Worcester 42CDI problem

[meditek]

My Corgi registered fitter has just fitted a Worcester 42CDI Combi boiler. I have just read through a thread from last August where ideas were put forward but the original poster disappeared while the thread disintegrated into internal argument. Anyway, this is the problem :-

1. The 'commissioning' gas inlet pressure for this boiler is 18mbar at the boiler when full on.

2. when first installed the standing pressure was 21mbar (at the boiler) and the inlet pressure (at the boiler) on test was 14 mbar.

3. the plumber then fitted a wide bore feed from the meter to the boiler with 28mm pipe with 4 right angle bends only approx 6 metres long

4. this raised the standing pressure at the boiler to 28 mbar.

5. Under test the inlet pressure (at the boiler) only rose to 16-17 mbar!

Reading the previous post it would appear that if the regulator is not faulty then it could be a partially blocked feed. Is it the case that Transco should ensure 19 mbars 'working' pressure or not? Should my fitter contact Transco and insist on a proper working test? Do I have any rights to a proper gas feed?

Thanks,

david

 

[cross thread]

what is the working pressure at the meter ? it should be 20 mb +/_ 1. if not supply prob

 

[Rob]

what is the working pressure at the meter ? it should be 21 mb +/_ 2. if not supply prob

Fixed it!

 

[meditek]

what is the working pressure at the meter ? it should be 20 mb +/_ 1. if not supply prob

It is 20 mbar. rob884 what do you mean by 'fixed it'?

Can one persuade Transco or whoever to measure the pressure at the meter with a 4.4m3/h flow which is the boiler's max? I realise standing pressure of 20 mbar are possible with a pinhole feed.

Thanks

 

[WDIK]

what the hell are you talking about?

a standing pressure will not be affected by pipe work size.

where are you getting this aledged reading?
if at the appliance 17mb then lucky you

 

[meditek]

WhatdoIknow, you are quite right. It just so happened (rather than was caused by) that by the time he'd fitted the larger pipe the standing pressure was 28. Must be a local pressure variation or something. My mistake.

Anyway, despite the bigger pipe and the increased pressure the test only showed an increase of 3-4 mbar which was insufficient to commission the boiler. 17 just isn't good enough!

 

[corgiman]

Then you have to increase the size of the pipe

the gas supplier is required by law to provide a minimum of 12 Mbar and no more

 

[Corgigrouch]

One of my customers has a similar problem... The supply from the main in the street is only 18Mb with the Worcester 28Si 2 running.. Pressure tested before entering the meter.. It took three Transco guys, 3 vans one blocked street, several gallons of tea and two tins of biscuits for them to test the supply and to decide that nothing could be done..

 

[bengasman]

the gas supplier is required by law to provide a minimum of 12 Mbar and no more

Where does it say this? Though I find it hard to believe, I would not wager a single pound on it being not true. Would not be the first complete balls up that prescot’s staff had allowed. Boy, am I glad that dimwit is gone.
And more interestingly, how am I supposed to commission a boiler if the MI states the burner pressures must be 15, which means an inlet pressure of at least 17?

 

[Agile]

It is 20 mbar. rob884 what do you mean by 'fixed it'?


Thanks

Dear OP,

You have to learn that the STANDING pressure is of no relevance!!!

What needs to be measured is the working pressure at the meter and at the boiler as measured at the same load on the supply!

There should be no more than 1 mB difference between the WP at the meter and at the appliance!

In practice that transposes to a WP at the boiler input above 18 mB !

Tony

 

[meditek]

It is 20 mbar. rob884 what do you mean by 'fixed it'?


Thanks

Dear OP,

You have to learn that the STANDING pressure is of no relevance!!!

What needs to be measured is the working pressure at the meter and at the boiler as measured at the same load on the supply!

There should be no more than 1 mB difference between the WP at the meter and at the appliance!

In practice that transposes to a WP at the boiler input above 18 mB !

Tony

He didn't test at the meter last night because it was dark. He will do that on Monday.

Thanks, there a couple of things that appear odd to me and that maybe could affect the WP difference :-

a) the 28mm pipe is connected to the meter by a short length of 22mm pipe and the flexible hose in the meter seems like 22 too. At the boiler end there is also a short length of 22. It would seem that the 28 is throttled at either end. Is this normal practice? Seems that the boiler does not have provision for a 28mm direct connect.

b) the previous boiler (greenstar 400) was fed by 22mm pipe. This was tried first before the 28. WP at the meter was 20 and 14 at the boiler. After fitting the 28 he connected the old 22 via a T-joint to the 28 just below the boiler, presumably thinking it would give a better feed. However, it seems possible to me that this is having the opposite effect as some of the gas leaving the meter is being directed over an inefficient route. What do you think?

Thanks,

David

 

[Rob]

the gas supplier is required by law to provide a minimum of 12 Mbar and no more

Where does it say this? Though I find it hard to believe, I would not wager a single pound on it being not true. Would not be the first complete balls up that prescot’s staff had allowed. Boy, am I glad that dimwit is gone.
And more interestingly, how am I supposed to commission a boiler if the MI states the burner pressures must be 15, which means an inlet pressure of at least 17?

I heard slightly different a few weeks ago from Transco guy. He said they have to supply a minimum of 14mb. So if you ring them up, they come out and say it's ok then it gets left on.

 

[sooey]

That's what I thought, minimum, w/p of 19 mb with a 4 mb allowance through the meter and 1mb through the pipe work equals minimum of 14 mbar at the appliance. How the hell your supposed to commision is another story though.

 

[ChrisR]

Da's right Sooey. The 4mbar bit is contentious though. I haven't got it quite straight who says exactly what yet, which is a task I have cos I teach the stuff!

14mbar miinimum is at the outlet of the meter, though, without a load being specified as far as I'm aware.
I've complained about standing pressures below 19mbar, and found it higher later after they'd turned it up miles away. Water in the pipes can cause the gas carrier big problems.

OP many meters have 3/4 = 19mm outlet connections and many boilers 15mm inlets, but you have to use whatever you need to use in between to keep the resistance low. It could be 35mm. Everyone accepts that the gas pipe sizing tables are wrong, Even when you're as careful as you can be on the joints, etc. In practice, sulphides from the gas, roughness, dirt, burrs, compounds and the rest mean you get more pressure drop than you might think.

 

[meditek]

Further to my last questions (answers would be appreciated) the outlet in my meter is actually 28mm. The 22mm is joined to that then the 28mm supply to my boiler.