A hot tub heater can be damaged very quickly if it is energized without enough water moving through it. This is commonly called a dry fire.
At Bath & Spa Parts Online, we see this problem repeatedly when spas are restarted after draining, opened for the season, refilled after service, or fitted with a replacement heater. The spa may look completely full, but if air is still trapped in the plumbing or the circulation pump has not been properly primed, the heater can still be sitting partially or completely dry.
The key point is simple:
A full hot tub does not automatically mean the heater is full of water.
Before turning the heater on, the plumbing must be filled, the pump must be primed, trapped air must be removed, valves must be open, and water circulation must be established.
What Is a Hot Tub Heater Dry Fire?
A dry fire occurs when the heating element becomes energized without enough water flowing across it.
Under normal operation, water continuously carries heat away from the element and transfers that heat back into the spa.
If there is little or no water around the element, the element temperature can rise extremely quickly.
This can damage:
- The heating element
- The heater tube
- High-limit components
- Sensors
- Gaskets and seals
- Wiring or terminals in severe cases
A badly dry-fired element may fail almost immediately or may suffer internal damage that causes premature failure later.
When Are Dry Fires Most Likely to Happen?
Dry fires are especially common immediately after the spa's plumbing has been emptied or opened.
Pay extra attention after:
- Refilling a drained hot tub
- Opening the spa for the season
- Replacing a heater
- Replacing a circulation pump
- Replacing a main therapy pump
- Cleaning or replacing plumbing components
- Replacing a filter or filter housing
- Opening unions or valves for service
- Winterizing and later restarting the spa
- Any repair that allows air into the plumbing
The most dangerous assumption is: "The tub is full, so the heater must have water in it."
That is not necessarily true.
1. Fill the Spa to the Correct Water Level
Begin with the water level recommended by the spa manufacturer.
If the water level is too low, the pump may pull air through the skimmer or suction area instead of maintaining a solid column of water.
This can cause:
- Poor circulation
- Air in the plumbing
- Pump cavitation
- Flow errors
- Loss of heater flow
Do not attempt to compensate for a low water level by simply running the pump longer.
Correct the water level first.
2. Open All Water Valves Before Startup
This is one of the easiest mistakes to make after servicing a spa.
Many hot tubs have slice valves, gate valves, or other shut-off valves around the:
- Pump
- Heater
- Filter
- Circulation plumbing
These valves are often closed during service to prevent water loss.
If one remains closed when the spa is restarted, the pump may run while little or no water passes through the heater.
Do not confuse water shut-off valves with air-control valves used by the spa's jet system.
3. Prime the Pump and Remove Trapped Air
This is probably the single most important step in preventing a dry fire after refilling the spa.
When a hot tub is drained, air replaces the water inside the:
- Pump housing
- Heater tube
- Suction plumbing
- Return plumbing
- Filter housing
Simply adding water back to the shell does not guarantee that every air pocket disappears.
The circulation pump must establish a solid flow of water.
How to Know the Pump Is Primed
Signs of proper priming usually include:
- Strong, consistent water flow from the jets
- No surging or sputtering
- No loud pump cavitation noise
- No repeated air bubbles coming from the returns
- No flow-related error code
Many spa control systems include a dedicated Priming Mode immediately after power is restored.
During that period, the pumps can be operated while the heater remains disabled.
Use that time to make sure the pumps are completely primed before allowing normal heating operation to begin.
4. Bleed Air From the Plumbing When Necessary
Sometimes a pump will not prime because a large air pocket is trapped in the plumbing.
This is commonly called an air lock.
Possible signs include:
- The pump runs but very little water moves
- The pump sounds louder than normal
- Jets sputter or produce mostly air
- The spa displays a flow error
- The heater will not engage
Some systems provide an air-relief valve at the filter housing.
On others, a technician may carefully loosen a pump or heater union enough to release trapped air before retightening it.
Always follow the spa or equipment manufacturer's startup procedure.
5. Confirm Actual Water Flow Before Allowing the Heater to Run
Do not rely solely on the fact that the pump motor is making noise.
A pump can be running while moving very little water.
Before heating begins, verify that you have real circulation.
Look for:
- Steady water movement
- Normal jet flow
- Normal return flow
- No obvious air lock
- No closed valves
- No flow or pressure error
If the circulation does not look normal, shut the system down and correct the problem before allowing the heater to operate.
6. Check the Filter and Other Flow Restrictions
A dry fire is not caused only by an empty heater.
Severely restricted water flow can also allow the element to become dangerously hot.
Common flow restrictions include:
- A dirty filter cartridge
- A collapsed filter
- A blocked suction fitting
- Debris inside the plumbing
- A partially closed valve
- A failing pump
- A clogged circulation line
If you are troubleshooting a low-flow condition, one useful test on many spas is to temporarily remove the filter cartridge and see whether normal circulation returns.
If flow improves significantly with the filter removed, the filter may be dirty, damaged, or excessively restrictive.
Do not operate the spa without the filter as a permanent solution.
If you need help identifying the correct replacement cartridge, use our Filter Finder.
