our expansion tank is leaking from the nuts and bolts at the bottom when the hot water is on?

[Fluffycloudland77]

Any ideas?

5 year old system, condensing boiler.

It's probably under a 10 year guarantee with the boiler installation so get the company back because it seems like this part has failed.
Before you call them out make sure you haven't done anything that would invalidate the warranty- check it's not over-pressurised

Ok, how do I do that?

There's a similar shaped red tank next to it with a valve on it, if you turn it a pressure gauge goes up. The house builder said to keep it on about 1 so that's what I do.

So if you are sticking to the instructions to keep it at about 1 then you are fine- next step is to phone the installation company

Is this a white metal thing, somewhat apple-shaped, next to your white unvented hot-water cylinder?

If so, you need a heating engineer with the G3 qualification to repair or replace it, because it is part of the safety system for the cylinder. White ones are made of stainless steel and it is fairly unusual for one to go wrong. There is an safety valve that will open if the pressure goes too high, and they have a rubber container inside that holds air so it can be compressed or expand to accommodate changes in pressure. The air can be pumped up like a tyre.

Perhaps it has a loose connector that just needs tightening up.

Whoever services your boiler will (should) have the cylinder qualification, and should check the cylinder during your regular boiler service.

A similar red vessel is used for the radiator water, but is usually concealed inside the boiler casing, and can be serviced by any gas-safe qualified boiler engineer.

p.s.

the pressure in the white one is not the same as the pressure in the red one, there is a separation in the hot tapwater (white) and the radiator water (red). The white one is stainless and very clean because your brush your teeth and wash your vegetables in it.