How many relays for an HVAC / thermostat controller?

Discussion in 'The Projects Forum' started by blueroomelectronics, Nov 18, 2009.

  1. blueroomelectronics

    Thread Starter AAC Fanatic!

    Jul 22, 2007
    I've noticed simple*wall thermostats have a single relay with fan & AC/heat done with a pair of switches. So it seems that as few as three relays could automatically control a typical homes AC & furnace.*
    On the other hand I've seen high end HVAC thermostats with as many as six relays to run zoned systems and/or heat pumps...
    So the question is would a four relay be enough for most people or would a six relay device offer much more flexibility?
    Also how much current? All the thermostats I've had a look at use 2A relays is there any advantage to a 5A relay?
    Lastly how many SPST relays vs SPDT, I was thinking 4 SPST & 2 SPDT would be about right.
  2. Duane P Wetick

    AAC Fanatic!

    Apr 23, 2009
    Nowadays, it is a much simpler and a lot less $$$ to just buy a smart thermostat (~$30.00) made by Honeywell & others; 7 day programmable, heat & cool, 2-1.5VAA battery power, program on your lap when watching TV, plug it in to its socket and it works!

    Cheers, DPW
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Hi BRE,

    don't get confused with multi-zone systems unless you are thinking of a centralised control with remote sensors.

    A typical multi-zone system has one thermostat (of whatever complexity) in each zone.

    The various levels of complexity could be, for example:
    Heat only.
    Heat & Cool.
    Two stage heat or heat+cool
    Any of the above with Fan control.

    That gets you to possibly five outputs: Two levels Heat, Two levels Cool, Fan.
    (Plus I believe some have delayed heat-pump control?)

    Some of the top end ones then add humidity sensing and humidifier + dehumidifier outputs.

    The power rating of the output relays depends on what it's to control; American systems appear to use 24V AC for HVAC thermostats, so you are only switching other power relays.

    In the UK (and Europe?) the thermostats typically switch 230V directly to the boiler (furnace) and / or motorised valves.
  4. blueroomelectronics

    Thread Starter AAC Fanatic!

    Jul 22, 2007
    Well the question was part of something bigger I had in mind... More a controller with Ethernet.
    The controller would be similar the grey box controller pictured with an RS485 remote thermostat as pictured below (a TR16 - RS485 Serial Thermostat). The TR16 is a five relay device all SPST.
    The Aprilaire 8870 RS422 thermostat has six relays 5 are SPST and one is SPDT
    As an added bonus I'm thinking of adding a 433MHz RF receiver as they appear to be compatible with some Oregon Scientific RF thermometers & humidity sensors.

    On further reading I think it would be a good idea to include an onboard LM34 temperature sensor as a freezing indicator (the centigrade LM35 doesn't go below freezing without modification).