Replacement pump arrived.
Will it work ?
, will it even switch on ?
, or go up in a puff of smoke ?
, what will SWMBO say if I have spent a grand on some scrap stainless steel ?
Connected the final pump and for test purposes, the hot water loop is a builders bucket with 35 litres of water in it so it acts as a thermal store. Its raining so the river flow is more than average and the river temp is also higher than normal at 17 degrees. So conditions are about as favourable as they could be.
Switched on the unit, it makes a strange noise then falls silent.
Then the ground loop pump starts up, followed a minute later by the hot water loop pump. Soon after the compressor starts up and is reasonably quiet. I had screwed the unit down to a wooden plinth and added rubber feet just in case of excessive vibration, but I neednt have bothered. Almost instantly the gauges on the exterior of the unit move in opposite directions and the hot water output feels slightly warm, with the ground loop getting colder with condensation forming on the ground loop pipes.
Wow, it actually works, and 17 mins later
..
..
A constant increase of 2 degrees per minute in the 35 litre builders bucket, from 18 degrees all the way up to 58 degrees in 17 mins when the high temperature tripped out the compressor. I have calculated that equates to a constant 5.7kW of heat output. The electricity consumption gradually increases in line with the compressor pressure, thus reducing the COP value. The ground loop rapidly drops in temperature for the first 5 minutes, then becomes constant. On the first run it stabilised at 10 degrees out and 14 degrees return.
I have since re-ran the test each day, each time with differing pump flow rates and with different river conditions. From this I draw a number of conclusions.
1) With a slower river flow the ground loop stabilises at a lower temp, first run was out 10 degrees and return 14 degrees (10 / 14). Subsequent runs with less river flow were 5 / 9 and 3 / 7. But the outcome was the same, no measurable difference in heat output or electricity consumption. I have read somewhere that the ground loop needs to go considerably below zero to make a difference.
2) I slowed down the ground loop pumps and although the temperature difference between flow and return widened from 4 to 5 degrees, there was little measurable difference in heat output or electricity consumption.
3) I slowed down the hot water loop pump and although the temperature difference between flow and return widened from 4 to 5 degrees, there was little measurable difference in heat output or electricity consumption. In fact the heat pump manual says for efficiency, the temp difference should be less that 6 degrees for optimum operation.
4) The headline spec of 8kW seems to be a fair bit of an exaggeration.
5) The COP figures compare favourably with the findings of the Energy Savings Trust heat pump trials which published real life COP values.
6) My crazy copper heat exchanger design seems to work as the ground loop temp is stabilising at above zero temperatures.
Now I just have to work out what I am going to use it for. 
