Programmable Network Room Thermostats

If you'd like to have individual control of the temperature in each room, and be able to extend that control to a mobile phone or the internet, there are plenty of companies offering a solution.  Most of these however, are just re-branded versions of products from heatmiser.

There are two things I really like about the heatmiser networked programmable room thermostats:  they are fully featured stand alone programmable thermostats and the protocol to talk to them remotely has been published by the manufacturer.



Full Standalone Functionality

There is a range of devices available with slightly different features, which include:

• 7 day programming with 4 changes per day
• Self learning optimum start
• Holiday facility
• Automatic back-light
• Touch screen versions available

Open Control Protocol

The protocol documentation is available on heatmiser's website.  In summary it's half duplex RS485 at 4800 bps.  The protocol packets have CRCs and are acknowledged.  All transactions are initiated by the master (not the thermostats), however it is conceivable that multiple masters could co-exist.  The protocol allows you to query any information and change any configuration that you could do at the thermostat.

Cable Requirements

Most of the thermostats require 3 pairs of connections:

1. Power: 12V DC
2. Heat Demand Signal: Volt-free contacts rated at 16V
3. Data: RS485 at 4800bps

The recommended cable for this is Belden 9538, which has 8 stranded cores (not twisted pairs) and an overall screen.  Alternatively, CAT5 FTP can be used.  In both cases, the screen is only connected at one end, so is being used purely as a screen.  The third wire (signal ground) is in the form of the negative power supply connection.

I'm already using lots of Pink C-Bus cable, which is CAT5e UTP with mains rated outer insulation.  I'd like to use this for the thermostats too.  The power and volt free contacts will be fine with almost any cable.  The only question is over the RS485.  At such a low speed, and a total length of <50 m I think unshielded twisted pair should be perfectly adequate.  I've seen it working at higher speeds (56Kbps), over longer distances (200 m) and in harsher environments (railways) with whatever cable was already installed.