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Hot Tub Not Heating After a Refill? Here’s Why + How to Fix

A hot tub not heating after a refill is a common problem, irrespective of the brand, model, and age of your spa or jacuzzi. Generally, the problem is simple and thus easy to solve, but you may also have a complicated issue. So, I will discuss all the typical causes and the respective fixes. 

A hot tub not heating after a refill is most likely due to an airlock caused by air trapped in the plumbing system. A hot tub may also have clogged filters or a blockage in the pipes. Otherwise, check the pump, heater, heat sensor, thermostat, and flow and high limit switches. 

Check for an error code displayed on the control panel and if the indicator light is on. Having an error code eliminates guesswork, but you may not have any, and the indicator light might be off. Keep reading as I explain the causes and easy fixes for your hot tub not heating after a refill.  

Why Your Hot Tub Is Not Heating After a Refill

More than a dozen components must work perfectly for your hot tub to start heating after a refill. While any critical part may fail, it is practical to investigate the typical issues before inspecting the complicated problems. Most of the common issues have inexpensive and easy fixes.

Here are the reasons why your hot tub is not heating after a refill: 

Your Hot Tub Has an Airlock in the Plumbing System

Hot tubs have a self-contained plumbing system. The network of pipes, pumps, heater, jets, and diverter valve should be filled with water, but nothing else, not even air. 

Tiny air bubbles are not a problem in this recirculatory unit because hot tub pumps can generate more than sufficient pressure to eliminate them. However, the plumbing system cannot have an airlock. 

An airlock is essentially trapped air inside the plumbing system that effectively blocks the flow of water, thus incapacitating the pump and disabling the heater. Generally, airlocks happen when you refill a hot tub or fill it for the first time. 

Here is how your hot tub develops an airlock when you refill it with water:

  1. Your hot tub had water throughout its plumbing system before you drained it. 
  2. As you drained the water out and its level dropped, air seeped into the jets, through the filters, and eventually the drain holes.
  3. When the hot tub has no water, air will find its way throughout the plumbing system, even to the pumps.
  4. As you refill the hot tub, the water flows into the plumbing system through the drain holes, jets and filter tubes.
  5. The water level in your hot tub rises, but the air inside the plumbing system is still there.
  6. The water pressure increases as you refill the entire hot tub, thus pushing the trapped air further into the plumbing. 
  7. Eventually, all the trapped air blocks a part of the plumbing, usually at the pump. In some cases, the airlock is at the pipe connected to the heater. 
  8. This airlock deprives the pump of the water flow necessary for it to work. The impeller of a water pump is not designed to work with air nor circulate it.
  9. Therefore, your hot tub pump won’t work if there is an airlock. The hot tub’s heater will not start working without sufficient water flow and a functioning pump.
  10. If the airlock is next to the heater, that will also prevent your hot tub from heating. The heater will not start if the flow switch cannot sense water pressure due to the airlock.    

Now, you may encounter a few variations from this standard process of how an airlock forms. 

  • In some hot tubs, the pump may function, but the heating still won’t work after a refill. In this instance, you need to investigate other problems. 
  • Hot tubs with two pumps may have one functioning alright. However, the second pump may have an airlock. 
  • Many hot tubs have a circulation pump. Like the primary pump, this low-power unit can also have an airlock.

If all the pumps are running as expected and the jets & waterfall are working gloriously, you will have to inspect the other components of your hot tub. 

The Water Flow Through the Pump and Jets Is Obstructed

In some cases, the water flow through the pump and jets is obstructed due to a clogged filter and other blockages in the plumbing system. So, even if you do not have an airlock, check that the water pressure is optimal through all the jets.

If the water pressure through the jets and waterfall is weak, you may have a clogged filter, or the plumbing has some debris buildup. You should also check the skimmers to see if they are dirty. 

You should consider such inspections before you suspect another critical part is preventing your hot tub from heating after a refill. 

One of the Critical Hot Tub Components Is Not Working

A hot tub not heating after a refill could be due to a:

  • Broken flow switch
  • Faulty heat sensor
  • Fried thermostat
  • Blown high limit switch
  • Bad heating element

The indicator light on the control panel could be off or flashing, and an error code should tell you if there is a flow, dry, sensor, or other typical problems with your hot tub.

However, your hot tub was probably working until recently, and it is not heating only after a refill. Thus, you should try to remedy a probable airlock and address the water flow problems before testing the switches, sensors, and other elements.  

7 Easy Fixes for Hot Tub Not Heating After a Refill

You may need only one of the following easy fixes depending on the causal problem in your case. 

However, there are instances when an airlock is not the only issue. Older hot tubs can also have failing or malfunctioning parts, and it is possible that a new spa or jacuzzi may have a faulty part, too. 

1. Use the Pump’s Bleed Valve To Resolve an Airlock

An easy way to fix an airlock is by using the bleed valve on the hot tub pump. Many modern pumps have at least one bleed valve. Some pumps have two or three. 

Here are the steps to use the bleed valve to resolve an airlock:

  1. Refer to your hot tub manual to easily locate the pump.
  2. Remove the housing panel to access the pump.  
  3. Look for the bleed valve on the pump, typically a threaded screw.
  4. Take a flathead screwdriver and rotate the valve to loosen it a little, or use a pair of pliers or a wrench if you think a screwdriver will damage the plastic.
  5. There will be a hissing sound when the trapped air escapes.
  6. You may notice some water drip out through the bleed valve.
  7. A little water pouring out is a sign that the airlock is no longer there.
  8. Wait till the hissing sound stops, and you feel some water pressure.
  9. Tighten the bleed valve to restore the seal on the pump. 

