help with expansion vessels

[samohtom]

Hi. I've been having an issue for a while now with my unvented system discharging through the discharge/overflow pipe to outside. To give some info, it's a heat pump system with a 300 litre cylinder consisting of two 150 litre cylinders, one for hot water at the top, one for the radiator circuit at the bottom (not really being used for another month or two). I think my system pressure is too high, as for example I run a bath, heat pump kicks in to heat the hot water tank back up and the system pressure rises (as read on the pressure gauge marked P on my crude diagram) to around 40 psi. This is just enough for a small amount of discharge to occur via the Pressure and temperature relief valve(s). These are rated at 90 degrees C and 4 bar though, from looking on my gauge 4 bar is about 58 psi?! I don't think the temperature could be the cause, since with it being a heat pump system the hot water is heated to 45 degrees C and once a week the immersion kicks in to heat to 60-70 to kill bugs. I checked the pressure on the red expansion vessel with a good tire gauge and it was just over 40 psi. Is this correct given it's positioning? I tried draining the hot water tank a bit and checked the expansion vessel was zero psi, then closed everything and repressurised but I've just ended up where I started - 43 psi (I did it when the tank was cold). The pressure of the gauge shown reads around zero to 6 psi most of the time, rising to 40 psi when it's heating as mentioned. I don't know how to stop the discharge to outside, do I need a bigger expansion vessel perhaps? The two I have are 8 litres each.

Please help this ignorant fool!

Tom

 

[SimonJohns]

The vessels should both have a pressure of 0.4bar above their normal working pressure when the system is drained.

 

[samohtom]

The vessels should both have a pressure of 0.4bar above their normal working pressure when the system is drained.

So are you saying if when the system is cold the pressure is 2 bars at the point just below the red expansion tank (next to the valve), i.e. my incoming mains water pressure is 2 bars, in order to set the correct expansion vessel pressure I should drain down enough to get zero pressure at the expansion vessel and then precharge it to 2.4 bars before refilling? Just want to make sure I'm understanding correctly :confused5:

 

[Reg Man]

I recommend you get a G3 qualified engineer in to give it a look over. These systems can be extremely dangerous in unqualified hands. Hence the building Regulations requirements for competent people to install and maintain them.

 

[SimonJohns]

So are you saying if when the system is cold the pressure is 2 bars at the point just below the red expansion tank (next to the valve), i.e. my incoming mains water pressure is 2 bars, in order to set the correct expansion vessel pressure I should drain down enough to get zero pressure at the expansion vessel and then precharge it to 2.4 bars before refilling? Just want to make sure I'm understanding correctly :confused5:

Yes. And I don't think g3 apply IMO. It's on the heating

 

[Reg Man]

2 X 150 litre stored unvented vessels, one for the hot water one for the heating. I think G3 does therefore apply.

 

[SimonJohns]

2 X 150 litre stored unvented vessels, one for the hot water one for the heating. I think G3 does therefore apply.

It's a red expansion vessel. Non potable

 

[Reg Man]

Still stored unvented. The early domestic stored unvented systems had red expansion vessels. The manufacturers changed the domestic unvented (Potable) to Blue, only because the Red ones frightened customers. Same principles apply whether domestic hot water or heating.