Below are tips from HTP's National Solar Manager, Rod Hyatt:
Here’s a service tip for those whose systems use an HTP drain back tank – specifically on the maintenance or replacement of the sight glass. The sight glass is the transparent tube that lets you know the water level of the drainback tank when both the pump is pumping and when the system’s drained down.
I received a call last week from a man who installed his solar system about two years ago. He’d noticed that a slight crack had formed in the sight glass, just below the water level. And as time progressed, the sight glass started to drip. He shut down his system and removed the sight glass, but was unable to repair it. So he ordered another one. Meanwhile, the system’s not working until the replacement arrives.
I suggested a repair – which can be either permanent or temporary – that has worked on my system: Replace the glass with a piece of ½-inch pex tubing that is not opaque.
The pex fits perfectly into the same connections as the factory sight glass. Pex has a temperature rating of over 400 degrees with no pressure, so it’s an excellent, industrial material for this application. I used Wirsbo pex for the 10-gallon drainback on my system. The length you’ll need depends on the size of the tank, but my tank required about 11 inches.
Earlier HTP drain back models seemed to have some flexing in the stainless steel tank and occasionally would break the sight glass. Also, sometime after years of exposure to heat, the sight glass simply needs to be replaced.
-Read more from Rod Hyatt's blog at: www.askrod.com