Oil boilers - nozzle size vs litres used per hour

ilikecookies

Hi folks,

I should probably give up trying to calculate theoretically how many litres of oil per hour my boiler uses but I had my oil boiler serviced for the first time this year (following its installation this time last year) and I couldn't resist asking the question.

As part of the service the engineer was explaining to me how the size of the nozzle impacts the number of litres used per hour.

What he explained at the time made perfect sense but upon reflection I'm now confused.

Basically he put in a new 0.85 nozzle (which was a like-for-like replacement against what was in originally). He said a 0.85 nozzle would burn 0.85 US gallons of oil per hour. This equates to 0.71 UK gallons per hour which in UK or US terms is 3.22 litres per hour.

I guess you then have to factor in that boilers are not 100% efficient and so working on the assumption that a boiler is 95% efficient this means a 0.85 nozzle would actually burn at least 3.4 litres per hour (probably slightly more as I understand that modern condensing boilers may not always be in condensing/high-efficiency mode 100% of the time).

Anyway this figure of 3.4 litres seems awfully high. In the height of winter last year I think we used more like 2 litres per hour.

Is the real usage much less as ultimately the boiler isn't actually on at "full blast" all the time? Eg. on start-up from cold it may run flat-out for the 1st hour or two to get the water temp/house up to heat and so use 3.4 litres/hour but then it will only fire up again as and when it needs to and so this figure drops dramatically?

Thanks in advance to anyone who can explain!

Cardew

Instead of complicated nozzle sizes etc you can look up the output(in kW) of your boiler. A litre of oil contains approx 10.2kWh.

So that 3.4 litres an hour is approx 35kWh which is about right for mid size oil boiler.

However that rate of consumption is only if the boiler working is flat out(heating both hot water and CH).

Boiler output modulates down when water is up to temperature set on boiler. So for instance if you only had one small, say, towel rail switched on, the boiler would only use a couple of kWh an hour, so a fraction of a litre of oil.

ilikecookies

Thanks Cardew - I was hoping you'd reply as your words are always wise

jrspigot

I found this old post while looking for something else.


You have made some incorrect assumptions, and the 'efficiency' of the boiler doesn't come into it at all.


It's also not a question of whether it is working 'flat out' or not.


The nozzle is rated to deliver a fixed flow rate at a fixed pressure, so as long as the pump for vapourising the fuel is set correctly , and the nozzle is not worn then for as long as the burner is burning you will be using fuel at that rate.


The fact that you don't think you are using it at 3.4 litres/hour is because you are not burning continuously.