A few questions

[bikerbas]

Can someone explain clearly a few questions.Im currently doing my portfolio and starting to revise reading the new bpec book.
I know how to tightness test.purging and gas rating ,pipe sizing etc.
Q1 when you calculate your pipe sizing to achieve 1mb drop over all appliances how to you test this once everything connected?
Q2 How do you test working pressures is at the meter with all appliances running?
Q3When you read some MI on boilers it will say it needs 18.5mb at inlet so if you had 21mb at meter and 19mb at the boiler could you sign off the appliance even thou youve got a 2mb drop but are in MI settings?
Cheers

 

[aceplumber]

Can someone explain clearly a few questions.Im currently doing my portfolio and starting to revise reading the new bpec book.
I know how to tightness test.purging and gas rating ,pipe sizing etc.
Q1 when you calculate your pipe sizing to achieve 1mb drop over all appliances how to you test this once everything connected?
Q2 How do you test working pressures is at the meter with all appliances running?
Q3When you read some MI on boilers it will say it needs 18.5mb at inlet so if you had 21mb at meter and 19mb at the boiler could you sign off the appliance even thou youve got a 2mb drop but are in MI settings?
Cheers

Q1 do working pessure test at meter hopefully 21mbr working (an appliance on ) and then test at appliance (when on ) , hopefully no less than 20mbr Q2working pressure at meter should be 21mbr + or - 1mbr Q3 you will get a large pressure drop through boiler from inlet to test point , if you were to cut a tee piece in just before the boiler , your pressure reading would be about 21 or 20 mbr

 

[bikerbas]

Cheers for that understand now

 

[JamesC]

well my views are

Q1 Do working pressure at the appliances with ALL apliances on full rate.

Q2 Working pressures are done 1) at the meter with ALL appliances on at full rate. You want 21mbar + or - 2mbar.
2) At P1 of the multifunctional control valve on boilers. Not before it enters the appliance or anywhere else. Also with ALL appliances on full rate.
Other appliances vary so check MI's

Q3 No you cant sign it off you've got a 2mbar drop and you can only have 1mbar.

 

[bikerbas]

So for Q1 would you have to install a tee piece before the gas valve to check the 1mb drop,Because if you test at gas valve you can lose a couple of mb through the valve?

 

[diesel_ltd]

Q3 you can sign off the appliance if it states in the MI 18.5mbar so long as your getting what the MI says.

Always go by the MI as they will have to do all the tests on the boilers to make sure its safe
So if it says 18.5mbar then its safe to run at 18.5mbar

 

[GrahamM]

So for Q1 would you have to install a tee piece before the gas valve to check the 1mb drop,Because if you test at gas valve you can lose a couple of mb through the valve?

The working pressure is taken at the meter test point with all appliances at full rate. This must be 21 mbar +/- 2 mbar. (19-23mbar)

The operating pressure of an individual appliance is taken at the appliance inlet pressure test point, again on full rate. Most appliances will have a means to test this. You are only allowed 1 mbar drop from the working to operating pressure so this will be in range of 18-22 mbar.

So if your working pressure is 21 mbar, the operating pressure must be not less than 20 mbar. If it is not, the pipe installation may be suspect.
If you were to be getting 19 mbar and the MI are looking for an operating pressure of 18.5 then this should be ok.
If the MI are looking for 20 mbar operating pressure and you have 19 mbar but the burner pressure and all other checks are ok then the installation is NCS.
If the burner pressure is affected by the low operating pressure then the install may be AR or even ID depending on the appliance.

Now the even more complicated bit. If the new appliance was fitted to previously installed pipework and there is more than the 1mbar drop between working and operating pressure but the appliance runs as it should then it will be NCS and is at your discretion if you sign off.

If it is a new pipework install you are fitting to and you have more than the 1mbar drop then it would normally point to poor workmanship. More than likelly inadequate pipe sizing and this may be reportable. And should certainly not be signed off.

 

[bikerbas]

I thought it was 1mb over all appliances not individual so if you had a boiler ,cooker and gas fire they all would be on full power than measure? so the 1mb drop is shared over all the appliances?

 

[chelsea]

i was taught its 21 + or - 2, not 1,just looked inmy corgi and logic book and that is what is says

 

[JamesC]

I thought it was 1mb over all appliances not individual so if you had a boiler ,cooker and gas fire they all would be on full power than measure? so the 1mb drop is shared over all the appliances?

Your allowed a 1mb drop at each appliance with all appliances on full rate.

 

[GrahamM]

i was taught its 21 + or - 2, not 1,just looked inmy corgi and logic book and that is what is says

Think you are getting confused with the working pressure and the permissible pressure loss across the pipework.

The meter regulator is set to give a working pressure of 21 +/-2mbar(19-23 mbar) with all appliances at full rate. There is a permissible drop of 1 mbar drop from meter to each individual appliance allowed. Any more generally points to bad pipe sizing, or blockage or even too many fitted joints.(form bend always prefered)

To sum up: if you have a working pressure of 21 mbar at meter, then you cannot have any less than 20 mbar at the appliance.

 

[revolt3k]

How many installations do you guys find to have more than 1mb loss between meter and appliance?

If the gas rate and burner pressure is ok - NCS
and
If the gas rate and burner pressure is out (more than 5%) - AR and turn off/label etc

 

[JamesC]

How many installations do you guys find to have more than 1mb loss between meter and appliance?

