'tank in a tank' hot water tank issue - is it broken?

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1 Nov 2025
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Hi all,
Moved into a house built in 2008 with a ground source heat pump that does both the hot water (with immersion heater also) and wet underfloor heating. The hot water tank is a 'tank in a tank design' where there is an inner tank for the domestic hot water and an outer tank for the underfloor. I recently had the immersion heater changed. Since then I've noticed the expansion tank making a lot of noise - i had thought this was the expansion volume but checked the pressure and it was okay. I've also since noticed that the underfloor heating pressure was high. I released some of the pressure using the relief valve, with water flowing out - but the pressure slowly refilled even though the manual filling pipe was closed. I'm worried that there's a hole between the two tanks...
I appreciate this is an unusual design (it seems a bad design to me!) but any advice or experience would be hugely welcomed!
thanks
Hugo
 

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