Repeat this process if you have another pump. If you have a circulation pump, check if it has an airlock, and follow the same procedure.

Here’s a fantastic video tutorial from The Hot Tub Shop to help you find and use the pump’s bleed valve:

However, some hot tub pumps do not have a bleed valve. So, you need another way to purge the air out of the plumbing system. 

2. Loosen the Pump’s Lock Nut To Purge the Air

Check your hot tub pump, and you will find a lock nut. Generally, the lock nut is aligned vertically and upwards from the pump or horizontally and sideways in a hot tub. The pump’s lock nut may be white, black, or gray.

Your hot tub plumbing system has more than one lock nut. However, you need to use the lock nut on the pump. Additionally, pumps can have two lock nuts. So, you must select the lock nut that is at a higher point.  

Here are the steps to use the pump’s lock nut to purge the air:

  1. Turn the pump’s lock nut to loosen it.
  2. Use Channellock pliers if the nut is stuck too tightly.
  3. There will be a hissing sound when the airlock dissipates.
  4. Some water will drip as all the air is forced out.
  5. Wait till you do not hear the hissing sound any longer.
  6. Tighten the lock nut snugly.

Here’s a quick video from Jasyn Love to help you prime the hot tub pump:

You can use this method to purge air out of any lock nut, even those connected to the heater. However, the process works best when you choose the highest lock nut in a plumbing setup. Air rises when you relieve the pressure in a sealed system, even if it is an airlock.   

3. Clean the Filters or Replace Them

Ideally, you should clean or replace the filters when you refill a hot tub. A dirty or clogged filter is often the reason for a hot tub not heating, regardless of a refill. Filters with a lot of gunk impair the water flow and pressure, thus affecting the heater and pump functions. 

4. Clean the Skimmers and Flush Out Debris

Like the filters, clogged skimmers will adversely affect the water flow and pressure. Inspect and clean the skimmers.

You may have debris stuck inside the plumbing if your hot tub does not start heating even after you eliminate an airlock. This problem could be evident when you turn on the pump, and a few jets don’t function normally, or the water flow is weak. 

You can use a plunger to flush out any debris stuck in the tubes or pipes. Some people use an appropriately sized plunger to get rid of airlocks, too. However, purging trapped air at the pump is much easier than trying to resolve an airlock through the suction of manual plunging.

In rare instances, a hot tub may have a rigid blockage inside the plumbing. The only option in such cases is draining the refilled hot tub and using a pressure washer or another powerful way to flush the system.  

5. Increase the Heat Setting To Test the Heater

Suppose you have eliminated the airlock, the filters and skimmers are clean, and the hot tub has no blockage in the plumbing. If the heater still does not work, check the temperature or heat setting on the control panel.

Increase the heat setting by a few degrees. Sometimes, a hot tub heater does not start heating when the heat setting is only a degree or two higher than the water temperature. 

6. Replace the Faulty Switch, Sensor, or Thermostat

If none of the easy fixes explained till now resolves your problem, you may have a faulty flow switch, heat sensor, high limit switch, or thermostat. 

Here’s how you can test a hot tub flow switch:

  1. Turn off the power and circuit breaker.
  2. Disconnect the flow switch from the two wires.
  3. Connect the two ends with a short wire (same gauge) to complete the circuit.
  4. Turn on the circuit breaker and power.
  5. Select a heat setting on your hot tub.
  6. See if the indicator light turns on.
  7. If the heating works, you have a faulty flow switch.
  8. Otherwise, the flow switch is fine, but another component is bad.

You can test the resistance of a heat sensor and high limit switch to know if they are working. However, both these components have safety implications. A high limit switch is essentially a safety feature that shuts off the heater when the system overheats. 

Thus, you should consult a certified technician to test and replace the high limit switch, heat sensor, transformer, thermostat, or control board.

7. Replace the Heating Element if It Is Not Working

Hot tub heaters do not last forever. So, the control board, thermostat, and all the switches, sensors, and pumps may be working fine. Still, your hot tub will not heat after a refill due to a bad heating element. In this case, the only solution is to replace the heater altogether. 

However, reset the heater to see if it works before considering a replacement. Most hot tub heaters have a reset feature, a membrane, button, switch, or control panel option. Check the manual for your hot tub and reset the heater.

Always Refill a Hot Tub Through the Filter Tube

You can prevent an airlock and other flow or pressure issues in a hot tub during refill. 

Do not refill a hot tub from the bottom up. Instead, put the hose into the filter tube. This way, you will have water flowing into the plumbing system before refilling the hot tub. The water flowing into the pipes will flush all the trapped air out through the jets and drain holes of the hot tub. 

Therefore, you won’t have an airlock, and your hot tub will start to heat after a refill. 

Here’s a comprehensive video tutorial from Spa Man to help you refill a hot tub perfectly:

Final Thoughts

Many people operate the pump and switch from low to high speed and back to eliminate an airlock from the hot tub. You may try this method, but there’s a genuine chance of damaging the pump, a much costlier scenario than the easy fixes I have shared to eliminate an airlock. 

Always refill your hot tub through the filter tube with all the jets open and the diverter valve in its neutral position. This approach will prevent an airlock in the first place. Therefore, your hot tub will not have an issue with heating after a refill.


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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