Why Your Pool or Spa Won't Heat (and Whether It's Worth Fixing)
You flip the spa on for an evening soak, come back twenty minutes later, and the water's still cold. The pump's running. The lights are on. But nothing's warming up.
I get this call more than almost any other, and it almost always lands the same way: everything looks like it's working, so it's confusing when it isn't. The heater is the one part of your system you never think about until the day it quietly stops doing its job.
Here's the thing most people don't realize — your pool and your spa usually run off the same heater. So when it goes down, you don't just lose a warm soak. You lose the ability to extend your swim season on either end. That's why I treat heater problems as their own category, separate from the usual water and equipment stuff.
Let me walk you through what's actually going wrong when the heat won't come, what our hard water out here does to these units over time, and how to tell whether you're looking at a quick fix or a replacement.
How your heater is supposed to work
You don't need to be a technician, but it helps to know the basics so the failures make sense.
Most pools and spas around here use one of two setups. A gas heater burns natural gas or propane and pushes that heat into the water as it flows through. A heat pump pulls warmth out of the air and transfers it into the water — slower, but cheaper to run. Standalone hot tubs usually heat with an electric element, more like a giant kettle.
All three depend on one thing above everything else: water has to be moving through them properly. That single fact explains most of the no-heat calls I get.
The usual suspects when nothing heats up
When I show up to a pool or spa that won't warm, it's almost always one of these:
Low water flow. A dirty filter, a clogged pump basket, or a closed valve starves the heater of flow. Most heaters have a safety switch that simply refuses to fire if water isn't moving fast enough. The heater isn't broken — it's protecting itself. This is the most common cause, and the most overlooked.
A scaled-up heat exchanger or element. This is the big one in our area, and I'll come back to it. Mineral buildup coats the surfaces that transfer heat, so the unit runs and runs but the water barely changes. Eventually it overheats and shuts itself down.
Ignition or gas problems. On a gas unit, a bad igniter, a dirty flame sensor, or a gas supply issue means the burner never lights. You'll often hear it try — a click, a whir — and then nothing.
A failed thermostat or temperature sensor. If the heater can't read the water temperature correctly, it either won't turn on or shuts off early thinking it's already done.
A tripped high-limit switch. This is a safety cutoff. When it trips, it usually means something else went wrong first — low flow or scaling — and the heater got too hot. Resetting it without fixing the cause just sets you up to do it again next week.
Corrosion and age. Heaters live outside, take a beating, and have a real lifespan. Sometimes the honest answer is that the unit has simply run its course.
Why Placer County is hard on heaters
Here's the local piece nobody warns new pool owners about.
Our water is hard — high in calcium and other minerals — and hard water is rough on anything it heats. Every time water passes through a hot heat exchanger, a little scale gets left behind. Over a few seasons that scale builds into a layer that chokes off heat transfer. The heater works harder, runs longer, costs more to operate, and wears out faster. It's the single biggest reason heaters in this region don't last as long as the brochure promises.
It plays out a little differently depending on where you are. In the newer builds around Rocklin, the combination of municipal hard water and long stretches of triple-digit heat means equipment runs hard for months. Across Roseville, Lincoln, and the bigger backyards of Granite Bay, it's the same mineral story playing out on a lot of well-used pool-and-spa combinations.
And then there's well water. A lot of properties out in Loomis, Penryn, and Sheridan run on wells, and that water can carry even more minerals — plus iron and sediment that municipal supplies filter out. On those properties, scaling and corrosion show up sooner, and older pool equipment on rural lots tends to take the brunt of it.
None of this means you're doing anything wrong. It just means heaters out here need attention before they fail, not after.
A few things you can check before you call
I'd rather you save the money on a service call when the fix is simple, so here's what's worth checking yourself first:
Clean or backwash your filter. A dirty filter is the number-one cause of a heater that won't fire. Start here.
Empty the pump and skimmer baskets. Clogged baskets choke flow the same way a dirty filter does.
Check your valves. Make sure the valves directing water to the spa or heater are actually open and set correctly.
Confirm the gas is on at the unit and the supply line — and that the breaker hasn't tripped.
Look at the thermostat setting. It sounds obvious, but a bumped dial or a setting below the current water temperature gets people more often than you'd think.
If you've cleared all of that and it still won't heat, the problem is inside the unit — and that's where it's worth bringing someone out.
Repair or replace?
This is the question I get the moment a heater acts up, and the honest answer depends on a few things.
A heater that's a few years old with a bad igniter, a failed sensor, or a tripped safety switch is almost always worth repairing — those are normal wear parts. But once a unit is well past its expected life, or the heat exchanger is so scaled and corroded that it's leaking or barely transferring heat, you can end up pouring money into a part that's about to be outpaced by the next failure. At that point a replacement often costs less over the next few years than a string of repairs.
I don't make that call for you sight unseen. What I do is come out, diagnose exactly what's happening, and then tell you plainly which way I'd go if it were my own backyard. Our service calls are free — we schedule a time, figure out the real issue, explain it in plain language, and quote the repair before any work happens. No surprises, no pressure.
A few quick answers
Why is my spa not heating even though the pump is running? Usually low flow or a tripped safety switch. The pump moving water doesn't guarantee enough water is reaching the heater — a dirty filter or a half-closed valve will stop it from firing.
Is it worth repairing an older pool heater? If the failed part is a normal wear item and the unit is otherwise sound, yes. If the heat exchanger is heavily scaled or corroded and the heater's near the end of its life, replacement is often the smarter spend.
Why does my heater turn on and then shut off after a minute? That short-cycling is usually a flow problem or an overheating safety cutoff doing its job — almost always traceable back to flow or scale.
The bottom line
A heater that won't heat is rarely a mystery once you know where to look. Most of the time it's flow, scale, or a worn part — and out here in Placer County, hard water and long hot summers mean scale is doing quiet damage to almost every unit, whether you've noticed it yet or not.
If your pool or spa won't warm up and the basics check out, don't keep resetting it and hoping. Get it looked at before a small fix turns into a full replacement.
We handle pool repairs and spa repairs across Lincoln, Rocklin, Roseville, Granite Bay, Loomis, Penryn, and Sheridan, and we're happy to take a look. Give us a call at 916-975-7370 or reach out here — and if you'd rather just stop thinking about your equipment altogether, ask us about weekly service and spa service so problems like this get caught long before your water goes cold.
