How to Control an Air Source Heat Pump with Any Hot Tub Spa Pack

Last updated: September 2026

With energy prices where they are, it’s no wonder so many hot tub owners are looking for cheaper ways to keep their tubs warm. You could use the tub less, but a better long-term answer is an air source heat pump. On my own tub it saved about 66% on my running costs. I showed how in How I added an air source heat pump to an existing hot tub.

The question I get asked most is: “How does my spa pack control the heat pump?” In this post I’ll show you how to control an air source heat pump from any hot tub spa pack, with no complicated wiring, no relays and no need for matching brands. It’s all done with water flow.

Do you need a heat pump that matches your spa pack?

First, a common misconception. People often think the brand of the heat pump has to match the brand of the spa pack. Not true. I have a Balboa spa pack and a Comfortline air source heat pump, and they work perfectly together. I’ve also fitted a Comfortline to a friend’s Gecko system.

Brand-matched heat pumps do exist. Balboa’s Clim8zone range connects straight to certain BP packs and shows up on the topside. But they only work with specific pack and topside combinations, and a lot of existing tubs aren’t on the list. I’ve reviewed them in Balboa Clim8zone review and Balboa Clim8zone II Mini review. For everyone else, flow control is the way to go.

How flow control works

When your hot tub wants heat, the spa pack turns on the pump that feeds the heater. That’s the circulation pump, or pump 1 on low speed if you don’t have one. Normally the pack would then switch on its electric element. With a heat pump, you disconnect the element and let the heat pump do the work.

1. The pack calls for heat. The water is below the temperature on your topside, so the pack runs the circulation pump.
2. Water flows through the heat pump. Inside there’s a flow switch that senses the movement.
3. The heat pump waits, then checks its own thermostat. Most units wait for steady flow first. On the Comfortline it’s about 3 minutes. If the water is below the heat pump’s set point, the compressor starts and it heats.
4. The pack reaches temperature and stops the pump. No flow means the heat pump switches off too.

That’s it. The pack doesn’t know the element has been disconnected. It “calls for heat” exactly as before, and the heat pump answers. In my experience with Balboa and Gecko packs, you don’t get an error code on the topside.

Setting the two thermostats

The heat pump has its own thermostat, separate from your topside. This is the one thing to get right.

Say the heat pump is set to 40°C but you’ve turned the tub down to 30°C while you’re away. Every time the pack runs the pump for a filter cycle, the heat pump sees flow, sees water below 40°C and starts heating. That’s heat you didn’t ask for, and it adds a little to your running costs.

The fix: keep the heat pump and the topside at the same temperature. When you change one, change the other. If your heat pump has Wi-Fi (Comfortline sells a Wi-Fi adaptor), you can adjust it from your phone alongside the tub.

If you have a 24-hour circulation pump

Many tubs run the circulation pump around the clock. In that case the flow never stops, so the pack can’t switch the heat pump off by stopping the pump. The heat pump’s own thermostat becomes the one in control. That’s fine, but it means the heat pump’s set point is the temperature you’ll get, whatever the topside says. Set the heat pump to the temperature you want, and set the topside the same or a touch lower.

If you have one two-speed pump

On tubs with no circ pump, pump 1 runs on low speed to filter and heat. The principle is the same, but check the low-speed flow is enough to make the heat pump’s flow switch close reliably. I’ve covered this in Can I add a heat pump to a hot tub with a dual speed pump?. If you’re thinking of adding a circ pump, see How to add a circulation pump to a hot tub.

Balboa heat modes

On a Balboa pack in Ready mode, the pump runs regularly to check the temperature, so the heat pump gets plenty of chances to heat. In Rest mode, the pack only heats during filter cycles, so the heat pump will only run then too. Pick the mode that suits how you use the tub. There’s a full explanation in Balboa heat modes explained.

Keeping the element as a backup

Most people unplug the electric element completely. If you leave it connected, it will run alongside the heat pump whenever the pack calls for heat. That’s handy in a hard frost, when the heat pump’s output drops, but it uses more electricity and puts more load on your supply. Some people reconnect it for the coldest weeks only.

Powering the heat pump

Flow control means the pack doesn’t need to switch the heat pump on and off electrically. The heat pump just needs a permanent supply.

You’ll often hear that you can tap into the main incoming cables at the spa pack. I used to suggest that myself, but I wouldn’t now without an electrician checking it. The heat pump’s cable has to be protected by a breaker sized for it. A 2.5mm² cable run straight off a 32A hot tub circuit isn’t. Check the spec sheet: the Comfortline 12.8kW, for example, calls for a 15A breaker and 2.5mm² cable, with a maximum draw of 7.74A.

• UK: 30mA RCD protection (Type A as a minimum), SWA cable outdoors, a local isolator, and a registered electrician. New outdoor circuits are notifiable under Part P, according to Total Skills.
• USA: GFCI protection and a disconnect within sight, to NEC Article 680 and the manufacturer’s instructions.

There’s more in Hot tub wiring explained.

Why flow control is the best option for most tubs

• Simplicity: no relays, no control wiring and no brand compatibility to worry about.
• Flexibility: it works with almost any spa pack, old or new, Balboa, Gecko or anything else.
• Efficiency: the heat pump only runs when there’s flow and the water needs heat.
• Easy to reverse: reconnect the element and close the isolation valves, and the tub is back to how it was.

The trade-offs are small. The topside won’t show what the heat pump is doing, and you have two thermostats to keep in step.

Troubleshooting

Problem	Likely cause	Fix
Heat pump never starts	Not enough flow to close the flow switch	Check valves, bypass, filter and air locks
Heats when the tub is turned down	Heat pump set higher than the topside	Match the set points
Tub overshoots the topside temperature	24-hour circ pump, so the heat pump thermostat rules	Set the heat pump to the temperature you want
Never quite reaches temperature	Pump stops before the heat pump starts, or it’s undersized for the weather	Check the start delay and filter cycle lengths; use a good cover
Flow error on the heat pump	Flow too low, or starting and stopping	Shorten the pipe run, reduce bends, check the pump

Still stuck? Work through Help! My air source heat pump is not heating my hot tub or Help! My hot tub is not heating.

Ready to make the switch?

If you’re ready to go, the full install is in my step-by-step guide to adding an air source heat pump, and it’s worth reading 5 things to consider before you fit one. For the bigger picture, see my ultimate guide to air source heat pumps for hot tubs and how to choose the right one.

You can check out the air source heat pumps and connection kits here. In the USA, head to parts4tubs.com. Building a new tub? My DIY hot tub plans include the full plumbing layout, so you can plan the heat pump in from day one.

FAQs

Can any heat pump work with any spa pack?

Almost always, yes, if you control it by flow. The heat pump only needs to sense water moving when the pack runs its circulation pump.

Do I need a relay or control cable between the spa pack and heat pump?

No. The heat pump’s built-in flow switch does the job. Only brand-matched units like Balboa’s Clim8zone use a direct connection.

Which temperature should I set on the heat pump?

The same as your topside. If you have a 24-hour circulation pump, set the heat pump to the temperature you actually want, because it’s in charge.

Will my topside show an error with the heater disconnected?

In my experience with Balboa and Gecko packs, no. The pack still runs the pump when it wants heat.

Can I keep the electric heater as well?

Yes, but it will run alongside the heat pump whenever the pack calls for heat. That uses more electricity and needs a supply big enough for both.

Got a question about controlling a heat pump on your tub? Drop it in the comments below. I read them all.

Cheers,
Andi

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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