One Thermostat Control for your House

One Thermostat Control for your House

It is impractical to have separate thermostats and other equipment in each office for individual comfort control. That is why you only have one thermostat in your home. That one thermostat controls the temperature for every room in your house. The area or rooms that are controlled by a single thermostat is referred to as a “zone”. If you only have one furnace at home, you only have one thermostat and therefore you only have one “zone”. In an office building There will be several zones per floor. In each zone a thermostat controls the temperature in several offices. It operates a zone box, usually a “VAV” (variable air volume) box. In our model we are going to use a VAV box to illustrate how the thermostat controls the office temperature of your Air Conditioning system.

A VAV box has a damper inside that is operated by an actuator controlled by the thermostat. This damper meters the amount of cool (50 – 55 degrees) air delivered to the office. The position of the damper that meters the proper amount of air to the office is determined by the thermostat. The thermostat detects the temperature of the air in the office. If the temperature is higher than the set point (the temperature you have the thermostat set for), then the thermostat causes the damper in the VAV box to open enough to allow more cool air to enter the office to absorb the additional heat that caused the temperature to rise. The temperature is then lowered. If the temperature goes below the set point, the flow of air is reduced by the damper. The reduced volume of air permits the heat load in the office to build up causing the temperature to rise. Obviously these changes are very subtle. The thermostat is always trying to reach an equilibrium or balance between the temperature in the room and the set point. When this balance is reached, the damper does not move until the thermostat detects a change in temperature.
There are many elements in place working together to maintain a comfortable temperature in a space. This is necessary to make certain the proper volume of air is always available for the VAV box to do the job the thermostat asks it to do. This includes maintaining a constant static pressure in the supply duct serving the VAV box.
Static Pressure is a term you hear frequently and do not have a clue to what it is. It must be bad or it must be good depending on the context in which it is used. Engineers use the term like everyone knows what it is. Static pressure is very important because without it we could never get the air where we want it.
It helps to think of air as a fluid just like water is a fluid. Air is a gas and water is a liquid, but they are both fluids.

The principals applied to a fan or blower is pretty much the same as the principals applied to a pump. They both move a fluid and they both put that fluid under pressure. If the nozzle on your garden hose is turned off, you feel the hose swell a little and get more rigid. This is because the pressure in the hose is pressing against the sides or walls of the hose even though it is not flowing. If you had a gauge in the hose you could see how much pressure is in the hose. The same thing applies to the air in the ducts. It is called static pressure, and now you know. This pressure is what gets the air through the duct (hose) and out the diffuse (nozzle). I am sure you have been in the shower and someone flushes a toilet, starts a dishwasher or turns on a faucet and suddenly the shower water gets very hot. You yell, curse a little and adjust the hot water. About then they turn off the faucet or the toilet tank finishes filling and your shower gets very cold. This is because of variations in the flow in the cold water side of your shower. More or less gallons per minute are being used causing the pressure to change. The same thing will happen in a VAV (variable air volume) system. Think of a VAV zone box that controls the temperature in an office as a valve that opens and closes more or less to supply the required amount of flow to satisfy conditions.

As discussed above, on a mild day the offices are satisfied easily and the thermostats throttle down the VAV air flow. This could cause the static pressure to increase (remember the hose when you turned off the nozzle). If it is a hot day, the VAV boxes may fully open to get all the air flow to the offices that they can. This could cause the static pressure to drop drastically effecting the ability to maintain the needed flow to satisfy the offices. Obviously Static pressure regulation is needed and becomes very important. Static pressure must be maintained during the peak flow demand (cooling) yet be controlled at the same level during low air flow demand (heating) otherwise high static pressure could seriously damage duct work, VAV boxes and other components. High static pressure also drastically increases the cost of operating the fan system.
There are several ways to control static pressure through all air flow demand ranges. The most common method is with vanes or dampers on the inlet of the fan or blower itself. The Static Pressure is monitored by a sensor (a kind of gauge) mounted at some strategic place in the duct system. The signal from the sensor is transmitted to a controller that responds by modulating (opening or closing at a controlled rate) the vanes or dampers on the fan inlet. As the demand for air flow goes up, the vanes open more to permit more air to be supplied by the fan to keep the static pressure from dropping. If the demand for air flow goes down, the vanes throttle closed to reduce air flow and prevent the static from rising.
The next time you hear an engineer mention static or static pressure, hopefully you will understand and will not be intimidated by the term.
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