Baxi Combi's

[Leo21]

sorry dont agree with that if you have in the case of a ideal 14mb when its pulling dhw not only is that appliance under gassing but the gas hob also running in the kitchen will be extinguished

Thatll be why it says in mi,s check this is ok for other apliances then. Condensers appear to under gassed because if the negative oressure the fan causes. You can gas rate them and they can be within tolerances and your hob and fire are pulling 19 to 20 as required.

 

[Mike Jackson]

I'm with gas man on this. The fan pull will be registered on the burner pressure test point, not the inlet test point. The lower presssures noted for certain boilers are for losses across the inlet controls. These have been calculated to add to the maximum 1 mbar drop across the pipework.

I've had a Worcester 24ri working with an inlet pressure of 2 mbar. It even gas rated correctly. Doesn't mean it was correct though and the gas fire went out when the boiler fired.

 

[Leo21]

Fair enough, but according to Vaillant engineers they have been told its all about the negative pressure of the fan.
And why should the losses be any different for inlet controls on a condenser to a standard efficiency boiler? The bit of pipe between the gas valve and the gas tap look almost identical and I can't see that accounting for a 6 mbar drop in Ideals and a 4 mbar drop in Vaillants. There's more to it than that.

 

[mfgs]

The original duo tec seems to be an alright boiler. I however don't like fitting them due to the extremely poor back up from Baxi, they will try absolutely everything to get out of honouring the warranty which is just not acceptable.

 

[Mike Jackson]

Fair enough, but according to Vaillant engineers they have been told its all about the negative pressure of the fan.
And why should the losses be any different for inlet controls on a condenser to a standard efficiency boiler? The bit of pipe between the gas valve and the gas tap look almost identical and I can't see that accounting for a 6 mbar drop in Ideals and a 4 mbar drop in Vaillants. There's more to it than that.

It's not a 6 mbar drop in Ideals. Read the instructions correctly. Look at http://idealheating.com/downloads/manuals/204978.pdf

Page 8 of the manual talks about the gas supply. It states that the boiler will work with a minimum inlet pressure of 14mbar but also states that BS 6891 must be followed which allows for a maximum 1 mbar across the pipework. There is also a chart giving pressure losses across the gas cock according to the heat input of the boiler. The worst case scenario with a 40Kw boiler is a 2.5 mbar drop across the gas cock giving a total allowable pressure drop of 3.5 mbar between the meter test point and the inlet test point on the boiler. Given that the working pressure of the meter can be between 19 and 23 mbar that gives us a working pressure at the boiler of between 15.5 and 23 but never more than 3.5 mbar below the working pressure at the meter.

The 14mbar is a bit of a red herring, they are just stating that the boiler will work down to 14mbar inlet pressure but go on to emphasise that BS 6891 must be followed a second time. By stating that it will work safely at 14mbar all they are doing is allowing you to NCS an undersized installation rather than AR it. If you fit a boiler with undersized pipe you could end up having to rectify the work at your own expense if Gas Safe get involved as they will not accept an NCS on a new install.

 

[Leo21]

Ok but we're still talking about a 2.5 mbar drop being acceptable through the boiler. Yes?

 

[Mike Jackson]

2.5 mbar through the boiler on the 40Kw. It varies on the boiler output. I had big arguments with a warmfont inspector over this a few years ago. Had a 35Kw boiler with a fairly long gas run but 28mm within 100mm of the inlet. Had a drop of about 2 mbar between the meter and the inlet test point (this was before they put the test point on the gas cock). The inspector was insisting that the pipework was undersized and that we needed to upgrade it to 35mm for some of the run despite calculations howing that the drop over the pipework was less than 1/2 mbar. There had been issues with this before and the company had rolled over and upgraded the gas pipe. I refused to do this and instead cut a test point just before the boiler. Testing at this showed that I was right and really ticked off the company as they had spent thousands upgrading gas runs previously, probably unnecessary in 50% of the cases.

 

[Leo21]

So why does the loss occur if its not because of the induction method? Gas cock to gas valve is virtually the same as a se boiler.

 

[hammers4spanner]

I am a member of the works scheme and its the worst scheme i have been involved in the back up is utterly shocking nearly as bad asthe ga i installed last year 2 pcb's and a gas valve have gone !!!

i have fitted a few he's and have had no call backs as yet but the heat team back up is probably the worst i have come across in the heating industry as yet

 

[hammers4spanner]

Also as a fit perspective they have a utterl carp jig !!!

 

[Linkyplumb]

i have fitted a few he's and have had no call backs as yet but the heat team back up is probably the worst i have come across in the heating industry as yet

Not dealt with Ferrolli then?

 

[hammers4spanner]

Yep ferrolli is easy to deal with

cust - hi my boilers broken down

me- what boiler do you have?

cust - its a ferrolli

me- you need a new boiler !!

jobs a goodun

 

[AWheating]

So why does the loss occur if its not because of the induction method? Gas cock to gas valve is virtually the same as a se boiler.

