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Thread: Advanced thermostat - is this feature available?

  1. #1
    DIY Member dirtmover's Avatar
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    Default Advanced thermostat - is this feature available?

    The issue is that I want to be able to equalise the temperature of my home without running the furnace fan continuously and in particular control the differential between upstairs and downstairs. This is especially problematic overnight in the summer when the AC tends to cycle less and I can end up with a ~7F-10F differential.

    What I would like is a thermostat/controller that takes input from multiple sensors and uses the average reading to determine when to run the heat/AC. When the differential temperature, max-min, of all the sensors is above a certain threshold the fan will be run.

    Simple? Is such a beast available? If so what words should I be using when searching for one?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Honeywell and others make themostats with multiple sensor inputs. But, if you don't resolve the return and supply balance issues, you may not be satisfied with the results. ANother possibility would be a zone setup, if your minimum air flow can be obtained with part of the system shut down for part of the time.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Lightwave's Avatar
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    I'm not aware of any thermostat that can do what you want.

    I wouldn't bet on the fan being able to correct an 7-10 degree spread between floors, either. You might need zoning.

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    DIY Member dirtmover's Avatar
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    I understand that balancing is a factor but I don't believe the system is too far out because when the AC is having to work, during the day, the temperature is fairly even. At night, unless it's very warm outside, the AC tends to stop cycling and this is when it ends up far warmer upstairs.

    Actually the fan does a good job correcting the temperature difference but I don't wish to have it running 24/7.

    Also, there are substantial periods of time throughout the year when neither AC or heat are required. It would be nice to be able to automate fan operation with the goal of equalising temperature. I realise you can get thermostats that will cycle the fan but without temperature differential feedback run time cannot be optimised.

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    Moderator & Master Plumber hj's Avatar
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    A rudimentary way of doing it would be to use a heating thermostat downstairs and an AC one upstairs wired in series. Set them at your normal setting, or the downstairs one a little lower and the upstairs higher. IF the upstairs gets too warm, it will activate the circuit, but the fan will not run unless the downstairs is cool enough to have energized its thermostat also.

  6. #6
    DIY Junior Member Rat's Avatar
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    Quote Originally Posted by dirtmover View Post
    I understand that balancing is a factor but I don't believe the system is too far out because when the AC is having to work, during the day, the temperature is fairly even. At night, unless it's very warm outside, the AC tends to stop cycling and this is when it ends up far warmer upstairs.

    Actually the fan does a good job correcting the temperature difference but I don't wish to have it running 24/7.

    Also, there are substantial periods of time throughout the year when neither AC or heat are required. It would be nice to be able to automate fan operation with the goal of equalising temperature. I realise you can get thermostats that will cycle the fan but without temperature differential feedback run time cannot be optimised.
    Why don't you want to run the fan in continuous mode? It is much better for your home overall to do this. Filters are more effective, hot and cold spots are less likely and fresh air (if equipped) is constantly being cycled. For the times of the year when neither the A/C or heat is needed, but you want to run the fan, temperature feedback is not needed; the fan just runs.

    Using something like the C-750 (CC-750 comfort control center) to sloww down the RPM to 400-600 RPM is a great way to keep continuous fan operation while using less energy to do it.

    If you have a system with an ECM motor (or X-13) you can use the Honeywell IAQ stat to control RPM in the same way (Honeywell Vision Pro, IAQ). this model will accept a remote indoor sensor, but will not average the sensors. If you have multiple sensor locations (min of three required) it is possible to wire them in series parallel (they are just resistors after all) to achieve the same thing as true averaging. For true averaging you will need to step up to a commercial grade stat.

    Averaging is not going to be the best option though; however, if you only need it at night (and you sleep upstairs) it will probably work ok for comfort although you may see higher utility bills.

    The best option is to find a good HVAC contractor who is WELL versed in multi-stage equipment zoning. You will see both better comfort and lower utility bills. Depending on your location I may be able to make recommendations.

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