If the water temperature is warm or hot-ish, but not quite the skin searing temperature you like – you’ve come to the right resource.
When your spa heater takes too long to heat the hot tub, or if your hot tub won’t get hot enough like it used to – here’s the troubleshooting steps to take. Some Hot tubs can heat-up to as high as 105°, although the recommended temperature for healthy adults is 104°.
Let’s Assume that you are receiving No Error Codes on the display panel. Everything seems normal, but the water is not as hot as it used to be, in the past.
Thermometer is Incorrect
First Off, thermometers can be wrong – they are not usually “precision instruments”. Even digital display temp readings can be wrong (see #5 below), and off by a few degrees. A digital spa thermometer can be considered more accurate than the rubber ducky spa thermometers.
Hot Tub Cover is Inefficient
An economy spa cover is not going to provide the type of heat trapping efficiency of thicker and denser spa covers. The R-value of the best spa covers can be 3x the R-value of a basic spa cover.
Secondly, as spa covers age, they can start to take on water, and sag in the middle. other covers can begin to rip on the edges or along the fold. If you see any steam leaking out of the sides of your spa cover, this can be enough heat loss to reduce overall spa temperature.
Thermostat is Mis-Calibrated
On older gas-fired spa heaters, and old hot tubs with mechanical thermostats (without any digital panel display), the spa thermostat can be adjusted. These thermostats have a copper wire and capillary bulb used to sense the water temperature. On the end of the switch is a 1/8″ hex head adjustment screw. Turn it 1/4 turn clockwise, and give it a few hours to see how high the temperature rises.
Test water temperature before using and be careful not to raise the temperature above 104° – which is possible to do on some hot tubs. Adjusting the set point too high can be dangerous or unhealthy for spa users. It’s also possible that the thermostat is defective, they don’t normally just go out of adjustment by themselves.
Outside Temperature Too Low
Some spas are just not able to overcome low outside temperatures. Especially for 110V plug-in portable spas, or spas built without a lot of insulation, a small 1-3 kw spa heater can not heat up fast enough to overcome heat loss.
Also true for spas and hot tubs that have small heater elements, under 4 Kw, or 4000 watt. The fact is – less expensive spas will have more trouble keeping up with low outside temperatures.
Using a top quality spa cover, floating spa blanket and improving insulation underneath the spa, even wrapping the outside of a wood hot tub, can all help to compensate and correct for low air temperatures. Spa heaters can also be up-sized.
Bad Temperature Sensors
Modern spas use electronic temperature sensors and high-limits to constantly check water temperature, inside and outside of the spa heater. These are connected by wires to a plug-in on the main control panel.
On digital spa packs, you will usually see an Error Code (Sn, Sn1, HL, Hot, OH), when a temp sensor is causing the heater to shut off, but if they are off a few degrees, a temperature sensor or thermostat can shut off the heater, thinking the spa is hotter than it is.
Using the Air Blower
Using a forced air blower or opening the air intake knobs will always cool the water, because the air temperature is much colder than the water temperature.
Spa Heater Not Running Long Enough
Spas and hot tubs heat slowly, some as low as 1 degree per hour, although most can do 2-4 degrees per hour. If the timer is not set to run long enough each day, it can have trouble keeping up, especially with low outside temperatures.
To bring it up to speed, run the circulation pump and heater continuously. It can take up to 24 hours – depending on starting water temperature, outside air temperature, spa cover efficiency and most importantly, the size of your spa heater.
Spa Filter is Dirty
Earlier in the article, we agreed to assume that there are no error codes – and a dirty spa filter should produce an Error Code (FL, Flo, FL1) if the pressure switch is sensing low flow and keeping the heater off.
You can remove the spa filter (spa cartridge) to see if flow improves because of a dirty spa filter. You may need to hit the heater element Reset button in this case. Replace spa and hot tub filters every 12-24 months to keep the water flowing and filtering well.
Spa Was Just Drained and Refilled
For spas that have been drained and refilled, you may want to run the heater continuously for a day or two, until the water gets hot, then reset the timeclock to run for 4-8 hours daily, or as much as it needs to maintain most of the heat.
Also, be sure that the spa circulation pump is fully primed, and not in an air lock condition, or drawing in air – both of which will cause the heater to overheat and shut off. You may need to hit the heater element Reset button in this case.
Spa Water Level is Low
If your spa skimmer begins to draw in a steady stream of air in a vortex inside the skimmer, or gulps down air because of a stuck skimmer door or thermometer, this will cause the heater to overheat and shut down. You may need to hit the heater element Reset button in this case.
Add water regularly to your spa to keep the level from dropping too low and drawing air into the suction intakes.