Just when you think you've seen it all - I have a client with a zone where, as the supply temp drops, the return temp rises! Since he's using our controls, I had to take a look. Here's a graph. Supply is the magenta line at the top of the chart - top of storage. Return temp from the zone is the red line. As the top of storage temp drops, you can see the return temp go up from about 100 to almost 110.
So what's going on? The zone in question supplies heat to a greenhouse through a flat plate heat exchanger. We control the speed of the circulator on the hot water side to maintain a glycol temperature of 120 (yellow line) on the greenhouse side. As the supply temp drops, the circulator has to speed up from about 20% to about 25% (green line). The higher speed means that the water doesn't lose as much heat passing through the HX, so the return temp actually increases. Very cool and unexpected. Here's a plumbing diagram. We're varying the speed of P15:
![NoFossil gets surprised NoFossil gets surprised](/talk/proxy.php?image=http%3A%2F%2Fwww.nofossil.org%2Fphotos%2Fhx-surprise.png&hash=16e2ff98a8a3fcc7a5e9be7947b1616c)
So what's going on? The zone in question supplies heat to a greenhouse through a flat plate heat exchanger. We control the speed of the circulator on the hot water side to maintain a glycol temperature of 120 (yellow line) on the greenhouse side. As the supply temp drops, the circulator has to speed up from about 20% to about 25% (green line). The higher speed means that the water doesn't lose as much heat passing through the HX, so the return temp actually increases. Very cool and unexpected. Here's a plumbing diagram. We're varying the speed of P15:
![NoFossil gets surprised NoFossil gets surprised](/talk/proxy.php?image=http%3A%2F%2Fwww.nofossil.org%2Fphotos%2Fimage09.png&hash=c14d57f436d0a39f5a1e4250248c0ec4)
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