Hot Tub Not Heating Enough? 10 Reasons Why

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hot tub heat

If the water temperature is warm, but not quite as hot as you like - you've come to the right place! Some hot tubs can heat up to 105°, although the recommended maximum temperature for healthy adults is 104°.

Let's assume that there are no error codes on the display panel. Everything seems normal, but the water is not as hot as it normally is. At this point there are a few troubleshooting steps you can take to identify and fix the problem(s) in your hot tub.

1. Thermometer is Incorrect

First off, thermometers are not usually "precision instruments" and can give an inaccurate reading. While they generally provide a close range for reference, they may not be perfectly calibrated. Even digital display readings on your spa's control panel can be off by a few degrees (see #5 below).

2. Hot Tub Cover is Inefficient


An economy spa cover is not going to provide the same 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 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.

Finally, you have to keep the hot tub cover on the tub while heating, or the spa will never heat up. For extra heat trapping, use a floating spa blanket.

3. 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.

4. Outside Temperature Too Low

cold weather

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 4,000 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.

5. Bad Temperature Sensors


Modern spas use electronic temperature sensors and high-limit switches 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.

6. 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. If this is causing problems during cooler weather, you may want to turn the blower off.

7. Spa Heater Not Running Long Enough

Spas and hot tubs heat slowly - some as little 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 your hot tub 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.

8. Spa Filter is Dirty

Earlier in the article, we agreed to assume that there are no error codes. However, a dirty spa filter will usually 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. Clean spa and hot tub filters every 3-4 months and replace every 12-24 months to keep the hot tub water flowing and filtering well.

9. 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 again. Once heated, reset the time clock 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 air locked or drawing in air. Both of these conditions will cause a heater to overheat and shut off. You may need to hit the heater element reset button in this case.

10. 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 if this happens.

Add water regularly to your spa to keep the level from dropping too low and drawing air into the suction intakes.

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