Oil furnace burning 'rich mixture'

I had my central heating oil boiler serviced last year, it had a bearing replaced in the motor.
Since then it seems to burning a 'rich mixure', i.e. there is a very obvious kerosene smell from the exhaust. And the oil fill seems to be used quicker.
There is a mixture screw/valve in the side but when I tried to adjust it the burner stops and refuses to start again of its own accord unless I increase the mixture.
Is it the quantity of fuel or the lack of air? Or am I completely on the wrong track.
I can't remember the burner model number but if it helps I can look it up to-night.
regards
MJW
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The smell is not forced to be due to a rich mixture. Unlikely though it sounds a weak mixture also creates smell in the exhaust gases. There is a fairly narrow band in which the mixture is "correct". In this band the efficiency peaks, outside it the efficiency falls off fairly rapidly. Rich mixtures invariably leads to soot formation which manifests itself as visible smoke and soot deposits around the flue discharge. Weak mixtures smell sour and the excess air causes the heat produced to be swept through the boiler and out of the flue too fast to be absorbed by the heat exchanger. In both non-ideal cases the fuel usage goes up.

Depending on the screw it might be the pump pressure adjuster. Is it on the burner casting or on the oil pump?

See above. It may be pertinent that the burner motor had a new bearing fitted as this would involve removal of the fan and cleaning off of the blades during reassembly. This gives fan delivery performance improvement and weaker mixture results.

It would be a help, together with the make and model of the boiler
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Hi
Thanks for the info. I will have a look at the Flue to-night for soot.
Your suggestion about a weak mixture might well be right, The fan was wire-brushed and alot of muck was removed from it so it may well be supplying more air at this stage.
The valve I talked about is part of the 'pump' which is mounted on the end of the motor. The oil feed line comes in one side and there are two hex-nuts on it. One of them is used to bleed the pump when I run out of oil and I have an air lock. The other hex nut (about 10-12mm in size) has a screw in its centre about 4mm with a screwdriver slot in it. I took it out once and there is a small spring inside it so I guess it is probably the oil pressure adjuster.
What should I do to fix a meak mixture? I will get the make and model of the boiler also when I get home and post it later on to-night, I think it is a Riello but I will check to make sure.
regards
MJW

