I think the cooling is getting sorted Pekka because . . . . .
I've been looking at heat exchangers that will isolate the furnace cooling water from the cold source water in the borehole. Many available on eBay, most seem to be Chinese, but many coming from Poland and Czech that also could be originally Chinese. Lots of amazing specifications, remarkable low prices and quality promises, and often with free delivery - but what spec do I need.
Well, in this application, erring on the large size is no disadvantage - I can't think over cooling the furnace water is an issue, and if it is I can easily throttle the cooling side feed water accordingly, so when a HUMONGOUS heat exchanger popped up on eBay, UK made, and brand new never commissioned it seemed too good an opportunity to pass up. Manufacturer was still in business, the H.E. was still a current 'off the shelf' model and the full specification was available and emailed to me - now could I win the auction

Half the issue was that it was 'collect only' and the seller wasn't responding to my requests for contact to involve couriers.
OK auction was ending today at about 5 pm, so I put the item on Shiply last night to get courier quotes, and then put a 'best offer' to the seller of the original starting price LESS my delivery quotes

Surprise phone call this morning from the seller accepting my offer. Shiply courier happened to be close to the sellers location (Esher), promptly collected it and has just delivered it - well actually THEM as it turns out that there were two

So what is it? - it's a UKE BR205-50C 50 plate copper brazed heat exchanger intended for an air source heat pump. 54 mm soldered copper inlet ports and 2" BSP outlet ports.
It weighs 34 kg ! and is 528 mm x 246 mm x 134 mm so I don't think there is any danger of it being under sized, and I have a spare

So I still need to source a pump for the borehole, a tank for the furnace water reservoir (might come from the original chiller) and work out quite a bit of plumbing lay out with big pipes.