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Pneumatic Bonnet Steam Valve

Owen Member Posts: 147
I run seven boilers for a school district. We have four systems that have pneumatic controls. There are about 50 unit ventlators in classrooms. Lots of fin/pipe and fin/tube radiators also used in the buildings in halls and such. The uni-vents all have a pneumatic bonnet which modulates between full open and fully closed as dictated by the thermostat. Some also have an odd venting scheme with 1/4" copper no one around here understands anymore.

Question 1. Does a modulating steam valve (the bonnet) require additional pressure at the boiler (above Dan's 2# maximum, try selling THAT around here) because it is often less than full open? In other words, if only 1/2 open, pressure is 1/2 that at the boiler? (I was told that by a vendor's rep.)

Question 2. What is the 1/4" piping and what does it do and how does it work?

I'll post photos tomorrow.
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