[RE-wrenches] SHW expansion tank pressure

[Carl Adams]

Andrew, Daniel, August,

Thanks for your suggestions.
Andrew, the PR valve had gone off only twice that I could tell.  When I
checked it again on Saturday the pressure  was again at zero but I used the
PR valve to vent some residual pressure/fluid from the system.  It seems
sound.

Daniel,
On my previous visit I had also assumed a system leak and so checked
connections at the collectors, at the line set transition, and there was no
evidence of a leak.  The pressure tank was clearly marked with the set
pressure, commissioning date, and initials of my lead installer, so I
"assumed that was good".  After all the system had been in service without
 issue for approximately 1 year.  Wrong!  After relieving the fluid in the
system I checked the expansion tank pressure, and it was at zero.  My
thought was either the bladder had ruptured or the Schrader valve had a
slow leak.  When I pulled the valve out there was no fluid (thanks for the
suggestion to check this August ), so I assumed a leaking valve.  I lubed
and re-seated the valve and re-pressurized the tank.  I then recharged the
system, bled the air,and 4 days later the pressure is rock solid after a
couple of good solar days.

I had never had an expansion tank lose pressure before, so my take away was
to add "Verify the expansion tank pressure", to the
 troubleshooting/service checklist.

Daniel,
I was unaware that the corrugated line set had capacity for expansion , but
witnessed this on the recharge cycle.  As I pressurized the system, closing
the drain valve, to allow pressure to build,  I noted the insulation on the
return line set slide up the pipe 2 inches or so and then slide back as
pressure was released.  It makes sense that the line set can expand along
the length of the pipe. Good to know.

So in reviewing things last Friday evening before my service call Saturday,
I went back and verified the pipe, collector, heat exchanger, and expansion
tank  volume.  I was sure the exp tank was adequate but still wasn't
thinking empty expansion tank.  In doing this I pulled out the Calefi
idronics 3 manual and did some reading.  The appendix there offers some
calcs for determining expansion tank volume (attached).  Tha calcs showed
the 18 liter expansion tank to be oversized for the  system (so good to go
on this front).  The appendix also offers calcs for expansion tank charging
pressure.  These calcs suggest a system charging pressure of 18 PSI (rather
than the 26 psi  suggested by the Schuco table I posted previously. The
Calefi calcs also seem to be independent of the charging pressure, where as
the Schuco table clearly correlates the two.  If anyone has any insights on
this please share.

I rechecked the expansion tank pressure after filling the system.  What I
found was that  the expansion tank pressure had increased from 26 psi to 35
psi, with the system pressurized to 30 psi.  My expectaion was that the
expansion tank pressure would have increased to match the glycol charging
pressure.  I thought at first this was a gauge offset, but on second
thought I considered it might be the glycol is an incompressible fluid
whereas the air is a compressible fluid.  Again, If anyone has any insights
on this please share.

Thanks again for your replies,
Carl Adams
SunRock Solar
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