along with the temperature and pressure relief valve.
Should these parts fail you have a potential bomb on your hands:
As the water heats up, it starts to expand, if the expansion vessel has gone flat, the expansion has nowhere to go,
so it keeps building and the temperature pressure relief valve should open and discharge the pressure.
The main part of a unvented hot water cylinder service is to check the expansion vessel, on the
Megaflo it is to reset the internal gap. If you read the paper work that came with the tank it will tell you how to
commission the tanks correctly. On many of the tanks it has a label explaining the sequence.
The expansion vessel has to be set to the incoming water pressure, every property is different.
It’s very important to set the vessel correctly also the discharge pipe is of the correct size and fitted so as not to harm anyone should the tank discharge.
The way to service a unvented hot what cylinder is to isolate the incoming water to the tank,
open a hot tap to release all pressure,
once all the water has stopped running out of the tap, you need to turn to temperature and
pressure relief valve to check no pressure if left. Now check the expansion vessel setting.
You should find a valve under a cover that just either unscrew or pulls off.
The valve is just like a car tyre valve.
You can then connect a foot pump with a gauge to the vessel and check the pressure. Reset the pressure to whatever you need and you are nearly done.
With the Megaflo, you isolate the water like the others, open the hot tap and the as before open the temperature and pressure relief valve but this time you keep it open until the gurgling stops. This is called resetting the air gap. Now you can switch the water back on, making sure you leave the hot tap open. Once the full flow of water comes out of the tap, you can then switch off and you are done.
We have been to many jobs where the client changed the temperature pressure relief valve because it was dipping, then called us in because it was still dripping after replacing the valve.
The best time to have your unvented hot water cylinder serviced is when you have your boiler serviced. Make sure though that the engineer is gas safe registered for the boiler and is registered for unvented. If you check the gas safe card it will state on the back if the engineer is registered for unvented as well. It is against the law for anyone to work on unvented hot water unless registered.
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