Gas Rate 28i Jnr

[GreenArmy]

Situation:- Service to Worcester Greenstar 28i Junior.
All good as far as CO2% (9.8 Max 9.2 Min), condense trap cleaned, flue and terminal good ect, ect.
Then when i do the gas rate to start with i only got 20.95kw (nett), i need 28.57kw.
OK, i check standing pressure at meter (23.5mbar) then check working pressure (20mbar) and then pressure at appliance (19mbar), so all good on that front.
Check gas rate again and get 26.77kw nett, still not enough. Check again and again and still get 26.77kw. So i check the pipe sizing, 8.5 meter pipe run with 16 (yes 16) elbows, so this makes equivalent of 16.5 meter pipe run (22mm), this is still OK and should give me enough gas.
Cant get through to worcester at the moment. Anyone any idea what might be wrong?

 

[newbie1]

poss gas valve problem what is burner pressure and inlet pressure
is the burner flame good
how old is boiler
have you taken a flue gas reading

 

[GreenArmy]

Flue gas reading as per opening post, inlet pressure 19mbar. Cant see burner flame, boiler installed August 2007.

 

[newbie1]

by the fg reading i meant the co/co2 ratio
inlet only tells you what pressure at inlet side is what is the burner pressure reading as this will confirm gas pressure going to burner

 

[GreenArmy]

Ok, ratio is 0.0001 at min and 0.0007 at max. The gas valve is a zero governor type.


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[newbie1]

well they are pretty good figures as the trigure point is 0.0008 as zero rated governor doubt you will get a burner pressure reading
i can only assume burners possibly and i say possibly need clearing/injector..19mb at inlet is good so u have
1/air
2/gas pressure
3/clear flue
4/ignition
only other are is poss the burners
sorry cant help more

 

[Plucky]

So your pressures are fine.

Without pointing out the obvious I assume your are definately setting the boiler into max rate?

I have had a brand new worcester that has given me funny gas rates and turns out the incorrect fan was installed at the factory

 

[GreenArmy]

Oh yeah i was in service mode on max and the boiler had been running like that for ten minutes whilst carrying out the flue tests. What was strange is the first time i gas rated it the rating was very low (20.95KW). Ive serviced dozens of these and can practically do it with my eyes closed. Im going to strip it down and clean the burner and h/e tomorrow. Thanks for the replies.

 

[wnjswozo]

do test with hot water running ,so at max rate,26.77kw net,not far off,i would,nt worry to much,fga is spot on near enough,and as you are aware that checks the combustion so from your fga reading its within tolerance

 

[Plucky]

Your actually not far of the 5% tolerance. You might be using a gas rate chart with a high calorific value whilst the supply of gas may be particularly low, or vice versa.

 

[so-solar]

Are they modulating gas valves on this model?

 

[payney1974]

do test with hot water running ,so at max rate,26.77kw net,not far off,i would,nt worry to much,fga is spot on near enough,and as you are aware that checks the combustion so from your fga reading its within tolerance

X2 . Couldnt of put it better myself

 

[oldplumber]

you should check the fan pressure first before stripping down as if the reading is greater than (dont quote me) -4.5mb, then the burner heat exchanger is clean enough and you save the price of a new replacement gasket, otherwise i'd be happy with the fga readings.

 

[GreenArmy]

Ok lads, thanks for responses. Of course i did have the hot water running and c/h whilst in service mode. As stated you cant argue with all the readings (apart from gas rate) so i didnt strip down, i called worcester who said as long as all my readings/measurements and pipesizing is how i stated then im OK. I have stated on the paperwork that although Gas Rate is slightly low, everything has been checked and worcester have given me advise that its OK (just to cover myself in case our Quality Assessor picks me up on it).
Old Plumber:- fan pressure was -6mbar.

 

[coley]

@newbie1
"well they are pretty good figures as the trigure point is 0.0008 as zero rated governor doubt you will get a burner pressure reading
i can only assume burners possibly and i say possibly need clearing/injector..19mb at inlet is good so u have
1/air
2/gas pressure
3/clear flue
4/ignition
only other are is poss the burners
sorry cant help more"

Just to clarify should the the trigger point be 0.0080

Thanks
Coley

 

[amy]

Ok lads, thanks for responses. Of course i did have the hot water running and c/h whilst in service mode. As stated you cant argue with all the readings (apart from gas rate) so i didnt strip down, i called worcester who said as long as all my readings/measurements and pipesizing is how i stated then im OK. I have stated on the paperwork that although Gas Rate is slightly low, everything has been checked and worcester have given me advise that its OK (just to cover myself in case our Quality Assessor picks me up on it).
Old Plumber:- fan pressure was -6mbar.

Hi, Just read about checking the fan pressure, exactly how is this done. THX.

 

[diesel_ltd]

there is a test point inside the boiler you need a electronic manometer to read nagative pressure which should read greater than -3.9 on a 28i junior on natural gas

 

[amy]

Thx for that, they didn't teach me that in college,lol, AMY.

And thx for that GrahamM, much appreciated, AMY.

 

[GrahamM]

Hi, Just read about checking the fan pressure, exactly how is this done. THX.

Some boilers, Worcesters in particular have a test point at fan and you attach a digi manometer and read the negative pressure reading. Comparing it to the manufacturers spec will give some indication of how dirty the combustion chamber may be and if a more intensive clean may be required.
Depending on model you may be looking for readings somewhere in region of -3 to -6 mbar at full rate.

 

[Bazz]

Is it always best to gas rate appliance on hot water mode and not heating mode?

 

[lame plumber]

apart from being a very old post, you test in service mode at max an min rates and some bolers require you to run hw to stay in the max rate mode to disipate the heat. if you dont have a service mode, hw will give you max rate heating wont for long enough.

 

[gassafe]

The Worcesters say to put in max & run hot tap, but I always find hot tap runs cold & it blasts round ch

 

[arran197]

Worcesters will diverter between mid position and CH. Always put them in max for gas rate. You're allowed a tolerance of 10% under and 5% over. So in the example above, the 26.77 kw he was getting, assuming the boiler is exactly 28kw (which they never are, they're usually about 27kw, but are badged higher), you're allowed a min of 25.2kw, so he would actually be ok to leave it as is.

 

[JBP Heating]

26.77 sounds not bad to me. Have you checked the fan pressure though? Should be a little red cap up the top near the fan.

 

[GrahamM]

This is a 2 year old post people are replying to.

The Worcesters say to put in max & run hot tap, but I always find hot tap runs cold & it blasts round ch

If the heating hits its max temp the boiler heat will divert to hot water and will not modulate down or cut off if you open tap in service mode.

 

[JBP Heating]

Ha Ha, so it is!

If the heating hits its max temp the boiler heat will divert to hot water and will not modulate down or cut off if you open tap in service mode.

Installed a 38cdi classic last week and the mi's suggest removing the diverter motor and running the hot tap when testing max rate.