Central Heating Controls

Central heating can be a waste of money – if effective controls are not fitted to the system. There is a wide range of controls, ranging from thermostatic radiator valves to sophisticated multi-programme controls. System controls ensure that unnecessary heat is not expended -and will help to keep your heating bills down.

Controls on heating systems are desirable for reasons both of economy and comfort. They cannot normally be fitted to ‘uncontrolled’ solid fuel appliances, where restriction imposed by controls could be dangerous.

Controls which restrict the gravity circulation from non-thermostatic solid-fuel appliances, where this pipework acts as a ‘heat leak’, should not be used.

Control systems enable you to conserve heat, for a single degree of heat greater than that needed-undetectable from a comfort aspect-could add up to five per cent to fuel bills, building up to a sizeable amount. Manual controls on a boiler alone may vary by a swing of as much as 6°C or more in temperature.

Primary and secondary

There are two types of controls-primary and secondary, or ‘comfort’ controls. The primary controls are fitted to a fuel-monitoring appliance; these also provide a combustion safeguard. The primary controls concerned with the basic appliance function are fitted during manufacture.

Secondary controls are intended to meet the comfort requirements of the home, consistent with the lowest possible running costs.

Heating controls should be of the ‘failsafe’ variety-that is designed to shut off automatically in the event of a fault. Most boiler controls are of this type.

There is a wide range of secondary controls, from simple time clocks and more sophisticated programmers, to thermostatic valves, room, cylinder and frost-sensing thermostats.

Control methods

The simplest controls are the handwheel and lockshield valves on radiators. The handwheel provides full local control, so that a radiator or other heat-emitting appliance can be either fully or partly shut down. The lockshield should be set during the balancing of a circuit; further adjustment is not normally needed.

There are two basic ways of controlling the temperature of the radiators which, in turn, govern the room temperature. First, a centrally mounted room thermostat, which senses the temperature of the air, and switches the pump on and off; and, second, thermostatic radiator valves, used in conjunction with a constantly running pump.

Using an air thermostat, the entire system is governed by the actual air temperature in the thermostat. It is, therefore important to pay attention to:

• Correct sizing and balancing of radiators;

• Correct choice of room thermostat;

• Its correct siting;

• Its correct wiring.

A more economic and efficient service can usually be provided by operating the heating and hot water services at different temperatures. Hot water is normally fed to the radiators at a much higher temperature than that of the domestic hot water.

By controlling the water temperature solely from the boiler thermostat, boiler ‘short cycling’ will occur. The boiler will keep firing at frequent intervals, merely to replenish its own heat losses, not to provide any useful heat. This is obviously a highly wasteful and uneconomic state of affairs.

The boiler thermostat’s function is to maintain the temperature of the heat exchanger at a predetermined level and also to act as a top-temperature limit.

As long as the air temperature in the flue is higher than the incoming air temperature, air flow will occur and this will cool the heat exchanger to the point where the boiler thermostat will call for heat again.

Similarly, water circulation by gravity in the primary circuit will result in cooler water entering the boiler and this will also cause the boiler thermostat to call for heat.

With modern, low-water content boilers, this cycling occurs more frequently than with boilers employing large, cast-iron heat exchangers with a high thermal mass.

The solution is to control the boiler from where there is a genuine demand for heat-the cylinder and the room. The boiler thermostat should be set at its maximum temperature to act merely as a top limit.

The efficiency of a boiler falls rapidly as the boiler thermostat setting is reduced. Below 60’C the risk of condensation increases; this can have a serious effect on the life of the heat exchanger.

Thermostatic radiator valves Thermostatic radiator valves are individually fitted to radiators and enable selective control to be achieved over given parts of the heating layout. These automatically control the amount of water flowing through each radiator, by sensing the local temperature of the air. With the exception of a single make, these are not suitable for use on single-pipe systems.

While this may appear to be a desirable arrangement, there are two drawbacks: As no electrical signal is involved, the operation of these valves cannot be linked to the boiler firing or time control. Temperature sensing is arbitrary and does not reflect the true air temperature in the room.

An air thermostat is best, because of its ability to be wired as an integral part of the whole control.

10. November 2011 by admin
Categories: Featured, Handyman Tips | Tags: , , , , , , , , , | Comments Off on Central Heating Controls

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