Heating Controls - There has to be a better way

Every year, around this time, I get interested in improving my central heating control system. With time off work, and heating being very important over the Christmas break, it's easy to start thinking that there must be a better way to improve the average home's heating controls.

UK homes, are by European standards, very poorly insulated and the average heating controls are very primitive. This combination makes for poor heating efficiency, and in times of rising fuel prices the cost of heating a UK house starts to get expensive.

A basic heating controller consists of little more than a timeswitch and a room thermostat. Very often the thermostat has been located in the hall, and often close to a radiator. This is the worst possible arrangement. Nobody spends much time in the hall, and as soon as the hall is at the set temperature, the heating will turn off - often leaving the other rooms of the house poorly heated.

The thermostat should be located in the room that you spend most time in, generally the living room, and it should be located at a suitable position and height to give an accurate reading of room temperature, free from draughts and the direct effect of any heat sources. If you live in an older house with solid brick walls, the thermostat should not be fixed to an outer wall - as these can be colder than the rest of the room, or an internal wall.

With the advent modern wireless devices, it is now common to have a wireless thermostat.  This makes it possible to sit the thermostat in any convenient position, and if you spend a lot of time sitting on the sofa in winter, then having the thermostat on a coffee table, or low bookshelf is a convenient and practical place to put it.

Thermostats are becoming more sophisticated, often with the timeswitch function built in. These combined units are called wireless heating controllers and are generally battery powered - however they perform the same basic function, of turning on the boiler at a time when heat is demanded.

I've had a wireless thermostat for about the last 8 years, since I put in a new boiler in 2005.  It's a simple device, by Drayton, and every 15 seconds sends a wireless packet to a receiver device, the packet corresponding to either a "boiler on" or "boiler off" request. As a safety feature, the receiver will automatically turn off the boiler if a valid packet is not received for a given time period. This is to prevent the boiler being left on if there is a block in the wireless signal, or if the batteries in the sender unit are exhausted.

The Drayton unit was easy to hack, and it was just a case of replicating the two boiler control packets at the right baud rate, using an ON-Off_Keyed (OOK) 433MHz transmitter, and the Drayton receiver was easily fooled into thinking that it was seeing packets from a legitimate Drayton thermostat. This quick hack has served me well, and I have had an Arduino controlling my heating since Winter 2009.

However, in the last couple of years, there have been some great advances in open source hardware, and with new devices available, it's time to revisit the home heating arena, and see what can be done to improve the system.

As heating fuel costs rise by some 10% per year, an external insulation is not really an option right now, the only way to reduce bills is to have the heating on for less time.  This can be done by a combination of the following means:

1. Turning down the thermostat,
2. Reducing the time of the heating periods,
3. Heating less rooms - or adjusting TRVs to lower individual room temperatures
4. Not heating the water unless you really need it, and only as hot as you need it
5. Limit the heating in milder weather
6. Make small lifestyle changes to help reduce heating demand

Over the Christmas break, or at weekends and other times of high home occupancy there will be noticeably different demands put on the heating, compared to normal workdays where the house is seldom occupied during the day.  However, I found that when I was working from home for long periods between 2003 and 2009, it was easier to maintain a better control over the heating and minimise gas consumption.

In older houses, it is often better to keep them heated at a low temperature than cycle the heating on and off twice a day. Once the fabric of an older house has cooled down, it takes considerable heat to get it back to a comfortable temperature.

So what features would make a better heating controller?

1.  User friendly graphical display with simple user interface
2.  Bath Boost - heats just the hot water tank and tells you how long it will take to reach temperature.
3.  Comfort Indicator - RGB LED lets you know if your room is in the comfort zone.
4.  Simple button interface  "Increase Comfort"   "Reduce Cost"   - the choice is yours
5. Link to remote web interface

Not all of these requirements are easily met with one device, and so a practical system is likely to be based around two or possibly three modules. This modular approach allows the basic system to be set up first, and then further sophistication added later, if desired.  I anticipate a system with three main devices:

1. A wireless operated relay unit to switch the boiler and any motorised valves for selecting heating or hot water or both.
2. One or more wireless thermostats located in the main rooms. One will have a graphical display and user interface.
3. A web interface for remote monitoring, control and update of the time and temperature program.

The simplest system consists of the wireless thermostat and the wireless relay unit. This is the equivalent of the most basic of commercial systems, but will be lacking in any real user interface.

The next level would be to replace the single wireless thermostat with a combined graphic display unit with user interface. The interface would be limited to a few buttons, so without an elaborate menu system might prove a little tricky to program.

The top level system would include a web connected microcontroller, providing a browser based user interface and full functionality for programming, remote control and monitoring. One possible, and popular candidate for this would be the Raspberry Pi - fitted with a suitable wireless interface so that it can communicate with the thermostat and the relay board.

The Hardware.

The open source hardware community is good at coming up with useful building blocks from which to construct a better heating controller. Central to this is a proliferation of low cost wireless enabled hardware, usually based on Arduino clones.

One particularly well matched unit is the GLCD module from openenergymonitor.org  This is an Arduino compatible backlit LCD Graphic display fitted with a temperature sensor, LDR light sensor, 3 user buttons, red-orange-green LEDs and a RFM12B wireless device - compatible with the JeeNodes library.  This will form the basis of the User Display.  It can be located in the living room, in a suitable position to act as the thermostat, temperature display and user interface.

Secondly, we need to replace the existing time-clock/controller with a pair of wireless controlled mains relays. These allow the boiler, circulation pump and motorised valves to be switched, depending whether heating and hot water are needed.

This requires a special relay board to be designed - an early draft layout of this is shown below:

This board is still in final design stage, but is expected in a month or so.

It provides three mains relays, and a simple button and LED interface - for local manual control of the relays.

Additionally it can support a variety of temperature sensors - including thermistors and DS18B20 one-wire sensors.

It has wireless module and real time clock, and will accept a variety of microcontrollers, including the ATmega1284 and the RFMPi69 module from Open Energy Monitor.