Pressure relief valve on vented HW cylinder

[Byersb18c]

Hi

I'm in the process of getting a multifuel boiler stove installed. The stove will be heating a 134 litre gledhill Indirect vented HW cylinder with a separate pumped CH circuit as its a 4 port boiler on the stove.
My question is in regards to pressure relief valves with a tundish and discharge pipe work. I have had conflicting advice from a couple of HETAS engineers, one said that a normally open 2 port valve on the gravity flow pipe before the coil, triggered by a cylinder stat set at 80°c to disconnect the gravity hot water to the cylinder (and thereby stop it overheating) is adequate to meet building regs. However the other Hetas engineer Informed me that I must also have a pressure relief valve, tundish, and discharge pipe to the outside.
I've had a read through of building regs doc g3 and can't work out for sure whether the PRV is required or not, any help would be much appreciated as the fitting of PRV and associated pipework will add quite a bit more to the installation costs. (forgot to add when the cylinder stat triggers 2 port valve to motor shut, it will also trigger CH pump to start circulating water from the stove round the 4 rads upstairs)

 

[SimonG]

The 2 port doesn't motor shut. It's a normally closed so in the event of a power cut the cylinder is rendered safe.

 

[Byersb18c]

Thanks for the reply mate, if it's normally closed how will the gravity circuit circulate through the coil and heat the DHW? Would it be a case of wiring it to a Cylinder stat that is constantly energising the valve to hold it open, and breaks on rise at 80°c? So the valve will close once the DHW is at 80°c but will also failsafe to closed in a power cut? Sorry for my limited understanding, I'm not a plumber, however I do like to try and acquire a basic knowledge of this kind of thing so that when I'm discussing it with tradesman I don't stand with a vacant expression on my face everytime they tell me something! Haha!

 

[chris watkins]

I think this section of Part G 3 makes it clear.

In the Secretary of State’s view Requirement G3(3) will be met for a hot water storage system that has a vented storage vessel if:

1. the storage vessel has a suitable vent pipe connecting the top of the vessel to a point open to the atmosphere above the level of the water in the cold water storage cistern and over it; and,
   
   
2. in addition to any thermostat, either the heat source, or the storage vessel is fitted with a device that will prevent the temperature of the stored water at any time exceeding 100 ̊C; and
   
   
3. the hot water system has pipework that incorporates a provision for the discharge of hot water from the safety devices to an appropriate place open to the atmosphere where it will cause no danger to persons in or about the building.
   As your stove is unlikely to have a device installed to stop the water boiling the cylinder would require one ie. A T&P would be required.