Hot Tub Not Heating? A Step-by-Step Troubleshooting Guide for UK Owners

Canadian Spa Company

As the evenings draw in and garden temperatures drop, a hot tub that won't hold its temperature is one of the most common problems owners face. The good news is that many heating problems aren't heater failures at all. They usually come down to a setting, a dirty filter, low water or trapped air, and you can fix all of those yourself in a few minutes.

This guide takes you through the checks in the order we'd do them, from the simplest to the most involved. It also shows you where to stop and call a professional.

Safety first: Always switch off and isolate the power at the RCD or isolator before removing cabinet panels or handling equipment. Never open the control box yourself, because it contains no user-serviceable parts. Any electrical fault should be investigated by a qualified electrician or spa engineer.

Steam rising from a hard-shell hot tub with its control panel in the foreground and autumn trees behind

Step 1: Check the set temperature and heat mode

It sounds obvious, but it often catches people out after a power cut.

  • Set temperature: Make sure the set temperature on the topside control is higher than the current water temperature. The heater only runs when the water is below the set point.
  • Heat mode: Many control systems have Standard, Economy and Sleep modes, often shown as ST/STD, EC/ECN and SL/SLP. In Economy and Sleep, the heater only runs during the programmed filtration cycles, so the water can sit well below the set temperature for hours. If you want the spa to heat to temperature and stay there, set it back to Standard.
  • Heater indicator: Most panels show a heating light or icon while the heater is running. If it's on, give the spa time. On a fresh fill it can take many hours to reach temperature, and a long time in cold weather.

Step 2: Read the display for error messages

Your hot tub protects its heater by switching it off when something isn't right. Messages vary between control systems, but common examples include:

  • FLO or LF: low or restricted water flow through the heater
  • dr or dRY: the heater may not have enough water in it
  • HH or OH: overheat protection has been triggered

Look up the exact code in your owner's manual. Many are available on our Owners Manuals & Downloads page. Fix the cause before you clear the message. Our hot tub error codes explained guide covers each message in more detail.

A dr or dRY message means the control system hasn't detected enough water flowing through the heater. Once it appears, the spa shuts itself down and won't restart until you reset it manually. Before you do, check that the water level is high enough to flow freely over the filters, that the filters are clean, and that all jets and valves are open (Steps 3 to 5 below).

For an overheat message, don't use the spa. Leave the cover off so the water can cool, and follow the reset procedure in your manual.

Step 3: Clean or replace the filters

A clogged filter is one of the most common causes of poor heating. The heater relies on a steady flow of water across its element. When oils, lotions and debris block the pleats, flow drops, the control system senses it and the heater shuts off.

What to do:

  1. Isolate the power and remove the filter cartridges.
  2. Rinse them thoroughly with a garden hose, working between each pleat.
  3. For a deep clean, soak them overnight in a bucket of warm water with Canadian Spa Hot Tub Filter Cleaner. Then rinse well and let them dry completely before refitting.
  4. If the pleats are matted, discoloured or brittle, or the cartridge won't come clean, replace it. We recommend a new Glacier filter every six months.

If you're replacing filters, make sure you buy the right cartridge for your model:

For the signs that a cartridge needs replacing, read when to replace a hot tub filter.

You can see every size in our Hot Tub Filters collection.

Step 4: Check the water level and clear any airlocks

Water level: If the level drops below the filter intake, the pump draws in air instead of water, and the heater will cut out. Top up to the level shown in your manual. As a rule of thumb, the water should sit above the highest jet.

Airlocks: Air trapped in the pipework is common after draining and refilling. Signs include a pump that hums or runs while little or no water comes out of the jets, or water that surges and splutters.

To reduce the chance of an airlock, fill the spa through the filter housing with the hose. If you already have one:

  • Turn the jets on and off a few times, switching between low and high speed if your spa has a two-speed pump. This often pushes the air through.
  • If that doesn't work, follow the priming procedure in your manual. On many spas this involves bleeding air from the pump union. Always isolate the power first.

Step 5: Make sure valves and jets are open

Closed slice valves on either side of the pump or heater will restrict flow completely. With the power isolated, check that all valves are fully open.

Closing lots of individual jets can also reduce flow, because water backs up in the pipework. Open them all while you diagnose the problem. If the jets themselves feel weak, our hot tub jets troubleshooting guide covers the flow checks in more detail.

Step 6: Understand how your spa shares its power

Some spas are designed not to run every component at full power at the same time.

  • 13 amp plug-and-play spas: on many models, the heater pauses while the jets are on high speed, so the spa stays within the limits of the supply. After long jet sessions, let the spa heat with the jets off.
  • Portable spas: on some models, running the bubble blower stops the circulation pump and heater, and heating resumes when you switch the blower off.

Your manual will explain how your particular model behaves.

Heats up when idle but cools during use?

