I've moved into a house that has a Vaillant Solar system for helping heat the water. We had various issues that resulted in a Vaillant engineer visiting, who pointed out various problems, including a missing air separator.

The installer returned and added the air seperator, and I thought this would be the last of it :)

However; we've noticed that over time the pressure has been dropping; and rising again when hot (this rising makes me think it's not just a leak). When the last work was done, the pressure was 2-3 bar even when cold. The pressure relief valve is 6bar. Today, the pressure is 0.5bar at 30-40 degrees; but rises to around 2bar at 60-70 degrees.

After checking [the manual](http://www.vaillant.co.uk/mediaarchive/2013/10/auroSTOR-commissioning-maintenance-troubleshooting.pdf) again; I believe there are two issues; but I'd like to know the impact of them (whether they're the likely cause) before embarking on the mission that is trying to get Taylor Wimpey and/or the installers to fix stuff; because it's a world of pain, and nobody seems to have a clue about how this system works.

Here's a picture showing the system and where the air separator should be installed (page 28 of [the manual](http://www.vaillant.co.uk/mediaarchive/2013/10/auroSTOR-commissioning-maintenance-troubleshooting.pdf)):

![Solar System](https://i.stack.imgur.com/VuxIJ.png)

I've marked some things in red relating to the positioning of the air separator. The things that appear to be incorrect in my system are:

1. The air separator is on the incoming hot fluid from the panels; which is shown here with a big X through it. This means the fluid going through the air separator is hotter than if it was on the pipe returning to the panels. **Could this be the cause of pressure loss after hot days?** (the system is supposed to shut off at high panel temperatures, so I would expect the fluid here to never be steam; but since the manual has a big X I'm wondering if there's a good reason for this?)
2. The pipes into the coil in the water tank are crossed over - our incoming hot fluid goes into the bottom of the coil, not the top as shown here. I'm not entirely surprised by this, as the incoming pipes from the panels were crossed further up too causing another issue they had to fix. I'm not sure if this problem should really have an impact on anything (in fact, to me, this seems more sensible, pushing hot fluid up instead of down; but I known nothing about any of this stuff!)

So; in short:

1. Are these problems enough that we should be fighting with Taylor Wimpey again?
2. Are either these problems the likely cause of gradual pressure loss?