"jim_in_sussex" wrote in message
oups.com...

Fatboise wrote:
Just wondering if anybody can help me out here. I have an oil fired
boiler for the central heating and hot water.


The boiler is 120000 Btu (35Kw) with a 0.65mm
nozzle in the burner.

The oil pump pressure is included in the heat production equation. The 0.65
usgph is the oil throughput at 100 psi but your burner may be set anywhere
from 90 to 200 psi. what does your manual specify? I'd guess a 120000BTU
boiler would have started life with a 1.00 usgph nozzle and someone has
"turned it down" although this is only speculation on my part.


A few days ago I timed the cycling on the boiler when it was up to
temperature and found that the burner was coming on for 2.5 mins and
then of for approx. 2.5 mins which I thought a bit excessive.


These boilers (at least for mine & several others I know) use an
optical flame/ignition sensor. The sensor on mine lies several inches
behind the ignition electrodes.

Your short cycling is a symptom off the sensor glass being sooted up or
dirty.

This assertion staggers me - on every other pressure jet burner the symptom
of a dirty photocell is locking out of the burner. There is no way I can
conceive of a dirty photocell causing shutdown without the burner going to
lockout and requiring manual intervention to reset. The signal is read by
the burner sequence control, this decides whether flame is present or not
and either allows the burneer to run or locks it out, no ifs no buts.

On mine I have to unbolt the Riello unit (amazingly only 1
nut!) from the front of the boiler and clean the glass. I use a long
child's poster paint brush. At the same time the whole of the
combustion chamber should be scraped, brushed & vacuumed clean of soot
at the same time On my boiler, access is from the top via several lift
off plates & a few more screws/bolts.

A full service would include an instrument check on combustion
efficiency and oil supply pressure. Going rate here is 50-100gbp for a
full service.

Only do 1 or 2 DIY cleans before getting a full service, because the
condition of the electrodes and jets can deteriorate significantly over
a longer time span. Replacing these is not difficult but you do need
the correct gear. Cost of the right instruments is 200gbp up plus
regular calibration fees.


Jets are advised replacement at annual service although 2 years is not
unrealistic. The boiler should NOT be making soot.

To the OP Short cycling as described is likely to be related to the rate of
water flow through the boiler with a possibility of a poorly adjusted
bypass. The number of radiators is meaningless, it is the sizing (load)
which matters