Central Heating Pumps

The heart of a modern heating system, the pump, is still referred to at times as an accelerator or a circulator. More important to the user is the improvement in its reliability. In the early days of small bore the pump was the most temperamental item in view, if not actually breaking down then noisy. Nowadays all that a pump needs is to be protected from too much foreign matter in the circulating water and it will generally go on working indefinitely. We will not attempt to explain the internal arrangements of a pump since this is rarely relevant to the user or even the installer. Pumps may be ‘in line’ fixed, and consequently supported by the pipework and brackets; or they may stand on the floor. The former arrangement is by far the most common, particularly since pumps have become more compact. They do require adequate support, usually achieved by pipe bracketing but it is not necessary to use flexible connections as was being advised little more than 10 years ago! Makers’ recommendations should be followed. For example some makers require that their pumps shall run with the spindle horizontal, which means fitting the pump in a vertical pipe line.

Many modern pumps have variable output adjustment, so that the pump, up to the limit of its capability, may be adjusted to suit the requirements of the installation. Broadly speaking a pump must be able to achieve the design requirements, which may be measured as causing a drop of 10 degC or 20 degF across the circuit. It must not do so much that it tends to push water right out of the circuit through the vent pipe. This condition, known as’pumping over’, encourages the water to dissolve air before it returns to the system. In that state it is responsible for massive internal corrosion of the system.

A pump which does not have adjustment built in may be suitably controlled by regulating the gate valve on the outlet. All pumps should be fitted with gate valves on inlet and outlet, so that the pump may be serviced without draining down the system. If an outlet gate valve is used as a regulator it is very helpful to attach a label to it, stating the amount of closure applied, e.g. ¾ turn, so that if shut for any reason the gate valve may be reset without a trial-and-error period.

Other forms of pump now available are the two-speed pump and the twin pumps in parallel. The first may have some application as a slow speed runner when the system is, relatively speaking, idling, as in late spring. Otherwise, the claim that it is suited to both large and small systems is not likely to benefit the householder if he has to pay for such versatility but never needs it. The other, the twin pumps, may suit those of a nervous disposition who believe in being prepared. It is however a development which would have been much more welcome ten or more years ago.

An important, indeed fundamental decision in circuit design is where to put the pump in relation to the boiler; and since the purpose of this concern is to relate the pump correctly to the vent and the cold feed, the decision really involves all three. Let us first dispose of one theory which was devised more as a money saver than on technical grounds, namely that it is satisfactory to combine the vent and the cold feed. It is not satisfactory and is bad practice.

The pump is more often fitted in the return than in the flow. In a small bore unit which comes fully equipped including a piped-in pump, the pump is almost always connected to the return. The return-fitted pump suits the average installation, in which the feed/expansion cistern is in the loft, and the highest point of the heating circuit is no more than about 0.6 metres above the bedroom floor level. There is then a distance between the cistern and the highest point in the circuit greater than the head of the pump. This creates a positive head of water sufficient to counteract the ‘suction’ effect which the pump has on a considerable length of circuit, which would otherwise encourage air infiltration. If the vertical distance between the water level in the cistern and the highest point of the circuit is small, and less than the pump head, then the pump must be fitted in the flow pipe.

The cold feed and expansion or vent pipes must then be connected both on the same side of the pump. The air vent pipe is taken off the second boiler flow tapping and is an extension of the hot water flow primary. Like the cold water feed it is effectively on the pump suction. This is an appropriate point at which to mention another detail of design. We have already mentioned that the force causing gravity circulation is a very small one. It does not take a lot to neutralise or even to reverse it.

If the pump works for, say 10 minutes in every 20 or 30 minutes, the hot water circuit is unlikely to settle down to work. Where no such arrangement exists the solution is to make both returns enter on the same side of the boiler, which means using a common pipe. By the use of a swept or pitcher tee the flow of water from the pump is used to induce a flow in the other pipe, and if the pump is not running a natural circulation can carry on.

We must now come to consider the appliances which are placed in a wet system circuit as heat emitters, and we start with the most common one, the radiator.