This link has some answers regarding what may be causing the problem:

http://homerepair.about.com/od/heati...t_boiler_6.htm

Check out the possible causes and click on the various underlined links on
the page.

For example, you may just need to drain some water out of the expansion tank
above the heater to correct the problem.

wrote:
I have an apollo heating system;which uses hot water to heat with.
my pressure relief valve is dripping.because it is an apollo system
the valve must be replaced by an apollo relief valve. my question is:
how dangerous(?) is waiting for 10 days until i return from a trip
that
is neccessary? the dripping occurs when there is NO call for heat.
also the dripis not a gushing flow of water,just a drip like a faucet.