Quote Originally Posted by Gary View Post
This is not an even trade. Lower flowrate will give you higher temp water, but less heat transfer. It is a mistake.
This is true, for the condenser heat-balance, as we saw in Drew's swimming pool heat-pump.

The best way to actually test the real effect of these changes is to measure the heat-up time for water cycling around a heat-pump loop. In practice, I've actually found little noticeable difference in tank heat-up time using slower, or faster flows.

What happens is that the lower temperature of the entering water can be set to maintain the same log-mean-temp-difference across the condenser, so that the condensing temp is not actually affected.

In Drew's case, my test was set a Tc,sat=50'C, then the water flow was closed off - the Tc,sat rose to compensate. This is not a realistic test for circulating water, though. What typically happens is that at a similar Tc,sat~70'C the whole heating cycle finishes - with less passes through the heat-pump circuit, but more dTw in each pass.