New Central Heating System: Heat not reaching last 2 rads

[mayfel]

Hello!

I've just installed a new central heating system. Due to the wierd and complicated natuire of our building, the route for the pipework is pretty strange (see the diagram below).

I set it running yesterday and started balancing it. The first 7 radiators are heating fine but 8 and 9 aren't heating at all. In fact, the flow pipe just beyond the 7th radiator isn't even hot. The first seven radiators have had the LSVs altered to reduce the flow and the last two are fully open.

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• mayfel
• 27 Nov 2008
• 1

Can anyone tell me what's happening here and what I can do to help it?

Many, many thanks in advance!

 

[D_Hailsham]

Could you number the radiators, so we know which ones you are talking about. Also give the order in which you balanced them.

Do you have TRVs on any rads?

How did you measure the flow and return temps on each rad?
What size is each rad?
What size are the pipes?
What do the numbers 2.5m, 3m etc mean - and why are they significant?

 

[mayfel]

Hello there!

I can answer a couple of questions now and I'll go an check on the others.

Here are the radiators numbered. I balanced them (not technically, just using my hands to feel warmth) from 1-7:

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• mayfel
• 27 Nov 2008
• 1

- All radiators have TRVs which are currently set to highest seting (rooms are all cold so no chance of TRV being shut)

- The pipes are all 15mm. I know I should have put 22 at the beginning but the guy who commissioned the boiler a couple of years back (when we just used it for DHW) said that with the power of the boiler it should be ok with 15mm even if it'd be better with 22.

- The numbers are floor heights. I thought that maybe the pump wasn't powerful enough to get the water up such a lot of hight differences (with rads 8 & 9 requiring the extra height for the return flow). I thought it might be useful to someone more plumbing-minded than myself!

I'll check the Rad sizes now!

Thanks for your time!

 

[mayfel]

Okay, Rad sizes are:

1) 1600 x 400 Single Panel
2) 1800 x 600 Single
3) 1600 x 400 Double
4) 900 x 400 Double
5) 1300 x 400 Double
6) 1200 x 600 Double
7) 1300 x 400 Double
8) 1400 x 600 Single
9) 1100 x 400 Double

Cheers!

 

[mercuryman]

looking at the amount of double radiators you have on the system you will proberly have to put an extra expantion vessel on as the one in boiler works by volume of water in the system, not by heat output.

 

[D_Hailsham]

Here are the radiators numbered. I balanced them (not technically, just using my hands to feel warmth) from 1-7

Thanks for that, You used the correct order to balance the rads, but "feel" can be somewhat inaccurate. If you want to do a proper job, get an infrared thermometer from Maplin for about £30.

All radiators have TRVs which are currently set to highest setting (rooms are all cold so no chance of TRV being shut)

You should take the TRV head off when balancing. If the rooms never get up to temp, so the TRVs operate, either the rads are too small or there is not enough heat coming from the boiler.

The pipes are all 15mm. I know I should have put 22 at the beginning but the guy who commissioned the boiler a couple of years back (when we just used it for DHW) said that with the power of the boiler it should be ok with 15mm.

The size of a pipe is determined by the heat which it has to carry and the flow rate through the pipe. If you have used 15mm where you should have used 22, the pump may not be able to circulate the water. Having a more powerful boiler will just make the problem worse!

The numbers are floor heights. I thought that maybe the pump wasn't powerful enough to get the water up such a lot of hight differences (with rads 8 & 9 requiring the extra height for the return flow).

I thought they were heights; I guess you equated that with the pump "head". Unfortunately that is a frequently misunderstood term, particularly when it is measured in metres/feet.

The pump head has nothing to do with the height of the radiator above the ground. It is a way of measuring the pressure loss in the pipes due to friction. This is determined by length, number of bends, valves, flow rate, temperature drop etc. A head of 4 metres means a pressure loss of approx 40kPa.

 

[D_Hailsham]

Rad sizes are:

1) 1600 x 400 Single Panel
2) 1800 x 600 Single
3) 1600 x 400 Double
4) 900 x 400 Double
5) 1300 x 400 Double
6) 1200 x 600 Double
7) 1300 x 400 Double
8] 1400 x 600 Single
9) 1100 x 400 Double

Do you know their outputs in kW?

 

[gas4you]

From your diagram it could simply be a nasty airlock.

Have you tried turning off all rads except number 9?

 

[mayfel]

Cheers for all the replies!

Unfortunately I don't know the heat outputs of the rads in kW. They're just your bog standard radiator.

Haven't tried turning all the rads off except 9. Will try that tomorrow. If it is an airlock how do I clear it? This has been my suspicion because of the abrupt halt of heat in the pipework.

Hmm... don't really know where to go from here. Would the extra expansion vessel prevent this problem do you think? I suppose I should look into the airlock first before doing anything drastic.

If the pipework is too small would it work to put a second pump in the circuit and have it operate on a pipe thermometer or timer?

Thanks again for your thoughts and time everyone!

 

[gas4you]

By turning off all other rads the full force of the pump should push any air lock into the rad.

You could also try pressurising the system to max temporarily to help remove any possible air lock.

 

[D_Hailsham]

Unfortunately I don't know the heat outputs of the rads in kW. They're just your bog standard radiator.

OK

Take a look at the Stelrad Elite Catalogue and you should be able to identify rads which are close enough to yours. Then post the kW information given. I can then work out the approx pipe sizes required.

 

[namsag]

Do what dave has suggested turn every rad off except the furthest one that is not heating .This will force the air out. Then turn that one off and do the other one,
Taking pressure up as he has also suggested will assist in moving your air lock