DCW and DHW

1. DCW and DHW Model Low Pressure System Plumb Set Group TM August 2003

2. Plumbset TM August 2003 Domestic Main Cold water feeds Main rises through Domestic Stop Cock (1) A drain off valve will be installed in line immediately after the stop cock Mains water is supplied to the kitchen sink (2) to provide a potable supply. Mains water rises to the Domestic Cold Water (DCW) storage cistern (3) (normally located in the roof space). Water level in the DCW cistern (3) is controlled by a float valve. As water is drawn from DCW cistern (3) it will be replenished from the rising main. There is an externally venting overflow mounted 25mm below the main feed into the tank (green)

3. Plumbset TM August 2003 Domestic Low Pressure Cold Water Water contained in the DCW Cistern (3) is fed at low pressure to various installations such as the bath (5), sink (6) and toilet cistern (7) The actual pressure will depend on the static head. Pipe work is normally in 22MM copper though it is likely to rise in 15MM section to the toilet cistern (7) and possibly to the wash basin (6) for neatness. An Isolation valve is installed in line on the rise to the toilet cistern, to enable ease of float valve maintenance.

4. Plumbset TM August 2003 Domestic Hot Water feed The Domestic Hot Water (DHW) Cylinder (4) is fed directly from the DCW Cistern (3). A shut off valve (probably a gate valve - not shown here) will normally be installed in the area of the DHW Cylinder. The Low pressure cold feed to the DHW Cylinder always enters the cylinder near the bottom because: Cold water is heavier than hot water and so would sink anyway and If cold water is introduced at the top of the cylinder it will cause a swirling action and mix with hot water already stored there. Water would then become tepid.

5. Plumbset TM August 2003 Domestic Hot Water Heating Circuit Hot water leaves the boiler (8) via the 22MM Primary Flow copper pipe which takes it to the top of the DHW heat exchanger (worm) immersed in the water held in the DHW Cylinder (4). As the Primary Flow passes down through the Worm, it conducts heat through the copper walls of the Worm, and heats the water contained in the DHW Cylinder (4). The water in the Worm never comes into contact with the water in the DHW Cylinder (unless a fault has occurred). Water leaving the Worm (Primary Return) has given off much of it�s heat to the Water in the DHW Cylinder. It returns to the boiler (8) to be reheated so that the process can continue.

6. Plumbset TM August 2003 Domestic Hot Water Supply Hot water flow depends on the opening of a tap or valve (such as a hot water valve on a washing machine). For a hot water flow to commence, the open tap or valve must be situated below the top of the water level in the DCW cistern (3) Hot water contained in the DHW cylinder (4) naturally rises to the top of the cylinder and the hot water exit is therefore at the top.

7. Plumbset TM August 2003 Domestic Hot Water Supply (cont) When a tap is opened, the hot water is pushed out of the DHW cylinder by the cold water from the DCW Cistern (3) which enters the DHW cylinder (4) at the base. Remember - the hot water is pushed out by the cold water entering the DHW cylinder (4) because there is a �head� of cold water. That is to say that the water level in the DCW cistern (3) is higher than the water level in the DHW cylinder.

8. Plumbset TM August 2003 Balanced H&C Water - safety features Boiler high temperature thermostat will limit temperature in primary flow. If this fails and the primary flow overheats water will expand and either cause a pressure relief valve to open or water will overflow back to a feed and expansion (F&E - not shown here) tank if fitted. DHW Cylinder thermostat limits DHW temperature (set to 65 degree C) If hot water in the cylinder expands too much it is vented back into the DCW cistern. If this results in the cistern becoming overfull the external overflow from the cistern will operate. Note the position for the connection of the DHW supply vent pipe work (into the DHW outlet but at least 450MM from the DHW cylinder).

9. Plumbset TM August 2003 Balanced DCW & DHW Schematic

10. Plumbset TM August 2003 Balanced Domestic H&C Water Advantages Hot and Cold pressure is the same at all installations except the kitchen sink A limited cold water supply is maintained if the main supply is temporarily cut The call on the the main is reduced at peek times Lower specification valves (such as gate valves) can be deployed on Low Pressure pipe work Low Pressure supply is quieter in operation. Small flaws such as loose tap washers and long unsupported pipe runs are unlikely to pose noise problems. Cold water supply to the kitchen sink can be maintained whilst the Low pressure DCW system is being serviced. Disadvantages Low pressure DCW supply may not be potable A Low pressure system is more prone to freezing Toilet cisterns take longer to fill than those supplied by the main Have to locate and close DCW cistern Low Pressure supply valve, to work on Low Pressure cold water applications. Where no valve exists, have to drain DCW cistern

11. DCW and DHW Model Low Pressure System Plumb Set Group TM JULY 2003