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Say I set my thermostat to 72 F. (Yes, I'm in the crazy U.S. and use their strange measurement system). I am under the impression that an HVAC system will wait for the temperature to be off by a certain threshold before turning on and then it might even overheat/overcool by a degree or so to prevent short cycling. For example, if it's summer, my AC might kick on at 73/74 and cool to 71.

First question - is that accurate?

Next, I am wanting to know about dual zone. I see two possibilities here:

  1. Two thermostats trigger a cooling cycle or heating cycle when needed. The furnace/AC is turned on to handle the requested zone(s) at the time. This seems very inefficient as Zone 1 might get to temp and shut down while Zone 2 could trigger a cycle again 5 minutes later.

  2. Two thermostats communicate between the furnace/AC and themselves. In my first example, let's say that Zone 1 hits 74 and turns on. Meanwhile Zone 2 is at 72. A 'smart' system might open the dampers in Zone 2 to cool it down to 71 while it's also working to get Zone 1 down to 71. Even though the thermostat at Zone 2 would not have triggered this itself. These dampers would only open for a short time but would prevent Zone 2 from requesting a cooling cycle shortly after Zone 1's cycle turns off.

Which of these is it? Or is it something else?

Last question - does it require special thermostats?

  • The multi-zone system at my prior residence had the concept of master and secondary, the master zone got priority. – Tyson Aug 11 '17 at 0:28
  • Are we talking about a variable air volume zoning system, or a variable refrigerant (water) volume zoning system (such as multi-splits or hydronic zoning)? – ThreePhaseEel Aug 11 '17 at 0:53
  • My multi-zone hydronic heat system only fires when there is a call by any of the room/zone thermostats and the boiler control finds the loop temperature is below the desired heating curve, based upon outdoor temperature, the optional "min temp" selection, and the adjustable hysteresis. – Upnorth Aug 11 '17 at 4:48
  • HVAC control systems are quite dumb, relying on simple relays to turn things on/off. As companies seem keen on making "smart" thermostats, I'm sure they could design them to communicate with each other. This would allow the thermostat in zone 2 to turn on if it's close to the threshold temp, and the other zone is already calling for heat/cool. However, I'm not sure how much efficiency that could gain, since I don't think anybody is already pursuing that tactic. – Tester101 Aug 11 '17 at 14:24
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Conventional multi-zone arrangements work independently. They don't know or care about each other. They're tuned individually to achieve the desired outcome. Sure, there's some minor inefficiency, but not usually enough to warrant extra equipment costs and complex control algorithms.

Also, such systems would be difficult for homeowners to manage and adjust on a seasonal basis. Heck, many don't even know to change their filters or how to manage humidity properly. How are they going to work sophisticated thermostatic equipment?

  • Yes, a system can become complicated if you want to fine tune everything as conditions change, but many systems incorporate outdoor temperature sensors that automatically adjust for "seasons". – Upnorth Aug 11 '17 at 5:05
  • Too bad, I was hoping for the other method. I don't think this is too much for a normal user. Personally, I would set my thermostat to 22 C (72 F) and be done with it. For me that temperature should be static year round. – Paul Aug 14 '17 at 15:54
  • In my area we don't cool to the same temperature that we heat to. The dramatic seasonal differences in humidity, as well as the different perception of our bodies from summer to winter, calls for about 70 in winter and 74-76 in summer. This also involves adjustment of vents for the different flows of heated and cooled air. This is already too much for many folks. – isherwood Aug 14 '17 at 15:57
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Recently on this site there was a discussion of the fact that some thermostats installed in the US allow setting a temperature difference between when the a/c turns on and when it turns off. The purpose of this is to reduce wear on the compressor by reducing the number of on-off cycles. A disproportionate amount of compressor wear occurs when the compressor starts.

The idea is that if the thermostat is set to 22 C, then the thermostat waits until the temp is 23.5 C before it calls for compressor on and it stays on until the temperature is 20.5 C.

My 26-year-old Honeywell programmable thermostat does not do that as far as I know. It has a negligible gap.

  • This is becoming more common as sophisticated controls become more common. For some reason I can't remember the name of the setting; but I had to go through a whole building and set all the thermostats in the rooms to only activate the valves at specific temperatures that were higher/lower than the setpoints. – JMac Aug 11 '17 at 12:45
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The answer really depends on the technology and design of the HVAC system. I have a residential multi-zone forced air system. The individual zones are controlled by motor dampers in the supply ducts. These dampers are controlled by an intermediate piece of logic, which the thermostats talk to. This controller talks to the furnace & A/C compressors, and blower.

When I bought the house, this was a piece of relay logic. The thermostats where simple mercury on/off switches, and the mode of the system (heat,cool,fan) was controlled by a rotary switch on the furnace next to the controller.
When one thermostat calls, the system turns on, opens that damper, and supplies. There was an option (slide switch) allow either or both zones. (eg: either 'stat opens for itself or both) There were problems with that controller in that it closed the dampers while the blower was still in cool down. Modern dampers actually open at the end of the cycle and the off zone closes at the beginning.

I replaced that with a Honeywell TZ-4 controller and new dampers. This system allows modern programmable electronic thermostats (I use RTH6300Bs). It listens to both, and after a delay decides what to do. If both are in the same mode, it opens the appropriate dampers and proceeds. If they disagree on mode, there are rules for what it does. Basically first call wins, and it will change modes to satisfy the other call (after first satisfied or 20 minutes)

  • Looks like it's not really supported, however with some thermostats I can write my own scripts (Nest allows this). – Paul Aug 14 '17 at 16:49

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