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Top 5 Hot Tub Heater Problems


Hot Tub Won’t Heat?

hot-tub-not-heatingA Hot Tub without heat, is … cold! And no fun for anyone. Hot Tubs are meant to be HOT, and if you’ve ever been in a Warm Tub, you know it’s just not the same.

If you’re in charge of the hot tub at home, you hear the complaints when the hot tub heater is not working properly. You need a quick solution to the problem of no heat, or not enough heat in your spa or tub.

Originally posted March, 2013, this post is New And Improved! More solutions to hot tub heater problems, and more pics and links to help you fix your hot tub heater!

Top 5 Spa Heater Problems


Low flow in your spa or hot tub is most commonly associated with a dirty spa filter. If your spa heater won’t heat, remove your spa cartridge and clean the filter(s), to see if you have a pressure or flow rate problem. Low water level, or a clogged pump impeller, closed valves, or clogged pipes or blocked spa drain covers. Some hot tub heaters have screens on the inlet side, to trap any debris that gets past the filter. You may notice reduced water flow coming into the spa or hot tub.

A spa heater relies on sufficient water flow to operate. A pressure switch, screwed into the heater chamber, senses when the water flow is too low to properly protect the heater. Pressure Switches break the electrical circuit powering the heater element, and the heater shuts down, and will begin to heat until proper water flow is established. Most pressure switches can be tested by using a jumper wire to connect the two wires together, bypassing the switch. Many spa pressure switches can be adjusted with a thumb wheel, or small slotted screw, to close the circuit at a particular pressure rating, for example, some are adjustable from 1 to 5 psi. You can make small adjustments to the pressure switch by turning the wheel or screw, but the factory calibrated setting is safest for your spa heater.

Some spa heaters use a flow switch instead, or in addition to a pressure switch. Flow Switches sense water flow, not water pressure, and when water flow is great enough, two paddles are pushed together, to close the circuit. When flow is less than required, the flow switch remains open and the heater will not operate. Some flow switches can be tested by using a jumper wire to connect the two wires together, bypassing the switch. Harwil type mechanical flow switches can also be unscrewed from the tee housing, and inspected for scale or corrosion. Flow switches are not adjustable, but sometimes do need adjustment – be sure that surfaces  are clean of scale, and the paddles are not bent. Harwil type flow switch paddles must also be perpendicular to the water flow for full operation.


Spa Thermostats and Temp Sensors shownThe thermostat is the dial that you turn to crank up the heat. Most new spas use a temp sensor connected to a circuit board, and the topside control panel is just a remote control. Spas of 20 years ago mostly used potentiometers and solid state probes and for spas 30-40 years ago, mechanical thermostats with a capillary bulb are common. If you have a thermostat “knob”, instead of a lighted red arrow, you can test your thermostat to see if the unit is faulty internally, or if the sensor bulb has become corroded.

For newer spas and hot tub heaters with a topside control panel, the thermostat is usually replaced by a temperature sensor, which plugs into the main circuit board on your spa controller, and the probe end slips into a thermowell. Inspect the cord and probe for damage, and be sure that it is plugged into the panel snugly. If your topside control panel is displaying incorrect water temperature, it’s likely a bad temp sensor, but if it’s not displaying properly or does not respond to input, or appears to be water damaged,  you could have problems with a topside control panel.


selection of Hot Tub High Limit Switches shownThe High Limit is another switch, similar to the pressure switch and thermostat discussed above, in that it is part of the safety circuit. Its purpose is to prevent a run-away spa heater – one that won’t shut off, and could overheat (OH) and create scalding water, or suffer a literal meltdown. High Limits are calibrated with a preset maximum temperature (e.g., the upper limit or high-limit), at which the switch will open, and short the electrical circuit carrying power to your spa heater element.

Some hot tubs use two high limits, one monitoring temperature inside the heater, and one outside of the heater. High Limit errors will normally display a HL code or OH code when they are the cause of the problem. On older spas, the red button will pop-out when the high limit has been reached. Nuisance spa high limit tripping can be related to low water flow (causing higher than normal heater water temperature), incorrect voltage, malfunctioning element, loose wire connections, damaged wires, and finally – a faulty high limit switch.


3 hot tub heater element styles shownHot tub heater elements are similar to an electric hot water heater element, and as such, spa heater elements burn out very quickly if operated without cooling water surrounding it. Hot tub elements can also be tested to determine if there is a short in the coating surrounding the heating element. Use a test meter set on Ohms Ω to measure spa heater element resistance. A good element should display 9-12 Ohms usually – but if it pegs to infinity, or keeps rising slowly, there is a short in the element and it should be replaced.

Spa heater elements can also develop a scale buildup, from hard water or sanitizing with salt systems. When a spa element develops scale on the outside of the element, it will reduce the element’s heat output, and could lead to element failure. Spa heater elements look like a stovetop heater coil, and as such, if the outer casing becomes cracked, a new element is needed. For hard water areas, using a sequestering agent to keep calcium scale in solution, and running alkalinity on the low side, 70-80 ppm, can reduce scale formation on heater elements.

diagram of spa heater showing leads, switches, unionsFor those of you with newer spas, you’ll often find that your spa heater element is housed in a sleek stainless steel chamber, with unions for easy removal. On these complete spa heaters, you can test the element, high limit and pressure switch for resistance, as measured in Ohms. When testing with a multi-meter or ampmeter, an “OPEN” is when the meter spikes to a high reading. A “SHORT” is when there is little to no activity on the meter. When there is no resistance, the current is leaving the circuit, known as a “short-circuit”.



This last category causes profound heartburn to many of our customers. These spa heater problems are causes that you normally don’t think about, but can be a quick solution, and isn’t that what I promised you? Check these causes of spa heater troubles first, for the quickest solution that will have you shaking your head.

  1. GFCI tripped. Look for the electrical outlet on your spa pack. The one with the red TEST button. If it’s popped out, push it back in firmly.
  2. Door Interlock open. Many spas have a cabinet switch or spa pack cover switch, to prevent operation unless doors are tightly closed.
  3. Spa Cover needs to be replaced. Warped, broken and ill fitting spa covers can allow as much heat to escape as is being put into the tub.
  4. Loose Wires – Connections must be tight and not oxidized. Chewed wires (rodents), melted wires or crimped wires are also possible.
  5. Blown Fuse – A power spike or surge, or other incorrect voltage may have sacrificed the fuse on a control board.


I have sincere hopes that this information has helped you heat your hot tub up again. If you’re still baffled, leave a comment below for me, or give our customer support hotline a call at 800-770-0292.


Happy Hot Tubbin’!


Daniel Lara


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