I have recently purchased about a 10 year old spa from a family friend. It worked trouble free for them. I have had it working great for about a month, but recently noticed the temp. down at 75 when we usually have it about 105. I turned the thermostat up and the heater didn't kick in. So with a bit of trouble shooting I was able to determine nothing, accept the heater will turn on when the pump is on high speed. and the spa will heat up, but as soon as I put the spa to low speed the heater turns off and of course the spa cools off. I spoke to the previous owners and they suspect it could be the magnetic contactor for the heater. What does this part do and Will this wear out over time? Or should I be looking at something else. oh yeah it is a 1.5 hp 2 speed pump with a 240v supply.dura-pak with a 4 kw heater.

The problem is more likely to be that the flow/pressure switch isn't being made at low speed (heat mode) based on your description of the problem. In this case you either developed a blockage somewhere restricting the flow (remove filters and see if the problem goes away, if so, clean/replace filters) or your flow/pressure switch is weak. If it is the style that has a knurled knob on it's top, you could try rotating that knob a turn counter-clockwise to see if that helps.

Thanks guys,
I took my filters out, and instantly my tub started working properly. Put the new clean filters in and never looked back. How often should I change my filters? And is cleaning them with filter fresh a good practice? How long do they usually last cleaning them. As you guys can see I'm new at this. Again thanks for the help.

Filters should be cleaned monthly, or as needed if sooner, first by spraying with a good pressure water stream, then by soaking in a filter soak (I use 1 part muratic acid with 4 parts water) followed by a final rinse. Depending on the filter manufacture process and how hard you are on them, they can last a long time. Mine has been going for about 3 years now. Always a good idea to have 2 so you have a spare ready to go while the other is being cleaned.