Replacement radiator is colder at bottom than it is at top.

11 Nov 2004
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United Kingdom
Hi there

I have a new radiator put in to replace the old single radiator. It has significantly heated up the room but I noticed that the top of the rad is really hot but the bottom is luke warm. ie I can keep my hand on the bottom of the rad but not on the top.

Is this normal? No other rads in the house have this problem and I have also replaced another 4 rads in the house with no problems.

Many thanks if anyone has any advice

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It needs balancing! BUT if you have a modern boiler there should be a differential of 15-20° C across flow to return.

So Its normal to have a feelable difference in temperature.

You dont say anything about how this came to be fittted. Any professional SHOULD have balanced the rads as part of the job. Builders and some Polish dont though.

See the FAQ on this site to do it yourself.

Hi Agile

The rads have been fitted over the last few months by the plumber at my work. I did ask him what the prob may be. He says they may need bled, but I thought that would only be if the rad was cold at the top.

I have a look in the FAQ within plumbing and can't see where it says to balance rads. Could you point me in the right direction?

I checked the rads again last night. Two of the rads are definately luke warm at the bottom but boiling at the top. The rooms are still heating but I just don't know if this is something which will cause a long term problem or could I just ignore it?

Thanks for your reply

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It's microbore. It could just be that the rad is cooling. I did think of this as the hot water would naturally rise to the top - would I be correct in thinking this? :confused:

Anyway, still don't know where I can find the bit on how to balance the system. Any clues guys?

:?: Thanks

Morning guys

Got really fed up with the cold living room last night so tried to balance radiators through trial and error rather than via the thermometer method (as I did not have one available at that point). I have two problem rads which were newly installed. One large one in the dining/living room 600 x 1400. One in the bedroom 600 x 1100.

My boiler is in the kitchen. The kitchen rad was fully open on the return side. Have turned this back to 1/4 turn. Have turned hall and small living room rad back to 2 x 1/4 turns. I turned the various other rads in the house back to about 1/2 open. The only rads fully open now are the 2 problem ones. However, they still are not heating up very much at the bottom.

I spoke to the plumber at work today. He says that I don't need to balance rads as it is a combi boiler and that the water which came from the old rads was not sludgy and just had the inhibitor in it. Also said that I should not be having cold spots.

He suggested that I turn all TRV's off and allow the water to be blasted through the rads that are not heating up properly.

Don't know if this will help, but hopefully it will.

Any other thoughts would be appreciated.

I spoke to the plumber at work today. He says that I don't need to balance rads as it is a combi boiler

tell him he is talking complete and utter rubbish

course it needs balancing :rolleyes:

turning the hot ones off might be a start incase you have some trapped air

You will need to balance the rads and the fact it's a combi boiler has no bearing on the fact.

The problem is that it may not be possible to balance if the pipework to the new radiators is not up to the job or is sludged up.

Is it 10mm pipework or heaven forbid 8mm? :(

It depends what horrors are lurking below the floorboards but with 10mm you should be OK after a bit of flushing.
So much for NHS plumbers. I will try to balance them tonight.

Jackthom - it is microbore - quite a thin pipe and not sure if it comes in different sizes.

Thanks again for the reassurance. Didn't want to have another go at balancing if not necessary.

Hi guys

Still problems. I have listed below what I did:-

CH put on. All TRV's turned off except for the problem rad in the dining room. TRV taken off of the rad in the dining room as well. The radiator still did not fully heat up even after blasting this one rad. The top of the rad got very hot. It is the bottom 1/3 which is not heating. The boiler kept cutting off and on (I am assuming because the rads were not calling for heat as the majority were switched off).

I then let heating cool down when I went to fireworks. When I returned I:-

- Took off all TRV's except one in the spare bedroom. This TRV would not turn completely off when the plumber tried to switch it off the other week. I was not sure what would happen when I took it off, so I just opened it fully.
- Opened all LS fully.
- Put on boiler. Heat when to all flow pipes at approximately the same time on the majority, but I think in this order:-
+kitchen (boiler is in kitchen)
+Dining Room
+Living Room (dining & living room are all one room with an archway between)
+ Bathroom (upstairs)
+Hall (downstairs)
+ Daughters Room
+ Our Room
+Spare room

- Flow at boiler 58, return at boiler 44.

- Turned off boiler.
- Closed all lockshields except kitchen.
- Started boiler and worked around rads in the above order. Readings as follows:-
Kitchen Flow (F) 42 Return (R) 36
Dining Room F 47 R 35
Living Rm F 54 R 42
Bathroom F 51 R 40
Hall F 52 R41
Daughters Room F 55 R 42
Our Room F 54 R 44

After doing all of the above (took about 3 hours), the rad in the dining room is still the same with the bottom 1/3 not as warm.

The surface temps on this rad where top of rad in the middle 47, middle of rad in middle 45, bottom of rad in middle 39.

Does this make any sense to anyone?

The problem rad was a new replacement put in in September?

Could it be a valve


- Flow at boiler 58, return at boiler 44.
Make and model of boiler?
Have you set the boiler temperature to 58 or is it automatically determined by the boiler?
If you set it to 58, why?
If you can adjust the boiler temperature, try setting it to 70 or 75 and check the rad temperatures again.
Do you have a room thermostat?

Balancing of microbore systems will always be tricky as the radiators are not in parallel, like the rungs on a ladder. This pic shows the difference -

Assuming you have a two storey house, you will probably have one pair of manifolds for the ground floor rads and a second pair for the first floor rads. The pipes from boiler to manifold will probably be in 22mm; only the pipes from manifold to rads will be 8mm or 10mm.

All the rads fed from one pair of manifolds will heat up almost instantaneously, though the smaller rads will tend to heat up quicker. Adjusting the LS valve alters the flow rate through the radiator. The flow rate is directly proportional to the radiator output (flow for a 2kW is twice that for a 1kW). But this does no mean the LS valve has to be open twice as much, due to the design of the typical LS valve.

Try adjusting the rads in size order - largest first - and have the LS open as little as possible to give the required differential. You will have to adjust the LS by an eighth of a turn or less at a time and leave sufficient time for the temperatures to stabilize.

- Closed all lockshields except kitchen.
- Started boiler and worked around rads in the above order. Readings as follows:-
Kitchen Flow (F) 42 Return (R) 36
Dining Room F 47 R 35
Living Rm F 54 R 42
Bathroom F 51 R 40
Hall F 52 R41
Daughters Room F 55 R 42
Our Room F 54 R 44
Are these temps after you have adjusted the LS valve?

Most of them have a differential of between 10 and 12 degrees; the exception is the kitchen where it is only 6 degrees.

A difference in the flow temperature at each rad is to be expected as there will be some heat lost in the pipe from manifold to rad; but that does not explain the large drop for the kitchen and dining room radiators.
Hello again

I got the chance to speak to one of the engineers at work. He thinks that the flow from the boiler should be higher than 58 deg when at full - therefore probably a boiler fault - which is beyond my capabilities. I will need to double check the temperature again tonight.

For info I have an Alpha CB28X boiler.


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