Anyone know what this is?

[Paul.]

The large metal rod on the roof, next to the chimney exhaust. I thought it was a lightening rod for the steel roof, but today it has started spewing hot water out. When I turn the hot water on in the house, it stops. I'm thinking some sort of heat conduction rod that has split?

[SteveB]

It's the pressure relief for the hot water cylinder. If the water in the cylinder gets too hot, it expands and overflows out that pipe. It's a second line of defense. The thermostat should stop heating before it gets to that point and there's a valve that stops water being added to the cylinder when the pressure reaches a cutoff point. Ideally, there would never be anything coming out of that overflow. In practice, it will have a bit of a spit and gurgle every so often. That's normal. If it does it continually, get a plumber to have a look at it.
I'm not a lawyer or a plumber and my plumbing advice is worth as much as my legal advice.

[Peter Andrews]

Yep hot water overflow pipe - had the same issue last year and had to replace the inlet valve under the house cost about $90 with a plumber doing it!

[Jos]

Check the thermostat on the hot water cylinder, 55C is just right they say. Cosey looking place Paul!

[Paul.]

Thank you all for the replies, confusing stuff, all this house owning :/