Hot water heating control...? What is it?

DanBee

I came across this during a site visit to replace a boiler. Residential Open hot water gravity system
the item pictured has Honeywell stamped on it
My guess is that it contains mercury in the bottom section and operates a check valve.
When the return water gets hot enough the mercury would rise and close. When the water would cool down, the mercury would drop and open the valve.

Zman

That is the jist of it. Lot's of mercury, so be careful.
Dan has a better explanation in "Classic Hydronics"

DanBee

Thanks for the Reply Zman. i remember seeing something similar years ago, I wasn't 100% on it. thanks for confirming.
DB

pecmsg

Zman said:

That is the jist of it. Lot's of mercury, so be careful.
Dan has a better explanation in "Classic Hydronics"

and Asbestos!