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

[tamz]

true indeed I was told at one of their courses that they allow 2.5mbar to be lost at inlet

That is a big get out of jail card for some :wink:

 

[mickie]

increasing the working pressure at the meters govner will increase the working pressure at the boiler, who ever said it wont is wrong because while the gas board was out i got him to up it a little while he was upgrading it, which did help a lot

 

[boilerwoes]

Ignoring the hob.

The pressure drop you have is down to more than you are stating.
A 15m x 22mm run with 10 elbows to the tee
then
3m x 22mm + 1m x 15mm + 1 elbow from tee would give an overall drop of around 1.68mb (it would be around 2.34mb allowing for a 7kw hob)

The 15mm part somewhere under the floor has never been upgraded. Get it done again and done right this time.

Thanks for the calculations. I do think it is a bit odd as before the building work, the working pressure at the boiler was 19mbar, so they must have screwed something up, but what? I am pretty sure 22mm pipe was added to the run for the hob (as the 22mm pipe was tee-d off the pipework going to the boiler). But I am wondering whether there is a blockage in the system. Maybe something is in the pipework that could be causing this significant pressure drop? Are there standard ways of flushing out debris, etc from gas pipework?

I guess the worse case scenario is to re-lay pipes etc, and I will need to have a think as to whether I want them in the walls or under the floors.

 

[GQuigley67]

it would yes but the pressure drop would still be the same

 

[boilerwoes]

increasing the working pressure at the meters govner will increase the working pressure at the boiler, who ever said it wont is wrong because while the gas board was out i got him to up it a little while he was upgrading it, which did help a lot

Is there a maximum working pressure at the meters governer that the gas board will allow? I am wondering whether to ask them to increase the pressure (if possible) and then resign myself to the fact that there is a large pressure drop, but at least the working pressure at the appliances are higher? Thoughts on this approach?

 

[boilerwoes]

it would yes but the pressure drop would still be the same

I am assuming you are referring to my question about the gas board increasing the pressure at the governor. In terms of the pressure drop being the same, I could live with that as long as the pressure to the appliances is high enough. So my next question is whether having a large pressure drop in the system is allowable as long as the appliances have their minimum required working pressure?

 

[mickie]

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.

 

[fuzzy]

I am assuming you are referring to my question about the gas board increasing the pressure at the governor. In terms of the pressure drop being the same, I could live with that as long as the pressure to the appliances is high enough. So my next question is whether having a large pressure drop in the system is allowable as long as the appliances have their minimum required working pressure?

doesnt matter if you can live with it or not, there should be no more than a 1mb drop across the system. there are reasons why the working pressures are controlled at the meter governor, if not you could run it in 10mm and just boost the pressure, but then its not safe

 

[bill577]

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.

 

[boilerwoes]

doesnt matter if you can live with it or not, there should be no more than a 1mb drop across the system. there are reasons why the working pressures are controlled at the meter governor, if not you could run it in 10mm and just boost the pressure, but then its not safe

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?

 

[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.