On our college campus, we have quite few buildings with VAV, reheat systems. We are in a colder climate (Ave winter temp 25F).
I am trying to determine how cold I can make my night setback temperature without running into problems. Would love to hear from some who have played with lower setpoints.
This greatly depends upon your heat loss of the facility and the mass. Typical school and college buildings in Ohio are have interior and exterior walls that are masonry block and on average tend to tolerate a setback of 62 degrees overnight for a 2 hour recovery to setpoint in those early 10 degree F. mornings. These buildings are also typically slab on grade single story or two story buildings.
Other affects are proper envelope construction, vapor barriers, and insulation. I have found many a school facility lacking proper R ratings and vapor barriers over the years.
The difficulty I am having with your question is that it assumes that a single value setpoint is the answer. Logically, this cannot be true.
A building that is in design temperature conditions will never be able to raise space temperature (provided the design engineer has not grossly oversized equipment). Therefore it follows that the colder it gets outside the less capacity there is to raise space temperature; ergo, your setback temperature should be reset upwards by colder outside temperature.
Further, the coldest time of the day is typically when a building is trying to recover space temperature. Add in the fact the internal gains such as lighting and plug load will not be there and that there is no solar gain... well, you can see that a low night setback temperature is an invitation to a rash of cold calls first thing in the morning.
The good news is that energy savings can occur during those 95% of hours when the building is not at design conditions and that there are more hours during the warmer times than there are at colder. A night setback reset should properly maximize savings when it is available and minimize complaints when it isn't.
Oddly, most (if not all) canned software solutions from BAS manufacturers do not offer a setback temperature reset and solely focus on managing (optimizing) the start-up of systems to recover from night setback. Therefore, implementing a reset strategy will reqire a bit of custom programming (if that is your situation).
One final unoccupied cycle feature that I have heard is effective with VAV/reheat systems with supply and return fans is to run just the return fan at low speed when it get very cold outside. The circulation of air, as I understand it, evens out the building temperature and minimizes cold spots in the building without significantly increasing cost.
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