Location of Automatic bypass valve with unvented cylinder?

[caravanboy]

Hi,
I've just had my central heating boiler relocated and a new unvented cylinder professionally installed. I was surprised at the hot water temperature coming out of the taps was so hot even thought the cylinder stat is set at 50C so I started looking at the pipework. When the programmer was NOT telling the boiler to heat the water the cylinder's coil pipes still seemed very hot. I let everything cool down overnight and have run the system up and concluded that the Salus Automatic Bypass Valve is definitely letting heat through to the cylinder even when the system should only be heating the radiators. All of the radiator TRVs are fully open (since the house is still quite cold and <14c) so I assume the plumber has not set up the automatic bypass valve correctly? I've seen other threads on how to set it, but he'll be coming back for a small leak anyway. HOWEVER, my worry is that given tonights Googling to try and understand this I've just seen a post that says the ABPV should NOT bypass the DHW valve on the cylinder coil circuit if the cylinder is unvented. My boiler has an external pump so I assume there is no "overrun" control and the system is an S-plan with two 2-port valves. Any advice on if I need to complain that he's piped it up incorrectly?
Thanks.

 

[picasso]

The ABV shouldent bypass the hot water zone valve especially on an unvented cylinder, it can bypass the central heating zone valve but normally is installed between the flow and return after pump and before the zone valves.

 

[Norcon]

What boiler do you have?
If its a twin burner cooker with overrun on the oven burner for example then by passing the unvented two port valve is an option.

A thermostatic mixer will blend the temperature down to a safe level which your plumber should have fitted anyway to comply with the amendments to The Building Regulations 2000, Approved Document G – Sanitation, hot water safety and water efficiency, enacted on April 6 2010.

 

[caravanboy]

Thanks for the feedback so far. I can only assume he thought the ABPV would dump the excess heat into the cylinder. I've adjusted it today (following advice on another forum thread, I shut down all TRVs and I have two rads without TRVs. I turned the ABPV to full at 0.5bar and it did NOT pass any heat, I then turned it back until it did start passing heat at around 0.4bar. When I opened all the TRVs it then seemed to stop again as I'd have expected. The plumber had set it at about 0.2bar. I guess that will give me a but of peace of mind until he returns and I can discuss it.

My worry is still that if it stays as it is and the CH was on for a while with ABPV activated there is nothing to stop the unvented tank getting hotter and it could potentially overheat, although then I presume its safety valve would cause it to discharge at ~90c?

When he comes back should I insist he changes it to bypass the CH zone valve rather than the DHW zone valve?

Also, my boiler is a Worcester Bosch Greenstar HE 29R (circa 2004). I don't believe I have a thermostatic mixer that you mentioned? There's nothing in the cylinder installation manual that I can see about that either. It has pressure reducing valves, balanced-cold and various safety valves but nothing thermostatic that I can see. Can you explain what it is for?

Thanks.

 

[CBF]

it should be on the boiler flow & ret after the pump but before the zone valves, it is there so that when the valves close & the pump carries on (on pump over run) it has somewhere to pump ie around the boiler, if it is after the zone valves it's in the wrong place so he needs to change it, i hope he holds the G3 unvented cert (did you check ?)

 

[gaswizzard]

If the ABV is after the zone valves then it wont be acting as a by-pass.

 

[caravanboy]

If the ABV is after the zone valves then it wont be acting as a by-pass.

It is definitely before the valves, my concern is just that it is pypassing to the DHW circuit rather than the CH circuit. I understand what it is for and that it should (normally) bypass on the CH circuit, I just need to know if there are any (rare?) reasons that the plumber may try and claim that it is acceptable to bypass into the cylinder instead. From what I've read and what's been said so far I can't imagine there are but I just wanted to know before I discuss it with him. Thanks.