They installed a larger air source heat pump today (Mitsubishi ZubaDan). Previously, the smaller ASHP didn't have the capacity to heat the required throughput of water to the required temperature, and therefore, amongst other things, wasn't able to get sufficient heat over to the sunroom. Also, since it was running at 100% load permanently, that is a recipe for breakdowns.Early indications are that the ZubaDan is up to the job. The sunroom is getting heat, the restrictor on the flow rate has been removed, and the pump in the garage is making considerably less noise.
We'll know for sure after running it for a few days.
The cost of this change were borne by the plumbing contractor.
