Fitting Tanks and Pumps for Central Heating Systems

Having positioned all the radiators and the boiler, with its controls, the hot water cylinder and the F & E tank have to be included along with the pipework and pump to complete the system.

Hot water cylinder

The majority of cylinders are copper with a 127-litre capacity. They are usually of the indirect type, which means that the water fiorh the boiler flows through copper tubes known as a heat exchanger coil which prevents hot water from coming into contact with the water in the cylinder. This prevents furring-up of the system. A cylinder is usually put in the airing cupboard and if it is above the boiler can be gravity fed from it.

Gravity feed circulation

As the water in the pipes on the feed side of the boiler is heated it expands and becomes less dense. It therefore rises and is displaced by cooler water coming from the return side into the boiler, so beginning a circulation.

This system requires large pipes and is mainly found in older installations. To heat up a Cylinder by gravity feed would take up to four hours.

Modern systems use pumped circulation which reduces the time taken to heat up the cylinder to 20-60 minutes depending on the size of pipe, usually 15 or 22mm, and the pressure produced by the pump.

Feed and expansion tank

This is always positioned above the highest point of the heating circuit, and if this happens to run into the roof space then the F & E tank must be above it. This is to ensure that the whole of the system is filled with water, which prevents air locking, which in turn would stop water circulation.

It is also sized to take the expansion of water from the open vent and is usually 45 litres nominal capacity. It can be galvanised steel or polyethylene.

Pipework, sizing and systems

From the sketch plan the pipework can now be shown connecting up the various pieces of equipment, and it can be sized.

Sizing involves intricate and lengthy calculation, but for most domestic heating the sizes of pipework can be based on the following loading for a given radiator layout and output:

Up to 3kW 15mm pipe

3kW to 10kW 22mm pipe

10kW to 18kW 25mm pipe

These sizes are based on a temperature difference on the flow and return pipework of 11°C. Smaller sizes should be avoided as they can give high water velocity which causes noise.

The best system of pipework uses two pipes. If sized correctly this gives complete control of the heating load.

The flow pipe is usually at 81°C and the return at 70°C, which means that each radiator connected will have the same temperature of flow and return water. The pipes can be installed in almost any position but take care to prevent air locking.


Nearly every heating system has a pump to circulate water to each unit. It is important that you position it correctly in relation to the F & E tank and the heating open vent.

The pump produces a positive or negative pressure which can introduce air into the heating system or push water up the open vent to discharge over the F & E tank. These problems can be overcome as in the examples below.

Pumps should always be fitted with an isolating valve on each side so that the pump can be serviced without touching the rest of the heating system. Most pumps have an air cock on top to allow any air trapped inside to be discharged.

Pump sizing

Before any pump can be sized, the heating circuit with the highest radiator output or the longest run of piping has to be determined. This is known as the index circuit. From this the head of the system is calculated.

Head is the total pressure drop across the circuit. It is not necessary for this to be calculated in a modern domestic heating system since the domestic pumps usually have up to four selections of head ranging from zero to 45 kN.

The other factor you need to know is how much water the pump has to circulate around the system.

Air in the system

No matter how carefully you make your installation air is always present, and it is vital that it is dispersed as quickly as possible. It causes these problems which can seriously affect the system.

1. Air locking which Can restrict the circulation of water, and result in some radiators not working.

2. Corrosion. A heating system has a mixture of copper and steel components and any air in the system will cause oxidation of the steel — rust.

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