How do you know if a heater is compatible with your whirlpool tub?

Parts for Maax Spas‎

www.spaguts.com/‎
(714) 764-0014
Pumps, packs, heaters, controls,
ozonators for Maax hot tubs.

# 27 Hot Tub Heater Replacement for 240 volt hot tubs ...

? 6:27
https://www.youtube.com/watch?v=xNWjM8c_b-8
Oct 10, 2011 - Uploaded by softcellelectrical
27 How to replace an in line or flow through heater for an above ground ... I just changed out the water in my spa when I discovered the heater element ... Is there a hose that should fit on it? ... I figure it will run similar to a whirlpool setup. ... tests on the unit...but, it sounds like 1. your sensors may be DC and

You should be abl to find a PVC reducer at Lowes or Home Depot. If they don"t have the exact size they have rubber reducing boots that have strap clamps attached to them. If that doesn't work then look up a place in your area that specializes in repairs for pools and spas, they should be more than willing to sell you what you need.

This is a bathtub heater. it is only 1500 watts. they are not meant to heat the water in a bathtub. They are considered a maintenance heater ONLY.
This style heater will maintain whatever temperature you put in the bathtub. It will only produce about 2 degrees of heater per hour.
solution: start with the water temperature where you want it to be when bathing and this heater will keep it that temperature while the pump is running and the tub is full of water.

if the light on the heater is not coming on when the tub is full of water and the pump is on then the heater is not working.
check the circuit breaker to make sure it is on.
the heater and the pump cannot be on the same circuit. They have to be on separate circuits.
Make sure they are on separate circuits.
Check to make sure you have the heater installed in the correct place.
there is a pressure heater that mounts anywhere after the discharge of the pump.
there is a vacuum heater that mounts in front of the pump in the suction line.
if you put on or the other in the wrong place it will never come on.

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.

If you currently have hot water coming to other parts of the house, and you have patiently waited for the hot water to reach the tub, then you don't have a hot water line connected to the faucet. Also, because a hot tub or whirlpool can hold more water than your water heater holds, an inline heater is often used. It attaches to the plumbing system on the whirlpool.

probably already solved this but yes, you can put a heater on any bathtub that has water jets.

When you say turns off, is it tripping the safety circuit?

all heaters have a thermostat (or multiple) and heating elements. 
Start by removing the elements and seeing if they are burnt through.  If so, replace. 
If they look fine then replace the thermostat(s).
Both are relatively easy and cheap to perform.
Remember to turn off the power and water to the heater and drain the unit completely before doing any servicing.

don't know the whirpool duet cos i'm in uk not us.
i assume your machine is a front loader
places to look for heater are
remove back plate and look for heater at bottom of rear face of tub
sometimes heater is mounted on front plate of tub in which case tilt machine backwards and see if you can see heater mounted towards bottom of front face.
most likely place however is on the rear plate of the tub