Hot tub / spa won't heat

A cold tub is a letdown on a winter night, but the most common cause is simply low water flow — a dirty filter or a closed valve — and that's a clean, free fix before anything electrical is on the table.

tuneWhat are you noticing?

Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.

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In Canada, start with water flow — it's the most common reason a spa won't heat. Pop the filter and hold it to the light: if the pleats are grey and packed, flow is choked and the heater's flow switch won't let it fire. Confirm every valve is open and that the pump is moving water without gurgling. Cleaning or replacing the cartridge, opening the valves, and purging trapped air is a clean, near-free fix.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order

Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check the flow

With the pump running, watch the jets and the return: strong, steady flow is fine; weak or sputtering flow points to a restriction or trapped air.
Turn the spa off, lift out the filter cartridge, and hold it to the light. If the pleats are grey and packed rather than open and white, it needs cleaning or replacing.
Check that every water valve (often near the pump/heater) is fully open — a valve left closed after a clean or refill is a common flow-killer.
If the pump gurgles or runs dry, purge the air lock: follow your spa's "priming" routine, or briefly loosen the pump union to bleed trapped air, then retighten.

buildgarden-hose schedule20 min

handymanHow to fix it

Clean or replace the filter cartridge, confirm every valve is open, and purge any trapped air so the flow switch sees good flow and lets the heater fire again.

Turn the spa off at the panel, lift out the filter cartridge, and rinse it between the pleats with a garden hose. If it stays grey or is more than a season or two old, replace it with the same model number.
While the filter is out, check the housing and skimmer for debris, and make sure every water valve near the pump and heater is fully open.
Reinstall the clean or new cartridge, refill to the correct level if low, and restart the spa.
If the pump gurgles or won't prime, purge the air: run the spa's priming mode, or briefly crack the pump union to bleed the air lock, then retighten and confirm strong, steady flow at the jets. The heater should now fire.

schedule30 min savings$0–30

Spa filter cartridgeMatch the exact model number printed on the end cap (diameter, length, and hole/thread type all vary)

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Cleaning or replacing a spa filter, opening valves, purging air, and adjusting panel settings are routine homeowner maintenance in every province.

visibilityHow to check the reset

report Reset once — never bypass a GFCI that keeps trippingA hot tub runs on 240 volts in a wet location, and the GFCI is the device that will cut the power in a fraction of a second if current starts leaking to ground — that is what stands between a fault and a shock in the water. Resetting a one-time nuisance trip is fine. Resetting a GFCI that trips again and again, or wiring around it to "make it stay on," is not: it removes the protection and can be lethal. If it won't hold, leave it off and call a licensed electrician.

Find the GFCI breaker or spa-panel disconnect that feeds the tub. If the handle is in the middle/tripped position, the heater has no power.
Locate the spa's high-limit reset — usually a small button on the heater or control pack, sometimes behind an access panel. If it has popped, it has cut the heater on over-temp.
Reset each one ONCE: push the GFCI firmly to OFF then ON, and press the high-limit button until it clicks. Then watch the tub come up to temperature.
If it holds and heats, you're done. If the GFCI or high-limit trips again, stop — do not keep resetting it. A repeat trip is a real fault and a call to an electrician or spa tech.

buildNo tools schedule5 min

verifiedWhy this one goes to a pro

The heating element, its relay, the flow switch, and the circulation pump all live in the spa's powered equipment bay, on a 240-volt circuit surrounded by water. Working in there means real shock and electrocution risk, and getting the bonding or GFCI protection wrong afterward leaves that risk hidden. Have a licensed spa/pool technician or electrician diagnose and repair it — they can confirm the fault quickly and leave the tub safe. Your job stops at the filter, the panel, and a single reset.

gavel Check your province first AB: licensed professionals only · BC: licensed professionals only · ON: permit + inspection required · PE: licensed professionals only · QC: licensed professionals only
Line-voltage work on a bonded, GFCI-protected 240V spa circuit is licensed-only; confirm the provincial electrical authority's rule before publishing any DIY framing.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

verifiedWhy this one goes to a pro

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

sort

Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.