If these solutions do not solve your problem, consider replacing your entire water heating unit on the hot tub.
If you are not experienced working with electricity, contact a service repairman. Hot tubs vary greatly in their model types and configurations. It is difficult to diagnose the problem and may be dangerous to resolve. Think twice before repairing this yourself. Proceed at your own risk.
There are some simple steps you can take before you call in a repairman to fix your hot tub. If the water is circulating but your hot tub will not heat up the problem is very likely your heating element. If the water is getting too hot, there are several adjustments you can make to cool it down.
Unplug your hot tub prior to working on it. Wear rubber-soled work boots to ground yourself. Water and electricity are a very dangerous combination. If you are not experienced in working with electricity you might consider contacting a repairman. See warning section below for more information.
Clean or replace your hot tub filter cartridge. If your water heats up, but very slowly, you may simply need to change the filter. A heater that continuously turns on and off also indicates the filter must be changed. Hot tubs that are used often will need to have these cartridges changed more frequently. If you are unsure how to change the filter on your brand of hot tub, see your manufacturer's guide, contact the retailer or find the guide online. (See References for a link to online product manuals.)
Open and close the doors on the spa cabinet. This is suggested if your hot tub is not heating.
Determine if the pilot light went out. If your hot tub is gas powered and the pilot light is out, relight it to see if it solves your problem.
Reset the hot tub time clock if the tub is not filtering and heating for a long enough period of time.
Repair or replace insulating cover. Damaged insulating covers may effect the heating of your hot tub.
Check to see if the high limit switch has been tripped. This switch is designed to shut down the heat on your hot tub once the temperature reaches or exceeds 120 degrees Fahrenheit. If the hot tub has been drained recently this switch may trip due to air in the lines, leading to heating problems. If you have recently emptied your tub it is recommended that you run the tub a few times and the problem should resolve itself.
Locate the red button on your spa pack. Press this button to reset the high limit switch. If this switch keeps tripping and you have not recently emptied your hot tub you may need to contact a repairman.
Determine if the spa thermostat has been calibrated too high. Check your product manual to determine if your hot tub can be re-calibrated by the screw located on the thermostat. If the hot tub is getting too warm it may need to be calibrated down. Some hot tub models can be re-calibrated down by turning the flat or small Allen type screw located on the thermostat a quarter turn clockwise. If this does not work the thermostat may need to be replaced.
Check to see if the hot tub temperature probe is pushed down into the thermal well. If the probe is sticking out of the water the temperature is not being accurately read. Push the temperature probe back into place. If you do not know where the probe is located, consult your owner manual. (For additional temperature probe repairs see References.)
Look inside the thermal well to see if there is any calcium or mineral building on the sides of the thermal well. If this is the case, you should replace the thermal well.
Determine if the water flow is slower than usual. Low or slow water flow can impact the heating of your hot tub. Check to see if the gate valves are fully open. Also replace the filter cartridge. Low water flow may also be a pump problem. (See Resources for pump troubleshooting tips.)
Deb Katula has written and researched for Societe Generale, FIMAT, Nikko Securities, Chicago Mercantile Exchange and Arthur Anderson. She holds an MBA in economics and finance from the University of Chicago; a Japanese language fellowship from Harvard; and a Bachelor of Arts in business/psychology/Asian studies from Augustana College.