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Comment by jpatokal | Hacker News Reader

So people would use a mercury seal to put pressure on the water, allowing the system to run hotter without boiling.

I didn't quite follow this part. Where in the system does the mercury go, how does it add pressure, and does it have anything to do with Wikipedia's description of using mercury in glass-to-metal seals?

https://en.wikipedia.org/wiki/Glass-to-metal_seal#Mercury_se...

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This page describes that system, I think:

http://www.oldhouseweb.com/how-to-advice/gravity-hot-water-h...

(the "Honeywell Heat Generator")

I've now linked that from the article; thanks!

(I read that page when I was first reading about this the night of the spill, but had forgotten about it. Holohan is amazingly knowledgeable about old heating systems.)

Did your home inspector note the upfeed system in the pre-purchase report? Did you sign a waiver limiting the inspector's liability?

Just reread the inspection report; they didn't notice it.

Wow. If there’s anyone deserving the title of a hacker, it’s the author of this page.

It's a pressure valve. Here's the patent:

https://heatinghelp.com/assets/documents/141.pdf

I don't think I've a clear enough understanding of how it works to try to explain it.

It's essentially a pressure regulator I think.

Here's an EPA document which shows some pictures, look at page 2 for the unit the OP has.

http://www.epa.gov/mercury/pdfs/EPA-905-F-11-008.pdf

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