Gas rate app

[tamz]

The calculations will vary region to region according to the calorific value of the gas. Unless you know and use that in your calcs you are working to an approximation.

 

[acecharly]

I have all mine on 2 sheets of paper just cross reference job done.

 

[Orry Clayton]

The calculations will vary region to region according to the calorific value of the gas. Unless you know and use that in your calcs you are working to an approximation.

I was thinking same, how many check bills for correct CV?

 

[tamz]

Very few if any.

 

[zeebeedee]

No mention of checking the cv of the gas in any text book or reg I have seen. The difference is probably that minimal it has no affect.

 

[kay-jay]

No mention of checking the cv of the gas in any text book or reg I have seen. The difference is probably that minimal it has no affect.

Up to 5%

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[zeebeedee]

Probably not far of that in human error on the timing or readings

 

[zeebeedee]

Up to 5%

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So on a 24 kw boiler you could be 1.2kw out.

 

[Nostrum]

Sutely the badged input rating of the boiler would have to make an assumption on the CV of the fuel as well though, so in effect it doesn't matter. Only gas rating to determine the correct quantity of fuel is being used by the appliance, not to actually calculated heat production.

 

[EPH]

Well if BG say so then I'm ditching mine :willy_nilly:

Be better teaching some of the lads the back boilers need ventilation!

 

[Doodlebug]

Well if BG say so then I'm ditching mine :willy_nilly:

Be better teaching some of the lads the back boilers need ventilation!

Are you suggesting British Gas engineers work unsafely?


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[EPH]

Are you suggesting British Gas engineers work unsafely?


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I'm saying I regularly go to CF installs with no ventilation that used to be under a BG contract since installation but they can't afford it, when I tell them what needs doing I normally get asked to leave because "British Gas never mentioned that".

I can only speak from e pertinence and I wish I had a pound for every time it's happened.

 

[villa_tom]

I'm saying I regularly go to CF installs with no ventilation that used to be under a BG contract since installation but they can't afford it, when I tell them what needs doing I normally get asked to leave because "British Gas never mentioned that".

I can only speak from e pertinence and I wish I had a pound for every time it's happened.

Ive never been to a job anywhere for anyone where an open flued boiler has had no vents, blocked up vents yes, but no vent at all, no.
I for one know that safety is drummed into engineers at BG and there lads wouldn't miss this and if they did then it would be a massive defect and heads would roll.

 

[tamz]

The calorific value can make quite a difference to the calculations.
The cv of gas supplied through the grid varies from between 37.5 - 43MJ/m³ and the quoted values vary constantly.
No matter how you choose to work it out there has to be a margin for tolerance as you will never have the exact figures.

The calculation is MW ÷ cv = m³/s or simplified to the units we use (kW and m³/hr) we use kW x 3.6 ÷ cv = m³/hr
To do it from m³/hr - kW the formula is m³/hr x cv ÷ 3.6 = kW

As an example, say on a boiler burning 3scm/hr
If the calorific value of the gas was 37.5 the HI would be

3 x 37.5 ÷ 3.6 = 31.25kW

same boiler burning 3scm/hr at 40.5 MJ/m³

3 x 40.5 ÷ 3.6 = 33.75kW

same again at a cv of 43MJ/m³

3 x 43 ÷ 3.6 = 35.83kW

A bit of a difference.

BG wanting to ban the use of apps is not because they are wrong. They are not. They will just want to standardise their info they receive back by having there guys use the same method.

 

[GrahamM]

BG wanting to ban the use of apps is not because they are wrong. They are not. They will just want to standardise their info they receive back by having there guys use the same method.

Exactly! As I touched on previously. :wink_smile:

 

[Doodlebug]

British Gas aren't likely to approve the use of a gas rate app which has passed no industry tests now are they. ;-)


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[Nostrum]

The point i'm trying to make is the badged heat input on the appliance will have been based on a calorific value used at the time of the test. So even if you can find out the C.V of the gas, you can't compared egg's with egg's.

By gas rating the appliance you are only confirming that the appliance is using the right amount of fuel in the given time. Because the gas consumption information is rarely available on site, the industry seem to have decided to adopt the technique of converting f3/hr or m3/hr into KW in an attempt to simplify the process. The reality is it makes for a pretty crude and inaccurate calculation for heat input but is a workaround for consumption.

