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Unvented Hot Water Cylinders:
A Fault-Finding Guide for Engineers

This fault-finding guide has been designed with engineers in mind to help determine any faults that may occur on the Unvented Hot Water Cylinder.

Important: Any work that is undertaken on an Unvented Cylinder MUST always be performed by a G3 qualified engineer.

For a fault finding guide geared towards home owners, please see the following link: A Fault-Finding Guide for Homeowners


Unvented Set Up

Answer:

When installing your Unvented Controls on the Unvented Hot Water Cylinder you need to always ensure that they are set up in ascending sequence. The following example is for the most common stainless unvented cylinder:

  • Pressure Reducing Valve: Set at 3.0 Bar
  • Expansion/Pressure Relief Valve: Set at 6.0 Bar
  • Pressure & Temperature Relief Valve: Set at 7.0 Bar

These should always be installed in this order on an Unvented Cylinder set-up with the pressure reducing valve being installed first, followed by the pressure relief valve and then the pressure & temperature relief valve (which will come factory fitted). If any of the valves above to do not follow sequence, then this will cause problems to the operation of the unvented hot water cylinder.

Tip: When installing a combination valve with a multiblock place it at the same level as the top of the cylinder. As this makes it easier when it comes to servicing in the future.

Answer:

We offer a wide variety of cylinder spares from over 70 manufacturers.

See our full range here: Click here!


Expansion Vessels

Answer:

When water is heated it expands. An expansion vessel is designed to take up this additional volume of water so it cannot escape from the unvented cylinder system. The additional volume of water created through this expansion is forced inside the diaphragm. When the water cools, it is forced back into the cylinder by the pressure contained within the expansion vessel on the other side of the diaphragm.

Answer:

Start by moving the expansion vessel from the system and test it remotely. This is because if you test or re-instate the expansion vessel when it is in situ, you may receive a false positive. This is the only way you can accurately test an expansion vessel unless it’s been installed with a proper service valve.

An Expansion Vessel will have a thread on one end and on the other end there will be a Schrader valve. The Schrader valve, which is protected by a cap, can be used for testing the current pressure or to re-instate the designated pressure required for your system. Normally the pressure is set to match that of the pressure reducing valve, which is determined by the set working pressure recommended by the cylinder manufacturer.

Answer:

All expansion vessels used on unvented hot water cylinders should be for Potable use (water which is safe enough to be consumed by humans or used with low risk of immediate or long-term harm). Always ensure that the expansion vessel being used on the cylinder is a minimum of 10% of the overall capacity of the cylinder. I.e. an unvented cylinder of 250 Litres should use a 25 Litre expansion vessel.

For a full guide on how to size a vessel for your cylinder please see our Expansion Vessel Sizing Guide. (Click here!)

Answer:

If the membrane has been pierced or if the charge in the expansion vessel has been depleted, it will not accept the expansion from the heated water as the expansion vessel will already be full of water.

Pressure then builds up in the cylinder until it passes somewhere between 6.0 to 9.0 bar depending on the rating of the expansion relief valve. When this occurs, it will cause the expansion valve to open and discharge the excess water.

Answer:

Please make sure ONLY a G3 qualified engineer is working on an Unvented Cylinder.

To test an expansion vessel for a fault it is recommended to remove the vessel from the system completely, this will ensure that your results are accurate. If you try to test an expansion vessel while it is still on the system, it could give inaccurate readings.

To remove the expansion vessel, the cylinder will need to be drained. If the expansion vessel is positioned up high, you will only need to drain the vessel up until the T&P valve. Once this has been done, you will need to use the Schrader valve to pump the expansion vessel back up to its required pressure and then leave for a suitable time (1-2 hours) to see if the pressure depletes.

Please be advised that if the vessel was to have a pin sized hole then this would obviously take longer to notice when testing.

Tip: Expansion vessels should be installed on hard pipe and not flexi-hose as this will help prevent against dead lead and legionella.

Answer:

See our full range here: Click here!


Pressure Reducing Valves

Answer:

A fault with an installed Pressure Reducing Valve could cause several issues within the system. These include.

  • Full or reduced mains pressure into the cylinder increasing the system above the pre-set bar rating of the pressure reducing valve.
  • It could reduce the water flow below the recommended pressure for the system.
  • It could also ensure a continuous flow of cold water going through the system.

Answer:

A cylinder comes complete with a multibloc, which incorporates a Pressure reducing Valve, a check valve and a relief valve. This multiblock that comes with the system is only designed to protect the cylinder. It is not designed to regulate the pressure for the whole property. We often identify issues where the multiblock has been installed to a property’s mains pressure. Please do not do this as this WILL cause the multiblock to fail.

Answer:

When replacing Pressure Reducing Valves, it is imperative that it is replaced with the exact same bar rating to avoid invalidating any warranties. The pressure reducing valve supplied with the unvented cylinder is to be fitted as close to the unvented cylinder as possible, preferably within 3 meters.

If you are trying to restrict the flow of water pressure to the property, then you should use a separate pressure reducing valve to be installed near to the stop cock.

Answer:

See our full range here: Click here!


Pressure Relief Valves

Answer:

As the expansion relief valve or pressure relief valve is designed to relieve water at a certain bar rating, when it is discharging you should consider that it could simply be performing its job. A high number of pressure relief valves returned to us are tested and found to be NOT faulty. This is because it is replaced due to it relieving water which is its primary function.

