You have by-passed the pressure switch which is there to PROTECT the heater from activating under undesirable conditions, NO WATER FLOW.
You mention the circulation pump, but it is your primary 2 speed pump that is providing water flow, but it is limited to a 20 minute time frame for safety reasons, but then so is the pressure switch.
I am not convinced the problem is your pressure switch, it is just telling you the water is not moving under pressure as it should be. It could be the filter is clogged, simply remove it, it could be your "circulation pump" IF you have one and it is not working. It is a small pump that provides continuous circulation, which you are not getting water flow to close the pressure switch, to allow the heater to work.
Reconnect the pressure switch, and allow the system to operate as moving water will not freeze, and then find out why your pressure switch is not closing.
What kind of spa do you have?