room temperature control

I have a project that envolves AXB-TEMP?s.I have one sensor in each room of the house. In each room of the house I have heating/cooling systems which I control through relays.
Can anyone give me an example for temperature control algorithm with a hysteresis function ? Now I?m controlling the room by comparing the room temperature to the setpoint, but the client is not satisfied, because this type of control doesn?t consider the inertia of each hvac system(and it takes a long time for a room to heat or cool...and in the client's own words " this should be more than an on/off control"

thanks,

Liviu

Comments

  • yuriyuri Junior Member Posts: 861
    you should do something that checks how far the current temperature is "away" from the setpoint.
    The bigger the difference, the harder the your HVAC should heat or cool...
  • ericmedleyericmedley Senior Member - 4000+ posts Posts: 4,166
    liviu.cioc wrote: »
    I have a project that envolves AXB-TEMP?s.I have one sensor in each room of the house. In each room of the house I have heating/cooling systems which I control through relays.
    Can anyone give me an example for temperature control algorithm with a hysteresis function ? Now I?m controlling the room by comparing the room temperature to the setpoint, but the client is not satisfied, because this type of control doesn?t consider the inertia of each hvac system(and it takes a long time for a room to heat or cool...and in the client's own words " this should be more than an on/off control"

    thanks,

    Liviu

    One thing to keep in mind is that heating / cooling systems usually need some kind of power down cycle or you may risk damaging the unit. for example some gas furnaces need to run on 'fan only' a few minutes after a heating cycle to allow for the unit to cool down a bit.

    In some cases it is the T-Stat's responsibility to deal with this.

    Also, you should find the manufacturer's specs on how much temperature variance is recommended. For example. heat cycle should go to (for example) 2 degrees above heat point set and now cycle back on until temp reaches 2 degrees below heat point set. That way your system isn't cycling on every 5 minutes bringing the temp up one tick and shutting off after 30 seconds of operation.

    Not that this effects you, but we tend to not do this kind of thing (your relay system) since several companies make communicating thermostats. These devices can run just fine on their own and are designed with HVAC specs in mind. However, the control system can do anything you'd practically want to such as running the system remotely, programming and intelligent adjusting. It's a legal liability thing. We find that even with 'normal' T-Stats in the system, the HVAC people are very quick to try and blame us for any problems that come along.
  • viningvining X Member Posts: 4,354
    Some t-stat can also be switched so that it calculates the time to turn on to achieve a particular temperature by a certain time not the traditional method of turning the system on at a specific time until it reaches temp.

    In a large system this calculation to achieve a set temp by a certain time can be very tricky depending on the number of zone, how many zones are calling during this time & outdoor temp. I would think they just consider an average of how much time it has historical taken to reach desired set point in previous attemps.

    You could write a little program using persistent vars or write to flash that records the amount of time it takes to get from point A to point B and possible track how many other zones may be running at the same time that may have an affect on this time. You may possible need to record theses times for every combination of calling zones unless a multiplication factor can be determined.

    No zones calling a room takes 5 minutes to come to temp, additonal zone running takes 8 min, 2 additoinal zones take 12 minutes, etc.

    You should also include on delays & off delays for post purge/cool down as eric mentioned to reduce rapid cycling of the boiler/furnace.
  • AuserAuser Junior Member Posts: 506
    FWIW, check out http://en.wikipedia.org/wiki/PID_controller.

    This is probably the most appropriate approach to what you're trying to achieve, but it's a little [ahem] complex. Good luck!
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