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Unvented Hot Water Cylinder Fault Finding Guide

Domestic Unvented Hot Water Cylinder – General Fault Finding Guide

This short fault finding guide is to help determine any faults that may be occurring on your Unvented Hot Water Cylinder. Please note that for certain points you MUST always use a G3 qualified engineer.

Consumer Check List

  1. Check the circuit breakers in the consumer unit; is the circuit breaker for the hot water cylinder in the ON position as required? Depending on the model of cylinder some may have 2 circuit breakers; one is the main immersion heater and the other would be the boost immersion heater. If the circuit breaker is in the OFF position please turn it to the ON position. If the circuit breaker does not stay in the ON position, do not hold it or try to force it to stay on the ON position. Leave it in the OFF position and contact a qualified engineer as there may be an electrical fault, this could be a faulty immersion heater.

  2. Is the supply RCD (Residual Current Device) protected? In addition to the circuit breaker which is supplying the cylinder may be RCD protected. If this is the case then try to turn it on. As before with the circuit breaker if the RCD does not stay in the ON position then do not try to force it or hold it in the ON position. Instead turn off all the circuit breakers protected by the RCD and turn the RCD back on. Now try to switch the RCD to the ON position. If it still will not stay in the ON position contact a qualified engineer. If it does stay in the ON position then next try to switch each circuit breaker on one at a time to identify which circuit breaker is tripping the RCD. This problem can be a result of either a fault on the cylinder or it could be a fault on another circuit. If the fault is with the cylinder then contact a qualified engineer. If the fault is with another circuit e.g. the Utility Room then you will need to unplug all appliances in that area, then turn the RCD and breaker back on and then plug each appliance back in one at a time. There could be a fault with a tumble dryer that is causing the RCD to trip.

  3. Check the controls for the water heating. Is the programmer/controller switched to the ON position or is it set up correctly for timed use. It may simply have been moved to the OFF position. If this is the case then set back to the ON/Timed position.

  4. Checking the 13amp fuse. Has the 13amp fuse in the fuse spur blown? If it has then this will need replacing.

  5. High demand water useage. Lack of hot water. Has there been any unusual demand for hot water than normal. I.e. have any guests stayed over which would have caused more hot water to be used than normal? If so there may not be a fault with the cylinder.

Faults with an Unvented Hot Water cylinder which require work to be carried out by a G3 qualified engineer

The faults which are listed below must be carried out by a qualified engineer. As the faults with the unvented hot water cylinder are a potential danger as it includes use of mains pressure equipment you must contact a qualified engineer.


  1. Water being discharged through the tundish. A tundish is a visual device to show when water is passing through to highlight the fact there is a possible fault with a safety device on the unvented cylinder. There are a number of 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.

  2. Fault with the pressure reducing valve (Stand alone or part of multibloc). A fault with a pressure reducing valve could allow full or reduced mains pressure into the cylinder increasing the system above the pre-set bar rating of the pressure reducing valve. All unvented cylinders will have safety control devices which will be set at different pre-set bar ratings. Depending on the manufacturer these will differ. You must ensure that any valve replaced is replaced with the exact same bar rating as this could invalidate 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. Please note a faulty pressure reducing valve could also reduce the water flow below the recommended pressure for the system.

  3. Fault with the expansion relief valve. As the expansion relief valve is designed to relieve water at a certain bar rating, when it is discharging you should take into account that it could simply be performing its job. A high number of expansion relief valves returned to the manufacturer as faulty are tested and found to be NOT faulty. This is because it is replaced due to it relieving water which is its primary function. Other faults which could cause this are the pressure reducing valve not holding its pressure. If the pressure reducing valve is faulty and starts to pass more pressure this will 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.

  4. Faults with a pressure & temperature relief valve. A pressure & temperature relief valve has 2 main functions. It will be set at 95 Deg C and it will also be set at a pre-set bar rating. One fault which can occur with a pressure & temperature relief valve which would cause the valve to open is that it is opening at normal operating temperatures below 65 Deg c. Always ensure that the bar rating is always the same when replacing a valve.

  5. Checking the unvented control set up. Always ensure that the unvented controls on the unvented hot water cylinder are all in sequence. The example which follows is for a stainless unvented cylinder:

    Pressure Reducing Valve: Set at 3 Bar
    Expansion Relief Valve: Set at 6 Bar
    Pressure & Temperature Relief Valve: Set at 7 Bar

    These valves above are in order as they would be found on an unvented hot water cylinder. The pressure reducing valve is first followed by the expansion relief valve and then the pressure & temperature relief valve. All of the bar ratings must always be in sequence as above, 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.

  6. Faults with an expansion vessel. An expansion vessel is normally spherical in shape and contains a membrane or bladder. It will have a thread normally 1/2" or 3/4" MBSP on one end and on the other end there will be a Schrader valve which can be used for testing 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. 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 sealed unvented cylinder. 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. If the membrane has been perforated 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 is passes somewhere between 6 to 9 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. The valve closes when the pressure in the cylinder reduces below its operating pressure. 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. 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. You will need to drain the cylinder down and remove the expansion vessel. Using the Schrader valve you will need 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.

  7. Fault with thermostat and high level cut out. This particular fault allows water to become heated to 95°C. The immersion heater thermostat should have removed the electrical supply to the heating element at 65°C. When this failed the high level cut out should have operated at 90°C when this fails the last line of defence is the TPR (Temperature and Pressure Relief) valve. This opens at 95°C and prevents the water from turning into steam. It should be noted that in this instance the water inside the cylinder is still being heated as the source of heat has not been removed. The water in the cylinder is cooled by cold water from the main supply replacing the hot water that is being discharged. This fresh cold water will start to heat up as soon as it enters the cylinder. This fault could result in a large volume of near boiling water being discharged under pressure over a long period. This reinforces the need for adequate discharge pipes e.g. made of copper to withstand extremely high temperatures and of a suitable width when hydraulic resistances are taken into account. It also highlights the importance of ensuring that the discharge pipe is correctly terminated and not likely to discharge hot water onto occupants or people within the vicinity.

  8. Faulty immersion heater. An immersion heater that does not heat the water inside the tank, after the above points one to five above have been checked, may be a fault relating to either the immersion heater or the thermostat controlling it. This is best checked with a voltmeter and an ohmmeter. To replace the immersion heater the water cylinder would need to be drained down.

  9. Consumer box circuit breaker keeps tripping. Often when an immersion heater burns out the heating element bursts out of the protective sleeve that prevents it from coming into contact with 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"