Yes I do and it works like a charm. I have a water to water HX installed in my return line to my tub right before it enters the tub. This way, all the other components of the tub, like heat probes flow sensors filters........really don't know the HX is there. I have boiler water coming into the HX with a 3 way 24V taco valve. I have 2 modes of heating my tub, electric or wood via a dbl pole dbl throw (on-off-on) 50 amp switch (looks just like a wall light switch) mounted to my tub skirt. You need to find which 110 hot lead to the heater shuts off when the tub stops calling for heat. This is on a 240volt heater and tub. Take that wire off the heater and run to switch. Take "wood" side of switch to a 24 volt transformer, run the "elec" side of switch back to the terminal of heater element. Now, when the tub calls for heat, either the 24volt transformer wire (which opens 3way valve to allow boiler water into HX) becomes hot, or the element like normal works, depending on what I'm heating with. With no power to wire (tub up to temp) the 3way valve goes into "bypass" and no boiler water goes to HX. My tub has a 24/7 circulator pump so it works perfectly. It sounds complicated (especially me trying to explain it) but it really is not. I did all the work myself. My tub holds 480 gal of water.
70K Hot tub HX $319.00 (make sure it can take chemicals, that's why so expensive)
1" 3way 24V Taco valve $89.00 (e-bay)
Underground pex left over from install but calculated when purchased
50amp on-off-on switch w/ outdoor mounting box $95.00 (special order from electrical supply)
2- 20 amp ground fault breakers for OWB circ pumps $30.00ea. ..........I figure a very "remote" chance that one of my circ pumps would short and "electrify" my boiler water which in turn would "electrify" my hot tub.....not a good thought so ground fault breakers on pumps will eliminate that scenerio. I would HIGHLY suggest this.
Any questions just ask away.