It's suprising what you can thread through a small hole! My latest, ongoing project is using a 100 litre electric water heater as the body of a heat exchanger.
So far I have made a coil (same form as MK2 heat exchanger, 3 coils 1/4" etc....) but with both entry and exit protruding from where the heating element used to live.
I have it piped up to a 5Kw Mitsubishi external unit. It works but at the moment suffers from overheating of the compressor. I'm planning to install a TXV to better control it. I may not progress much though until the autumn as it's getting pretty warm here in the day and I'd have to be out working on the roof in the sun... maybe a (rare!) cloudy day.
My idea is to have this exchanger in series (and before) my oil boiler as an 'accumulator'. My thinking is that I can pre-heat the water going to the boiler thereby reducing the consumption of oil.