If the gas rate and burner pressure is ok - NCS
and
If the gas rate and burner pressure is out (more than 5%) - AR and turn off/label etc

1 in the last year with more than 1mb drop.

 

[revolt3k]

Not many then

 

[gbgas001]

Hi Guys

The working pressure at the meter test is only to make sure that the meter regulator is set properly and gives a pressure between 19 and 23 mb. We also do the standing pressure to make sure the regulator closes and this must below 30mb.

When checking pipe sizes are ok,
1 You would turn on the boiler then record the working pressure at the meter
2 then at the appliance inlet... in theory there should only be a maximum of one mb drop in the pipe line.

This is a British Standard... but in the case of most condensing appliances (manufacturers instructions) they may accept a minimum of 17mb at the inlet due to the design of the burner. In this case the manufacturer is always correct and the British standards can be ignored...ish.

Step one and two can then be completed for all other appliances

At no time are you asked to turn on all gas appliances in a house and record the pressure at the meter. This would be a foolish test as the pressure may well be less that 21+ or - 2mb, in this situation it would be ok... looking at typical demands how often is more than one appliance on together for any length of time the answer is minimal. At those times this may make the appliances slightly under gassed and as we all know undergassed is not a combustion fault...


Hope this clears thing up

gb-gas

 

[Toddyplumb]

However the 1mbar drop on the pipeline cannot be ignored no matter what pressure the boiler demands.

 

[BLOD]

The MI for the Ideal icos state "the icos HE range is able to deliver it's full output at inlet pressures down to 14mb".
So if you had 15mb at the appliance and 19mb at the meter would it be ok to commission and ncs for the pipework ?

blod

 

[Toddyplumb]

I wouldnt ! What then happens to that 15mbar when other appliances fire up . I Know we have just been discussing that we do not test for this , but you know its gonna drop even futher

 

[gbgas001]

The MI for the Ideal icos state "the icos HE range is able to deliver it's full output at inlet pressures down to 14mb".
So if you had 15mb at the appliance and 19mb at the meter would it be ok to commission and ncs for the pipework ?

blod

This type of appliance has a pre mix burner, this is sucking the gas out the pipe so the inlet pressure you are reading is not the actual pressure it could be 2 to 5 mb less due to the vacume effect.

Even the older valliant boilers recognised this with there normal fan flued units. when doing a burner pressure you connected one side of the manometer to the test point and the other to the combustion chamber casing to give you the true burner pressure.

Even running 28mm to these types of boilers may not give you the 1mb max drop you are looking for... would that make it NCS when you know the pipe work is well oversized.

You are correct though in that the judgment is yours and in most cases you can tell if the pipework is too small by experience... you could always do the pipe size calculations for it to make the correct call for NCS.

gb-gas

 

[Toddyplumb]

I had this on a gas safe inspection drop on gas line ( 28 - 22 mm) 1.2ish to a pre mix burner , had to extend gas line futher in 28 mm . Using 1 mbar pipe calculations I reconed we had 2 metres of 22 to play with

 

[gbgas001]

I had this on a gas safe inspection drop on gas line ( 28 - 22 mm) 1.2ish to a pre mix burner , had to extend gas line futher in 28 mm . Using 1 mbar pipe calculations I reconed we had 2 metres of 22 to play with

I may get slated here.... but ..... a lot of gas safe inspecters dont know much more than the average engineer... its all fine and handy knowing exactly what the British Standards say, but being able to figure out the gray areas is the hard part....

These grey areas eventually become the new British standard as they did when soundness testing moved to tightness testing.

I bet the manufacturers instructions said the pressure you had was ok?

We really need a knew set of pipe sizing tables to compensate for pre mix burners, if we are made to stick to this 1mb drop across the pipe line.

gb-gas

 

[oldplumber]

may be wrong here but if you run several appliances such that there is a dRop in pressure on the gas supply that is very much a combustion problem as for one the burner pressure could be such it goes out ie on a hob and allows a gas escape or poor combustion on a boiler due to a pressure drop and the explosive risk that can occur. If an open vent boiler isnt provided with the correct inlet pressure of 20mb say and then another heater say an andrews water heater comes on at full load, causing a further pressure drop, my simple interpretation is you have a potential for a disaster and you should sort out what the reason behind this is. and issue warning notices if neccessary.

Also if you fit a new appliance on old pipework that isnt sufficient to supply the required gas it isnt ncs, but id and i would riddor the rgi who installed the new appliance or certainly give him a bell mentioning the issues ive found such that he might like to make good and ive found 2 of these this week one on a yr old combi running badly on 15mm pipework, and onother older combi on a rather long run of 15mm that hadnt been coping to well judging from the burner problems it was now showing.

only my opinion, but its worked for me over recent years

 

[gbgas001]

No i agree to an extent... the decision is always with the engineer... but as for the gas going out the pipe work would need to be hugely undersized... such as 8mm pipe to the cooker... being realistic undergassed is not a combustion fault... it will never produce carbon or carbonmonoxide... as for the combis the old rule of 22mm to the last metre of pipe will never work....... GREY AREAS... IF WE DIDNT HAVE THEM THEN WHAT WOULD WE EVER TALK ABOUT LOL

gb-gas