[jeff5may]

If you are bechmarking the unit, you could run all of the power drawing equipment through a kill o watt meter and log the hx temp Delta t of the indoor hx as a function of time. The flow rate is not so easy unless you have a good flow meter. Since you have one, I would definitely use it if I trusted it.

Measuring the bucket temp will introduce errors when or if you include them into your calculated heat values. You're better off using source water that is relatively constant in temperature. The closer to the output of your hx you can measure its discharge temperature, the better. This will exclude any heat loss in the plumbing.

Running off tap water, you can set the flow rate with ease. Testing the unit at different flow rates and calculating your heat transfer will help you zero in on the "sweet spot" of your setup. If the hx is oversized compared to the compressor capacity, there will exist a flow rate where you will stop gaining raw btu's. Above this flow rate, there will be a balance point where your pump power catches up with the increase in COP. Depending on your goals, your optimum flow rate will lie somewhere between these two points.