A small drop is normal. You'll lose a degree or two as soon as the cover comes off, because the air is cooler than the water. Most spas have a temperature sensor that picks this up within a few minutes and switches the heater back on to return the water to the set temperature.

If the temperature keeps falling during long sessions but recovers once the spa is left alone, nothing is broken. The spa is simply losing heat faster than the heater can replace it. To help it keep up:

  • Keep the cover on between sessions, and let the spa heat with the jets off after a long soak.
  • Use the air blower sparingly. It pushes cooler outside air through the water and takes a lot of heat out of the spa.

On some spas, depending on your electrical supply, an engineer can reconfigure the control system so the heater keeps running while the jet pumps are on, or fit a larger heater. Both draw more power, so a qualified electrician must first confirm that your supply cable and fuse are rated for the extra load. This is never a DIY job, and you should never open the control box yourself.

Step 7: Stop heat escaping through the cover

If your spa heats but can't hold its temperature, or takes longer every week, the cover is the first thing to inspect. Heat rises, and a cover that's waterlogged, split or no longer sealing lets it escape fast.

Warning signs:

  • The cover has become noticeably heavier.
  • The middle sags or dips.
  • You can see steam escaping at the hinge or edges.
  • The vinyl is torn.

A heavy cover usually means the foam core has absorbed water, and a waterlogged core insulates poorly. If yours is past its best, browse our Replacement Hot Tub Covers. To protect a cover that's still in good condition from rain, UV and winter weather, fit a Weather Guard / Spa Cap sized to your cover. On the Swift Current portable spa, a Floating Thermal Blanket adds an extra layer on the water's surface.

Step 8: If the RCD trips, stop and call a professional

A heater that has failed will often trip the RCD, either straight away or when the heater tries to come on. Don't keep resetting it. If the RCD trips again after one reset, leave the power off.

Our guide on a hot tub that keeps tripping the RCD explains what to note down before you call.

  • Ask a qualified electrician to test the supply and the installation.
  • If the supply checks out, contact us to arrange a service visit.

Testing heater elements and measuring whether the control pack is supplying the correct voltage to the heater terminals are jobs for a qualified electrician or spa technician, not for owners.

Prevention: Keep your heater healthy

Most heating problems can be prevented with a good routine:

  • Clean your filters regularly and replace them when they're worn. For timings, see our guide on how often to clean your hot tub filter.
  • Keep your water balanced. Test regularly with 3-in-1 Test Strips, which check chlorine, pH and total alkalinity. Water that's out of balance encourages scale. If your water has turned cloudy, foamy or green, see our cloudy, foamy or green hot tub water guide.
  • Protect against limescale. Calcium deposits on the heater element reduce efficiency and can shorten its life, especially in hard-water areas. Use Control (Stain and Scale Inhibitor) to help keep calcium in check. Our hard-water hot tub guide explains how much to dose and how to keep pH in range.
  • Look after your cover. It's your spa's main defence against heat loss.
  • Book an annual service. Our Spa Gold Full Maintenance Service includes a pipe flush and a filter soak. Our technician also electrically tests the pumps, blower and heater and checks the circuit board.

Quick diagnosis table

Use this to jump to the most likely step for what you are seeing:

What you notice Most likely cause Go to
Display shows a lower set temperature than you expected Set point changed or economy/rest mode on Step 1
An error code on the display Flow, sensor or overheat fault Step 2
Heating slowly, weak jets as well Dirty filter restricting flow Step 3
Not heating after a refill Airlock or low water level Step 4
Some jets dead, heat not rising Closed valves or jets Step 5
Heats when idle, cools while you soak Power sharing on a 13 amp model Step 6
Heats overnight, loses it by morning Worn or waterlogged cover Step 7
RCD trips when the heater comes on Heater or electrical fault Step 8, then call an engineer

Heating FAQs

How fast should my hot tub heat up?

As a rule of thumb, about 1 to 2°C an hour with the cover on. Our guide on how long a hot tub takes to warm up gives figures by model.

Can I keep using the tub while it is not heating?

If the water is still clean and balanced, yes, but do not keep resetting a tripping RCD. If the RCD trips, leave the power off and call an electrician.

Will a new filter fix my heating problem?

Often, yes. A clogged cartridge is one of the most common causes of slow heating and flow errors. Glacier filters are best replaced every 6 months for best performance, or up to 12 months for economy, and sooner with heavy use.

Is a heater fault covered by warranty?

It depends on your model, its age and how it has been looked after. Call us on 01293 824094 with your model and any error code.

Still not heating? We're here to help

If you've worked through every step and your hot tub still won't heat or hold its temperature, it's time for an expert. At this point the cause is likely to be a faulty component, such as the heater itself, the circulation (low-speed) pump, the pressure switch or the thermostat, and these need diagnosing by a spa engineer. Visit our Hot Tub Servicing page or contact us. Have your spa model and any error code on the display ready.

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