Why boiler outputs have a range?

[Bogart]

Probably been asked before but cannot see where so here goes again. Just was thinking re my oil boiler it is rated at 25kw to 35 kw currently fitted with a nozzle that supposedly can manage up to 31kw.
What is the reasoning I can fit one of 3 nozzles to obtain 3 maximum outputs? Why not just fit the biggest nozzle and be done?

 

[Last Plumber]

Because your system may only need 25 Kw (for example) and there is no point over egging the pudding just to warm birds bums, so to speak.

 

[Last Plumber]

Hope that makes sense?

A lot of people think, the bigger the better but that is not the case. The central heating system design, output, pipe sizes etc, dictate what the maximum flow rate will be.

 

[oz-plumber]

The real reason would be the manufacturers don't want to make 3 or more boiler sizes.
So they give you and the installer the option of range rating their boilers to suit.
Saying that, all gas boilers are rated at a certain kW output but can be adjusted to suit kW output required.

So at the end of the day it comes down to the installer setting the boiler up to the required kW output.

 

[Ric2013]

Biggest nozzle means boiler would have to switch on and off more because it is always providing more heat than needed. Size it properly and, at least sometimes, it can run continuously.

 

[Bogart]

Well having worked out my loading it is about 25kw with everything fired up rads and water. So would not a boiler with a max output of 25kw be suitable?

 

[DaveyHep]

Well having worked out my loading it is about 25kw with everything fired up rads and water. So would not a boiler with a max output of 25kw be suitable?

In theory yes but.....what if your calculations a bit out? Or you later want to add extra rads? I would recommend adding something for wriggle room. Get something with (say) a 30kw max output and initially size the nozzle at 25kw. See how you get on.

 

[Best]

Well having worked out my loading it is about 25kw with everything fired up rads and water. So would not a boiler with a max output of 25kw be suitable?

No. If you had an oil boiler that had a maximum output of exactly same as your house requirement, then that would mean the boiler is always over firing.
Oil boilers are actually rated too high for their claimed maximum outputs. You will notice that they come factory set (to some extent) for a lower output - usually the middle one if there are 3 possible outputs.
If on highest output it is highly likely the baffles will get damaged eventually.
Oil boilers don't modulate, so whatever the output the choosen nozzle and oil pressure gives, is a constant. Your oil boiler needs to run at an output for highest demand. Any higher would give quicker heat up and recovery, but will cause cycling and wasted fuel.
As has been mentioned, you also want a boiler that can be slightly uprated if extra radiators are added in the future.