Hot Tub Not Heating Enough? 10 Reasons Why
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If the water temperature is warm, but not quite as hot as you like, you've come to the right place! If your hot tub is not heating properly, there are a number of things that could be going on. Whether it's low pressure, power issues, faulty parts, or even insulation issues, it's important to identify the root of your heating problem.
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. There's an Issue With the Heater
If your heater's not heating, the first thing to check is the heater itself. Most hot tub heater elements have a lifespan of about 5-10 years. Check wires and connections to the heater, and ensure all connections are tight. Look for signs of frayed, chewed, or burned wires, and make sure that there is power coming to the heater. If you notice that the coating around the filament of the heating element has any cracks or chips, the element should be replaced.
DIY TIP: Rule out power problems by testing for voltage coming into your spa pack, then verify that the voltage is reaching your heating element. Sometimes a bad contactor can prevent voltage from reaching the element.
If you are comfortable testing voltage, you can check the volts on the heater terminals with a multi-meter or AC volt meter. Set the meter to AC volts, with a setting higher than 240V. With the thermostat turned up and the pump running on high, touch or clip each meter lead to each terminal (at the same time), and you should see 220 volts or something pretty close (215-225V). If you get 220V but no heat, this usually means that the heater element is bad. If you don't get any volts while testing the heater leads or terminals, check that GFCI outlet or circuit breakers have not tripped. Make sure the thermostat is turned up. If you still get nothing, the circuit board is likely bad.
Another thing is to check for continuity. With power off, place your multi-meter on the lowest Ohms setting, and touch each lead to each terminal at the same time. If the meter doesn't move, the element is bad. If the meter does move and shows steady resistance, the element is probably good, but there may be a short. Again, inspect the element for cracks or chips.
2. Hot Tub Cover is Inefficient
In some cases, if it seems like your spa or hot tub is not heating enough, the problem could actually be your your cover. 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 economy level 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 steam seeping 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 cover on the tub while it's heating. Otherwise, your spa or hot tub will never heat up. For increased heat trapping efficiency, use a floating spa blanket.
3. Thermostat is Not 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°. Although some hot tubs have safety measures in place to prevent this from happening, it 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 is Too Low
Some spas and hot tubs are just not able to overcome low outside temperatures, which can make it seem like it's not heating enough. Especially for 110V plug-in portable spas, or spas built without a lot of insulation, a small 1-3 kw spa heater can not warm the water fast enough to overcome heat loss. This is also true for spas and hot tubs that have small heater elements under 4 Kw or 4,000 watts. 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, improving insulation underneath the spa shell, and even wrapping the outside of a wood hot tub can all help to compensate for low air temperatures. Spa heaters can also be upgraded and up-sized.
5. Bad Sensors or Switches
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 the sensors aren't calibrated and are off by a few degrees, a temperature sensor or thermostat can shut off the heater, thinking the spa is hotter than it is. Occasional tripping of the high limit switch is a bit of a nuisance, but not overly uncommon. However, frequent tripping issues can indicate low flow, a malfunctioning heater element, or sometimes even a bad high limit switch.
When your spa filter, suction intakes, or return jets are clogged, blocked or obstructed, the pressure switch will stay closed and keep the heater from working. The same happens if your pump is on low speed, or if valves are closed. Many spa pressure switches are adjustable from 1-5 psi, and when the internal diaphragm fails it requires replacement.
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 often 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 heat up as little as 1ºF per hour, although most can do 2-4ºF per hour. If the timer is not set to run long enough each day, the heater can have trouble keeping up. This is especially true when outdoor temperatures are low.
To bring your water temperature up to speed when it seems like the hot tub isn't heating, run the circulation pump and heater continuously on high speed (some heaters will not operate with the pump on low speed). 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. 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. In this case, you may need to hit the heater element's reset button. 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.
DIY TIP: If your filter is clean and in good condition, try troubleshooting other reasons for low flow rate in your hot tub. If your circulation pump has a strainer basket, check to make sure it's clean. Obstructed suction inlets, clogged pipes, or clogged pump impellers can all cause flow problems, as can obstructed outlets, closed valves, closed jets, and air in the pump. If all else fails, low flow errors are sometimes the result of a faulty pressure switch (see #5 above).
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. If this happens, you may need to hit the heater element reset button.
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 if it 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 so your hot tub can start heating again.
Add water regularly to your spa to keep the level from dropping too low and drawing air into the suction intakes.
There you have it! When you're having problems with cool hot tub or spa water, these 10 issues are likely to be the culprit. If you're looking for more information about various issues with hot tub heaters, take a look at our popular blog post, "Top 5 Hot Tub Heater Problems."