I have an Apollo hydroheat system, and my heater wasn't working. I called the plumber who had installed the water heater, and they told me without even looking at it they couldn't do anything, and referred me to a heating place.
The heating place, also without looking at it, told me the only thing I could do was replace the furnace for $4000; I could try Benicia Plumbing if I wanted to, but because Apollo is no longer in business replacement parts aren't available. Decided to take my chances and have Benicia Plumbing come out. An hour and $165 later, my heater was fixed--wasn't a problem with the heater at all, it was the check valve on the water heater.
I should have called these guys in the first place! Thank you Benicia Plumbing!!!