Baseline equipment modulation control

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Hello all,

I'm having some trouble interpreting what is required in regards to
modulation w/ heating and cooling equipment. I have always trusted the
curves in the Trace ASHRAE library members however, I have recently seen
some conflicting documentation and model reports that are confusing me.
Per Trace curves, all equipment is inherently capable of modulation. At
100% load, a boiler consumes 100% of the power and at 1% load a boiler
consumes 1% of the power. It seems like Trace has this wrong.

The only thing I have been able to find in ASHRAE refers to cooling
part-load efficiencies, and states that minimum efficiency requirements
need only be adhered to if the compressors are capable of modulation.
However, there is nothing that defines whether or not compressors should be
modulating, so it would appear that the baseline is on/off. There are no
specifics outlines in regards to boiler/furnace modulation that I have been
able to find either.

Can you tell me in your experience, what the requirements are for baseline
equipment modulation for all equipments types? And does Trace really have
this wrong with all their ASHRAE library members or am I missing something?



*Jason Dietterick, CEM*

The Stone House Group
Suite 400


301 Broadway
PA 18015
tel. 610 868 9600 | dir. 484 548 4007
fax 610 868 2272

Jason Dietterick's picture
Joined: 2015-08-13
Reputation: 0

Almost all boilers modulate to prevent overheating. There is some minimum turndown and at that point the boiler would cycle. Fuel use of a boiler is generally linear if you exclude jacket losses radiated off the boiler.

Fuel use of a boiler that cycles at 50% load for 20% time (0.5 x 0.2 = 10%) = Fuel use of boiler that runs continuously at 10% load.

Even if Trace is ?wrong? the result is the same. One item to check is ?Miscellaneous Accessories? for the boiler which may be constant energy use when boiler is running (e.g. make-up water pump, combustion fan, controls, fuel oil pump, etc). Compare these in the baseline and proposed since they may create an artificial benefit or penalty to heating plant energy use.

HW pump modulating versus constant speed riding the pump curve is where you will see part load energy savings on the heating system. For LEED (90.1-2007), the baseline HW pump must have a variable speed drive if building is 120,000 SF or more.


sparker's picture
Joined: 2010-10-13
Reputation: 1