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I'm pretty sure it's historical. Older homes here were set up to use gravity to provide the water pressure, so the obvious thing to handle boiling over (which,

I'm pretty sure it's historical. Older homes here were set up to use gravity to provide the water pressure, so the obvious thing to handle boiling over (which, yes, could be quite frequent) was to run a pipe through the roof with just a bit more head elevation than the supply. My home for instance was built in the 1950s, and in the attic has a copper "header tank" which has an overflow/vent to outside and a float valve to maintain a set water level in it. That would've supplied one or two heat exchangers in the wood/coal fires, and cold water at the same pressure. Now, the header tank is out of the circuit, the original fires were removed decades ago in a remodel (their water heating functionality replaced with a "low pressure" electric hot water cylinder which still vents through that same pipe), and we're now connected with city water. The current situation is like many homes of similar vintage and has a couple issues: the cold water is substantially higher pressure than the hot, and we now have an Ajax valve which occasionally needs to be checked on in case it's leaking through and pumping electrically-heated water out on to the roof.

mindslight 5y ago
I do admire the passive elegance. It's interesting that your current water heater continues to use the old vent, rather than just ignoring it in favor of being higher pressure with a relatively inexpensive temperature relief valve. Are the hot water distribution pipes not rated for higher pressure? How big is that pipe that goes up to the roof? I assume it's covered with something to avoid random debris or animals from getting in?
- bacon_waffle 5y ago
  As is discussed in the article, mains pressure hot water cylinders are more expensive than low pressure ones. Also the installation (ed: of the first mains pressure HWC) is more expensive since it's supposed to be done by a certified plumber where (I'm fairly certain) a homeowner is allowed to replace like-for-like. Anecdotally, it does seem like there is a hesitancy in switching to mains pressure hot water, due to old valves and such failing, but I think the biggest reason is simply that a low pressure hot water system means less heated water going down the drain. From memory, the pipe is just a piece of 1 inch copper. There's nothing on the top, it just points straight up to the sky.

mindslight 5y ago

I do admire the passive elegance. It's interesting that your current water heater continues to use the old vent, rather than just ignoring it in favor of being higher pressure with a relatively inexpensive temperature relief valve. Are the hot water distribution pipes not rated for higher pressure? How big is that pipe that goes up to the roof? I assume it's covered with something to avoid random debris or animals from getting in?

bacon_waffle 5y ago
As is discussed in the article, mains pressure hot water cylinders are more expensive than low pressure ones. Also the installation (ed: of the first mains pressure HWC) is more expensive since it's supposed to be done by a certified plumber where (I'm fairly certain) a homeowner is allowed to replace like-for-like. Anecdotally, it does seem like there is a hesitancy in switching to mains pressure hot water, due to old valves and such failing, but I think the biggest reason is simply that a low pressure hot water system means less heated water going down the drain. From memory, the pipe is just a piece of 1 inch copper. There's nothing on the top, it just points straight up to the sky.