Adding 3 metres of 28mm pipe at the end of a 15 metre 22mm run

[boilerwoes]

All,

First time poster, long time reader!

I need some advice before I get some quotes to resolve an issue. I found out a couple of days ago (from a boiler engineer) that the gas inlet working pressure at my boiler is too low (approx 13.5mbar). I haven't got precise figures, but apparently the working pressure at the gas meter is approximately 20mbar, but the boiler engineer was not precise about it. The system is gas tight, so no leaks. System has 22mm pipe to a boiler and to a gas hob. The 22mm reduces to 15mm for the last 0.5m for the boiler, and 1m for the gas hob.

Replacing the 22mm pipe with 28mm pipe from the gas meter is going to be hard due to obstacles etc. One option could be to increase the pipe size for the last 2-3 metres of the 22mm run, just before it enters the boiler, but am not sure if this will help reduce the pressure drop?

In terms of length of 22mm pipe - there is approx 15 metres of 22mm pipe between gas meter and boiler with about 10 corners and 1 tee. From the tee, there is approximately 3 metres of 22mm pipe before reducing down to 1 metre of 15mm pipe going into a hob.

Thanks in advance.

 

[boilerwoes]

The minimum that vaillant allow is 17mb, I would disconnect the boiler and hob and blast some compressed air through the pipes, as the pipes have been cut to install a tee for the hob the installers may have got dirt ect in the pipe.

Sounds like a sensible first option - is compressed air etc a standard piece of kit to expect RGI's to have with them?

 

[boilerwoes]

the working pressure is their to make sure a hob or another gas appliance doesnt go out while a boiler is running, also most boilers wont work properly with a working pressure of say, 15mb..

if you only have a boiler and nothing else, and the burner pressure is spot on, just the working pressure is down its only ncs, it wouldnt be dangerous.

I have a boiler and a 4-burner hob. What's the difference between burner pressure and working pressure? I thought they were the same thing? There is also a concept of standing pressure, right?

 

[fuzzy]

Thanks Fuzzy - that's why I asked the question as I wasn't sure if there are regulations that state it can't be more than 1mb or not. Sounds like you are saying that there are regulations that state there cannot be more than a 1mb drop, full stop. So even if they were to up the pressure at the governor (to say 25mb), to allow about 19mb at both appliances, there would still be a "breach" of the regulations as the drop is 6mb.

So it sounds like the only option I have is to fix the fact that there is a 6mb drop across the pipework (either find source of blockage or re-lay new pipe), and reduce that to no more than 1-2mb. Is that right?

not a direct reg as far as im aware. its purpose is for design. there incoming pressure is 21mb (+-2mb) so you should get 20mb at appliance. Although some appliances as previously mentioned can operate fine on less incoming pressure that is just that appliance. they need guidelines for design, they have said 1mb max drop or classed as ncs

 

[mickie]

burner pressure = appliance
working pressure = pipework

both got with appliance running

 

[fuzzy]

I have a boiler and a 4-burner hob. What's the difference between burner pressure and working pressure? I thought they were the same thing? There is also a concept of standing pressure, right?

burner pressure can be lower than working pressure. one is before appliance regulator and one after. you need say 20mb working pressure at appliance and mis for burner pressure, could be 14mb, bu reduced at source. the appliance regulator is designed to work with incoming pressures of around 20mb. if somebody boosted meter regulator then this incoming pressure could/would be more and the applaince regulator is not designed for that (not saying it wouldnt work, not sure why it wouldnt, unless seriously high but not worth risk)

 

[fuzzy]

burner pressure = appliance
working pressure = pipework

both got with appliance running

working pressure at appliance and at meter

 

[mickie]

feeling a bit uncomfortable now, a little too much info

 

[boilerwoes]

feeling a bit uncomfortable now, a little too much info

Don't be - I am not daft to touch anything gas related. To be honest, not even practical enough to sort out plumbing issues (I get a plumber to sort out leaks on rads etc), but I like to understand and know about issues (boiler, gas, water, etc) and potential resolutions. Having been messed around by dodgy tradesmen (RGI or otherwise), I want to make sure I know what's what first.

For example, I now know to push for them first trying compressed air to clear any blockage. I also know that the builders screwed something up as technically, there shouldn't be a significant pressure drop (based on the calculations posted earlier).

 

[tamz]

is compressed air etc a standard piece of kit to expect RGI's to have with them?

Definately not.

 

[bill577]

If you let the rgi know that you want the pipes cleared with c/air he should be able to get a hold of a compressor (I use my scuba cylinders if I want to clear pipes).

 

[REDSAW]

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.....

 

[boilerwoes]

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.

 

[Pssst]

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.

 

[boilerwoes]

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.

 

[gasmarc]

sounds like defo a partial blockage

 

[helix]

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.

 

[boilerwoes]

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?

 

[GQuigley67]

vaillant allow a drop of around 2-2.5mb at their valve so it sounds like your getting a correct reading.