So we have oil fired heating/water.. Main bits being a pretty big insulated water tank, 2.5m tall by 1.5m diameter approx. And the oil burner, which again is a pretty big unit, like the size of a washing machine.

The oil burner has various settings like temperature and timers .. But my question is between ECO and COMFORT.

ECO means that the burner will directly heat water on demand and deliver it to taps, shower, radiators etc.
COMFORT means that the tank will always be kept full of hot water so, delivered from the tank to shower etc and replenished.

In summer it seems to make perfect sense to use ECO, no heating, just showers basically.. My oil level virtually didn't move between March and October.

But what about winter .. Am I better to use ECO or COMFORT??
It's been on ECO and we've had heating on and off since October, I've gone through probably 400l of oil..

Does it make better sense to have the tank full of hot water in the COMFORT mode? I can't get my head around which is best -- and trial and error might burn through even more oil!!

What do you do?

how does it deliver directly to taps bypassing the tank? or does it have to heat up the whole tank on demand?

Dog knows.. The tank has loads of pipes in and out! It's pretty complex looking .. When it's on ECO.. You take shower, the flame comes on for the duration of the shower and then no more flame..

It goes via the tank still though.. Some kind of indirect heating loop I assume.

I can't really answer as all systems are different. I'd suggest running it in each mode for 7 days, and then compare the oil consumption. That should give you a reliable answer.

ECO mode will save oil and hence money.

The two drawbacks:

It may take longer for hot water to come through to the tap and a large hot bath may take a little longer to fill.

The boiler will have to heat the water at a faster rate as it is heating from cold and this could shorten the life of the boiler.

Swings and roundabouts...

It probably has a heatsink, an internal water tank or metal mass through which the water to be heated flows in pipes (like a reverse immersion heater).

Oil isn't as instant as gas, and it is more efficient and emissions friendly to burn less frequently but over longer periods. Also less likely to accidentally boil a large mass of water than the water in the heating flow pipes.

Hence the heat sink (or on our boiler strangely named "heat generator") is heated to a point above the necessary temperature for hot water or heating flow, and then when it has passed on the heat and is too cool it gets heated up again.

Our boiler allows me to see the current and target temperatures for hot water, heating flow and heat generator.

ps. we don't have instant hot water - given how well insulated hot water tanks are, not sure what the point is to have an option - instant just saves space, but if you have the tank anyway?