Hi, I'm new to this forum, just found you, so hello all.
I picked up a Thermospas unit from a friend who didn't want it.
It's a tradeout deal I'll have to pay up later.

Spa looks remarkably well, but has been outside in elements for 6 months or so with the wood side detached beside Spapack.

It had Pigtail wiring cut and contained 240 4 wire hookup, which I removed and ran new 4 wire 240 hookup to dedicated breaker.
Everything runs off of the pack

It's the smaller 2or3 person version. Possible model "penthouse"
1 3hp 2 speed pump
Blower Motor
Balboa Instruments 1900D Pack circa 1997
Straight 2'' pipe heater with unions directly under the control box.
Topside digital with up/down Temperature control, light, blower, and Hi/lowpump

Everything seems to be working fine, But no heat!
Let it run 6 hours or so and It lost 1 degree.
When it comes on it runs at low pump speed and stays there until I do otherwise.
Like I said, it's wired exactly as it was when disconnected from other setup. When it comes on, it has a delay that counts down before it starts up. On the readout there's no "Sn" code or anything displaying that there's a bad sensor or anything.
I've heard that Thermospas are overpriced 'Dogs' after researching it, and although I have not yet had time to try and call them, I hear they are not much help.
I read somewhere that if the flow sensor is out of whack the heater won't work. Anyone have any ideas? If I have left out any pertinent info, please ask.
Thank You.

Hopfully you will find this helpful.... the switch in your picture that you don't know what it does, is indeed your pressure switch (flow switch) and if you KNOW you have enough waterflow when heating, you can jump this switch out to see if that is the problem... otherwise you might want to double check your circulation pump (if you have one, it's small, about the size of a popcan, and has a 3/4" hose going to and from it). If that isn't circulating, your heater will never kick in (due to no flow on the flow switch).

Hope that helps you to find your problem

Jer