The Story: Got an old (~15 years) Vita spa for free but it had some electrical problems, otherwise in great shape (tub is perfect, blower, heater, pump all work). The previous owner got tired of replacing expensive boards so he built his own controls with switches and relays, it was done correctly and worked well for probably a decade. When I got it, he told me the problem was with the flow switch but he didn't feel like fixing it, just wanted it gone. Upon inspection I had to replace several dry rotted wires and the rusted out control switches, so now the relay setup is working flawlessly.

The Problem: I filled the spa and kicked it on, the heater lamp (and relay) would not come on, and the pump was not flowing well. On high the pump was acting funny until I realized the old filter was too clogged and not flowing enough water (pump sucking air). I removed the filter and the pump is working great. However, I have to put the pump on high to close the flow switch then immediately switch to low to maintain it and run the heater. If I just switch to low, the flow switch will not close. I ran the heater for two hours and the flow switch remained activated, but not if I start it on low.

Th Question: Is this a problem with the flow switch (I hope) or is the problem with the pump (I hope not), or is my problem something else. Is there anything I haven't checked? In the process of changing the wire, I replaced both gate valves and they are fully open. Thanks in advance for the help.

Yes of course. Some flow switches, particularly the Harwil brands with rubber stoppers and isolators will become 'hardened' and cease operating at low flow rates.

I'd replace the flow switch for sure.

Well I fixed the problem, at least temporarily, by modifying the flow switch (i slightly bent the swing arm on the microswitch). This gives the big swing arm (the one in the pipe) more leverage to actually trip the switch. I will replace the switch soon, but I want to get everything else operating properly first. Thanks for the reassurance though, I have a spare used pump, but I am not sure it even works, and a new pump is pricey.