Staged smaller boilers vs modulating bigger

[Waramanga]

Hi all,
What are your thoughts on a small radiant HHW system of roughly 65kW being supplied by two smaller or one bigger modulating boiler/s? Is there an efficiency dividend? My reasoning is that most of the time through the year here only one boiler would be needed apart from warm-up in the morning or really cold days midle of winter, so one could be running at full power and perhaps higher efficiencies for more of the time. The system will have a number of zone valves with the smallest zone potentially just 2.5kW, so a smaller modulating boiler will do less on/off cycles for the load so more efficiency gains?
Or, are these modulating boilers so efficient at all loads it doesnt really matter?
cheers

 

[ChrisConley]

Two smaller boilers gives you a some redundancy, which is nice.

Most mod-con boilers efficiency increases with low firing rates but not all, I'd check the efficiency curves at part load for the boilers you're looking at to get a better comparison.

 

[DRWeig]

Good answer, Chris,

I gotta add that the redundancy issue strikes a chord with me. I've been burned twice by boiler failure over the years - in each case, having two smaller units would have enabled me to maintain freeze protection (though not comfort) with one boiler dead...

Waramanga, let us know what you decide to do!

Good on ya,

Goober Dave

 

[cry22]

You need to look at condensing boilers with a wider delta T, especially for such a small load.
the colder the incoming water, the more efficient they are.
They are also more efficient at part load, so modulating two condenssing boilers at part load will get you more efficiency than one large boiler (condensing type that is).
You do need to watch the firing cycle limitation of the boiler manufactuer (such as Fulton), some have a maximum number of cycles per year at 12,000. 2 boilers at part load will cycle more than one boiler at full load. If you exceed the 12,000 cycles, you could wack your 10-year warranty within 6 month.