Pool Heater Not Working: The Quick Troubleshooting Checklist

If your pool heater isn't working, start by checking these five things in order: water flow, power supply, thermostat settings, gas or electrical supply, and water chemistry. About 80 percent of pool heater failures are caused by one of these, and most can be fixed without a service call. Work through this checklist before you book a technician - you'll either solve it yourself or show up with a useful diagnosis when the tech arrives.

Why Pool Heaters Fail: The Short Version

Pool heaters look complicated, but they're actually pretty logical machines. They need water flowing through them at the right rate, fuel or electricity to generate heat, and water chemistry that won't corrode or scale the internals. When any one of those three things is off, the heater either won't fire at all, will fire and shut right back off, or will run without actually warming the water. Most troubleshooting comes down to figuring out which category you're in.

Gas heaters (natural gas and propane) have a few extra failure points - pilot lights, ignitors, gas valves - that electric heat pumps don't. Heat pumps have their own quirks, including an outdoor air temperature floor below which they simply won't operate efficiently. Knowing your heater type before you start saves time.

Step 1: Check Water Flow First

Low water flow is the single most common cause of pool heater problems, and it's the one most people skip past. Every pool heater has a flow sensor or pressure switch that shuts the unit down if water isn't moving fast enough - this is a safety feature, not a flaw. If your filter is dirty, your pump basket is clogged, or you have partially closed valves somewhere in the line, the heater will either refuse to start or will fire and immediately shut off.

Check these things in order:

1. Look at your filter pressure gauge. If it reads 8-10 PSI above the clean baseline, clean or backwash the filter.
2. Pull and empty the pump strainer basket and the skimmer basket.
3. Confirm all valves between the pump and heater are fully open.
4. Make sure the pump is actually running - not just humming, but moving water.

If you've been chasing pool heater issues for a while and flow keeps coming up as the culprit, it may be worth revisiting your overall equipment sizing - an undersized pump for the plumbing layout is a recurring headache.

Step 2: Confirm Power and Thermostat Settings

Before assuming anything is broken, rule out the obvious. Check that the heater's breaker hasn't tripped - reset it once if it has, and watch whether it trips again immediately. A breaker that won't stay on points to an electrical fault that needs a licensed electrician, not a reset. For gas heaters, confirm the on/off switch on the unit itself is in the on position - this sounds too simple, but someone bumps it off more often than you'd think.

Set the thermostat at least 5 degrees above the current water temperature. If the pool is already at your target temp, the heater won't kick on, and that's normal behavior, not a malfunction. Also check whether the heater is in "pool" mode vs "spa" mode if your controller manages both - wrong mode selection accounts for a surprising number of "broken" heater calls.

Step 3: Gas Supply and Ignition (Gas Heaters Only)

If you have a gas heater and confirmed good flow and power, check the gas supply next. Make sure the gas shutoff valve at the heater is open. If you're on propane, check the tank level - a tank that reads "20 percent" in cold weather can actually be too low to deliver adequate pressure. Try another gas appliance in the house to confirm supply is live.

For ignition issues, most modern gas heaters use electronic ignition rather than a standing pilot light. If the heater is clicking but not firing, the ignitor may be dirty or failed, or the gas valve isn't opening. You'll often see an error code on the display. Look up that code in your owner's manual or on the manufacturer's website - specific codes narrow the problem fast. Ignitor replacement is a DIY-possible repair on many models, but anything involving the gas valve itself should go to a certified tech.

If you smell gas near the heater, stop. Turn off the gas supply and call your gas utility or a licensed professional. Don't try to troubleshoot an active gas leak yourself.

Step 4: Heat Pump-Specific Issues

Heat pumps extract warmth from outdoor air, which means they have a hard floor on operating temperature. Most residential pool heat pumps stop working efficiently below 50 degrees Fahrenheit - some units won't run at all below 45 degrees. If it's cold outside and your heat pump isn't keeping up, that may simply be the physics of the equipment, not a failure. Check whether your unit is sized correctly for your climate - if you've been wondering about that, our post on what size pool heater you actually need covers the climate side of that decision in detail.

If the outdoor temp is fine and the heat pump still won't run, check that the unit has clear airflow around it (12 inches of clearance on all sides), that the fan is spinning freely, and that the evaporator coil isn't iced over. A frozen coil usually means low refrigerant - that's a refrigerant technician job, not a DIY fix.

Step 5: Check Your Water Chemistry

This one catches people off guard, but water chemistry can absolutely knock a heater offline. Low pH - anything below 7.2 - creates corrosive water that attacks copper heat exchangers and can trigger safety sensors. High calcium hardness above 400 ppm causes scale buildup inside the heat exchanger that restricts flow and cuts efficiency dramatically over time. Maintaining pH between 7.4 and 7.6 and calcium hardness between 200 and 400 ppm isn't just good for the pool - it directly protects the heater. AquaDoc's pH reducer and calcium hardness products are calibrated for pool volumes specifically, which makes the dosing math easier when you're correcting a significant imbalance.

Low cyanuric acid (CYA) levels can also cause chlorine to burn off fast and leave water chemistry unstable, but that's a secondary issue. The ones that directly harm heaters are pH and calcium hardness.

Reading Error Codes Before Calling a Tech

Modern pool heaters display error codes for a reason - write yours down before calling anyone. The most common codes across brands relate to: ignition failure (gas), low flow, high limit (overheating), and sensor faults. Your owner's manual will have the full list. If you've lost it, search the manufacturer's name plus your model number and "error codes" - most manufacturers have the documentation on their site. Showing up to a service call with the specific error code can cut diagnostic time significantly and save you money on labor.

If you're still not sure whether your heater is the right size for your pool to begin with - which can cause chronic underperformance that looks like a failure - the pool heater sizing calculator on this site is a good starting point before you invest in repairs on a unit that was undersized from day one.

Frequently Asked Questions

Why is my pool heater running but not heating the water?

The most common cause is low water flow. The burner fires but the water moves through too quickly or the heat exchanger can't transfer enough energy. Clean your filter, check pump basket clogs, and confirm all valves are fully open. If flow is fine, the heat exchanger may have scale buildup from high calcium hardness.

Why does my pool heater keep shutting off?

A heater that cycles on and immediately shuts off is usually tripping a high-limit safety switch. This happens when water flow is too low, the unit is overheating, or there's a faulty sensor. Start with the filter and pump basket, then check for any closed valves restricting flow in the system.

Can bad water chemistry stop a pool heater from working?

Yes. pH below 7.2 creates corrosive water that damages copper heat exchangers and can trigger shutoffs over time. Calcium hardness above 400 ppm causes scale inside the heat exchanger that blocks flow and reduces efficiency. Keep pH at 7.4 to 7.6 and calcium hardness between 200 and 400 ppm to protect the unit.

What does an error code on my pool heater mean?

Error codes vary by manufacturer and model, but common ones flag ignition failure, low flow, high limit, and sensor problems. Check your owner's manual for the specific code - most manufacturers also post documentation online. Having the exact code ready when you call a tech speeds up diagnosis considerably.

When should I call a pool heater technician instead of fixing it myself?

Call a licensed tech if you smell gas anywhere near a gas heater, if the breaker trips again immediately after resetting, if the heat pump has a frozen coil (low refrigerant), or if the heat exchanger is visibly corroded or scaled. Gas lines and refrigerant systems are not DIY territory.

Most pool heater problems have a boring, fixable cause - a dirty filter, a tripped breaker, a thermostat set below the water temp. Work through this checklist in order and you'll either find the fix or arrive at a service call knowing exactly what you've already ruled out. That alone is worth the 20 minutes it takes to go through it.