Heating Controls

Heating system is only as good as the way in which it is controlled. As such an electronic programmer is now regarded as an essential com­ponent of a central heating system. It is the on/off switch for the whole heating system and overrides all the other controls (except safety features such as frost thermostats). Digital display has brought big improvements to programmers, offering switching for different parts or zones.

Look for ones with a boost (or ‘Extra Hour’) facility, which allows you to turn the system on at odd times.

Thermostats

These provide a secondary level of on/off switching, controlled by tem­perature rather than the clock. If the air or water temperature is higher than the thermostat setting, the thermostat will switch the heating system off, even though the programmer says ‘run’.

A typical heating system now has a number of thermostats controlling different elements:

• Thermostatic radiator valves (TRVs): temperature controlled valves that switch the flow of hot water into radiators on and off depending on the room temperature.

• Room stats do much the same job as TRVs but are mounted independently on the wall. They are frequently specified with underfloor heating: one per heating zone. Alternatively, they are often used as a whole-house thermostat, overriding all the radiators or underfloor heating zones.

• Cylinder stats control the temper­ature in hot water tanks. They are usually built in, but can be adjusted if you find the hot water isn’t the right temper­ature for you.

• Boiler stats control the tem­perature of water pass­­ing through the boiler, which is the hottest part of the sys­tem. Again, these are normally built into the boiler.

• Frost stats are a safety device used when part of the heating system – typically the boiler – is mounted outside the house and has to come on in order to avoid frost damage. Frost stats are designed to take precedence over a timer, but note that malfunctioning frost stats can be expensive to run and difficult to pinpoint.

Boiler Managers

Boiler managers, or weather compensators as they are alternatively known, aim to stabilise room temperatures whilst reducing the fuel consumption of boilers. You programme the manager to have your home heated to the temperatures you choose at the times you select and the manager decides the most efficient way of achieving this. A microproces­sor is fed data, monitoring outside air tem­perature and heating water temper­ature, and it is able to inde­pendently switch the boiler and the pump on or off to maximise efficiency. Their use is particularly rec­om­mend­ed with con­densing boilers where they are able to maximise the length of time that the boilers can work in condensing mode (which increases fuel efficiency).

Currently the most upmarket control option available, the use of boiler managers is slowly filtering down from large industrial buildings to housing. The price reflects the variety and complexity of controls from relatively simple systems like the Honeywell AQ 6000 to multi-zoned systems like Landis & Staefer’s RVP 75 or the DCD Heating Controller, which can cost more than £300.

The downside of sophisticated heating controls is that the operators – you and me – tend to find them very confusing to operate and to programme. This is particularly true of the more upmarket systems like the Landis & Staefer’s. If you lead an erratic lifestyle, in and out at all hours, the supposed efficiencies of a boiler manager will be wasted on you, but for homes with regular occupation patterns, a boiler manager should produce tangible savings.

Motorised Zone Valves

These are what the programmer operates; they are used in conjunction with pumps to switch the flow of hot water around different parts (or zones) of the system. The simplest valves switch all water either this way or that and therefore give rise to something called hot water priority, which means that the hot water stops being pumped around the radiators when the domestic hot water is being heated in the cylinder. There are more costly valves that are capable of opening in a middle position, which allows a flow of hot water both ways.

A standard set of heating controls (excluding TRVs) for a four bedroom house would cost around £200 (materials £100, labour £100). These controls are often sold in packages, which usually include a pump, motorised valve, a programmer, a room stat and a cylinder stat. Note that wiring must be provided between all these components.

Programmable Thermostats

The trend is for heating controls to become more complex, and the programmable thermostat is the first step along this route. Essentially, it combines the functions of the ordinary programmer (time switching) with that of the room stat (temperature switching). Most will allow you to set different temperatures at different times of day — a feature known as setback. This is particularly useful with underfloor heating, which can be run at a trickle during the night and brought slowly up to a comfortable temperature in the morning. Whereas ordinary time-only programmers can be placed anywhere that’s convenient, once you introduce a thermostat you have to think very carefully about its location and which temperatures it will be measuring.

Multi-Zone Heating Systems

An energy-saving idea which splits the space heating into separate heating zones – typically upstairs/downstairs – in addition to heating the hot water. It involves using at least a three-zone programmer. Again, the benefits very much depend on lifestyle, in particular whether you really use your house in a predictable way. If, for instance, you hardly ever went upstairs before 9.00pm then you could have upstairs heating off all day which could conceivably save 10-15 per cent of space-heating bills. You could achieve similar results with TRVs or room stats on an underfloor system but these would need daily manual adjustment. Zoning more complex than upstairs/downstairs is possible, but would probably involve longer and less-efficient pipe runs, which would tend to cancel out any advantage.

Further reading:

• The Death of Central Heating?
• How to Heat Your Home
• A Guide to Heating & Plumbing