How big should a central heating system be? Well obviously no bigger than it has to be. Over-sizing can result in waste of fuel and an unnecessarily costly installation. Under-sizing results in complaints of lack of temperature and too little hot water.
Complaints of lack of performance are very difficult to answer, particularly if you designed and installed the system yourself. For this reason there is a great temptation to play for safety and oversize everything. In fact the prudent designer will always allow a margin to take care of unforeseen problems and to provide for flexibility of use. The skilled designer will make sure that this margin is no greater than it has to be.
Heat loss calculations are undertaken to determine the size of the space heating load. These should always be worked out in detail using TJ’ values and areas of walls, floors etc. to determine the loss through the structure. Never rely on shorthand methods using factors and the cubic capacity of the rooms. Careful pre-planning is less expensive than remedial work to an unsatisfactory planning system.
An outside design temperature of – 1°C is normally used except in some areas where much lower temperatures are reached frequently. It has been the practice to use a variety of different internal temperatures for various types of room. However, because once a radiator has been sized to heat a bedroom at, say, 15.5°C, it will not be able to give a higher temperature should this be required under design conditions. A child’s bedroom will most likely be eventually used as a study room or even a bed-sit, and for sitting down a temperature of 21°C is required. If an elderly per-son uses the room the maintenance of a temperature of 21°C under all conditions could be essential. So, for these reasons, you should consider designing for 21°C overall in a domestic situation. It certainly makes the calculations easier as there are no differences between various rooms to take care of.
You must decide if the increase in flexibility achieved by this is worth the slight added installation cost.
Air change allowances are usually worked out on the basis of changing the air so many times per hour. Typical figures are 1.5 times for reception rooms and 2 times for bathrooms and circulation areas. A more accurate method is to use the crackage around opening windows and doors and the average winter wind speed.
When you have calculated the heat requirements it is normal to add a percentage, usually 10%, to them to enable the system to respond more rapidly to intermittent operation. This will give a large radiator which can heat the room up more quickly from cold. If the system is always to be operated continuously this margin can be omitted.
The next sizing operation concerns the circulating pipework and pump. It is a false economy to drastically undersize the pipework. Many of the complaints of underheating we investigate are caused by a lack of water flow rather than a lack of radiator surface. Additionally, a problem associated with some modern heating systems is noise, and one of the most annoying noises is water passing through undersized pipes at high velocity. Pump sizing is greatly facilitated by the fact that domestic circulators have variable outputs.
Before arriving at a total system load we must consider the domestic water heating requirement. In modern systems the primary pipework from the boiler to the coil in the storage cylinder is part of the pumped circuit, and recovery of the temperature of the domestic hot water after use is rapid. The current BS 1566 Part 1 type copper cylinder is fitted with a primary coil which, in conjunction with pumped circulation, can give a half hour reheat.
When a three port motorised valve is used which gives priority to the cylinder circuit, no additional allowance is made in boiler power for domestic water heating. My own preference, however, is to use two separate two port motorised valves, one to control space heating and one to control vater heating. With an allowance of about 2kW on boiler size this will give simultaneous performance on 30th services under most operating conditions.
If the D.H. W. cylinder has I capacity larger than 140 itres, consideration should De given to providing more spare boiler power for water heating, particularly if more han one bathroom is served. Rhe cylinder contents have to be raised from 10°C to 30°C in a given period of time, and the longer the time you will accept, the smaller boiler output you will require.
Final boiler sizing can now be undertaken. The boiler must be capable of coping with all of the load that it will have to meet at any one time. Let us examine this load in detail – particularly that due to space heating. The heat losses through the structure can all take place simultaneously. The heat losses due to air infiltration, however, cannot all occur at the same time. Rooms on the windward side of the building will gain air from the outside and heat it up. They will then pass this air on to the rooms on the down-wind side of the building from whence it will be lost to the outside again.
These latter rooms do not have to re-heat this air so the boiler does not have to meet the full amount of the calculated heat losses from them.
Providing the system controls can react separately in different rooms, I.e. thermostatic radiator valves, a proportion of the air infiltration heat loss can be deducted from the boiler size. The C.I.B.S. Guide Section A9 suggests that half of the infiltration losses can be deducted from boiler size. This is quite a high proportion but it indicates the reductions that can be made.
The boiler will therefore be sized to meet a load made up of its space heating requirement less an infiltration deduction and plus an intermittent operation allowance. Capacity will also be required to meet the losses from pipes in unheated areas and any domestic water heating load. Having taken all these factors into consideration do not oversize the boiler because it will then operate uneconomically. Remember that you are sizing the boiler to meet design conditions, I.e. – PC outside, and for the rest of the time it will have spare capacity in any case. There will also be internal heat gains from peo-ple and household appliances which a responsive control system can take advantage of.
Oversized plant operates at a lower efficiency than correctly sized plant so carry out your calculations carefully and do not add any extra margins.