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Discuss Adding 3 metres of 28mm pipe at the end of a 15 metre 22mm run in the Plumbing Jobs | The Job-board area at PlumbersForums.net

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feeling a bit uncomfortable now, a little too much info
yeh! About 10 post's ago.imo.
This is a public forum lads, you dont even need to be a member to read this.....
 
All,

Thanks for all your input. RGI is booked in, so will let you know what the outcome is once he has had a chance to accurately measure the pressure at all points and diagnose the problem. Am hoping it is just a minor blockage (a bit of grit, etc) which can be flushed through.

Cheers.
 
An interesting discussion..apologies,havent read all of it but will just add that IMHO...

Adding a larger pipe diameter anywhere along the run will lead to a decrease in pressure loss due to the reduced resistance to flow through that section of pipework. Of course,from a design point of view, we should be starting with the bigger pipe diamater at the meter and reducing as/when necessary.

What you do need to watch out for on long runs is the discharge volume.

For example...and to illustrate, if we have a known pressure at the meter outlet,say 20mb,connected to a length of 15mm tubing..lets say 10m..then there is a finite discharge volume of gas which will be available at the end of that pipework. You can find tables in various documents for discharge rates of given diameters of pipe for a given pressure differential.

So,in our example..if we have a great big length of 15mm and we then stuck in a section of 28mm on the end of that in the hope of reducing the pressure loss..we may still be on a loser because the discharge volume on the end of the 15mm length could well be insufficient to feed our appliances !


Meter outlet WP is typically 20mb +-2mb. It can be lower than this and often is due to network management and other issues.

It is set this way as most appliances save for central heating boilers, have no internal pressure regulation and so for example,cookers and fires rely on the meter regulator to maintain a safe working condition.

If your tempted to break the seal on a meter reg and wind it down to try and get your new combi to work better, think about over gassing the other appliances !



Once upon a time everyone had gas fires and cookers,then they had "normal" natural draught central heating boilers.

Now everyone wants a Megawatt combi.

No one is throwing lots of new pipework in the ground and increasing the diameters/pressures of the supply network so something has to give.
 
All,

Just as an update - I have had an RGI come over to investigate. Part of the pipe was cut in order for him to blow down it to see if there was any obvious resistance/blockage - nothing was apparent. The pipe is definitely a full 22mm run - no 15mm pipes at all. It is possible that certain parts of the pipe which were embedded in wall and weren't accessible may have a blockage, but he couldn't feel much resistance.

After doing this, the working gas pressure went up to 17mb (acceptable for Vaillants). So he may have dislodged something, but he didn't see what, if anything. However a further test 2 weeks later showed this drop down to 15mb. So it appears that there is more debris in the system... If anyone has any other thoughts, then chip in. I wonder whether they could have used compressed air, rather than blowing? Would that have been better?

Only solution now is to run a new pipe - and to be safe, it will be 28mm - at least this future proofs it. That will be scheduled in at a later stage.

Cheers.
 
Would it helps if the RGI uses a powerful vacuum machine to suck out the pipe work once it has been disconnected at the meter and the appliances? Just a suggestion.
 
Just thought I would give you an update on this. I had an RGI install a new pipe run - shortest run possible was about 10m of 28mm pipe to the boiler from the meter. This helped the situation a bit, but we were still seeing a pressure drop of more than 2mbar - so we were getting about 17.5/18mbar at the boiler which was surprising (at least to me anyway) as I was expecting virtually no drop. There were definitely no issues with the new pipework.

Do you think any "gunk" could have got into the boiler gas valve to reduce the working pressure?
 
vaillant allow a drop of around 2-2.5mb at their valve so it sounds like your getting a correct reading.
 
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