how much heating oil does the boiler use?
The simple answer for a domestic central central heating boiler is two to three litres an hour if it’s continuously alive and burning oil. In reality it’s half of this because when you turn on the boiler it runs until the system gets up to temperature and then it stops until the system cools. If I watch my boiler, it’s active for 40% to 60% of the time.
The calculation: find how many hours of heating you’ve programmed into your boiler. If you had 6 hours a day, half that to get 3 (=50% active) and multiply those 3 hours … by 2 litres for a flat/condo boiler or 3 litres for a house boiler. So the house boiler uses roughly 3 * 3 = 9 litres a day depending on weather and house insulation. Read on if you want to equip yourself to monitor your actual oil usage.
My need was to meter the use of oil for billing purposes and I met that easily and inexpensively. Below I describe the equipment available while on another page I have a daily display of how much oil is used. Alongside my cheap and easy equipment, I also have a device that measures the oil level in the supply tank that shows me the daily oil usage in my home automation hub (Home Assistant). But if you simply want a meter to divide up a heating bill say, read on.
Central heating boilers burn heating oil from a tank in the garden – we do this here because there’s no access to the ‘natural gas’ that’s piped around the UK. Oil is 25% more expensive than gas. As there are two households drawing oil from the same tank I was keen to find a way to divide up our use of fuel. Your need will likely be different.
So how much oil is used by a boiler? A first suggestion was to look for a diesel oil flow meter that would operate at around 3 litres an hour or 10 litres a day. The oil flow meters I found were suitable for 100 litres a minute and that’s too much. The mini-flow meter at Rapid (below) could however measure slow oil consumption. Another, from Adafruit seemed ideal – until I saw that its data sheet quoted its accuracy at 10%. The deal breaker that sent me looking elsewhere for a meter was that a flow meter needed plumbing into a fuel line – and I was hesitant to introduce a device into the fuel line that might one day block and cause the boiler to fail. So I changed tack …
My simple solution was a 240v hour meter. It’s a part you’d find on a machine in a factory where it would count the time until a service was required. This timer is mechanical so it doesn’t reset if the power fails. When it was time to service the boiler I got the boiler service man to wire the meter in parallel with the heating circuit so that the meter would only count the hours when the boiler was burning oil. You don’t want the meter to count hours when the boiler was simply ‘on’ as set by the heating programmer. The boiler man succeeded by trial and error. The meter cost £5 – which is a more appropriate price than paying up to £150 for an oil flow meter.
How is the oil meter calibrated?
The meter measures burning hours, and if we know the oil nozzle flow rate, we can turn that into litres. My calculation goes like this for two almost identical boilers:
The boiler spec sheet shows that a boiler nozzle size of 0.5 will lead to a certain flow rate – quoted in US gallons per minute. I calculate that as 1.895 litres per hour. If heating oil costs £0.50 per litre, the boiler costs 1.895 * 0.50 (about £1) to run continuously for an hour. For billing purposes, each hour the meter clocks up costs £0.95.
The boiler spec sheet shows that a boiler nozzle size of 0.85 will lead to a certain flow rate – quoted in US gallons per minute. I calculate that as 3.222 litres per hour. If heating oil costs £0.50 per litre, the boiler costs 3.222 * 0.50 (about £1.60) to run continuously for an hour. For billing purposes, each hour the meter clocks up costs £1.61
Given the above, verified by how much the oil level dropped over several days, I concluded that here was a way to apportion the cost of heating oil between the two households. I’m using the ratio of working hours towards dividing up the cost of the last tankful of oil.