The Icy Blast

Unusually cold weather this winter arriving earlier than expected has once again focussed the mind (well mine anyway) on the subject of efficient home heating.

The nights of 17th and 18th December were without doubt the coldest nights so far this winter with suburban Surrey temperatures dropping to -6C.

As I write this we are experiencing a second wave of Arctic winds bringing temperatures down well below zero, large flakes of snow settling on an already frozen ground. More chaos due on the roads and airports, just 3 weeks after the first blast of winter that caused widescale disruption to transport and infrastructure.

In Britain we have the awkward situation that we do not get severe winters each year, so little is done in advance to prepare us for wintery weather. Much of our older housing stock was built with very poor insulation, and the time has come to upgrade the older houses with measures such as draught-proofing, adequate loft insulation, double glazing, more efficient condensing gas boilers and ultimately whole house external insulation.

External Insulation

Now whilst it may be possible to reduce heating gas consumption by 25%, in an older property with no cavity with a suitable thickness of external insulation, the cost of this upgrade will run to appproaching £10,000. With a current gas consumption of just £500 per year, a saving of £125 per year and with an 80 year payback time make the expense of external insulation seem barely worthwhile. However, the price of gas has trebled in the last decade, and if it continues to follow this trend, or just double with each decade, it brings the payback to a more realistic 35 years. It is questionable whether one would benefit from that level of expenditure, and there may be cheaper and more cost effective means to achieve the same effect.

A Cheaper Option

Over the last few weeks of wintery weather, my day to day gas consumption has been around 110kWh, costing around £3.50 per day. Now suppose that across the heating season, you could achieve a 10% reduction in gas usage through a smarter control system, well that would save about £50 on the annual bill, or possibly up to £100 for some larger users. It could paypack within 2 years.

The average central heating controller is a very simple timeswitch, used in conjunction with an often poorly sited thermostat. It is a technology which has hardly changed for 30 years, and is certainly not best suited to today's lifestyle. With modern microcontrollers and better temperature sensing it should be possible to gain overall better control of the heating system and a higher degree of comfort for lower gas consumption. This was the motivation behind the Navitrino Heating Controller.

The S-Plan wiring schematic for central heating uses two motorised valves connected in series with the room stat and tank stat respectively. When the valves have reached their closed position the circulation pump and boiler are energised.

Many houses have this simple heating plan. The time controller often uses an industry standard backplate, making upgrading the controller relatively simple.

Modelling the System

Firstly it is important to gain a knowledge of the existing heating characteristics of the house. These will depend on type of construction, outside temperature, prevailing wind, and whether intercommunicating doors are left open or closed. For example, one night last week, the boiler stayed on for an unnecessary two and a half hours, solely because the door between the living room and the hall had been left ajar. As my boiler is generally running on average for one hour in every three, that extra 2.5 hours represented a significant extra gas usage. However, for a given outside temperature, a certain amount of heat will be required to maintain the principal rooms at the comfort temperature.

Older houses, without cavity walls need more heat than better insulated ones. You have 2 layers of brick to warm up before the house feels warm, and considerably more heatloss through the solid walls. Older houses were often fitted with open chimneys and sash windows and these too lead to extra draughts and heat losses. For any given house, there will be a certain amount of heat needed to bring it up to temperature, and then another rate of heating to maintain constant temperature determined by the difference between external and internal temperatures. This difference between internal and external temperature may vary widely in cold weather, and it is not unusual for an older house to use twice as much heating fuel on a very cold day compared to a milder day just to maintain a constant room temperature.

The effects of weather can be fairly predictable, for example a cold day in 2010, may follow a very similar temperature pattern to a cold day in 2009, and a knowledge of past temperature profiles could be used to optimise the response of the heating controller to any particular day's weather. The temperature profile of a day could be characterised by just the maximum and minimum temperatures, the average temperature, or a series of readings taken at regular intervals throughout the day and night. The daytime temperature is also reasonably predictable, in that it will generally rise after sunrise and fall after dusk, and the rate of change of temperature lies between certain credible limits in terms of degrees change per hour. Knowing the rate and direction of outside temperature changes would allow a controller to predict where it needs to be in order to maintain comfortable conditions indoors, without burning gas unnecessarily.

A controller could be given a model of a typical winter's day, expressed in likely temperatures, chosen from a short list of typical types. There might only be only a dozen different day models required, to match the temperature profiles of every day between September and April. If these day models are characterised by average temperature, and sorted in order of descending and rising again temperatures, then if you have just experienced a day which matches model type 8, for example, the next day is most likely to be another 8, or a colder 7, or a warmer 9. The controller should be able to anticipate from night time temperatures, taken in the early hours of the morning, say 4am, what the following day is likely to be, and then take the necessary action to ensure that the interior of the house is kept comfortable. If say by 8am, the outside temperature has sufficiently warmed, then the controller may decide that it should be following a warmer profile.

Interfacing to existing systems.

In order to achieve general acceptability, a new heating controller should be easily retro-fitted to an existing system, without the needs for an electrician, plumber or heating engineer - it should be self-installable "Plug & Play" and utilise existing wiring, pumps and valves.

Fortunately, most standard timer based central heating controllers use a common wiring backplate, which allows one controller to be swapped out with a new one or indeed one from a different manufacturer.

This backplate usually has 6 connections, including live and neutral supply connections, and separate outputs for selecting central heating on and hot water on. If heating is demanded because we are in a heating on period, a relay switches live to the heating switched live, and depending on whether the thermostat is closed (demand) this live then energises the heating motorised valve. When this valve has fully opened, a microswitch is activated which then energises the circulation pump and the boiler. Similarly, if hot water is demanded, subject to the position of the tank-stat, the hot water motorised valve is closed and the boiler and pump energised.

So the existing controller could be replaced quite simply with a microcontroller and a couple of mains SPDT relays. Often the central heating programmer is fitted in the most awkward of positions, such as the airing cupboard, where the pump and valves and cylinder are located. This is generally inconvenient and a better solution would be to have a combined programmer and thermostat, which is battery powered and portable and which can be placed within the room where the greatest degree of comfort is needed – such as the living room.

Using wireless technology, this control unit could readily communicate with the boiler control unit to schedule the heating and hot water as required, whilst additionally acting as a display device for showing heating trends, gas usage and offering functions such as hot water and heating boost. This central display would communicate with room temperature and outside temperature sensors, and possibly wireless controlled thermostatic radiator valves, to make a fully integrated zone heating control system.

Some of the work done by Jean-Claude Wippler of JeeLabs on his JeeNode might be of direct relevance such as the JeeLabs RoomNode which was designed with the aim of measuring temperature, humidity, light levels and occupancy via PIR using a simple wireless sensor. Such a sensor network using low cost wireless technology which could be extended at a later date to include other compatible sensors and actuators.