On the tools WATER PRESSURE
KEEP YOUR HEAD ON
In the second of this new series of domestic hot water technical tips, Apex Valves answers two common questions about water pressure. Q: What is meant by water head and how is it calculated? A: Head is another way of describing pressure. A water depth of 10m is the same as 1 atmosphere of pressure (1 bar) or 100kPa. If you dive under water, you will feel the pressure increase on your body. This pressure is generated by gravity acting on the depth of water above you. Exactly the same thing happens in a pipework system. The VERTICAL distance between two points equates to the difference in pressure.
The function of the Relief Valve (RV) is to protect the cylinder from over or under pressure. There is no requirement for a Temperature/Pressure Relief Valve (TPR) on low-pressure installations. Over pressure could occur if there was no Expansion Control Valve (ECV). Under pressure could occur if water is drawn off faster than it can come in. This would probably be if the cylinder was on a storey above the outlet and there was some restriction on the incoming supply. In this case, the air return flap inside the RV opens and allows air in to stop the hot water cylinder collapsing.
Q: In what applications do I use 3.7 and 7.6 Feed Valves—and what Relief Valves should I use?
**See note
150mm min
150mm min
250mm Heat trap TEMPERING VALVE (TV20)
BALL VALVE
COLD WATER INLET PRESSURE REDUCING VALVE (FV)
2. Fit a longer vent pipe (adjust up the Feed Valve and fit a 7.6m Expansion Valve if applicable). Note: To protect the cylinder, the end of the vent pipe cannot be more than 7.6m above the bottom of the cylinder.
Insulation
4.0m Maximum height above cylinder base
Insulation
TEMPERED WATER TO BATHROOM
FILTER STOP WITH NON RETURN (FS15)
1. Change to a full mains pressure installation
PRESSURE RELIEF VALVE
DRAIN
SIDE ENTRY
COLD WATER EXPANSION VALVE (EVT)
HOT WATER CYLINDER with Thermostat and Thermal Cut-out
PRESSURE RELIEF VALVE (RV) (VBN) RELIEF DRAIN *See note
NOTE:
DRAIN BOTTOM ENTRY
1.0m minimum Recommended between E.V.T & R.V
HOT WATER TO LAUNDRY & KITCHEN
That brings us neatly to another place a Relief Valve should be used. If the pressure of a low pressure, openvented system is to be increased to give a customer better flow from the taps, you have several choices:
*R.V and E.V drains may be combined provided discharge is via a minimum airbreak of 25mm. Drain must have a minimum size of 20mm diameter and be one size larger than the largest relief valve outlet. (Refer diagram) **1.0m minimum copper pipe length from cylinder to Tempering Valve. (Refer diagram)
ABOVE: Standard valve-vented low pressure system. NOTE: *RV and EV drains may be combined provided discharge is via a minimum airbreak of 25mm. Drain must have a minimum size of 20mm diameter and be one size larger than the largest relief valve outlet (refer to diagram). **1.0m minimum copper pipe length from cylinder to tempering valve (refer to diagram).
A: We all know that the Feed Valve (FV, or Pressure Reducing Valve) is used to control the incoming high (usually mains) pressure to a hot water cylinder. But did you know that the FV 3.7 and FV 7.6 have the same parts, so you can simply adjust the valve up or down to get either setting? Low pressure cylinders are often made of copper and will only withstand low pressure, normally 76kPa (7.6m head).
The RV is set at 6.5m head. The reason for this is that there is normally a 1m height difference between the location of the RV and base of the cylinder, so if you add these two figures together you get 7.5m total head. As you can see from the last point, providing you use the same units, you can add valve head and physical head together to get total head.
To improve hot water service when an exhaust pipe is already fitted. Note: Do not use in wetback installation and in frost areas. Inlet Valves required as for Valve Vented.
HOT WATER CYLINDER with Thermostat and Thermal Cut-out
ABOVE: Open-vented conversion to valve-vented.
3. On most houses, a 7.6m vent pipe is impractical or simply ugly, so a Relief Valve is fitted to the end of the vent pipe (adjust up the Feed Valve and fit a 7.6m Expansion Valve fitted if applicable). Note: To protect the cylinder, the total head must be less than 7.6m, so it is normal to use an RV 3.7. In the image above right, the total head is: RV 3.7 + 4m static head head for bend on top of the vent pipe (say 0.1m) = 7.6m
About Apex Valves: Apex® Valves are a New Zealand based company that specialise in the design and manufacture of tank, trough and plumbing control valves. These tech tips are drafted by David Crichton, Senior R&D Manager for Apex Valves, and based on customer questions and situations. If you have a technical query, email orders@apexvalves.co.nz or call 0800 500 484.
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