Extract from 'Central Heating - Fault Finding & Repair'

Introduction
When a central heating boiler fires-up the water in the boiler heat exchanger will be rapidly heated. This heated water is needed either at the domestic hot water cylinder to heat the water in the cylinder or is needed at the radiators to bring the room(s) up to a comfortable temperature. The most efficient way of quickly moving the boiler heated water to where it is needed is by using a machine. This machine will circulate heated water around the system allowing the water to give up its heat where it is needed. This machine is called a pump, circulator or less commonly an accelerator. To cover most situations the phrase ‘domestic circulating pump’ has been popularised.

Domestic circulating pumps are fitted to the pipe work or pre-fitted by the boiler manufacturer within the boiler unit. Depending on make and setting the electric motor rotates the shaft at speeds of between 700 and 2000 rpm with a circular veined wheel drawing water in through its centre and throwing it out by centrifugal force. As the water is thrown outward, it creates a pressure drop, causing more water to be drawn in. The faster the impeller spins the greater the centrifugal force which increases the water pressure in the system. To ensure they give satisfactory service they need to be of high quality. Leading manufacturers now produce circulating pumps that are robust, resistant to corrosion and come with a two year guarantee.

They operate within a system pressure range of between 0.1–10 bar (factory tested to at least 15 bar). Most central heating systems tend to have a working pressure of between 0.5 bar to 3 bar. The operating water temperature range for circulating pumps is large, typically between 10–110°C so they can easily deal with most conditions. The water contained within the circulating pump both lubricates the bearings and keeps the pump motor temperature within its designed operating range.

However, to avoid condensation in the motor windings the water temperature must always be higher than the ambient temperature or the condensation may short circuit the motor windings causing the circulating pump to fail.

Note: Circulating pumps should not be covered with insulation, linen, etc or failure may result due to
condensation or overheating.

Modern circulating pumps construction consist of a cast iron housing, stainless steel rotor can and rotor cladding with ceramic upper and lower bearings, a carbon based thrust bearing and rubber ‘O’ rings. The terminal box consists of plastics.

Pump Capacity
Circulating pumps must be able to circulate the total mass of water for all the circuits against a resistance equal to the pressure drop in the index circuit (index circuit is the circuit with the greatest functional resistance).

The mass flow is calculated:

            Q = M.Cp.T
so        M = Q/Cp.T

Where  M = Mass flow rate (Kg/s) Q = Boiler output (Kw)
           Cp = Specific heat capacity of water (4.18Kj/Kg °C)
            T = Temperature difference between flow and return (10°C)

Note:
a) Add a small margin on the flow rate of pump to facilitate balancing the system and future extensions
b) The design flow temperature should not exceed 82°C
c) The system design temperature drop should be 10°C (unless the boiler is a condensing boiler)

Alternatively, virtually all domestic situations can be accommodated by installing an automatic and fixed speed circulating pump
.
Electrical Supply
Circulating pumps require a 230V mains supply but have a low power consumption (similar to a standard light bulb) ranging between 35-100 Watts depending on speed. They must be fused at 3Amp and be connected using a 3 core heat resistant cable with a minimum c.s.a. of 75mm2 in each wire connected into either a 2 pole fused isolator with a minimum of 3mm contact separation in both poles or by using a 3 Amp fused plug and socket. They must always be earthed. Never remove the terminal box cover without first disconnecting the power supply.

Pump Over-Run
This is a timing device which enables the circulating pump to continue running for a short time after the boiler has stopped firing. This facility allows the pump to continue circulating water through the pipe work removing very hot water from the boiler heat exchanger. If the residual heat was not removed from the boiler the water may boil (causing a noise called ‘kettling’) or possibly eject water from the open safety vent. The boiler manufacturer installation instructions will say if over-run is required.

Note: Always follow the manufacturers recommendations.

Extract from 'Central Heating - Fault Finding & Repair'