Heater Turns On but Won’t Heat

Why Your Heater Runs But Water Temperature Doesn’t Increase

If your pool or spa heater powers on—fan spinning, pump running, indicator lights on—but the water doesn’t actually heat, it usually means there’s an issue with flow, gas delivery, internal components, or environmental conditions.

This guide covers the most common causes and fixes.

⭐ 1. Water Flow Too Low for Full Heating Output (Very Common)

A heater may turn on even if flow is barely above the minimum requirement—

but with low flow, the heater cannot transfer enough heat to the water.

Signs:

✔ Heater says “Heating” but temp rises slowly

✔ Salt system shows “Low Flow”

✔ Pressure gauge low or unstable

✔ Weak spa jets

Fix:

• Increase pump speed to 3,000+ RPM for spa
• Clean filter
• Clean skimmer/pump baskets
• Make sure all valves fully open
• Fix air leaks on suction side

Most “heater running, but no heat” cases are actually flow issues.

⭐ 2. Gas Supply Too Low (Gas Heaters)

If gas pressure is insufficient, the heater will ignite but produce weak heat or cycle.

Possible causes:

✔ Propane tank low

✔ Undersized gas line

✔ Partially closed valve

✔ Gas regulator malfunctioning

Fix:

• Confirm propane tank level
• Open all gas valves
• Ensure no other major appliances are running off same line
• Have gas provider check pressure

Spa mode requires higher gas volume than pool mode.

⭐ 3. Heat Pump Running in Cold or Windy Conditions (Heat Pumps Only)

Heat pumps depend on warm outside air. In cold, windy, or rainy weather, they will run but barely heat the water.

Signs:

✔ Running normally but 0–1°F per hour

✔ “Low Ambient” warnings

✔ Outdoor temps < 55°F

Fix:

• Run spa later in the afternoon
• Use gas heater as backup (if available)
• Add a wind barrier or equipment screen

This behavior is normal for heat pumps.

⭐ 4. Thermostat Issue

The heater will not heat if the target temperature is near or below the actual temperature.

Fix:

• Set temperature 5–10°F higher than water temp
• Make sure you selected SPA mode if heating the spa
• Ensure automation isn’t overriding setpoint

⭐ 5. Heat Exchanger Dirty or Scaled

Scale buildup greatly reduces the heater’s ability to transfer heat.

Signs:

✔ Heater sounds normal but heats slowly

✔ Temp rises only a few degrees

✔ Visible scale in plumbing or equipment

Fix:

Requires professional cleaning or acid flushing.

⭐ 6. Internal Bypass or Check Valve Failure

A stuck bypass valve allows too much water to flow through the heater without absorbing heat.

Signs:

✔ Heater runs continuously

✔ Water temperature barely increases

✔ Large temperature difference between heater inlet/outlet

Often caused by debris or worn internal valve.

⭐ 7. Heater Cycling On and Off

If the heater ignites, runs briefly, then shuts down repeatedly:

✔ Flow is borderline

✔ Gas supply fluctuates

✔ Temperature sensors failing

✔ Thermal limit switches tripping

This creates the appearance of “no heat,” because the heater never runs long enough.

⭐ 8. Automation System Not Calling for Full Heat

Hayward automation may show the heater “ON,” but if:

✔ Wrong body is selected

✔ A schedule is overriding heat

✔ Freeze mode engaged

✔ Heater interlock set incorrectly

…the heater will not run consistently.

Fix:

• Confirm SPA or POOL mode is active
• Raise setpoint on both the app + heater unit
• Reboot automation panel

⭐ When to Contact Professional Pool Company

Call us if:

✔ Heater runs but temperature does not increase

✔ Spa takes more than 20–30 minutes to heat

✔ Heat pump runs constantly with little gain

✔ Propane or gas usage suddenly spikes

✔ Heater cycles on/off repeatedly

✔ Flow seems normal but heating is poor

We can diagnose:

• Gas pressure issues
• Faulty bypass valves
• Dirty heat exchangers
• Bad sensors
• Automation miscommunication
• Heat pump compressor problems

⭐ Summary

If the heater turns on but won’t heat, the most likely causes are:

• Low flow
• Gas supply issue
• Heat pump running in cold weather
• Thermostat set incorrectly
• Internal bypass failure
• Scaling inside heat exchanger
• Automation not requesting heat correctly

Most “no heat” situations trace back to flow or gas pressure.

⭐ Need Help?

If your heater runs but won’t warm the water, contact Professional Pool Company. We can diagnose the heater, flow, and automation to restore proper heating.

Updated on: 29/11/2025