Sometimes only slight variations can cause problems in one installation that you’ll not see in another, so be careful, and if you decided to tackle a project like this, remember to think…. and measure… and re-think… and re-measure… BEFORE you cut or glue in that fitting!
 

On this spa, we are going to remove the older equipment configuration on the left, and replace it with the newer digital system on the right.  We are also replacing the blower assembly.
After all power is confirmed to be turned off, and the spa is drained, the first thing to do is to remove all of the existing equipment, including the pump.  Now’s a good time to go ahead and replace the pump seal assembly.
Removal of the side control assembly from the spa is next.  Usually 4 screws hold it in the spa shell, accessed by lifting up the plastic decorative cover of the side control.
In this case, I decided to move the pump to the lower level of the spa, and attach the suction directly to the valve where the skimmer and bottom drain are tied to each other.  This way I will have a lot of room to put the spa control pack itself.
Like I said before… think and re-think.  In this case I forgot to re-think, and installed the PVC U-bend.  All I really needed in this case was a simple 90 degree elbow to route the pump discharge to the spa pack.
Now, what I did at this point was to fabricate an ‘S’ configuration that will get the water from the heater discharge into the existing filter.  Without this arrangement, the filter would need to be replaced.  Here I used 2 1.5″ U-Bends, and 1.5″ PVC flex hose.  This usually works pretty well, much better than using 90 degree elbows.
Before I get too far on the plumbing, I’m going to go ahead and install the thermowell for the new temperature sensor.  I usually pick a flat spot somewhere easy to get to, at the half-depth point of the water level.  This will ensure that the temperature reading reflects more of an ‘average’ temperature of the water.  I use a 1″ flat wood bit to make the hole.	Then I apply a liberal amount of clear silicone sealant to the back of the thermowell.
Pushing the thermowell through the shell, the next thing is to apply silicone sealant around the back of the thermowell, where it meets the fiberglas shell.  Note how I removed the tub foam insulation to provide a positive sealing surface for the retaining nut.
	Top left, the retaining nut threaded on and tightened down finger tight.  Top, the temperature sensor inserted into the well assembly, and the retaining cover threaded into place.  Left.  The finished assembly, with the excess silicone sealant wiped off.
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