Joe: I am assuming that an aquastat is some type of thermostat. Have you found out any more about this?
DKW: I am surprised that a UPS would be a problem. Do you have any more details on what goes wrong, maybe the brand, what you think might cause it not to function.
I am now using APC Line-R voltage regulators on TV, computer, and did so because of a friend who kept losing appliances to bad electricity from the utility company. The Line-Rs have eliminated those problems.
I had a power supply go bad on a computer flat screen. It was on an APC UPS box. When I connected it directly to the mains, it ran okay and the screen powered up. This caused me grief and service calls over many months (until we eliminated every other possible cause and were left with only the power supply), and dragging around an old CRT screen to use when the flat screen was inoperative. So... I put it onto the Line-R as well. When new, the power supply worked okay for several months, but acted up after I moved it (but not immediately after I moved it). It may be a manufacturing defect, may be related to the UPS, or may be related to "country" electricity.
All this started me wondering whether the external voltage regulator (like the APC) might be a good thing to use on questionable applications/devices/appliances, when running on ST power.