Thursday, April 21, 2011

Balancing Energy Efficiency and Occupant Comfort

I received notice yesterday that our abstract for a talk at Greenbuild, to be held in Toronto this year, has been accepted. The subject will be “Can we have our cake and eat it too? Balancing Energy Efficiency and Occupant Comfort.” If you have been following my blog, you will know that I am concerned that the USGBC is putting global warming and energy use ahead of occupant needs, according to their hierarchy of importance. We will address the possibility that they are not mutually exclusive, as has been implied by some. I suspect that the LEED credit for 30% increase in minimum outside ventilation air will not make the cut for 2012. There is a lot of data, nonetheless, to indicate productivity is enhanced with an increase in ventilation rate. So, we will discuss sustainable designs that will allow for an increase in ventilation when there is no energy penalty, but it will likely not be in place all the time.

We will start with my article from December's ASHRAE Journal (a copy is available on the Krueger website ( and expand on that. The new Krueger DOAS fan terminal, ( along with today’s abundance of control options, allows for a very flexible response to weather and building loads. I envision a set of operating strategies, all of which are simple control variations.

1) At design load, the rooftop DOAS unit will deliver the minimum quantity of outside air in accordance with ASHRAE 62.1, in as efficient a manner as the rooftop make-up air unit can achieve. The zone DOAS Series terminal will meter the 62.1 minimum quantity of conditioned outside air to the space. The inlet temperature from the make-up air unit will likely be a function of both the local design day climate and the system employed. Additional zone cooling demand will be handled by the sensible cooling coil on the zone unit.

2) At conditions other than design, which are likely to be most of the time, the rooftop make-up air unit can flex to run in reduced compressor output (assuming a variable capacity digital scroll) economizer, direct evap, indirect evap, of a mix of all the above, again depending on the design of the unit. The zone unit, which has a pressure independent VAV inlet controller and an ECM fan motor, can do whatever is necessary to maintain conditions in the space. This includes increasing the quantity of ventilation whenever practical (when there is little energy penalty), which will have a payback on the occupant productivity side. Again, the sensible zone cooling coil will modulate to maintain comfort conditions in the space.

The really good news is that most of the hardware is presently available to provide an optimum balance between energy and ventilation rates. All that is needed is a set of operational sequences in the zone and roof control units.

Authored by: Dan Int-Hout, Chief Engineer Krueger