Interpretation of oil boiler efficiency

[garyo]

I've just had a service and my 20 year old non condensing oil boiler is running very nicely at 91% net efficiency and 86% gross.

Does this mean the most I stand to gain from a condensing upgrade (in terms of fuel consumption costs) is the difference between 86% and the ~94% gross quoted by Grant and the like in their brochures? Or is the additional heat recovery from the condensing function additional to this?

Thanks

 

[big-all]

100% getting all the energy out
you will get up to the difference
any figure will tend to be possible but a bit less should be expected
so at best you are looking at 8% possibly nearer 5%

 

[Burnerman]

Personally I'd leave things well alone.....old oil boilers were often built like tanks and are very long lasting. I don't think you'd ever recoup the expense of a new one.
My ancient Panda 55 must be nigh on 50 years old - and it stays until it starts leaking!
John

 

[garyo]

Okay thanks. A pointless exercise then unless the cost of oil quadruples.

 

[Malc]

Oil was always more efficient than gas so gas gains much more by condensing. But the problem with oil is that it smells. That's not a problem with a flue gas of 180C as the smells are up and away, just the pigeons go what a whiff . Condensing oil is a lot colder flue gas and you get plumes of wet and smelly exhaust. Where does your flue point? At you neighbour? Do you like him?..........

But if you want to work out the gain just look at the efficiency for you boiler and compare it with a similar kW condensing boiler. Plus bear in mind that you need oversized radiators to make full use of a condensing boiler. To run in condensing mode the return must 55C or cooler. As temperature difference is typical 10C this is 65C out, 55C return, 60C average rad temperature, 40C above a 20C room (Tdelta = 40). In the old days rads were sized on 60C Tdelta, 85C out, 75C return, 80C average rad temperature, Tdelta = 60. Hence unless you have replaced your radiators with large ones you would not get condensing mode as much as rad output at Tdelta=40 is 60% of Tdelta=60. FYI rads these days are quoted at Tdelta=50 (78%) and Tdelta=30 (41%).

 

[Steelmasons]

To run in condensing mode the return must 55C or cooler.

That will depend upon combustion settings..

 

[Malc]

55C is the accepted maximum return temperature for condensing to occur. See here or google it.

 

[oilboffin]

Anything between 83%and 93% is good with oil 83 is a b-c boiler and would Not be in your interest to
change the boiler.what make is your boiler.

 

[Steelmasons]

55C is the accepted maximum return temperature for condensing to occur. See here or google it.

...
Google dew point temperature in relation to CO2% in flue gas...

 

[Malc]

Maybe you could provide a link. I accept that the dew point varies with the amount of water vapour in the flue gas and that's same thing as saying CO2 since the CO2 and the water both come from the oil burnt. But if you have set up the oil boiler for maximum efficiency then the amount of CO2 is between fairly tight limits, hence so is the water vapour, and so is the dew point. 55C is a rule of thumb. The cooler you run the return the better the efficiency but there is a step change from condensing to non condensing and that point is usually taken as 55C.

 

[Steelmasons]

A rule of thumb doesn't suggest return temperature "must" be 55c or cooler..

 

[Malc]

OK. If you are lucky and have your boiler set not to the ideal set point and maximum efficiency then you might get condensation mode to occur at 60C rather than 55C. Happy?

 

[Steelmasons]

Happy???.. What have you said that would make me happy??..you should be happy as you've learnt something from it...now what can you do for me in return???

 

[oilboffin]

Let's look at oil you get10.something kw out of a litre of oil.If a litre costs 70p that's 7p a kw
At the moment our electricity is about 13p a kw so you see how cheap oil is.

 

[garyo]

Anything between 83%and 93% is good with oil 83 is a b-c boiler and would Not be in your interest to
change the boiler.what make is your boiler.

It's a trianco eurostar utility 125. The burner itself is a recently new replacement.