Water leaking due to faulty knob on jacuzzi whirlpool

Is this an outdoor hot tub?

If it is an outdoor hot tub...is it heating up or just running all the time?

This issue will have nothing to do with the jacuzzi tub. You will find the problem exists in the way your home was plumbed by the plumber.
Somewhere there is a clog in the water line going to the hot water faucet.
Find that and you will solve your problem.

Rod
US Parts Center

Take the unit off of the tub and see if there is a seal there,if there is a seal there try putting some silicone caulking around the seal or where the seal would be and then put the unit back together but don't over tighten the unit until the caulking is cured(24hrs) and then tighten the unit back up and this should seal it back up,unless the water is leaking past the actual switch itself,then you will need to replace this switch!Good Luck!!

Shut water off and remove knobs. Replace valves on side leaking. Go to Home Depo they will help.

The bath heater you have is not designed to HEAT water - they only Maintain what you start with.
You need to start the bathtub with the hot water you want. It will only heat about 1-2 degrees an hour.

the last time I looked at heaters for jacuzzi tubs they are designed to keep the water at the temperature you filled the tub and not heat up the water from cold to hot.

It could be back flowing into the hot water heater tank which is not full because of the leak. Even though the valve is closed on the cold water entry side, there is probably no valve on the exit side. That is how it is returning to the tank.

there should be a knob connected to your water lines that come from the wall, turn the hot to the left.

The only product I know of which will do what you ask is an item called Plast-Aid Acrylic and PVC repair kit. Google it and you will find the website. It is about $20. and it works like a charm. good luck

Depending on how you heat the hot water, whether gas or electric, is a missing detail in your problem, but if it is electric then your problem is the size, being only 40 gal, and needing probably about that to fill the tub, will always lead to your current position.
If the hot water heater is gas, the chances of it keeping up are slightly better, but again things like the temperature of the water leaving the tank, any mixing valves, etc. will effect the water temp at your tub.  Before raising any temps, make sure you follow local code, as not to scald yourself or others.