combi boiler pipe size calculations

[krav965]

Hi, just down the gas pipe calculations on a worcester bosch 28i junior combi a work buddy is fitting and it has posed a question...the boiler requires a rate of 3.02m cubed /hr.the existing gas supply pipework is made up of 3mtrs 15mm and 1.5 mtrs 22 mm copper...taking into account all bends etc the calculation on the existing pipework exceeds the requirement of the new boiler....yet the regulations require 22mm pipework be used on condensing combi boilers...???

Should we no longer pay any attention to pipe size calculations or do pipe size calcs still hold good and a combination of pipe sizes can be used providing the calcs hold good????????

 

[Gray0689]

there is only a regulation that the pipe has to be the correct size to feed the appliance with the required volume of gas at maximum rate with the correct pressure loss
Not just 22mm pipe

 

[REDSAW]

its against the regs to fit to n.c.s,
so if the regs require it, you fit it.
if the m.i. state 22mm then its 22mm.

theres no get out clause if the calcs are wrong because it adds up right but not to m.i's.
and thats min for future expnsion

 

[krav965]

Hi guys....the mi's state 22mm but then go on to give you a table of calcs from 15mm up to 28mm...also giving you the rate of 3.02m cubed/hr with the statement. I can understand the need for installing to mi's but in this case it makes a mockery of doing calcs in fact if it were to be done in 22mm as stated you would have a discharge of almost 6m cubed/h nearly twice the required volume....

 

[Billy Bob Bob]

22mm at the appliance you will find the rest of its 28mm from the meter

 

[krav965]

Hi Gas man...no it seems that its 22 at the meter ,dropping to 15mm..i knew i would create a little controversy sorry folks I thought I would throw the question out there

 

[That Extra Mile]

Worcester-Bosch states at least 22mm for all its boilers (except some of the smallest regular boilers). Jonathan,

 

[Billy Bob Bob]

Hi Gas man...no it seems that its 22 at the meter ,dropping to 15mm..i knew i would create a little controversy sorry folks I thought I would throw the question out there

is the existing appliance still in and working? if not you need to record the pressure at the inlet in dhw mode on the new boiler with all appliances working,if its a combi you are going wrong somewhere in your calcs if its a heat only different matter but mi,s overrule gas regs if its a combi 15mm will not be adequate

 

[tamz]

If the boiler has a 22mm connection you use 22mm. It still has to be worked out even if it has a 15mm inlet.
If you have 3m of 15mm and more than 1 elbow you are under with your sizing.

 

[krav965]

Hi Gas man...no new boiler not in yet...just checked my calcs and they seem ok...the boiler isnt far from the gas meter, I was being generous in my distance at 4.5 mtrs, , I only have 3 bends in the run ( allowed 0.5 each for those ) then 3 mtrs 15mm and 1.5 mtrs of 22 mm....still comes up to more than the required 3.02m cubed /hr that is required....but I am a bit knackered so i agree my calcs could be a bit wonky......fanx guys

 

[krav965]

Thanks Tamz...dont forget about the 1.5mtrs of 22mm..

 

[pobsdad]

Does anyone ever do the calcs on domestic,except in unusual circumstances?

 

[tamz]

A boiler on its own is easy to know at a glance but when there are other appliances i work it out. Only takes 2 or 3 minutes.

 

[tamz]

Thanks Tamz...dont forget about the 1.5mtrs of 22mm..

So definately undersized.
Stick a bit 22 in for all the time it will take.

 

[krav965]

Agreed Tamz...undersized as regards Mi but not in the calcs.....

 

[Billy Bob Bob]

Agreed Tamz...undersized as regards Mi but not in the calcs.....

but mi,s are the law so go with what they say

 

[krav965]

Agreed gas man.....posed a fairly interesting question though..

 

[krav965]

Thanks for your inputs guys..

 

[Billy Bob Bob]

Agreed gas man.....posed a fairly interesting question though..

not all regs make sense,its our job to do it right,document everything,leave it safe and cover our asses backwards

 

[AWheating]

If you look inside many combis they dont have 22mm pipework after the gas cock, many manufactures have a 22mm gas cock and state a min size of 22mm supply pipework because thats whats required normally. Also when they used to have 15mm gas cocks to many cowboys assumed that 15mm to the boiler was fine because thats what size the nut is at the boiler and this lead to many installs being undergased. If your calulations work out and 15mm is ok then i would phone the manufacture up to clarify what they will except, if they still state min of 22mm then do so to avoid call backs and warranty issues.

 

[joe feeney]

the MI is what you go by they over rule the regs if stats 22mm then you have to use 22

 

[capitanpugwash]

Some combi's only need 15mm for the gas supply. Vaillant Ecotec 824 combi states 15mm with the proviso of correct working pressure at the appliance inlet, which means if you do your sums 15mm can be ok....

Edited to add: that MI's obviously rule with minimum pipe size.