TB 115 worcester bosch inlet pressures.

states :the integral appliance isolation valve and boiler pipework could further reduce the OP by upto another 1.5mbar.

and note 7 states : gas safe register would recommend that the only way to ensure that pipework design and inlet operating pressure are accurate is to ensure the pipework design (pipe size) is correct. The correct design may be proven by installing a suitable test point immediatley upstream (before) the appliance inlet gas isolation valve to allow confirmation that pipework design is correct.



TB 129 also mentions pressure losses on isolation valve, pipework and filters can be between 0.5mb and 2.5mb on modern natural gas appliaces.


it is incorrect to choose your boiler based on the min inlet pressure the manufacturer requires, no matter the boiler make and model the installer is responsible to ensure that the supple pipework complys to the current requirements having no more than a 1mb from meter to appliance connection. Many engineers use this "grey area" (and its not really a grey area tbh) as a reason to not upgrade the gas supply when fitting new appliances. Infact its bad workmanship at best and very dangerous at worse.

the hard thing for responsible installers who want to work to the correct standards is you have to compete for the same work.

 

[Billy Bob Bob]

TB 115 worcester bosch inlet pressures.

states :the integral appliance isolation valve and boiler pipework could further reduce the OP by upto another 1.5mbar.

and note 7 states : gas safe register would recommend that the only way to ensure that pipework design and inlet operating pressure are accurate is to ensure the pipework design (pipe size) is correct. The correct design may be proven by installing a suitable test point immediatley upstream (before) the appliance inlet gas isolation valve to allow confirmation that pipework design is correct.



TB 129 also mentions pressure losses on isolation valve, pipework and filters can be between 0.5mb and 2.5mb on modern natural gas appliaces.


it is incorrect to choose your boiler based on the min inlet pressure the manufacturer requires, no matter the boiler make and model the installer is responsible to ensure that the supple pipework complys to the current requirements having no more than a 1mb from meter to appliance connection. Many engineers use this "grey area" (and its not really a grey area tbh) as a reason to not upgrade the gas supply when fitting new appliances. Infact its bad workmanship at best and very dangerous at worse.

the hard thing for responsible installers who want to work to the correct standards is you have to compete for the same work.

well said aw

 

[AWheating]

please ignore the spewling and grammer in my post, lost the edit button again 🙁

 

[Leo21]

My point was that Baxi want min 19mbar, when as you say Worcestor are happy for less.
The fact that I fit Vaillant who accept 16mbar means I don't have to upgrade the gas as I would a Baxi. At the end if the day I'm following the MI,s.
And let's be honest if the manufacturers say its ok its safe.
By choosing Vaillant I can be more competitive I can make more money and still adhere to the MI,s
All your comments are noted although I don't agree.

 

[AWheating]

you still must ensure the gas supply meets the regulations and is sized correctly tho, manufacturer's instructions only apply to the appliance not the gas supply to it and in any case manufacturers instructions require you to work to the current regs which is a 1mb drop.

the quicker manufacturers pull there fingers out there arses and put test points on the iso valve the better imo, not that it will probably happen but it would stop this poor excuse for undersized pipework imo.

 

[Leo21]

Fair enough, I always gas rate to within tolerances. Check other appliances have enough gas. So to my mind the the t,s are cossed and the I,s dotted.

From what I've been told by Vaillant it is that it's the negative pressure produced by the fan that causes the deficit, not filters and gas cocks.

Im finding myself fitting 28mm for the first 3 to 6m on most installs now, especially on anything bigger than 24 kw.

My intention is to fit apliances correctly and cost efficiently.

 

[Mike Jackson]

My point was that Baxi want min 19mbar, when as you say Worcestor are happy for less.
The fact that I fit Vaillant who accept 16mbar means I don't have to upgrade the gas as I would a Baxi. At the end if the day I'm following the MI,s.
And let's be honest if the manufacturers say its ok its safe.
By choosing Vaillant I can be more competitive I can make more money and still adhere to the MI,s
All your comments are noted although I don't agree.

In the instructions they still state that the gas supply must comply with BS 6891 though. So if you fit a boiler and have a working of 21 at the meter and 16 at the boiler the chances are the pipework is not to spec as the pressure loss over the gas valve is unlikely to be 4 mbar. In the absence of the figures for losses over the gas valve you need to rely on accurate calculations for the gas pipe sizing.

Not giving these figures means that without cutting a test point in before the inlet means you can't be 100% sure that the pipework is correctly sized.

 

[AWheating]

tbh ive never seen great drops caused by the boilers ive fitted. Ive only seen it as stated by the manufacturer on a vailiant boiler, which supprised me as the gas meter was less than 2 meters from the appliance.

one thing to remember is if you need to put in some 28mm you can put it in anywhere on the run, which can be helpful if the meters in a bad position.