If the bubbles only occur on pump startup for the first few minutes, that is normal and part of the solar panel priming process.

However, if the bubbles continue and never stop, then it is probably the solar vacuum release valve allowing air into the panels. This usually occurs because the pressure at the panels has dropped and that in turn is usually due to a dirty filter but it can be caused by other factors as well. Try cleaning the filter first and see if that helps.

I will second the cleaning of the filter first and if that fails, climbing up to the roof top and checking the vacuum relief valve that lets air out when the system is priming but keeps the water in and lets air in when the system is turned off.

Scott

It turns out the vacuum was completely stopped up -- a hunk of fibrous root was in there. The pressure must have tripped that valve. (I wrote my original post last night... noticed this morning that the vacuum wasn't running either.)