Here's a thing - the pipe going into the input side of the heat exchanger feels cooler than the pipe that comes out if I get hold of each of them the same few inches distance from the tank. Not by much, mind, they're both too hot to hang onto for long but the flow side is cooler than the return.

I'm assuming that it's because the flow goes in 8 inches lower down than the return and so meets colder water, thereby cooling the pipe whereas the return pipe emerges from 8" higher in the tank where hotter water will be, thus warming the pipe.

Just counter-intuitive, or are my pipes upside down ? ;-)

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Simon