Sizing a boiler - short cycling?

[Fatboise]

Just wondering if anybody can help me out here. I have an oil fire

boiler for the central heating and hot water. We've been in the hous for a few months now and we seem to be using a lot of oil and I' trying to see if I can get the boiler running more efficiently

Now it's a big house (2600 sq feet) and I understand that a house tha size will be hard to heat. The heating would be on inthe morning for a hour and in the eveniong for 2-3 hours. There is 15 radiators and th hot water cylinder. The boiler is 120000 Btu (35Kw) with a 0.65m nozzle in the burner.

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

I've tried searching for info but find it hard to find anything on th boiler cycling question. I had a look at the sedbuk website but don' have all the measurments to size the boiler - need to find the drawing again.

Any inputs much appreciated................

-- Fatboise

[Doctor Drivel]

You have three alternatives:

1. A new modulating boiler (expensive)

1. A heat buffer. A cylinder, can a cheap normal direct domestic cylinder, that the boiler heats up directly, not via a coil with two cylinder stats that prevent boiler cycling. The CH can be taken off the cylinder and TRVs on all rads using a Grundfos Alpha auto modulating pump. The DHW can also be taken off the buffer cylinder, or have a quick recovery coil cylinder, again with two cyl' stats and taken off the boiler directly.

2. If you have good mains pressure install an 'integrated' heat bank/thermals store, again with two cyl' stats, CH off the store directly with Alpha pump. The cold mains water is heated via the heat of the stored water. Explanation: 2 and 3 above the boiler comes on for one long efficient burn to heat the stored water. Only when the water cools off substantially does the boiler come in to fully reheat. With 2 and 3 above, install a magnaclean filter on the CH return to the buffer or thermal store.

[The Natural Philosopher]

If you have a 2500 sq ft house, with only average insulation, you could end up burning up to 600 quids worth of fuel a month.

The fact that you have a 35KW boiler shows that someone thought you needed one...

A 50:50 duty cycle implies you are needing 17.5KW of heat ..whilst the shortish cycling is not hugely good, the fact that a 50% duty cycle is needed shows you are indeed needing a LOT of power.

To get a linger on and off period, change the thermostat to one with more hysteresis.

[jim_in_sussex]

The boiler is 120000 Btu (35Kw) with a 0.65mm

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. 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.

HTH

[Donwill]

Do you know if it's cycling on the room thermostat or on the boiler's own thermostat?

Have you measured the temperature on the flow and return near the boiler?

Is the pump providing sufficient flow of water thro the boiler (when firing) so as to provide 11 deg C across the boiler?

Donwill

[John]

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.

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.

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

[Pete C]

Hi,

Are you able to measure the return temp to the boiler, eg with an IR thermometer?

cheers, Pete.

[Fatboise]

Thanks for the replies - very helpful! Pete, Haven't any way to measure the temp but will look into it thi week. I'll get back with any info that I find - once again thanks I have better understanding of the problem

-- Fatboise