Unit 316 session 6 Secondary circulation in domestic dwellings

​Secondary circulation in domestic dwellings

Open Ended

What do you think Secondary circulation is?

Open Ended

Why is secondary circulation needed?

​Secondary circulation is required where the length of any draw-off pipework is excessive.

​British Standard BS EN 806 and the Water Supply (Water Fittings) Regulations give the maximum length a hot water draw-off pipe may travel without a secondary circulation system being installed.

​Secondary circulation is a method of returning the hot water draw-off back to the storage cylinder in a continuous loop to eliminate
dead legs
by reducing the distance the hot water must travel before it arrives at the taps

Open Ended

Use the internet and research the meaning of a Dead Leg in a hot water system.

​​Dead leg:
when a hot tap is opened, a certain amount of cold water is usually drawn off and allowed to run to drain before hot water arrives

at the tap. This wasted, cold water is known as a dead leg. Under the Water Regulations, dead legs must be restricted, If this is not possible, then secondary circulation is required.

​In all installations, secondary circulation must use

forced circulation via a bronze or stainless steel-bodied circulating pump to circulate the water to and from the storage cylinder.

Open Ended

Why do you think we need to use a bronze or stainless steel bodied pump in a secondary hot water system and cannot use a standard heating pump?

​A secondary circulation pump is very similar in design to a central heating circulator.
The difference is that the secondary circulating pump has a body cast from bronze or stainless steel to eliminate discoloured water and subsequent contamination.

Bronze and is a non-ferrous metal that does not rust.


Central heating circulators have a cast iron or steel body that would rust if used on a secondary circulation system.

Under no circumstances should a central heating circulator be used.

​Secondary circulation installations on open vented hot water storage systems.

​With secondary circulation on open vented systems, the return pipe runs from the furthest hot tap back to the cylinder where it enters at about a quarter of the way down. A circulating pump is placed on the return, close to the hot water cylinder, Isolation valves must be installed either side of the pump so that the pump may be replaced or repaired.

​Secondary circulation installations on unvented hot water storage systems

​In most cases, a secondary circulation connection is not fitted on an unvented hot water storage vessel and, unlike open vented hot water storage vessels, it is not possible to install a connection of the vessel itself.

Where secondary circulation is required, this must be taken to the cold water feed connection using a swept tee just before the cold feed enters the unit.
To safeguard against reverse circulation, a non-return valve or single check valve must be fitted after the circulating pump and just before the swept tee branch.

​Time clocks for secondary circulation

​If secondary circulation is used on hot water systems, it should be controlled by a time clock so that the circulating pump is not running 24 hours a day.
The time clock should be set to operate only during periods of demand and should be wired in conjunction with pipe thermostats (also known as aquastats) to switch off the pump when the system is up to the correct temperature and circulation is not required and to activate the pump when the water temperature drops.

​Insulating secondary circulation pipework

If secondary circulation systems are installed, they should be insulated for the entire length of the system.
This is to prevent excessive heat loss through the extended pipework due to the water being circulated by a circulating pump.

​Electric trace heating uses an electric cable that forms a heating element.
It is positioned directly in contact with the pipe along the whole length of the pipe length.
The pipe is then covered in thermal insulation.
The heat generated by the element keeps the pipe at a specific temperature.

The operation of the trace heating element should be timed to a period when the hot water system is in most use (such as early in the morning and evening).
If the pipe is well insulated and installed with a timer, the amount of energy usage will be minimal.

​The use of trace heating instead of secondary circulation

Multiple Choice

What is the purpose of secondary circulation in a hot water system?

• To increase water pressure

• To reduce the distance hot water must travel

• To eliminate dead legs

• To restrict cold water flow

Multiple Choice

What is a dead leg in a hot water system?

• Excessive heat loss in the pipes

• Wasted, cold water before hot water arrives

• A malfunctioning pump

• A type of secondary circulation pump

Labelling

Label the diagram

Drag labels to their correct position on the image

Secondary return

Bronze pump

Cold feed pipe

Open vent pipe

Multiple Choice

What material is commonly used for the body of a secondary circulating pump?

• Cast iron

• Steel

• Bronze or stainless steel

• Copper

Multiple Choice

Why should a central heating circulator not be used in a secondary circulation system?

• It's too expensive

• It causes discoloured water due to corrosion

• It rusts

• It's not efficient

Multiple Choice

Where is the return pipe placed in a secondary circulation system for open vented systems?

• At the top of the cylinder

• At the bottom of the cylinder

• About a quarter of the way down the cylinder

• At the furthest hot tap

Multiple Choice

Why are isolation valves installed on either side of the pump in a secondary circulation system?

• To control water temperature

• To prevent reverse circulation

• To allow maintenance

• To enhance heat insulation

Multiple Choice

What should be fitted after the circulating pump to safeguard against reverse circulation?

• Isolation valves

• Non-return valve or single check valve

• Central heating circulator

• Timer

Multiple Choice

How should the secondary circulation system be controlled to optimize energy usage?

• Continuous operation

• Manual control

• Controlled by a time clock

• Temperature-based control

Labelling

Label the diagram

Drag labels to their correct position on the image

Single check valve

Secondary circulation

Bronze pump

Return connection

Multiple Choice

Why should secondary circulation pipework be insulated?

• To prevent water contamination

• To reduce water pressure

• To minimize heat loss

• To improve circulation efficiency

Multiple Choice

What is an alternative to secondary circulation?

• Forced circulation

• Insulation

• Discoloured water

• Trace heating