gas rate ?

gigz · 7 Oct 2006

Hi all, i've just recently passed my ccn1 and specialist subjects so go easy on me for my lack of experience, gas rated a vaillant turbomax 242e today for my mother (free of charge i might add, ahh) it took 42 seconds for one complete revolution of the dial so 3600x1042/42 gives me 89,142btu = 26.1 kw. Now the data table says 29.3 kw max (dhw) i know there is a tolerence of +/- 5% so its not right, What should i do ? I know some of you will be saying "you've passed your ccn1 you should know what to do" but i'm confused whether i've calculated it wrong I.E. gross or net. Thanks for reading.

chrishutt · 7 Oct 2006

gigz said:
What should i do ?

Nothing to the boiler until you've got CEN1 as well.

Most likely explanation is that the burner pressure on DHW is modulating down to maintain the HW temp. You may need to set the boiler on the chimney sweep setting which gives a constant maximum rate.

It's also possible that the boiler is burning gas at a lower rate than intended due to a very high resistance on the gas supply pipe. As you should know (with CCN1) the pressure drop from meter outlet to appliance inlet should be 1mbar when burning full rate (DHW without modulation), so ideally 21 mbar at meter outlet and 20 mbar at appliance inlet.

kevplumb · 7 Oct 2006

You may need to set the boiler on the chimney sweep setting

on a 242E

ChrisR · 7 Oct 2006

I thought it was +5% -10% , which you're still just outside. CLose enough though. When you've done the boilers module you can try to adjust it if you feel like it!

More accurate is to use the gas rate itself, book says 98.5 cu ft/hr.
Worth measuring the gas pressures at the boiler. Both the inlet pressure and the burner pressure, when it's burning max (hw). I bet you find the burner pressure is slightly low. It should be 10mbar +/- 1mbar
And quite likely, it won't adjust any higher!
Vaillant insist that unless you have a full 20 mbar at the inlet, you won't get full gas pressure (and therefore full gas rate) at the burner. Seems barking to me, but I've asked them more than once. They start objecting to half a mbar down.

Rob · 7 Oct 2006

gross or net refers to THEIR data badge look for the CE kite mark is it which would indicate if their figures are gross or nett.

your allowence is 5% above 10% below. so its only slightly out. Id be checking burner pressure next.

Paul Barker · 7 Oct 2006

The table in the corgi manual gives you both gross and net figures, the data badge should tell you if it's gross or net, but most boilers quoted as say 24kw are 24kw net.

On most modern band a boilers you can only gas rate it and check the working pressure.

You ideally want 19mb working pressure, but the Ideal Isar I fitted yesterday actually said in the instructions it was perfectly happy with 14mBar.

chrishutt · 7 Oct 2006

kevplumb said:
You may need to set the boiler on the chimney sweep setting

Click to expand...

on a 242E

You're right, I was thinking of the Turbomax plus - 824.

the plumbing miracle · 7 Oct 2006

i dont know why you lot take gas rates so seriously on room sealed appliances you are brainwashed into believing anything past a tolerance is a big problem you should know by experience a gas rate is only one of many tools in your armoury

gigz · 7 Oct 2006

Cheers for the info guys, i do have my cen1 (ckr1, wat1 and htr1) and i'm gaining experience all the time, i checked the gas pressures (working and standing) first and that was ok. The boiler is working fine i'm just getting in a bit of practice (i know very sad on my day off

) if you guys found a boiler this way, would you do anything to it

fitz1 · 7 Oct 2006

BG would condemn it and sell you a 5000 boiler swap.

gigz · 7 Oct 2006

Gasguru · 7 Oct 2006

Many combi's will modulate down unless you have high water pressure and a good flow rate (make sure the DHW temp is set to maximum and the heat exchangers are cold to start with).

This is often a problem timing unitary meters over the 1 minute and nearly always a problem with ultrasonic meters (over the 2 minutes).

If you work with +5% and -10% and your figure is close.

You will get to know which boilers you are likely to have to take the burner pressures instead - these tend to be the more powerful or with low mains pressure/restrictive flow restrictors.

The CE mark does not necessarily require the net rate to be displayed, it can be the gross rate - many boilers destined for the UK will still quote Gross rates.

and to save calculator time....

for diaphragm meters use

1100 / (time for 1 revolution) = gross Kw and then divide by 1.1 for net if required

and for ultrasonic use

(38.75 X (finish meter reading - start meter reading)) / time in seconds (normally 120) = gross rate and then divide by 1.1 for net if required

I've found gas rate as much as 50% over - worst culprits are Pott suprimas, Glowworm compact (S. Duvals)

Rob · 7 Oct 2006

Highest ive come across was 120kbtus on a performa 28

kevplumb · 7 Oct 2006

the plumbing miracle said:
i dont know why you lot take gas rates so seriously on room sealed appliances you are brainwashed into believing anything past a tolerance is a big problem you should know by experience a gas rate is only one of many tools in your armoury

cos we sleep at night

ChrisR · 7 Oct 2006

the plumbing miracle said:
i dont know why you lot take gas rates so seriously on room sealed appliances you are brainwashed into believing anything past a tolerance is a big problem you should know by experience a gas rate is only one of many tools in your armoury

It's only a measly 24kW combi. I wouldn't want to lose more than 10% of the output !

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gas rate ?

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