Boiler data - % efficiency at min/max modulation

[DJKent]

We all know that short-cycling is bad - and that it's good for a boiler to have a wide modulation range.

But I can find no facts in data sheets - is the boiler as efficient at it's lowest modulation level: as at full gas?

Has anyone ever had hard data from a manufacturer?

I'm wondering if we're fooling ourselves - like heatGeek's graph that shows we may be fooling ourselves about efficiency - his graph shows boilers with datasheet claims of ~ 95 % efficiency - instead run at ~85% efficiency when return temps are above 50...

 

[Chuck]

The full spec sheet for plenty of modern boilers give efficiencies determined under various operating conditions, e.g.

https://www.vaillant.co.uk/downloads/aproducts/ecotec-plus-48-64kw/ecotec-plus-tech-sheet-1-8-web-1205237.pdf

This, gives four values: PCDB seasonal efficiency, Part L2 seasonal efficiency, Net efficiency at 100% output (80/60), Net efficiency at 30% output (50/30). Not exactly what you asked for but quite close.

In practice, efficiencies at low modulation levels go up because, for a given emitter system, the return temperature is lower.

Modern boilers and controls limit "short cycling" to typically six (gas) or three (oil) cycles per hour. I doubt that anyone who isn't working in a manufacturer's design lab actually 'knows' what impact the has on the life of the boiler. Based on the long term warranty cover available for modern boilers I suspect that it's not that harmful to the boiler.

I don't like short cycling because the oscillating temperature of the radiators reduces the comfort level for the occupants who then increase the thermostat set point to compensate. Also the slip-stick noise of pipework expanding and contracting can be annoying.

 

[DJKent]

This, gives four values: PCDB seasonal efficiency, Part L2 seasonal efficiency, Net efficiency at 100% output (80/60), Net efficiency at 30% output (50/30). Not exactly what you asked for but quite close.

Thanks @Chuck - I checked those numbers and many boilers have better efficiency at the "30% load, 30 degree Return temp) level.
You quote those numbers as '50/30' degree C: where did you find the 50C Flow value - I only see the Return being given ?

Either way -those numbers are changing 2 factors (return Temp / % output).
It's widely know from heatGeek and etc that lower temp Returns makes for a big jump in efficiency.
But I haven't seen anywhere the % change as % output changes.

It's possible for example, that dropping output below 30% may, for some boiler designs, seriously reduce % efficiency?
I'd love to have hard facts!