Heating Primer

A simple hot water / central heating (HWCH) control system consists of a cylinder thermostat and a zone/room thermostat (see figure below). Thermostats are switches operated by a thermal sensor. The thermostats will switch on or off at a single set point. If the temperature drops below the set point in the cylinder or the room, the boiler will fire.

A simple addition is a timer. The timer (usually wired in series with the thermostat) will be set to switch on or off the HWCH based on your requirements (for instance, to be 'on' between 6am and 9am and between 6pm and midnight on weekdays and 6am to midnight on weekends) thus saving heating while nobody is around. Temperature is maintained at the single set point. A minor improvement on this is the 'night setback' function, this function gives a second (lower) temperature set point during the evening/night.

A fully programmable thermostat further improves on the timer plus thermostat-with-night setback. It adjusts the set temperature during the day, so for instance you may want the warmest temperature first thing in the morning, and a bit lower in the afternoon and lower still at night.

Typically, you will set times and target temperatures so you feel comfortably warm (but not too hot) at the times of the day you are around. In practice, this may mean that you are starting the heating cycle too soon and thus wasting energy. An "optimum start" programmable thermostat will not only monitor the whether the target has been reached or not but will also take into account the actual temperature gap. If the gap is large it will fire the boiler sooner. If on the other hand the gap is small, it will wait as long as possible thus saving energy.

A small (but important) improvement is automatic time synchronisation. This not only ensures automatic adjustments for summer/winter time but also ensures that the clock in the heating system does not lose time or is affected by a power failure.

No matter how good your programming is you will still need at least two manual features:

• Override. For the times you want the heating or hot water to come on regardless of the actual program. A standard feature is to have a toggle button which gives a one-off 1, 2 or 3 hrs hot water / heating window.
• Away setting. Again, you may want to do this manually (in our systems you access this either from a touch panel or as by product of another function - for instance, the arming of the security subsystem - as the 'away' setting impacts security, lighting and other subsystems). This setting leaves the heating at the required level while you are away.

A final tweak is outside temperature compensation. It is an improvement (albeit somewhat complicated) where the heating output is increased as the outside temperature drops during the 'on' cycles. Or, if temperature outside increases, heating output is reduced saving energy. This may be achieved by pumping more/less hot water at constant temperature (simple) or increasing/decreasing the temperature of the water at constant flow (not so simple, unless you already have a mixing valve in your installation). The former requires a pumping overrun delay (or a variable speed pump), the later requires a mixing valve. If there is more than one boiler, the same can be achieved by boiler sequence control, whereas more boilers (or stages) are brought online as the heating requirement increases.

Two other common features, "boiler interlock" and "pump overrun", are discussed in the Heating Glossary.

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Zoning

An obvious drawback of the schemes above is that the entire residence or space is set to the same temperature. Zoning further improves on this. It allows you to divide your residence into distinct areas so to accommodate different heating needs cost-effectively. Most residences will have 3 zones: hot water, upstairs heating and downstairs heating (four zones, if you have a basement and a further zone, at least, if you have an indoor pool). Larger residences may have multiple hot water zones and multiple heating zones. At the very least you should consider having a heating zone per floor (and if there are external buildings, one per building).

Below we set out typical comfort requirements for different types of rooms/activities to assist in defining your heating zones. But please note that these are indicative and each person has its hot/cold preferences!:

When zoning a residence you may also take into account other environmental factors such as orientation and prevalent winds.

The diagram below shows a schematic of a zoned system using under floor heating.

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Other heating features

Cycle control. Maximum number of times the boiler will be fired in a one hour cycle (typically between 3 and 9). This prevents the boiler from firing too many times.

Frost control or minimum room temperature. A set point below which the boiler will be fired regardless of set points or timers (ie. override). Use to protect pipes from freezing or ensure house does not drop below a temperature that could affect its fabric or its contents.

Tower rails. These are typically connected to hot water systems (provided the hot water has a return flow). Alternatively, they can be connected to the heating system and incorporate a thermostatically controlled electric heater to warm the rail when the heating is off (and you are not away).

And if heating is integrated with a lifestyle control system, then you have other options. For instance, you can automatically close the curtains if there is too much sun coming in or in the evening to better hold the heating.

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Boiler Sizing

Once you have identified your zones and heating requirements you need to ensure that your boiler can deliver the comfort that you want. Qualified engineers perform the calculations but this boiler sizing link can help you to double check their numbers.

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