Overconditioning
What is it and how does it work?
Zoning - Overconditioning
Overconditioning is used to send excess air into a zone that is not as conditioned as the rest of the home. This seems to be a point of confusion for many installers who expect it to work in place of an intelligent bypass damper.
An example of where this may be used. If a customer has a zone that is unoccupied regularly, such as a few bedrooms that aren't used or a conditioned basement, they will typically keep that zone at a lower temperature in the winter or a warmer temperature in the summer to keep costs down. If a home doesn't have unused areas of the home that are a designated zone, overconditioning is unlikely going to be a way to manage minimum airflow.- Zone 1 is set for 71 degrees and its holding steady there. It has an airflow limit set at 750 CFM
- Zone 2 is set for 72 degrees and its holding steady there. It has an airflow limit set at 650 CFM
- Zone 3 is set for 62 degrees and it too is holding steady. It has an airflow limit of 550 CFM
- Overconditioning is set for 5 degrees in one 3
The system has a 2 stage gas furnace with a minimum airflow of 1000 CFM.
An Overconditioning setting of 5 on zone 3 will falsely adjust the set point to 67 in Zone 3 when another zone calls. This will drive the damper open to assist the other zone. It will NOT modulate because it has a 5 degree demand. So if Zone 1 calls, it will remain open and Zone 3 damper will also remain open and Zone 2 will close once the furnace is firing. If the basement Zone reaches 67, it too will close. Most of the time this wont happen because hopefully zone 1 satisfies. Keep in mind as zone 1 gets closer to set point, it will modulate and more air will go into the dump zone.
If in this scenario all zones were regularly occupied and all set for 71 degrees, there is no zone that can be used to over-condition.
In almost all cases where a two stage furnace or condenser is used a bypass will be required. Don't fall into the mindset that overconditioning is a viable solution in a fully occupied home. Remember the lowest capacity of a two stage furnace or condenser is 70% of nominal. Only if you designed each of the zones to accept this much air would you be able to zone the system without a bypass.
There are areas where bypasses are not allowed in the traditional sense. In these areas, a bypass may be able to be ducted into the conditioned space somewhere but not back into the return.
A bypass also has it's limitations. If the furnace is running and dumping half or more of its warm air back into the return, it will likely cycle on it's limit or the bypass discharge air sensor may be tripped causing the bypass to close.
Traditional, non EcoNet zoning systems often use a bypass and at some point the same thing will happen. Either it will trip the discharge air limit, or the furnace limit. The difference is, it wont alert anyone. We should all know that limit switches are NOT operating controls, they are safeties and should not be used as operating controls to cycle equipment. Overheating the heat exchanger, blower motor or control board regularly will most certainly cause premature failure.
Zoning is best suited for variable capacity equipment such as the fully modulating furnace or variable capacity outdoor units.
Oversized furnaces will certainly cause problems too because they require more airflow.
An alternate option would be to set mechanical minimum positions on all the zones so that leakage can occur. This has to be done carefully or the system could over-condition those zones with no control.
Be sure to manage expectations. Two stage equipment should not have more than 2 or 3 equal zones and will likely require a bypass.
