tank in the attic causing huge condensation

That's the feed and expansion tank for your heating system.

It should not be overflowing like that unless there is a problem.

You need a plumber to look at it.

You have a problem with your heating system causing this.

Find a few local plumbers tomorrow and start phoning them.

If the house will be vacant tomorrow during the day then leave the attic hatch open to promote ventilation of the attic. The stack-effect should push the damp out the eves.

to me it would suggest corrosion and the problem is with the boiler unfortunately

(I'm not an RGI btw so best assume i'm an idiot 😉)

This is kinda interesting and I've often wondered about systems that have been converted from F&E to your general purpose system boiler, without actually removing the F&E tank.

So, in my non-expert opinion, the feed from a heat-only boiler comes out and the vent is attached at that point. The vent arcs up and over the tank (by about 1/2 metre - not sure why but I'd say to avoid startup surges feeding the tank). After the vent attachment point, the feed is attached (within 150mm apparently) and then the pump comes after that lot.

So you come along with yer system boiler, remove the pump and install the boiler. Now the pump is before the vent/feed. This strikes me as a bad idea. Could this lead to what the OP is seeing?

In my parents' place, the RGI capped the vent and put a one way on the feed. Mainly because he wasn't arsed removing the tank and it made it easier to put fernox into the system.

Does it only overflow when the heating is on? If yes, then it's the system pumping over - either a recent blockage, or it was incorrectly piped or pump speed set too high and could well have been happening for years. Eventually it'll rot the system as you're constantly introducing oxygen to the water which can rust the system.

Or does it always overflow regardless of the heating being on? If so, then it's most likely a leak in your hot water cylinder coil back feeding the heating circuit. That's a health hazard as the flow could reverse if your mains pressure dropped (if it's a mains fed cylinder) or your cold water header tank dropped below the level of the heating system header tank (if it's gravity fed).

Ditto - it's not the immersion expansion that's overflowing (that's a whole other issue if you see that - don't ever ignore a boiling immersion!) but the vent into the F&E tank.

Perhaps get a boiler checkout* from an RGI - someone who can see your entire system and give you a sane recommendation. It'd have been better in July but, you know, here you are.

* Good to do anyway if you haven't already. I told Bord Gais in July that "boiler's vibrating a bit, is that the fan?" and they said "no, it's a cracked flue collar" and I went "Oh bugger" and they went "Yeah. It's an extra 10 euro is that ok?" and I went "Instead of CO poisoning? Seems like a deal to me."

@[Deleted User]

Did you manage to get a plumber?

Great news, thanks for the update!