Heating Control
Control of central heating systems through Home Automation systems is a practical, straightforward and ecologically friendly!
For existing heating systems which have a room thermostat control, the easiest interface point is the thermostat switch itself. The thermostat switch is a dry contact, that, depending on the boiler design, will either be low-voltage switching (12-48V) or mains voltage switching (240V). If you assume that it's high voltage, you won't go far wrong. Automated control of the boiler then simply involves wiring a suitably rated relay in parallel with the thermostat contacts, and setting the thermostat to a very low temperature, so that the mechanical contacts never close. Once this is done, operating the relay will start the boiler provided that it has been set to run continuously on the time clock (if present). This approach has the benefit of leaving the original boiler controls untouched.
This job is made easier thanks to the fact that most thermostat manufacturers print a helpful circuit diagram inside the lid of the thermostat, so identification of the correct contacts is straightforward.
Relay mounting can be tiresome, and needs to be done properly, especially where mains voltages are concerned. A small DIN rail box and relay make for a professional job - these can be sourced from the likes of CPC, Comfort, and Let's Automate.
Controlling the relay itself by computer depends on the choice of Home Automation controller. If using Home Vision or Comfort, it is simply a question of connecting the relay coil to a suitable output. Interface to a PC using other I/O ports is beyond the scope of this article - however, so remember that the relay coil will generally need a diode in parallel with the coil to counter the destructive EMF spikes generated as the coil switches.
The relay approach may also be modified for use with X-10 controllers. In this case, the simplest option is to use a "wall wart" to power the relay via an appliance module. Sending an "on" signal to the appliance module will power up the wall wart, and operate the relay. X-10 also manufacture a switchable wall-wart, but this is less convenient than the appliance module solution because the switchable wall-wart consumes a whole X-10 house code by virtue of the fact that it switches off in response to any off command sent to the same house code as the device address itself.
For boilers which use an old-style mechanical clock and are plugged in (or wired via a fused spur), it may be possible to control the boiler directly using an X-10 appliance module. Kieth Doxey's approach is documented here.
Control of immersion heaters and towel rails is easily done with X-10 appliance wall switches. These are generally capable of switching the loads involved (check the rating of your immersion heater carefully prior to ordering), and also provide for local control, allowing human override if required.
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