Inherited with a house recently bought, I know little about this spa.

The jets will run and everything seems hooked up properly but it won't heat. It has 3 knobs for controls, one for a light, one for heat settings and one that is an on/off for the jets.

The previous owner had never, NEVER cleaned the filters and they were severely clogged. After replacing them I got the water circulating fine using the jets on/off switch. The reset switch on the heater was popped and I reset it. The spa does nothing with the heat all the way up, the only way it runsis when the jets are on. I don't believe the heater is doing anything then either.

Thoughts.

This spa does not heat when the jets are "on." You must keep the spa covered, turn the jets to "off".....and wait. It will only gain approx 2(F) degrees per hour, so be sure to give it ample time to heat..... Some troubleshooting tips: Keeping the jets in the "off" position, turn the thermostat knob all the way up and then all the way down. At some point while turning the knob, you should hear a slight "clunk" when the heater relay energizes. It will only do this when the jets are "off." If you're not hearing the "clunk" of the relay, there is a problem with either the t-stat, heater relay, or wiring inside the control box. If you hear the clunk, but still no heat.... a problem with either the heater, or heater wiring inside the control box. If after some time, you notice that the jets will not turn "on" again, and the heater high limit has again tripped, it's likely the small, silent, 24hour circulation pump has failed, and needs replacement. The silver lining in this scenario is that you know your heater is working " title="Smile" /> The spa will have alot of trouble heating without a cover, be sure you have one... Good luck

That sounds like a Hot Springs indoor/outdoor.
I just replaced that paticular model.

It is a 120 volt tub so the heater does not work when the jets are on.
Like "swine" said you should be able to hear the relay cycle when moving the temp control up & down.
Also there is a seperate circulation pump for the heater. so the problem you are having could be the heater or the small heater pump.
Be aware that Hot Springs parts are not cheap!!

Also when your heat is working be careful about setting your temp.
Mine was an 85 model & had no limiter on the temp & you can get it hot enough to hurt you!!!