In August you flipped the spa on, went inside for a drink, and it was ready by the time you came back. Now it’s the end of October, the sun is gone before dinner, and the same spa heater has been running for an hour and a half while the water sits at “fine, I guess.” Most of the time that’s not a dying heater. It’s the plumbing around it, and that’s the kind of thing our pool repair and equipment upgrade work spends a lot of fall chasing down.

A spa built into an inground pool is a different animal from a standalone spa. It shares the pump, the filter and the heater with the pool. A couple of valves decide where the water goes. When any of that sharing goes a little wrong, the heat you’re paying for ends up somewhere you’re not sitting.

How one spa heater serves two bodies of water

On a combo, there’s one heater on the pad. In pool mode, water comes out of the pool, through the pump, filter and heater, and goes back to the pool. In spa mode, two valves turn: the suction valve pulls from the spa, and the return valve sends the water back to the spa. The spa overflows into the pool over the spillover edge, and the pump pulls it back from the spa again.

If the valves only go most of the way, the spa heater is quietly working on the pool too.

The math on a spa heat-up

A British thermal unit, per the U.S. Energy Information Administration, is the heat it takes to raise one pound of water one degree Fahrenheit. The USGS puts a gallon of tap water at about 8.3 pounds.

Say your spa holds 500 gallons. That’s roughly 4,150 pounds of water. Say you have a 400,000 BTU gas heater running at 84 percent, which is what Pentair lists for the MasterTemp’s standard copper heat exchanger. That’s about 336,000 BTU an hour going into the water, which on paper is around 80 degrees an hour.

Nobody’s spa heats at 80 degrees an hour. The gap between paper and reality is losses: heat into the pipes, heat off the surface, and on a combo, heat running downhill into a pool that holds many times more water than the spa. If the spa used to heat in 30 minutes and now takes two hours, something in that list got bigger.

Valve actuators that stop a little short

Most combos switch modes with motorized valve actuators, the round gray motors bolted on top of the three-port valves. Jandy’s JVA actuator runs on 24 volts and stops where its cams tell it to stop. Jandy’s manual is blunt about what happens when the cams or the valve are off: “On a pool/spa combination, the problem would be spa draining or overflowing.”

An actuator that stops 20 degrees short of where it should will still look like it’s in spa mode. It isn’t. Some water is coming from the pool, and some heated water is going back to it. The other common failure is a stripped or broken diverter inside the valve, where the motor turns and the valve doesn’t. Jandy lists that one in the same troubleshooting chart.

Valves and actuators are one of the regular parts in our post on pool equipment that actually breaks, and they’re usually a cheaper fix than anything with a burner in it.

Too much spillover is heat running into the pool

A spillover looks great. It’s also an open drain from your warm spa into your cold pool, and every gallon that goes over the edge takes its heat with it.

On a lot of pads, there’s a manual valve or an automation setting that splits the spa return between jets and the spillover. If somebody cranked the spillover up in summer because it looked nice, it’s still cranked up. A gentle sheet over the edge is plenty. A waterfall is a heating bill.

Heat pumps and cold Ventura County nights

If your combo heats with a heat pump instead of gas, cold weather hits twice. The Department of Energy’s page on heat pump pool heaters says they’re most efficient heating air above 50°F and start to lose efficiency below that. A clear December night in Simi Valley or Moorpark gets there a lot faster than a marine-layer night in Ventura.

The DOE lists standard heat pump sizes as 75,000, 100,000 and 125,000 Btu. Run the biggest of those through the same math and it’s a fraction of what a big gas heater puts out, before the cold air takes its cut. Heat pumps are great at holding a pool’s temperature cheaply. Spinning a spa up from cold on a cold night is not what they’re best at, and our post on the cheapest way to heat a pool here gets into that tradeoff.

Wind, the blower and an uncovered spa

The Department of Energy calls evaporation “by far the largest source of energy loss” in pools and says a cover can save 50 to 70 percent on heating. Say the spa is at 102 and the air is in the 50s. That spa is steaming off heat the whole time, and a Santa Ana wind across the surface makes it worse.

The air blower doesn’t help either. It pushes outside air through the water. Great once you’re in, a bad idea while you’re heating. Leave it off until the spa is up to temperature.

Low flow makes the heater quit early

Heaters need a minimum amount of water moving through them. Pentair’s MasterTemp manual lists 40 GPM as the minimum for the 400 model and 20 GPM for the 175 and 200, and the factory pressure switch is set at 3 PSI. When flow drops under the minimum, the heater shuts off or never fires.

A dirty cartridge is the usual reason flow drops in fall. Leaves, sycamore fluff and a summer of sunscreen all end up in the filter. If your heater keeps cycling off with a flow or pressure message, start with the signs your pool filter is going bad before anything else.

Variable speed pumps add a wrinkle. A pump set to a low, quiet speed for pool filtering may not move enough water for the heater in spa mode. The spa mode needs its own speed setting, high enough to keep the heater happy.

When it really is the spa heater

Sometimes it is the heater. A scaled-up heat exchanger, a weak igniter, a gas valve or a control board can all make a heater slow or unreliable. Error codes, repair costs and the age where replacing beats fixing are all in our post on whether a pool heater is worth fixing, so we won’t repeat it here.

The quick test: if the heater fires, stays lit and the water coming out of the spa returns is hot, the heater is doing its job and the problem is somewhere else.

What to check, in order

Asked at the pad, every week

How hot should the spa go? Pentair’s heater manual says spa water should never go above 104°F, and it notes that spa thermostats can be off by as much as 4 degrees. Set it at 102 and check it with a thermometer.

Should I just keep the spa warm all week? On a combo, keeping the spa hot means keeping the heater and pump running in spa mode for long stretches. For most people who use it on weekends, heating it a couple of hours ahead is cheaper. If you’re on our weekly and monthly pool maintenance, we look at those schedules at the pad anyway, so ask.

Can I use the spa heater to keep the pipes from freezing? No. Pentair’s manual says plainly not to use the heater to protect pools or spas from freezing. That’s a job for the automation system’s freeze protection, which runs the pump.

Why does the heater keep running after I turn the spa off? It shouldn’t, but the pump should. Pentair says the pump should run for at least 5 minutes after the heater shuts off, so the heat exchanger cools down with water moving through it.

Send us the details before the first cold weekend

If your spa heater is crawling now that it’s cold, take a short video of the valves turning when you switch to spa mode, a photo of the filter pressure gauge, and a photo of the heater’s label. Send those through our contact page and we can usually tell whether it’s a valve, a filter or the heater before we roll a truck.

We work on Pentair, Jandy, Zodiac, Hayward and Sta-Rite equipment, or call (805) 586-2055.