Air Source Heat Pump Not Heating Your Hot Tub? Fixes to Try

Last updated: September 2026

It’s frustrating when you’ve been looking forward to a soak and your air source heat pump isn’t heating the tub. The good news is that it’s usually something simple, and you can often fix it yourself without paying for a call-out.

Air source heat pumps have surged in popularity, and because they’re still fairly new for hot tubs, there isn’t much good documentation out there. I’ve even had people contact me after an installer set their system up wrongly. I don’t know everything (though I like to think I do!), but I have plenty of hands-on experience with heat pumps and hot tubs, so here are the causes I see most often.

Air source heat pump not heating: it isn’t installed correctly

Installing a heat pump isn’t difficult, but if you or your installer get it wrong, it won’t heat your tub. If you’re still planning the job, my step-by-step guide to adding an air source heat pump shows the right layout.

Direction of flow

Heat pumps have a specific flow direction. Water from the tub must enter through the inlet (usually marked as the cold or “in” side) and leave through the outlet (the hot or “out” side), then return to the tub. If it’s the wrong way round, you’ll get a flow error on the display. On my unit that’s an E03, and other makes use different codes, so check the manual.

To work out the direction, trace the flow back to the pump. The pump draws water from the tub through its front intake and pushes it out of the top or side outlet. That’s the direction of travel. The connection order should be:

• Heat pump installed after the pump: Pump, then pipe, then heat pump inlet, then heat pump outlet, then back to the jets in the tub.
• Heat pump installed before the pump: Hot tub, then heat pump inlet, then heat pump outlet, then back to the pump.

Flow error or air lock

The next thing to suspect is an air lock. It’s a common cause if you’ve just installed the heat pump or recently refilled your tub. To check:

1. Is your circulation pump running?
2. Look for water movement in the tub. If there’s no visible flow, you probably have an air lock.

To clear it:

1. Turn off the hot tub at the breaker.
2. Loosen the unions at the heat pump until water starts to come out. You should hear the air escaping.
3. Do the same at the pump.
4. Turn the tub back on and check the flow. If the flow error has gone, that’s it.

If it persists, undo the union at the lowest point of the heat pump (with the tub turned off) and let a couple of buckets of water drain out. If all else fails, drain the tub and refill it through the highest point of the system, such as the filter or the top of the heat pump. There’s more on this in my article on air locks in a hot tub.

Air source heat pump not heating: the bypass valve is open

This is one of the most common and easiest-to-fix problems, and it’s the one people contact me about most.

The bypass on a heat pump setup relieves pressure if it gets too high, which can happen on single-pump systems where the jet pump is very strong. In that case, the gate valve needs to be open to give the jets proper flow. Under normal conditions, though, the gate valve should be fully closed, which forces all the water through the heat pump. Water always takes the path of least resistance, so if the gate is open, the water bypasses the heat pump and your tub won’t heat.

1. Trace the direction of water flow.
2. Make sure ALL the water is passing through the heat pump by closing the bypass valve.

If you have a single-speed jet pump and you keep running into flow or pressure problems, read about using an air source heat pump with a dual speed pump. A proper connection kit also helps (I stock one with a flapper valve for single-pump systems).

Air source heat pump frozen up or too much ice

If it’s cold outside, you may see ice on the outside of the unit. That’s completely normal. The heat pump pulls heat out of the air, and moisture in that air can freeze on the coil.

A thick build-up is different. Possible causes include:

• A malfunctioning defrost system or sensor
• Poor air flow, for example a blocked fan or a unit boxed in with no space around it
• Low refrigerant (for a qualified engineer to check, as it needs the right equipment and certification)

Too much ice stops air passing over the coil, so the unit struggles to collect heat and becomes less efficient. Use the manual defrost mode from time to time to clear it. See how to defrost a Comfortline for a worked example.

Air source heat pump not heating: it’s too cold outside

The warmer it is outside, the easier it is for the unit to collect heat. When temperatures drop below freezing (late 20s °F or lower), things get much harder. If your unit is only rated down to +5°C (41°F) and it’s -5°C (23°F) outside, don’t expect much heat.

Different models perform very differently in the cold. I tested my own unit, which is only rated to 0°C (32°F), in sub-zero temperatures, and it still worked (see the -4°C results). But that’s my experience of one unit, and it isn’t guaranteed for every model. Check your unit’s rating first. If you’re choosing a new one, read how to choose the right air source heat pump. My own Comfortline is listed as 20kW (formerly 21kW), and its extra capacity helps it cope when the air is cold.

Other things to check

Check	What to look for
Power	Is the breaker on and the unit showing on the display?
Set temperature	The set point needs to be above the water temperature, or the unit won’t run.
Mode	Make sure it’s on heating, not cooling or auto set to cool.
Spa pack control	The spa pack may need to call for heat and run the circulation pump. See how to control a heat pump with any spa pack.
Filter	A blocked filter cuts flow and can trigger a flow error. See cleaning a filter cartridge.
Fan and coil	Clear leaves and debris from the air inlet and fan.
Slow to heat, but working	Compare with the unit’s real output at your air temperature. It might just be undersized or cold.

If you’re still stuck after the checks above, I’m happy to help through the comments or my YouTube channel. For the wider heating picture, see hot tub not heating and the ultimate guide to air source heat pumps for hot tubs.

Need a heat pump or the parts to fit one?

I sell heat pumps, connection kits and fittings through parts4tubs.co.uk in the UK and parts4tubs.com in the USA. Planning a new tub with a heat pump built in? My hot tub plans give you the dimensions, plumbing and a parts list to build around.

FAQs

Why is my heat pump running but the water isn’t getting warm?

Usually because water is bypassing it (open bypass valve), the flow is low, or it’s too cold for the unit’s rating. Close the bypass, clean the filter and compare the unit’s output at today’s air temperature.

What does the flow error on my heat pump mean?

It means the unit isn’t detecting enough water flow. The usual causes are an air lock, a closed valve, a blocked filter, the connections the wrong way round or a circulation pump that isn’t running.

Is ice on a heat pump normal?

A light frosting in cold, damp weather is normal, and good units defrost themselves. A thick layer that isn’t clearing means a defrost, airflow or refrigerant problem.

How cold can an air source heat pump work?

It depends on the model. Some are rated to +5°C, some to 0°C and some to -10°C or lower. Always check the spec sheet for your unit’s rating and performance at low temperatures.

Can I fix a refrigerant leak myself?

No. Refrigerant work needs a qualified engineer with the right equipment and certification.

Got a question about your own tub? Drop it in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi