Central Heating Pipe Sizing

[nordberg73]

Hi,

Six years ago we had a new central heating system installed in our home. We have a Vaillant 831 combi boiler feeding 10 radiators in our 3 bed/2 reception semi-detached.

Recently, we've had some more work done that will ultimately require the boiler to be moved. When the builder who is doing the job pulled some of the ceiling/floor up, he was alarmed to discover that the majority of the pipe work is 15mm with very little 22mm. There is 22mm flow and return from the boiler, but they soon branch off into 15mm. He is going to speak to his plumber and see what he says, but I'd like to be a bit more informed when we come to discuss it.

I'm led to believe that there's no black and white when it comes to pipe sizing and that house size, radiator size, distances etc all play a part when sizing. Is this correct? If the 15mm is unsuitable, what would the symptoms be with central heating?

Many Thanks

 

[muggles]

Pipe sizes are usually calculated based on required heat output, but as a rule of thumb no more than two large or three small radiators should be fed off 15mm pipework. Trying to supply too many radiators will result in poor performance and potentially increased gas bills due to increased boiler cycling

 

[stem]

Larger pipes can carry more water and supply more heat. So, if the pipe system is undersized it won't carry enough water to get all of your radiators hot. In addition, long runs of 15mm also poses more of a restriction to the flow of water.

However, if for example; a 22mm flow and return has three runs of 15mm pipe branching directly off it, (not off each other) and each set feeds 3 radiators that could well be fine. But if one run of 15mm feeds 9 radiators, you may have a problem.

There are other considerations such as the size of the radiators, and number of bends in the pipe etc., but if your radiators get hot, then it is likely all is well.

 

[Dan Robinson]

You gotta love builders

 

[nordberg73]

Larger pipes can carry more water and supply more heat. So, if the pipe system is undersized it won't carry enough water to get all of your radiators hot. In addition, long runs of 15mm also poses more of a restriction to the flow of water.

However, if for example; a 22mm flow and return has three runs of 15mm pipe branching directly off it, (not off each other) and each set feeds 3 radiators that could well be fine. But if one run of 15mm feeds 9 radiators, you may have a problem.

There are other considerations such as the size of the radiators, and number of bends in the pipe etc., but if your radiators get hot, then it is likely all is well.

Thanks, the radiators do get hot and the hot water flow is fine also. We do seem to have quite high gas bills though which has always puzzled me. One of my concerns is that we have just added a loft conversion with en suite so the pipes go up another level and I imagine this could be an issue... I'm wondering if it would be worthwhile, whilst the ceiling is down and the floorboards are accessible, upping the main feed pipes to 22mm?

 

[oph]

15mm pipe carries just over 10K Btu's or 3KW.

From that you can then work out how many rads are loaded up on 15mm pipework

 

[nordberg73]

15mm pipe carries just over 10K Btu's or 3KW.

From that you can then work out how many rads are loaded up on 15mm pipework

Sorry, I don't follow that. Are you saying if I have more than 10k BTUs on a length of purely 15mm pipe, I have a problem? If that's the case, I have several problems!

 

[D_Hailsham]

It all depends on the system design temperature drop between flow and return.

Small Bore Heating Systems explains it all.

 

[oph]

15mm pipe carries just over 10K Btu's or 3KW.

From that you can then work out how many rads are loaded up on 15mm pipework

Sorry, I don't follow that. Are you saying if I have more than 10k BTUs on a length of purely 15mm pipe, I have a problem? If that's the case, I have several problems!

It will pass the energy but the system will take longer to heat up, the boiler will cycle on off more and wont modulate

 

[MJGas]

15mm pipe carries just over 10K Btu's or 3KW.

From that you can then work out how many rads are loaded up on 15mm pipework

 

[Dan Robinson]

*cough* dt20 *cough*