Upgrading to an Air Source Heat Pump: Chris’s Hot Tub Journey

Last updated: September 2026

An air source heat pump is a magical invention. It’s a far more efficient way to get your hot tub up to temperature. On my own tub it cut my running costs by 66%, and given the right circumstances it’s a really rewarding upgrade.

Fitting one can seem confusing, though, so here’s a real example. Chris got in touch with me by email, and we went through the whole process of adding an air source heat pump to his hot tub together, with a couple of interesting discoveries along the way.

Chris’s introduction

Chris emailed me to say he wanted to upgrade from his electric heater to an air source heat pump. As with all my clients, I was more than happy to guide him through it.

The first thing I need is pictures of the existing setup and the internal parts, like the one Chris sent in. Once I’ve reviewed them, I can tell you everything the project needs.

Chris decided he could do the upgrade himself. Armed with my plans and instructions, he ordered the air source heat pump and a connection kit from my parts4tubs.co.uk heat pump range. Once they arrived, he got started.

Note: be careful when choosing a connection kit, because there are lots of options for different setups and models. If you’re not sure which one you need, ask in the comments or get in touch, and I’ll help you pick the right one. If you’re still choosing the heat pump itself, start with how to choose the right air source heat pump for your hot tub or pool.

Chris’s setup

Chris’s existing setup has a circulation pump and a jet pump. The one we’re interested in is the pump that feeds the electric heater, because that’s where the air source heat pump connects.

The first thing we did was mark up the setup. I marked three pipes: purple shows where to cut the pipe, blue shows the cold feed to the heat pump, and red shows the hot water return from the heat pump.

I also sent Chris a diagram like the one below, matched to his layout, which describes the steps to install the heat pump.

For the full step-by-step, see can I add an air source heat pump to my hot tub? and how to add an air source heat pump to your hot tub.

Installing the air source heat pump

A few days after his order was delivered, Chris emailed me the picture below, wondering if it might be useful.

This was a surprise, because I’ve done hundreds of air source heat pump installations and had never seen a pre-plumbed bypass on a hot tub. Some newer hot tubs do come with this in place, which makes adding a heat pump later much easier.

After that discovery, the job got a lot simpler. Chris didn’t need to cut any pipes. All he had to do was plumb into the bypass. Once the pipes are in, you close the bypass and open the valves to the heat pump, and the water goes through it.

Don’t worry if your tub is an older model without a bypass. You can use your connection kit to cut one in.

After a few of the usual emails about the installation and setup, the emails stopped, and I thought that was another successful heat pump installation.

“I’ve got a problem…”

A few days later, Chris emailed to say he had a problem with his water flow. He was getting an E3 error on his control panel, which meant not enough flow was getting through to the heat pump.

This is usually caused by an air lock created during installation. To clear it, you open the highest point of the system and let the air out. In Chris’s case, though, water came out, meaning there was no trapped air. (If you do have an air lock, see how to clear a hot tub air lock.)

We were sure it had something to do with the heat pump or an air lock in the pipes, and tried lots of ways to fix it. One odd clue: the heat pump worked when Chris turned the jets on, but switched off again when he turned them off.

Eventually we worked it out. It wasn’t the bypass at all. The hot tub’s temperature wasn’t set high enough.

Why the set temperature matters

To understand this, you need to know how the heat pump is controlled. We control it indirectly, through the flow of water. The heat pump only runs when water is flowing through it, so the circulation pump has to be running to send water to it.

The circulation pump runs when the hot tub calls for heat. So the hot tub’s set temperature needs to be high enough to call for heat. When it does, the circulation pump comes on, pumps water to the heat pump, and the heat pump starts. If the tub’s thermostat is already satisfied, the circulation pump doesn’t run for heating and the heat pump sees no flow, which is why Chris’s only worked with the jets on.

In winter, we match the temperatures on the heat pump and the hot tub, and lower both, because heating costs more in the colder months. For other ways to link a heat pump to a spa pack, see how to control an air source heat pump with any hot tub spa pack, and if yours still won’t heat, help! my air source heat pump is not heating my hot tub.

The end result

Chris had no leaks and a newly installed air source heat pump. The setup looks great, and he now has a hot tub that heats up quicker and is a lot cheaper to run.

Thinking about a heat pump for your own tub?

My ultimate guide to air source heat pumps for hot tubs covers how they work, sizing and running costs, and 5 things to consider before fitting one will help you avoid the common pitfalls. You’ll find heat pumps and connection kits in the air source heat pump range on parts4tubs.co.uk, or on parts4tubs.com in the USA.

Building a new tub from scratch? Plan the heat pump in from day one. My DIY hot tub plans include the full plumbing layout.

FAQs

Why does my heat pump only run when the jets are on?

Usually because the hot tub isn’t calling for heat, so the circulation pump isn’t running and no water reaches the heat pump. Set the hot tub’s temperature high enough to call for heat, as Chris did.

What does an E3 error mean on a hot tub heat pump?

On Chris’s setup it meant not enough water flow through the heat pump. Error codes vary by make, so check your manual, but low flow is usually an air lock, a closed valve, a dirty filter or the pump not running.

Do I need to cut the pipes to add a heat pump?

Usually, yes. The connection kit lets you cut in a bypass. If your tub already has a pre-plumbed bypass, like Chris’s, you can plumb straight into it.

Got a question about adding a heat pump to 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