There are a lot of heating and cooling appliances on the market, and most of them come with horrible thermostats. You know the type: it require separate batteries, has a calculator-style LCD display that's difficult to read in low light, and lacks intuitive features.
Some manufacturers (particularly for split system air conditioning units) lock you into using the manufacturer's thermostat. Many don't.
Get ready for the future
In the future, all your home's heating and cooling will be co-ordinated by a small server appliance, situated out of the way in a closet or inside your home's data cabinet next to your broadband router. It won't consume much power, maybe around $2 worth of electricity per year. But it will run your heating and cooling appliances in a way that saves you money.
It will also make your heating/cooling system more practical. Instead of having to squint at a dedicated LCD display to access a limited set of functions, you'll be able to access a full set of features through your smart phone and PC: phones and computers are designed to allow for far better human-computer interaction than a dedicated thermostat could ever have.
Here's a phone screenshot showing the web interface for a prototype device we're working on, called THERMOSERVER:
Here you have an adaptive interface, where you can customise the panels on display, scroll down to access other features, program behavioural policies, and if you share your house with other people, you can specify restrictions to prevent them from running up your energy bill to high.
If you want a wall-mounted controller too, that's easy to do. Just purchase an inexpensive low-spec tablet computer next time your supermarket has them on special, plug it in to a power socket and stick it on your wall. If your house is cabled with ethernet sockets or your router is nearby, just add an ethernet dongle and you don't even need WiFi.
How a thermostat works
A typical central heating system with a wired thermostat on your wall works on a simple principle. There are two wires between the heating unit and your thermostat, that carry a low voltage (e.g. 25VAC). When your thermostat detects that heat is required (based on the temperature you've set and the ambient temperature in the room), it joins the wires to complete the circuit. When the room gets warm enough, it breaks the circuit again.
Cooling systems work the same way in reverse.
THERMOSERVER works using electromechanical relays to tell heating or cooling appliances whether they are required. This approach was chosen because it's common among third-party thermostats.
Some appliances use modulating thermostats, which don't just say 'yes' or 'no', they also communicate a power level that indicates how close the ambient temperature is to the target temperature (e.g. low, medium, or high). Such devices will typically allow you to flick an internal switch to 'non-modulating mode' allowing you to control them them with third party thermostats. For example, a Baxi hydronic heating system boiler comes with a wired modulating thermostat that can be moved from its original location on a wall into a special receptacle inside boiler unit itself if control via a third-party thermostat is required. In this case, the instructions say to switch the unit into non-modulating mode.
The efficiency benefits of running a system in modulating mode are debatable, and many high-efficiency heating units don't provide the capability at all. As the temperature inside your house nears your desired target temperature, a modulating heating system will switch to a reduced setting and keep running for slightly longer in order to reach the target cut-off temperature. A non-modulating system will simply reach the target temperature sooner. The laws of thermodynamics state that the same amount of energy is required in either case.
Insist on Interoperability
Some devices will only work with a supplied wireless remote control that is cumbersome to use and easy to misplace or step on.
Before you buy, always ask the manufacturer whether the appliance can be wired up to a third-party (and non-modulating) thermostat. For a dedicated heater or cooler this will be a simple 2-wire connection. For a combination heating/cooling unit (such as a reverse-cycle air conditioner), this ideally means one pair of wires for heating and another pair for cooling. Ask the salesperson too. If not, tell them it's a deal-breaker.
That way, you'll avoid locking yourself in to a single manufacturer's proprietary control system, and you'll have heaps of flexibility in the years to come.
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