Choosing and Fitting Central Heating Controls

The spiralling cost of energy makes it more and more important to have effective control of your heating system.

Control systems allow you to conserve heat and energy without noticing any loss of comfort: a single degree of temperature more than you need adds up to 5% on the fuel bill. Most controls are accurate to plus or minus a degree centigrade.

There are two types of heating controls that you will come across:

1. Appliance controls on boilers or storage heaters.

2. Room and hot water controls for central heating systems.

Appliance controls

Most controls fitted to appliances are designed to provide a fail-safe, as well as controlling the temperature of the water within the appliance.

Gas-fired boilers require a thermostat to control the water temperature. If this is faulty there should be a safety valve installed in the boiler or flow pipework to release the pressure if it rises too high.

Boiler control alone is not ideal as this causes frequent firing. It is recommended to keep the control thermostat at 60°C or above to prevent condensation and the possibility of corrosion in the boiler.

Modern boilers, such as low water content types, use copper or steel heat exchangers. This causes even greater cycling and needs precise design.

Room and hot water controls

The simplest method is to use lockshield valves on the return pipework. These balance the system, and once they are set, they do not have to be touched unless an extension is added.

The wheel valves provide local control, so that a radiator or other heat emitter can be fully or partly shut down.

There are two other methods of controlling the radiators which in turn control room temperature.

1. By using air-sensing thermostatic radiator valves (TRVs). These have become very popular. They have to be installed on the flow side of the radiators, or they can cause a noise like machine gun fire when the valve partly opens.

With this method the pump is kept running continually, and is switched on and off by a clock control. The disadvantage is that temperature sensing is arbitrary. On the plus side each room can be controlled to the temperature that the system was de signed for. Each or any room can be shut off without any detrimental effect to the overall system.

The hot water can be controlled on this system at a different temperature using a temperature limiting thermostat placed in the primary return, whether it is a gravity or pumped primary.

2. Room air thermostats control the whole house temperatures from one point.

The thermostat should be placed 1.5 metres (5ft) above floor level, away from radiators and other sources of heat and in a draught-free location.

Which is the best room is debatable. Many experts say that it should be placed in the hall, as this is usually the coldest area.

It is sometimes best to set the thermostat about 2°C higher than required so that anybody sitting in the living room will not feel cold.

The room thermostat can be used in many ways:

1. Switching a pump off and on.

2. Switching a boiler off and on.

3. Activating a zone valve.

4. Activating a diverter valve.

Heating system controls

In years gone by, systems had gravity primaries to the hot water cylinder, with a pumped heating circuit. Today it is more common and practical to have pumped pr imaries and heating. These are only some of the controls used. A fully pumped system can save boiler loads and provide a quick response to heating and hot water demands.

Diverter valve

This is the most common device. The diverter valve is a motorised valve under the control of two thermostats placed on the hot water cylinder and in the heating circuit. The valve has three ports — one inlet and two outlets. The inlet is the primary heating flow and the outlets are to the heating circuit and the hot water cylinder.

The function of the valve is to allow water pumped from the boiler to flow through the heating circuit or to the hot water cylinder, whichever has priority. This is usually the hot water cylinder — when the boiler is fired the water will heat up the hot water cylinder to the set temperature. Then the valve will open the port to the heating circuit and close the port to the domestic hot water cylinder. When both are satisfied the thermostats will shut off the pump.

Some diverter valves offer the facility of a mid-position where water can flow to both circuits.

The idea of diverter valves is to reduce boiler rating to a minimum, since the boiler output can be based only on the higher of the two circuits.

Frost thermostat

In conjunction with a room thermostat a frost thermostat can be used to prevent frost damage when the heating system is off for long periods. It is wired into the system to override the heating thermostat or clock when the ambient temperature around the frost thermostat approaches freezing point. It is usually placed on the outside of the house and set between 2°C and 5°C.

Zone valves using room thermostats

These work exactly as a room thermostat except that areas can be controlled inde- pendently. A typical application of this system would be to shut off upstairs bedroom radiators in daytime whilst heating downstairs living rooms.

Two-port valve system with full thermostatic control Two-port valves serve only as on/off controls and are very basic.

Programmers and clock control

These have different functions to perform and are not the same. They can, however, be incorporated in the same control box.

The usual purpose of the clock control is to turn the heating off when it is not specifically required in order to save fuel. The usual clock control has four tappets which can give two on and two off switchings to suit your heat needs.

For a working family it is usual to set one on for the early morning, say 5.30am to bring on the heating and give a warm house when getting ready for work. The off will be set for, say, half an hour before everybody leaves for work. In the evening the second on tapping can be set for one hour before arriving home, and the off setting for, say, 11pm before retiring.

A programmer is used in conjunction with a clock, but it allows you separate control of the heating and hot water. Modern programmers can be very complex with a range of perhaps six or 12 settings. The common settings are:

1. Heating and hot water continuously.

2. Heating and hot water, clock control.

3. Hot water clock control only.

4. Heating twice and hot water all day.

5. Heating and hot water off.

Many programmers are incorporated in the boiler panel and wired accordingly, being based on pumped heating and gravity fed hot water.

Fully pumped systems are normally controlled by a clock only.

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