Monday, December 20, 2010

Smarter Heating - Ideas for Low Cost Zoning and Weather Compensation

In the last post I began do describe some of the desirable features of an upgraded heating controller. Here's a recap:

1. Smart Relay unit retrofits in place of existing time controller - no changes to mains wiring
2. Uses a hand held combined display and programmer/thermostat connected via wireless link

The hand held display/thermostat allow the thermostat to be located in whichever room you spend most of your time in. In most homes this would be the living room for the evenings, but if you work from home, you may choose to have the thermostat in your work room or home-office during the day. Portability means flexability. If you are wanting to reduce fuel bills by partial heating of a property, best that you focus the heat and the temperature control into the room that you are most likely to be occupying, and let the thermostatic radiator valves prevent excessive temperature in other rooms.

Regardless of where you choose to site your thermostat it will communicate via a wireless link to the relay unit which controls the central heating and hot water circuits. As a display unit it will offer you real time display of your energy usage (similar to an electricity monitor) and also an efficiency indicator and an indication of mean outside temperature.

Low Cost Zone Heating.

Back around 2005, I discovered that you could use a small amount of heat applied to the wax cartridge of a thermostatic radiator valve, which would cause the wax to expand and shut off the valve. I experimented first with power resistors and then power transistors and found that approximately 1W of dissipated power would completely close a thermostatic valve in 15 to 20 minutes. It would therefore be possible to have a controllable resistance fitted to each radiator's thermostatic valve and shut down individual radiators when they were not needed. It would only take about 10W of electrical power to shut off 8 radiators and this could be done as a low voltage (24V) signal distributed along cable, such as telephone extension leads. When the boiler is not running, the valve control resistors would not have to be energised, allowing further economy.

For this to work well, the controller would have to pre-anticipate when it was due to turn the boiler on, and open the valves some minutes in advance. It would make sense to use a long boiler on time and off-time, so that the latency of opening the valves is insignificant compared to the boiler on times. A longer boiler on-time could be achieved by turning down the boiler water temperature, so that it heats more slowly. This would also have the benefit that the return water would be close to 50C so that the boiler works in condensing mode most of the time. Additionally by increasing the hysteresis from say 0.2 to 0.4C would double the boiler on time. In an ideal world, the boiler would run continuously at what ever kW output was needed to maintain the set temp, however this may be difficult to achieve with a 24kW boiler in a property that really only needs a 12kW unit.

The boiler output temperature will be a function of circulation pump speed. By dropping the circulation pump to the lowest speed, the boiler will lower its output accordingly. This can however be counter-productive, as some tests proved. A combination of low pump speed and low water temperature means that the radiator barely gives out enough heat to satisfy the room temperature demand of the thermostat, so the boiler runs for very extended periods at low power. Ironically this can lead to the use of more gas, than if it were allowed to run for say 30 minutes and then coast, until the lower hysteresis level of the thermostat.

Building on the above idea, another control strategy might be to have a fixed on period of say 30 minutes, allowing the temperature to rise, if necessary, above the upper hysteresis point and then turn the boiler off until the room temperature fallss below the lower hysteresis point. A small amount of temperature overshoot will not be noticed, and this strategy will lead to a decent length of on time and a longer off time.

Weather Compensation

This is usually performed with readings from an external temperature sensor, which allows the output of the boiler to be controlled in response to outside temperatures. For example, in cold weather, it might be desirable to bring the boiler on for longer if certain temperature conditions are required by a certain time. Similarly, in a property with high thermal mass, the boiler could be turned off earlier, for example in the late evening, if it is known that the night is milder and heat loss will be reduced.

With the Christmas break plus daily cold weather, I hope to code up some of these ideas on my Arduino mega based heating controller, and try them out.

1 comment:

Anonymous said...

Hi Ken,

As you know, I'm travelling a similar road. Here's an update on my progress todate As I'm siting this away from the boiler I'm using wireless relays (currently BBSB). Did you ever convert your PIC-based driver for RF3 relays? This might be a tidier solution. I haven't finished the weather compensation yet but that's next. I have a very simple algorithm in mind. Some experimentation will follow.

Regards, Paul