The offer of improvements to customer's central heating systems should be based on the provisions not only of new components but also the means of conserving energy.
This means, for example, that a boiler exchange should rarely stand alone, but that the system to which it is attached should be vetted with a veiw to eliminating those features which may cause problems in operation or cause the consumption of fuel to be unnecessarily high.
The selling and organising of a central heating improvement may therefore be more difficult than for a complete new system.
The aim of these guidelines is to assist in assessing the characteristics of a system so that the customer will recieve the best advice as to what may be offered to give trouble-free and economical runnuing after the improvement work has been carried out.
By far the most important factor which causes unnecessary waste of fuel in wet central heating systems is 'short-cycling' of the boiler, that is, where the boiler is not turned off by the system's control when heat is not rquired by either the space-heating or water-heating zones.
Instead the boiler is left to shunt on and off, under the action of the boiler thermostat, continually heating up the boiler water together with any local circuit. Such operation is very inefficient and the elimination of this characteristic forms the basis of very many proprietary energy saving devices offered to the public.
The provision of a cylinder thermostat and room thermostat, electrically arranged to shut down the boiler and pump when both thermostats are satisfied is an almost essential update for the system without them.
SAVINGS of 25% or more are common when these controls are added.
It is better, of course, if zone valves are supplied to shut down the water circuits when the cylinder thermostat or room thermostat is satisfied and this especially applies if gravity circulation can take place, albeit at a slow rate, when the pump is off.
Another major contribution to energy saving for many of the older systems is to change the gravity domestic hot water circuit to pumped primary. Many people still fail to appriciate that an ordinary indirect cylinder which gives a heat exchange rate of the order of 3kw when opertating with natural circulation is capable of using up to 15kw when the circuit is pumped.
Conversion to pumped primary is often not too difficult if the primary flow pipe to the cylinder is used as the open vent pipe and an alternative flow pipe is teed off from the central heating flow pipe. Further savings of 10% or more can thus be acheived by the reduced boiler cycling and quicker heat up, provided that a cylinder thermostat and hot water zone valve are used.
When carrying out such a change, it is necessary to ensure that the pump is in an acceptable position. Trouble can be caused by pumps located in the return pipe to a boiler encouraging air seepage into the system through radiator valve glands. Pumps, open vent and feed pipe connections should be checked and corrected if wrong.
The pump should be installed in the flow from the boiler.
It is a question of safety that the open vent pipe should be at least 22mm diameter and smaller sizes are not acceptable. The fact that a system may have operated satisfactorily for some years with a 15mm diameter vent pipe is no excuse for not changing it, the emergency condition which uses the pipe could happen for the first time next month.
Close coupled feed and vent pipe except where boiler manufacturers instructions specifically state otherwise, may be fitted provided that there is a cold water feed path available when all automatic valves are in the closed position. When this configuration is used the cold feed and open vent connections should not be more than 150mm apart.
Except where instructions state otherwise, the open vent pipe should be fitted to the flow connection or flow pipe from the boiler. The vent pipe may be used as part of the circulating system.
It is not unusual for complaints to arise due to the incorrect running of return pipework.
Waste can be caused by the domestic hot water (DHW) return being connected between heating returns to give a possible circulation of hot water through radiators when they should be off. The DHW return should be closer to the boiler than all of the heating returns to prevent this.
Customers often require the bathroom radiator to provide heat all year round. This can be acheived by making the bathroom a pumped circuit on its own not subject to any zone valves, and fitting a thermostatic valve to the radiator. This puts the radiator into operation whenever either heating or hot water thermostat calls for heat and turns the pump on. Although, of course, the radiator should still not heat very often with a well controlled system it does not become warm at times and will be hot after the bath has been used.
The advantage of a TRV is that it will prevent the overheating of a room which recieved heat from sources other than the heating system itself eg: from the sun, from TV sets, from people, or even from a too generously sized radiator.
The disadvantage is that, being non-electrical, it cannot affect the operation of the boiler and pump unless further controls are used, such as a flow-sensitive Salamander flow monitor.
Another more popular method is to use a room thermostat and a zone valve in one room only (a master room) or for the whole system. The thermostat can then be wired into the system electrics as in George Hannah's Superwarmth system, to shut off the boiler and pump when not required. The Trv's then act as temperature limiters in their rooms, and balancing should preferably slightly favour those rooms to ensure that they are heated before the master room is satisfied.
The essential features of our systems is that they provide economy by using pumped primary DHW circuits with thermostats and zone valves to prevent 'short-cycling' and unwanted water circulation.
Some advantage may be gained in dividing the premises being heated into different zones each controlled by a room thermostat to distinguish between living quarters downstairs and sleeping quarters upstairs for example.
Where this applies, our electrical wiring should ensure that the boiler ( and pump ) is turned off when all zones are satisfied.
Individual room control may be provided to a limited extent by the fitting of TRV's to radiators. Where TRV's are used consideration must be given to the overall control to avoid wasteful boiler cycling.
Further economy is, of course, provided by the use of time clocks or programmers to limit the periods of operation of heating and hot water. Programmers which allow the hot water circuit to be quite indipendant of heating times are preffered when our-type of controls are used.
There is now a wide choice of 'electronic' programmers with digital display but most of these do not effect the control system, merely providing a variety of time period options.
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