Gas Rate calculation question......

[Ryster]

Afternoon everyone, Im a touch confused at the moment.

Being the sad man I am, I got home today and thought I would look at the Commissioning checklist in my boiler MIs from when we bought out house.
So, I have a scan through it and come to the domestic hot water section and have a look at the numbers.

For the gas rate he has entered, on hot water only, a rate of 32.2 ft3/hr.

So, being even more of a sad man I thought I would check it. I have a u6 meter. So put the boiler into hot water only and ran the tap off. Took a reading from the meter and it was 35 seconds for one revolution of the test dial.

Now here is my problem, in my massssssssive gas folder I have had since college the following equation is to work out the gas rate.

ft3/hr = 1x3600 / 35 = 102.85


Now that to me is wrong, that is surely the BTU/H used?

I have always used the U6 gas rate table provided in this book instead of working it out (I can scan this if anybody would like it? Gives Gross and net input for Kw and Btu) and upon checking this table, for 35 seconds the BTU/H is 106.45.


So what exactly is wrong with the calculation in the book? Or because I have not actually calculated it on paper since college (5 years back) am I missing something out in the calculations?!




Any help would be appreciated, as its a bit boggling. Obviously the book could just be wrong, but I doubt it and think its more something im doing!

 

[mickie]

i got 106btu

31.35kw

 

[mfgs]

31.35kw gross / 28.50kw net is what I make it.

 

[Ryster]

Thats exactly what my table gives me.
So what exactly is going wrong with me using the equation given in the book?

 

[mickie]

3600x1040/35=106btu

btu/3412=kw

its missing out the cv of gas

 

[Ryster]

3600x1040/35=106btu

Number of seconds in an hour (3600) x CV of natural gas (1040) / seconds for one turn of test dial = BTU/h

BTU/h (106) / number of BTU in 1KW = KW


Im presuming thats how I should be doing it?

 

[mfgs]

3600x1040/35=106btu

btu/3412=kw

its missing out the cv of gas

Thats the right equation. just need to divide the final answer by 1.1 to give the KW in net

 

[mickie]

so you already knew all that stuff ? your book is saying a cv of 1, when you know its 1040

 

[Ryster]

so you already knew all that stuff ? your book is saying a cv of 1, when you know its 1040

Yeah, I know the standard Calorific Value and all the correct numbers. I was just getting totally confused when reading my book.
As I said, I normally use my table and have not had to work out on paper any of this since college. But I fancied giving it a go, just so I know exactly how to do it the long way if needed.

 

[mfgs]

I have the equations saved in my phone, so I always have them with me. Never use the tables in the books now.

 

[mickie]

ive got the chart laminated to make it easier for cp12s, but you really should be able to do it without if you get inspected

 

[Ryster]

ive got the chart laminated to make it easier for cp12s, but you really should be able to do it without if you get inspected

I totally agree, its just lazyness and workload that has meant over the past few years I just used the table.
I will be retaking my domestic in a few months, so need to brush up on the calculations.

All too easy to use tables and the like when out working on the job, but like you said, you should really know all this stuff.
Not going to use my table from now on, its a small calculation and one that if I use enough times will remember off the top of my head.

 

[fixter1]

3600 x 1040 / 35 / 3412 = 31.35kw gross / 1.1 = 28.50kw net

 

[Ryster]

3600 x 1040 / 35 / 3412 = 31.35kw gross / 1.1 = 28.50kw net

How would you explain the difference between net and gross?
Im not quite sure I believe the reason, or explanation given in my current gas book.



Whilst we are on the subject, I have a viper domestic gas book. However I tend to use my folders I got from college a few years back when I was doing my apprenticeship, I however appreciate these may be getting slightly long in the tooth though.
I have the gas safety regulations, and information on just about everything else. But my confidence has slightly been lost from this book/folder this afternoon, other than the Viper books can anyone else recommend a good up to date commercial gas and domestic gas book?

 

[GQuigley67]

I just do 1098/number of seconds = KW turns out almost exactly the same as the long calculation you are doing