Help! My Air Source Heat Pump Is Not Heating My Hot Tub!

Have you found yourself in a position where your Air Source Heat Pump is not heating your Hot Tub? I know it can be frustrating, especially on those days when you’re looking forward to a hot dip in your tub. Rest assured, in this article I will cover some of the possible causes for your air source heat pump not working and hopefully, by the end of this article you will be able to resolve the problem yourself. Hopefully, without paying for someone to come out to take a look at it because quite often, it is usually something simple.

With the global energy crisis upon us, Air Source Heat Pumps have surged in popularity. Unfortunately, since they are relatively new to the market, there is a noticeable lack of useful information and documentation available. In fact, I’ve even had people reach out to me after installers improperly set up their systems.

I’m not saying I know everything—though I’d like to think I do (yes, I’m that guy!)—but I don’t. However, I do have extensive experience with air source heat pumps and hot tubs, so hopefully, I can help you troubleshoot the issue either here on my blog or through my YouTube channel.

Air Source Heat Pump Is Not Heating My Hot Tub – not installed correctly.

The installation of an air source heat pump is not difficult. However, if you get it wrong, or your installer gets it wrong, then your air source heat pump will not heat your hot tub! Here are some key elements to the installation process.

Direction of flow

Installing an air source heat pump isn’t complicated. However, if you or your installer make a mistake, your heat pump won’t heat your hot tub! To avoid this, here are some key installation steps:

Direction of Flow

Air source heat pumps have a specific direction of flow. The water from the hot tub must enter through the “cold input”and exit through the “hot output”, then circulate back into the hot tub. It’s that simple. If the flow direction is incorrect, you’ll get an E03 or Flow Error message on your heat pump.

How to Determine the Direction of Flow

It’s easy. Just trace the flow back to the pump. The pump draws water from the hot tub through its front intake and returns it through the top or side outlet—this determines the direction of flow.

Correct Connection Order

• If your air source heat pump is installed after the pump, the order should be:
  Pump → Pipe → Cold Input → Hot Output → Back to Jets in the Tub
• If it’s installed before the pump, the order should be:
  Hot Tub → Cold Input → Hot Output → Back to Pump

Get this order wrong, and you’ll end up with a flow error.

Flow Error or Air Lock

The Next possible issue could be an air lock. If you’ve just installed your air source heat pump or recently refilled your tub, this is a common cause.

How to Check for an Air Lock

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

How to Fix an Air Lock

1. Turn off your hot tub at the breaker.
2. Loosen the unions at the air source heat pump until water starts coming out. You should hear air escaping.
3. Do the same at the pump.
4. Turn the hot tub back on at the breaker and check for flow. If the flow error is gone, the issue is resolved.

If the AirLock Persists

• At the lowest point of the air source heat pump, undo the union (with the tub turned off) and let a couple of buckets of water drain out. This usually clears the air lock.
• If all else fails, drain the tub completely. Then, refill it from the highest part of the system, such as the filter or the top of the air source heat pump.

Air Source Heat Pump Is Not Heating My Hot Tub – bypass valve open

This is one of the most common and easiest-to-fix issues when an air source heat pump either shows a flow error or fails to heat your hot tub.

Understanding the Bypass Valve

The bypass setup on an air source heat pump helps manage system pressure if it becomes too high—often an issue with single-pump systems where the jet pump is too strong. In this case, the gate valve should be open to allow proper flow to the jets.

However, under normal conditions, the gate valve should be fully closed. This forces all the water through the air source heat pump. Water always follows the path of least resistance, so if the gate is open, it will bypass the heater, preventing proper heating.

How to Fix It

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

That’s it—problem solved! Believe it or not, this is one of the most frequent issues I hear about from people contacting me.

Air Source Heat Pump Frozen up or too much ice

If it’s cold outside, you may notice ice forming on the exterior of your air source heat pump. This is completely normal.

Why Does Ice Form?

The heat pump extracts heat from the outside air and transfers it to your hot tub water. In cold weather, moisture in the air can freeze on the heat pump’s coils.

When Is Ice a Problem?

If there is excessive ice build-up, it could indicate an issue. Possible causes include:

• Poor insulation around the unit
• A malfunctioning defrost system
• Low refrigerant levels

Too much ice can prevent the heat pump from working properly, making it less efficient.

How to Prevent Excess Ice Build-Up

• Use the unit’s manual defrost mode periodically to remove excess ice.
• If ice accumulates too much, the heat pump will struggle to extract warmth because air must pass over the ice first, cooling it down and reducing efficiency.

Air Source Heat Pump Is Not Heating My Hot Tub – too cold outside

The ambient temperature plays a crucial role in how well an air source heat pump functions. The warmer it is outside, the easier it is for the heat pump to extract heat from the air. However, when temperatures drop to subzero levels (late 20s°F or below), things become more challenging.

Understanding Temperature Limits

If your unit is only rated to +5°C (41°F), but the outside temperature is -5°C (23°F), then the chances of it producing meaningful heat are low.

Different models perform differently in colder conditions. For example, I have tested my own unit—which is only rated to 0°C (32°F)—in subzero temperatures, and it still worked. However, this isn’t guaranteed for all models.

Check Your Unit’s Rating

If your air source heat pump isn’t heating, verify its temperature rating. This might be the reason it’s struggling.

To Sum it Up

If you’re still having trouble with your air source heat pump after trying these fixes, don’t worry—you’re not alone! Troubleshooting can take some time, but with the right steps, most issues can be resolved without expensive service calls. If you need further guidance, feel free to reach out, and I’ll be happy to help. 

Happy hot tubbin!

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

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