If the manufacturers printed the consumption figures on the date badge then the app / calculation wouldn't even be needed as you could just x the 2 min test period by 30 and you'd end up with the hourly consumption in m3/hr or ft3/hr (at max rate of course)

 

[tamz]

In reality the cv of the gas doesn't really affect your calculations as all you are checking is that the boiler is passing the correct amount of gas.
Injectors are sized to pass a certain amount of gas at a certain pressure and zpg's set to draw the correct amount of gas nearly no matter what the inlet pressure is (unless it is very low). That is (should be) the constant you are checking against what the manu says it should work out at.
You have no way of knowing the actual HI of the boiler as that is the variable (due to the cv).
It just means if you live somewhere with a high cv your combi will give a bit better hot water and if you live in London your onto plumbs as your cv is generally lower :lol:

Btw i had a job last year where the incoming pressure was below 10mb (water in the service) and the boiler gas rated and was running perfect! Work that one out¿¿¿

 

[kay-jay]

Btw i had a job last year where the incoming pressure was below 10mb (water in the service) and the boiler gas rated and was running perfect! Work that one out¿¿¿

was it running on fumes from a hungover buckfast swigging jock? :29:

 

[Nostrum]

In reality the cv of the gas doesn't really affect your calculations as all you are checking is that the boiler is passing the correct amount of gas.
Injectors are sized to pass a certain amount of gas at a certain pressure and zpg's set to draw the correct amount of gas nearly no matter what the inlet pressure is (unless it is very low). That is (should be) the constant you are checking against what the manu says it should work out at.
You have no way of knowing the actual HI of the boiler as that is the variable (due to the cv).
It just means if you live somewhere with a high cv your combi will give a bit better hot water and if you live in London your onto plumbs as your cv is generally lower :lol:

Btw i had a job last year where the incoming pressure was below 10mb (water in the service) and the boiler gas rated and was running perfect! Work that one out¿¿¿

And this is exactly the point I was making above.

 

[Doodlebug]

In reality the cv of the gas doesn't really affect your calculations as all you are checking is that the boiler is passing the correct amount of gas.
Injectors are sized to pass a certain amount of gas at a certain pressure and zpg's set to draw the correct amount of gas nearly no matter what the inlet pressure is (unless it is very low). That is (should be) the constant you are checking against what the manu says it should work out at.
You have no way of knowing the actual HI of the boiler as that is the variable (due to the cv).
It just means if you live somewhere with a high cv your combi will give a bit better hot water and if you live in London your onto plumbs as your cv is generally lower :lol:

Btw i had a job last year where the incoming pressure was below 10mb (water in the service) and the boiler gas rated and was running perfect! Work that one out¿¿¿

Why would you gas rate a boiler which has 10mb incoming pressure and water in the service.


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[zeebeedee]

Water in the service?????

 

[EPH]

Why would you gas rate a boiler which has 10mb incoming pressure and water in the service.


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Maybe he rated it before he found the water?

Normally put my gauge on the boiler to measure incoming pressure at the same time as a gas rate so they correlate, not un thinkable I'd imagine.

 

[tamz]

Why would you gas rate a boiler which has 10mb incoming pressure and water in the service.

Because i'd just fitted it and was commissioning it of course.

Guage on the boiler and fired it up. Dropped down to below 10 straight away. Guage on the meter (3 ft away), bouncing between about 8 and 12mb. I rated it out of curiosity. It was perfect.
Boiler was a 30si.
Water gets into the mains sometimes up here and can work itself into the service. If you disconnect the ecv you can hear it gurgling and sometimes you can get some out.
SGN cut the pipe in the street and blew it with the compressor.

 

[Doodlebug]

Because i'd just fitted it and was commissioning it of course.

Guage on the boiler and fired it up. Dropped down to below 10 straight away. Guage on the meter (3 ft away), bouncing between about 8 and 12mb. I rated it out of curiosity. It was perfect.
Boiler was a 30si.
Water gets into the mains sometimes up here and can work itself into the service. If you disconnect the ecv you can hear it gurgling and sometimes you can get some out.
SGN cut the pipe in the street and blew it with the compressor.

So it was just out of curiosity?
I wouldn't have bothered, you can't commission the boiler with only 10mbar at the inlet and water in the service.