In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
I received some great help previously getting started to understand my home's steam system ( http://www.heatinghelp.com/forum-thread/142756/Requesting-Help-to-Understand-and-Maintain-2-pipe-system ), and have a couple of questions about some next steps.
Summary: The system was installed when the house was built in 1937, and apparently is a vapor/vacuum system. Boiler is a Peerless JOT-5W installed in 1985.
Vaporstat - should I purchase a 0-1psi vaporstat or a 0-4psi model? Will I want/need to set the system pressure above 1psi? Or is it more important to have the finer adjustment capabilities of a 0-1 vaporstat instead?
Gauge - I will add a second, low-pressure gauge next to the vaporstat. Since the system develops vacuum during the heating cycle, do I need to find a 3-0-3 psi gauge, or will a standard 0-3psi gauge be ok?
Backflow preventer - there is a Watts 9D backflow preventer installed between the fill valve and the boiler ( http://media.wattswater.com/ES-9DM3_M2.pdf ) . I replaced it a couple of years ago thinking it was defective, but that didn't solve the "problem". After the thermostat is satisfied, there is a vacuum in the system, as confirmed by the 30-0-30psi gauge. Then I can hear air being sucked into the boiler through the vent/drain of the backflow preventer and water drips out the same opening. I can feel the vacuum and air movement by putting my thumb over the vent/drain opening. I'm not sure how much vacuum there should be in the system, but this "reverse venting" brings the vacuum down to zero. Is there a way to still have an inline backflow preventer but eliminate this problem?