Sounds like that one was rock bottom stuff. Have you got a filter on
it btw? I just wonder if there might be something in the pump.
It was OK but I was asking too much of it. I think the bearings just
got knacked from all the backpressure. If they weren't already shot,
it would be burbling away happily now that I've split the coil into 4
circuits.


You may even find 1/4hp to be more than you need, in which case you
can cut energy use/noise/increase life by reducing its run speed.
Whats your temp rise with the 1hp pump, with the present plumbing
setup and fast flow rate? It should be poss to make rough calcs from
there, if youre interested in slowing it down.

I'm not sure... maybe with the polythene on. It's just that when I
first measured it, I got 5 litres in 35 seconds with 7degrees rise.
When I split the coil into 4 circuits, I got the same temp rise but
with 5 litres in only 15 seconds. Admittedly the first measurements
were taken in less good conditions but I reckon that the black pipe
will only work efficiently up to a certain temperature - ie: not much
above ambient. So perhaps when I get my slower pump, I won't be seeing
10kw any more (although probably getting a better efficiency in terms
of electricity used / heat output).

I'll try the polythene at the weekend and post the before/after
measurements.


Well I'd be curious to see what happens if you ever space the pipes
out and put silvered mylar underneath, and polythene on top. I reckon
you'd seriously cook.
This is effectively increasing the collector size so I would imagine I
would get a large improvement.


Sounds like doing the same thing for DHW has to be on the cards, thats
some heat output! If you ever do I'd efficientise it as much as poss
though, since water output temp would have to be high, so all those
extra tweaks would be much more important.
I should imagine you'd definately need much better insulation
underneath, say polystyrene and maybe polycarbonate sheets over the
top (3m x 0.6m sheets can be had for about a tenner). I don't even
know if you'd be able to get DHW temperatures with MDPE pipe though.

I just bought 50m x 10mm copper oil pipe for about 60 quid (not for a
solar project!). You could use this on a black corrugated metal
collector surface - it could be well spaced out due to the high
conductance of metal.

Cheers

Nick.