Comment on FAQ 44/45

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Doctor Drivel

An Unvented cylinder is a pressure vessel and as such carries the danger that all pressure vessels carry so they are fitted with safety devices. The system is designed to FAIL SAFE.

This is wrong. Understand what failsafe means.

To prevent expansion vessel failure there are other safety valves fitted.

If the water overheated and turned to steam, the vessel could rupture, and the rapid expansion would cause an explosion. To avoid this happening a pressure reducing valve (pic below) is fitted to the inlet water to limit the pressure to about 3 bar. If the vessel fails the valve opens and the pressure is released to outside.

A pressure reducing valve will stop a rubber bladder rupturing? New to me.

Does he mean an expansion relief valve? I don't think he knows about these things at all, as he says unvented cylinders are failsafe when they are not. Failsafe is when all controls fail, it is safe. When all unvented cylinder controls fails it takes off like a rocket and takes down walls - it explodes.

No mention of an expansion relief valve and a pressure reducing valve does something magical.

Below: clearly shows the expansion relief valve on the cold feed.


"2. The Expansion Relief Valve is set to relieve the pressure in the system if it exceeds the 6 bar (approx 90 p.s.i) that it is set at. Normally this will only occur if the expansion vessel develops a fault or looses some of its charge, or if all 3 of the temperature-operated safety devices fail at the same time.

3. The Expansion Relief Valve can be directly connected to the Combined P.R.V/L.S./C.V or it can be installed independently, but it must always be sited between the Combined Valve and the Cylinder. A drain is connected to the Expansion Relief Valve to take away any water that does exit the valve should it operate. The drain discharges to an outside position via the Tundish.

4. The Expansion Vessel is designed to take up any expansion water that is created by the heating of the water. Water expands when heated. The expansion of the water when heated in this ‘sealed’ system causes the air in the expansion vessel to compress creating a space for the increased volume of water. The volume of water in the system will vary from 100% of its volume when cold to approximately 104% when heated to 65 degrees Celsius. The main purpose of the expansion vessel is to ensure that the Expansion Relief Valve doesn’t operate every time the water goes through a heating cycle, as this would contravene the Water Regs by ‘wasting water knowingly’. The Expansion Vessel can be screwed into the Expansion Relief Valve, although it is more normal to ‘T’-in a piece of 15mm pipe to the cold water side of the system, between the Expansion Relief Valve and the Cylinder, and run it up to a position above the cylinder and locate the Expansion Vessel at high level.

5. The Temperature and Pressure Relief Valve is designed to be both a back-up in case of failure of the Expansion Relief Valve and/or a back-up in case of a failure of the High-Limit Thermostat. The T&P Valve is located on the top of the Cylinder (and in Atlantic’s case, combined with the hot outlet connection pipe) as this is where the temperature of the water will be at its hottest. A drain is connected to the T&P Valve to take away any water should this valve operate. The drain discharges to an outside position via the Tundish "

This FAQ entry needs changing ASAP, as it is misleading to people who want good factual advice on safety aspects of appliances.
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