7. Never Bypass Flow or Safety Devices Just to Make the Heater Run
Modern spa systems may use:
- Pressure switches
- Flow switches
- Temperature sensors
- High-limit sensors
- Electronic flow-detection logic
These components help prevent the heater from operating when the system detects an unsafe condition.
If the spa reports a flow error, the answer is not to defeat the safety device just to make the heat indicator come on.
Determine why the system is reporting inadequate flow.
What About Flow Switches and Pressure Switches?
The original version of this article recommended installing a flow switch as a general dry-fire solution.
That advice is too broad.
Some heaters are designed to operate with:
- A pressure switch
- A flow switch
- Temperature sensors
- Electronic heater sensors
- A combination of these systems
You should use the safety system designed for the specific heater and control system rather than adding an arbitrary flow switch.
A properly functioning pressure or flow device can help prevent heater operation when circulation is inadequate, but it must be compatible with the equipment and installed and adjusted correctly.
Do Modern Spa Controls Prevent Dry Fires Automatically?
They can reduce the risk, but you should not assume they make a dry fire impossible.
Many modern control systems include startup logic that keeps the heater disabled while the pumps are being primed.
Some also monitor:
- Water flow
- Pressure
- Temperature differences
- High-limit conditions
These are important safety features.
But they are not an excuse to ignore proper startup procedures.
A misadjusted switch, failed sensor, wiring problem, incorrect replacement part, or unusual plumbing condition can still create trouble.
What Are the Signs of a Possible Dry-Fired Heater?
A heater that has been dry fired may show one or more of the following symptoms:
- The spa no longer heats
- The heater trips the breaker
- The element has an electrical short to ground
- The heater tube shows discoloration
- The element has visibly blistered or ruptured
- There is a burnt smell near the heater
- A high-limit error appears
- The spa heats briefly and then faults
Not every heater failure is caused by a dry fire, so proper electrical testing may be required before replacing parts.
Replacing a Heater? Pay Special Attention on the First Startup
A replacement heater is especially vulnerable during the first startup because the heater tube has just been installed empty.
Before applying power:
- Complete the plumbing installation.
- Install the correct gaskets.
- Fully open all required water valves.
- Fill the spa to its normal operating level.
- Check the plumbing for leaks.
- Prime the pump.
- Remove trapped air.
- Confirm normal circulation.
- Only then allow the heater to energize.
Do not install the heater, fill the shell, and immediately flip the breaker on without confirming water flow.
Why Filling Through the Filter Area Can Help
Many spa manufacturers recommend placing the garden hose into the filter compartment or filter standpipe when refilling the spa.
On compatible systems, this can help introduce water directly into the suction plumbing and may reduce the likelihood of trapping a large pocket of air around the pump.
Follow the filling instructions provided by your spa manufacturer, because plumbing layouts vary.
Do Not Assume Silence Means Everything Is Fine
A dry fire does not necessarily produce dramatic warning signs immediately.
You may not see smoke, sparks, or obvious external damage.
Sometimes the first symptom is simply that a brand-new heater no longer works.
That is why proper startup procedure matters more than waiting for an alarm or visible problem.
Hot Tub Heater Dry Fire Prevention Checklist
Before restoring normal heater operation after draining, refilling, or servicing the spa, confirm:
- The spa is filled to the proper level.
- All required water valves are open.
- The filter is properly installed and not severely restricted.
- The circulation pump is primed.
- Any trapped air has been removed.
- Water is flowing normally through the jets or returns.
- The pump is not cavitating or running dry.
- No flow or pressure error is present.
- There are no plumbing leaks.
- The heater safety devices remain connected and operational.
- You have confirmed circulation before allowing the heater to energize.
Water first. Flow second. Heat last.
Before a hot tub heater is energized, make sure the plumbing and heater are full of water and that the pump has established normal circulation.
Already Dry-Fired Your Heater?
If the heater was energized with little or no water flow, shut off electrical power and do not continue repeatedly resetting the system.
The heater may need to be electrically tested to determine whether the element has failed or shorted to ground.
If the heater needs replacement, make sure the original cause of the low-flow condition is corrected before installing and energizing the new heater.
Otherwise, the replacement heater can suffer exactly the same failure.
Use our Heater Finder to help identify the correct replacement heater for your spa.
If you are not sure what you have, call Bath & Spa Parts Online at 800-918-9143. Heater labels, part numbers, measurements, and photos of the existing equipment can all help with identification.
The Bottom Line
Most hot tub heater dry fires are preventable.
The biggest risk occurs when a heater is energized before the plumbing has been completely filled and the circulation pump has been properly primed.
After any drain, refill, heater replacement, pump replacement, or plumbing repair, take the extra few minutes to verify that the system is actually moving water.
Do not assume. Verify flow.
A few minutes spent priming the system can prevent destroying a brand-new heater.
This article provides general spa heater and circulation-system information. Startup, priming, plumbing, electrical, and heater procedures vary by manufacturer and model. Always follow the installation and startup instructions supplied by the spa, heater, pump, and control-system manufacturer. Electrical testing and repairs should be performed only by persons qualified to work safely on the equipment involved.