You’ve seen the advertising Wi-Fi thermostats that come with remote room sensors.
Some of the ads seem to imply that placing these temperature sensors in rooms other than the room where the main thermostat is located will allow you to individually adjust and control the room temperature in each of those other rooms.
Remote temperature sensors do just one thing – they read the temperature in the location they are in and relay that information back to the thermostat.
What the ad fails to say, but should say, is that in order to modify or adjust individual room temperatures, you’ll also need to install additional equipment not mentioned in the thermostat ad.
With a standard ducted system, it is not possible to achieve individual room temperature control by only installing temperature sensors in individual rooms.
A duct damper system or zoning system as they’re sometimes called will also need to be installed which controls the air flow in the ductwork to all of the rooms. Again, a remote sensor is only able to tell the thermostat what the temperature is in the remote room, it can do nothing more unless there is a damper / zoning system installed and interconnected with the temperature reading equipment.
Typical Home Layout
The typical one-story residential home heating and cooling system has one supply air duct system for distributing heated or cooled air to each of the various rooms and one return air duct system for returning / circulating air from the rooms back to the blower motor, located in the furnace, thereby creating a balanced system of air circulation. Some two-story homes have two furnaces and two AC’s and two duct systems – one system for each floor level. With a two furnace set up you’re able to individually control the temperature on each floor level, sometimes referred to as a home with two heating and cooling zones.
Basic Operation Example
For this discussion we’ll use a one-story home example. For a typical duct system, there is a larger trunk line or main duct coming from the furnace with smaller branch ducts coming off of the trunk line to service each individual room. To be able to control temperatures in individual rooms, you’ll need to install a zoning system with dampers in the duct system to regulate or control the air volume flowing through the branches of duct.
The zoning dampers are actuated by electric motors that open and close them and the actuator motors are controlled by either wired or Wi-Fi controls for each duct air damper. This gives you individual control of air volume for the rooms served by ducts that have dampers installed. Note there are manual damper systems installed in some homes and these require a person to physically / manually open or close each damper as necessary.
After installing an automatic zoning damper system there are other air related issues, created by adding the zoning system, that need to be addressed. When the home was initially built, the duct system was designed and sized to handle the calculated load for the entire house.
Again, the important point here is that the duct system was designed and sized according to the needs of the home and the same applies to the furnace blower in order to create a balanced system of air circulation – being delivered to and removed from each room.
Duct Sizing – Trying to Push More Air Through a Duct
Sometimes dampers are mistakenly thought of in terms of being able to close off one duct branch (damper) and force more air volume, CFM (cubic feet per minute) through another duct branch. The reason this is not accurate is because ductwork can only handle a given maximum CFM volume of airflow according to its size. Closing off one duct branch with a damper simply cannot force another duct branch to handle more CFM than its size allows it to handle. Closing off one duct branch will increase the air pressure within the system and instead of more air flow or CFM you’re actually creating back pressure within the duct system.
When multiple dampers are closed, you multiply the back pressure effect within the system. Duct system back pressure increases the work the blower motor has to do when trying to overcome the backpressure created by closing off dampers and move air through the system. This scenario shortens the life span of the blower motor.
Energy Efficiency of Zoning
To try to balance and alleviate this back pressure within the duct system created by the installation of dampers, it’s then necessary to also install a separate diverter duct from the warm/cool supply air side back to the return air side. The bypass helps relieve the built-up excess pressure within the supply air side by allowing air to escape back into the return air side.
This is highly inefficient and a complete waste of energy dollars because you’ve already paid to heat or cool that air that you’re now diverting / dumping right back into the return air side which will go directly back to the blower without it ever reaching and heating or cooling any rooms first. You’re paying to heat / cool air and then not using it.
The presumed energy savings from a damper system closing off individual (not in use) rooms and not heating or cooling them is largely a misnomer. The heated or cooled air that normally would have gone into those rooms is simply being diverted / dumped back into the return air side and wasted because the air pressure within the duct system has to be balanced.
The intent of sharing this information is simply to enlighten and inform the reader about what remote temperature sensors can and cannot do and is not intended to discourage interest in Wi-Fi thermostats. We hope this information is helpful for you. Please look for other Air-Smart helpful articles and new ones that are coming soon.