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Step 1
Get installation/ maintenance manual from boiler manufacturer this will tell you what oil pressure to set, as well as co2 and flue gas temperature (and smoke) .
Step 2
Acquire oil pressure gauge from HRPC (or other supplier) Cost about 35 with manifold. 0-300psi is likely OK.
Step 3
Check what Jet you have in boiler and change if more than 1 year old. Cost 7 or so. (HRPC) Quite often boilers accept a range of Jets to provide different heat outputs
Step 4
Install above. Now the problem is to set the air supply correctly. You need to find the adjustment for the air shutter.
I suggest that you ask the boiler manufacturer if they can advise you on the air setting for the particular jet you are using (if not mentioned in installation manual)
In an ideal world you should measure the % Oxygen in flue Gas, calculate the co2 % and compare to figures in manual. The problem is that an 02 meter will cost 200 + They also use sensors which have to be replaced every 2 years. You can use a chemical kit - similar initial cost.
You should also check the flue gas temperature. A thermocouple & meter to measure the flue gas temperature can be had for less than 60. Alternatively some electrical multimeters come with this facility.
It could be that if you start with the air shutter fully open and have a low temperature reading, you could progressively close the shutter until you get the correct temperature reading and thereby achieve a reasonably efficient set-up, but I have not tried this.
You can get equipment for checking the smoke for about 60.
Michael Chare
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Riello make a couple of basic models of burner. The old faithful which has a grey plastic control box sitting on top of the motor and is available in a range of outputs with slight detail changes for the various ranges, or the recently introduced RDB range with a black plastic control unit mounted to the left as you look at the burner from outside the boiler. Assuming your unit is the earlier of the two since it has needed a motor bearing then you can adopt the following procedure bearing in mind that this will need test gear to do it right.
1. Isolate the electrical supply and close off the oil supply valve Check the oil filter and renew paper filter elements. 2. Open up the flue access plate/doors to gain entry to the combustion chamber and flueways. 3. With brushes and a scraper clean all surfaces, removing any baffles or retarders to allow access to do this. 4. Vacuum out any deposits (soot and sulphur) you have dislodged. 5. Check the inside for corrosion or defects. 6. Reassemble, making sure all seals are in good condition and make a seal but do not overtighten and distort the joints. 7. Remove the burner from the mounting flange by undoing the retaining nut at the top of the mounting flange. 8. Withdraw from the boiler (on some boilers this may require you to dismantle some casing) 9. Loosen the blast tube retaining screws located at either side of the blast tube into the cast burner casing and remove the blast tube from the burner. This will expose the nozzle and electrodes for cleaning. Note carefully the position and clearances of the electrodes with respect to the nozzle, compare with the settings given in your manual. 10.With a nylon pan scourer clean up then electrodes and nozzle mount. etc. Renew the nozzle with one of the same capacity and spray angle (this is stamped on the flats of the old nozzle for typical example 0.65 US GPH 60 degree spray angle). Removal of the nozzle may require you to loosen the electrode block so be sure you know where to put it back again. 11. Clean out the aperture in the casing which you see behind the blast tube and note the photocell lens is clean 12. Reassemble the blast tube and secure with the two screws. (You did put the electrodes back didn't you?) 13. Undo the three screws holding the fan volute casing in place and expose the fan for cleaning with a stiff bristle brush. (Some models have a hydraulic ram which operates an air shutter over the air intake and this may need the mounting screws removing first to allow the volute casing to be removed). Clean the volute casing and the air intake apertures also then reassemble. 14. It is not normally necessary to remove the oil pump unless there is obvious sign/sound of bearing or coupling wear however if it is judged to be necessary then loosen the two (sometimes three) screws which clamp the pump into the end of the motor, remove the solenoid from the top of the pump, disconnect the oil pipe nuts and gently rotate the pump a little to disengage the oil pipes and withdraw it from the end of the motor. Check the bearings and the condition of the plastic drive dog which transmits rotation from the motor shaft to the pump. The motor bearings are sealed but a little light oil may be applied to inhibit corrosion. 15. Reassemble and check your oil joints. 16. At three yearly intervals renew the oil flexible pipe, sooner if it shows damage. 17. Obtain a suitable pressure gauge reading 0 to 250/300 PSI. Remove the hexagon plug below the pump pressure adjuster and fit the gauge to this port. 18. Refit the burner to the boiler and open the oil supply valve. Check for leaks and rectify if necessary. 19. restore power and set the controls to call for heat. The motor should start up and run for a few seconds before opening the oil solenoid with an audible click and the burner should fire. If it does not fire the controls will register lack of flame and the unit will shut down and go to lockout, lighting the lockout alarm lamp under the reset button. 20. Check the pressure gauge and adjust to the specified pressure given in the boiler/burner data sheets using the adjusting screw on the pump. Once set this screw should NOT be touched again. 21. Situated (usually on top of) the boiler shell you should find a flue gas sampling point. When the boiler has warmed up insert a smoke tester (Baccarach pump) and draw a smoke sample. This should be zero on the Baccarach scale in most cases (no smudge visible on the filter paper the sample is drawn through) 22. Insert an analyser probe into the sample point. Either an electronic tester or a Fyrite/Briggon wet tester sample pump tube. If you haven't got either of these you will be unable to go further and should seek professional assistance. Draw the sample into the tester and read the CO2 content of the flue gases. Adjust the air slide/shutter to give a figure in line with that given in the boiler specification sheet. 23. Recheck the smoke is still within tolerance 24. Insert a thermometer into the flue sample point - this may be included in the analyser probe if you are using an electronic set. Allow the reading to stabilise and record. By reference to tables or from your analyser set the nominal efficiency of the boiler can be obtained. 25. If the boiler has a conventional flue insert a draught gauge and record the flue draught for comparison with previous recordings which will indicate any possible problems developing in the flue integrity. 26. Remove all test gear, Check for leaks, Close up and clean down
It is a useful exercise if a firestop valve with a capillary sensor is fitted to test this by putting the sensor into a cup of hot water in excess of 65 degrees C
Typical flue gas analyser readings Smoke 0 to 1 Baccarach CO2 10 to 13 percent Flue gas temperature 190 to 230 degrees C Flue draught 0.02 to 0.05 inches water gauge or equivalent
HTH
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