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‘It sounds a little biased’
Excellent control of temperature serves the environment including health and protection, along with freedoms both economic and regulatory of the conservative. The best footprint respectfully uses the least possible resources and as it turns out saves the most money and provides the best environment.
Temperature set points are transition points of control. Other articles will explain in more detail how we control heat and cool functions. This article focuses on set points and set point bias.
Some assume a room to have one set point. There are three. The value most commonly understood as the room temperature set point is actually the ‘ventilation’ set point. Some control code mistakenly offsets this for the first stage of cooling transition. Most of its value is wasted by then.
Used properly, ventilation control is ‘analog’ (varies gradually rather than OFF/ON) and pre-acts inversely to the temperature demands of the controlled zone.
An initial example of this assumes cooling capacity from outside air, let’s say 50 degrees (Fahrenheit) and a room set point of 71 degrees. Under normal demands, introducing 68 degree air will maintain the room at 71 degrees.
As the room drops from 71 to 70 degrees we try to introduce 74 degree air, but it’s not possible because the most we can gain is 70 from the room, so the damper controlling the introduction of outside air closes to its minimum limit. Note: I defeat the minimum position (minimum = closed) if the room temperature is below 65 degrees.
As the room rises from 71 to 74 degrees we introduce 50 degree air (damper fully open to outside air at 50 degrees F.) in varying degrees (indexed as 68@71rt; 62@72rt; 56@73rt and 50@74rt). Used in this way, economizer cooling floods the zone with more outside air than indoor air quality control would. The room stays fresher longer.
Waiting until the room reaches 73 degrees prohibits the pre-active free cooling from effectively doing its job and the minimum position must be increased to achieve air quality exchange requirements, which will increase the demand for mechanical heating. So using ventilation as first stage cooling is wasteful and less healthy.
As you can see, proper introduction of free cooling and ventilating air is where 98% of the cost savings can be achieved and consequently where most of the DDC control code needs to focus. In this case cost savings equals environmental savings and nature takes care of air quality naturally.
Before we explain the other two set points, which are mechanical heat and cool transition points, we need to introduce the concept of bias.
Bias is a value based on seasonal outside air temperature. It modifies the ventilation set point reducing it by zero degrees in cold weather all the way to minus four degrees when outside temperatures project to exceed 110 degrees.
Early morning provides the most effective cooling air. Bias of the ventilation set point maximizes the cooling effect that outside air provides by asking for more of it at cooler temperatures. Building mass releases the free cooling gradually so that mechanical cooling is required much later in the day.
And that is the sole justification for biasing the ventilation set point. Again, it allows mechanical cooling to come on later in the day and saves lots of money and energy in doing so.
Additionally, since the heating set point is fixed a few degrees below the biased ventilation set point, the heat won’t come on unless the room gets up to four degrees cooler during what’s going to become an extremely hot day.
Pushing the heat set point back increases the cooling capacity of the building mass. Otherwise, ventilation cycle savings would be negated by mechanical heating costs.
Now that we have pulled the curtain back on the mechanical heating set point, we can add that the mechanical cooling set point is fixed a few degrees above the biased ventilation set point. Heat is typically fixed at 2.5 degrees below and cooling is typically fixed at 4.5 degrees above the biased ventilation set point.
This allows for a seven degree spread between the heat and cool set points. The bias effectively moves them beyond seasonally logical demands (64.5 for heat in the hot summer and 76.5 for cooling in cold winter). So, there are three set points (heat, cool and vent) that are influenced by a bias value and bias amplifies the effectiveness of the ventilation control.
Most economizer savings occur during the spring and fall. Mechanical heating and cooling are less natural than economizer control and thus less comfortable to the occupants.
Traditionally mechanical heat and cool cycles are digital (OFF/ON). Industry pursuits explore various analog answers to mechanical demand costs, but the most aggressive approaches cannot exceed the savings of not turning them on to begin with and that can only be delayed through superior economizer control… which is why it’s called “economizer” control.
This discussion targeted commercial temperature control. Future featured articles will explain why ASHRAE cycle-2 is best for traditional heat and cool control, while cycle-3 is most appropriate for ventilation (economizer) control and how to employ them.
Hospitality control is dedicated to occupant comfort dismissing economy for accuracy.
Residential control likely employs seasonal bias to keep energy costs down… you can make a personal choice of setting heat at 70 in winter, 67 in summer and cool at 78 winter and 74 summer. That’s exactly how bias works, but in commercial automation it can be programmed to happen automatically.
Energy costs, environmental impact and air quality can all be positively impacted through the proper use of biased economizer control. This should be everyone’s goal.
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Photo or Ion? Battery, battery backup or power? Which one do you need and why can’t you simply forget about them? Finally, why are the instructions the very last thing you find crammed into the very last little corner on the ‘instructions’ sheet?
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‘Beat the heat’
The summer rush will be here soon as the salmon migration of HVAC vans hit the streets. They are rushing out to fix someone’s air conditioner (AC), perhaps yours. The technician tinkers and mumbles something, makes a call then runs to the truck and returns with what looks like a stubby can of Red Bull, without the label of course.
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Pause at the door as you enter the building. Crack the door open just enough to slip a pencil in. Place your face near the opening and you should be able to feel a gentle breeze in your face from inside to outside. Then check from inside and you should feel nothing. In most homes you will likely sense no breeze either way. These are balanced conditions for residential and commercial buildings.