  1. Identify how much water is leaking through the tundish.

Test if its hot or cold water leaking through the tundish. If its hot water leaking though the tundish this means there is a problem with the temperature relief valve, and if its cold water the issue is on the opposite side. If its warm water, there could be an issue with the expansion vessel.

When testing for water you should be looking for a drip, weeping or if the caps open with water flowing through. If the water is flowing through there is an issue with the pressure reducing valve. If its weeping, there could be a problem with the expansion vessel. If its dripping, its likely there is something underneath the valve such as dirt or limescale. In this case, you may have to replace the relief valve.

Answer:

Other faults that could be causing the issues with your pressure relief valve are that the pressure reducing valve not holding its pressure. If the pressure reducing valve is faulty and starts to pass more pressure this will then cause the expansion relief valve to open.

After removing the expansion relief valve always check the inlet to the valve to ensure no debris or foreign objects can be seen.

Another reason could be a fault with the expansion vessel, please see below for faults with an expansion vessel.

Answer:

We offer a wide variety of Pressure Relief Valves from a range of manufacturers.

See our full range here: Click here!


Temperature & Pressure Relief Valves

Answer:

The pressure & temperature relief valve is designed to open for two instances; firstly, when the cylinder system reaches or surpasses a certain bar rating/pressure. Secondly, when the body of water in the cylinder reaches or surpasses a certain temperature, i.e 95°C.

One of the main reasons there is an issue with a temperature & pressure is that it has gone above 95°C. If you open the valve and it has stopped being white and flexible and instead is brown and rigid, you will need a new temperature and pressure relief valve.

When the valve opens up at 7.0 or 10.0 Bar and there is cold water coming out, there is likely to be something else in the system is drastically wrong, as this means the pressure reducing valve has failed and the pressure relief outlet is blocked. This could be down to poor installation or a blockage over time. A qualified heating engineer will need to preform a diagnostic to identify the cause.

Always ensure that the bar rating on the temperature & pressure relief valve is always the same when replacing a valve.

Answer:

See our full range of Pressure & Temperature Relief Valves here: Click here!


Tundishes

Answer:

A tundish is a visual device to show when water is passing from the Unvented Hot Water Cylinder. This highlights the fact there is a possible fault with a safety device on the unvented cylinder and there are several reasons that water could be discharging through the tundish.

Valves which could cause this are the pressure & temperature relief valve; the expansion relief valve, the pressure reducing valve or even the expansion vessel.

  • If hot water is being discharged via the tundish this would normally point to the pressure & temperature relief valve.
  • If it is cold water then you would need to check the pressure reducer & expansion relief valve (normally combined on an unvented hot water cylinder) or it could mean that the pressure in the expansion vessel is not holding.

Answer:

See our full range of Tundishes here: Click here!


Thermostat & High-Level Cut Outs

Answer:

Each Unvented Cylinder will be supplied with a thermostat/high-level cut out which protects the system from overheating and potentially causing serious injuries. Your thermostat will trip and depower the electrical immersion to stop the cylinder from heating/over-heating. A qualified electrician SHOULD be contacted in this instance.

If the stat has tripped press the RESET button to restart the system to its normal functionality. It may have tripped due to an electric circuit going elsewhere in the property.

Some stats are also susceptible to failing on high temperatures. This occurs when a tank is constantly calling for hot water from a cylinder and the tank will fill up with hot water and end up being discharged from the tundish. This is very costly and uneconomical and can cause issues further down the line.

Answer:

See our full range of Thermostats & High-Level Cut Outs here: Click here!


Immersion Heaters

Answer:

If you have a fault on your system and you have checked all of the fault testing for all of the other components on the cylinder, there is likely to be a fault on your Immersion Heater or the Thermostat controlling it. To test the Immersion Heater, use a voltmeter and an ohmmeter.

Please note: to replace the immersion heater the water cylinder would need to be drained down.

As an installer you should also take the immersion heater out and complete a continuity fault test on the immersion and the thermostat to make sure they are reading correctly. You will need a multi meter or a vault meter to complete this successfully.

Answer:

The easiest answer is you have not installed the correct Immersion Heater for your application. If you are in a hard-water area, it is recommended that you install a Titanium Immersion Heater to cope with the excess pressure.

If the homeowner lives in a hard water area and they are using a copper immersion, calcium carbonate will damage that immersion. In this case, we advise using an incaloy stat pocket and an incaloy immersion to prevent this.

If the homeowner lives in an area with an extremely high hard water area, a titanium immersion is recommended.

Answer:

Often when an immersion heater burns out the heating element bursts out of the protective sleeve that prevents it from encountering the water in the cylinder. As soon as this happens the RCD / breaker would trip. The solution here is to replace the immersion heater. Thread (Boss) sizes do vary with manufacturer; always ensure you use the same thread. Newer stainless unvented cylinders use a 1 3/4" thread whereas the older copper units used 2 1/4"

Answer:

See our full range of Immersion Heaters here: Click here!


Any further questions?

Our dedicated technical sales support team is on-hand to offer you advice and expertise.

Call us today on 01386 760066 or email at sales@advancedwater.co.uk