this 20deg difference that people go on about? I thought the temperature differential should be measured at the flow & return to the boiler - not each and every radiator - so that you get the best out of the boiler according to its commissioning instructions. Doing what you say seems entirely logical and foolproof, and if this results in the requisite temperature differential to suit the boiler, then fine. If not, then a bit of tweaking might be necessary, either to the rad valves or to the boiler setting (to be done by a qualified heating technician, of course).
One of my rads has two normal rad valves, do I adjust the vlave on the flow to balance this particular rad?


With two identical valves at each end of a radiator, partially-closing either one will cause reduced flow through the radiator. It would not matter if this were on the flow, or the return.
Adjusting the flow-rate to achieve a set temperature-drop is one way of setting the radiator heat-output to match the manufacturers declared output. These stated outputs are dependent on duplicating the test conditions with a stated flow-rate/delivery temperature at a stated ambient air temperature.
The driving force for giving up heat to the air is the temperature difference (water/air) and the volume (mass) flow rate.
If the flow through the radiator were a teaspoon-full/minute, it would be cool at the exit. Increase the hot flow to a bucket-full/minute, the water would give up more heat, but lose less temperature.
The temperature-drop method is fine if your boiler/pump output exceeds the output of your radiators.

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