In college I was taught that a secondary return pump should be on the return to the cylinder. This actually makes no sense, as I'm not sure how you'd guarantee that you didn't end up dragging in air from the taps and even my teacher was dubious about the claim that putting the pump on the return was the best way. But anyhow.
Hep does indeed say that its product is not suitable for secondary returns, but not why. Given that the same sudden shock from cold to hot would occur in a hot water distributing pipe whether or not a pump is involved, it can't be that. Nor can it, logically, be the temperature involved, as, if suitable for heating (radiator flow at 80°C), then carrying hot water for handwashing at 60°C should be a doddle.
All I can imagine is that Hep is not suited to the potential vacuum created by a pumped return.
Regarding mixing fittings, my feeling is that there is nothing technically wrong with it so long as the correct insert for the pipe is used (though someone I trained under insisted the insert should match the brand of fitting, not the brand of pipe), but I would still not do it as I have been unable to find a manufacturer's document that specifically allows it. My feeling is that although Hepworth/John Guest etc allegedly refuse to accept any liability anyway (local merchant claims they always find some excuse), at least, if it goes wrong, I won't be making myself look totally incompetent by mixing fittings. Based on that, I prefer to use copper in the first place.
Hep does indeed say that its product is not suitable for secondary returns, but not why. Given that the same sudden shock from cold to hot would occur in a hot water distributing pipe whether or not a pump is involved, it can't be that. Nor can it, logically, be the temperature involved, as, if suitable for heating (radiator flow at 80°C), then carrying hot water for handwashing at 60°C should be a doddle.
All I can imagine is that Hep is not suited to the potential vacuum created by a pumped return.
Regarding mixing fittings, my feeling is that there is nothing technically wrong with it so long as the correct insert for the pipe is used (though someone I trained under insisted the insert should match the brand of fitting, not the brand of pipe), but I would still not do it as I have been unable to find a manufacturer's document that specifically allows it. My feeling is that although Hepworth/John Guest etc allegedly refuse to accept any liability anyway (local merchant claims they always find some excuse), at least, if it goes wrong, I won't be making myself look totally incompetent by mixing fittings. Based on that, I prefer to use copper in the first place.