Can You Add an Air Source Heat Pump to a Hot Tub with a Dual Speed Pump?

Last updated: September 2026

Plenty of hot tubs run on a single pump. If you’ve got a 13A plug-in tub, that’s almost certainly you: one two-speed pump does the filtration, the heating and the jets. With electricity prices where they are, lots of those owners are looking at an air source heat pump to cut their bills, and the question I get is always the same. Can you add one to a tub with a dual speed pump?

I’ll let you into a secret. You can. But there are a few things to know before you take the plunge (see what I did there?), mainly around flow, jet power and the electrics.

What is a dual speed hot tub pump?

A hot tub with just one pump in the cabinet usually has a two-speed (dual speed) pump. On low speed it does the circulation: it pushes water through the filter and the spa pack heater and keeps the water moving during filter cycles. On high speed it drives the jets. It’s very common on 13A plug-and-play tubs that don’t need special wiring, and some older tubs were built with just the one pump too.

Bigger tubs usually have a separate small circulation pump for heating and filtering, and one or more jet pumps. If you’re not sure what you’ve got, open the service panel, turn the tub on and turn the temperature up. Whatever runs on its own is the pump (or the speed) that feeds the heater. There’s more on this in How many pumps do you need for your DIY hot tub?

How does the heat pump connect?

The layout is almost identical to a tub with a circulation pump. You cut into the return pipe just after the spa pack heater and take the water out to the heat pump and back again, with a bypass so you can control or isolate it. My step-by-step guide covers the cutting and gluing in detail: Can I add an air source heat pump to my hot tub?

The heat pump is controlled by flow, not by wiring. When the spa pack wants heat, it runs the pump on low speed. The heat pump’s own flow switch senses the water moving, and after a short delay it starts heating to its own set point. When the tub reaches temperature, the pump stops and so does the heat pump. That’s why it works with any spa pack. I explain it fully in How to control a heat pump with any hot tub spa pack.

On a single-pump tub, though, the same pipe that feeds the heat pump also feeds the jets. So my advice is to run 2in pipework right up to the heat pump and only reduce to 1.5in (or 50mm) at the heat pump’s own unions. We always want to keep as much flow as possible to minimise the impact on the jets.

The magic ingredient: a spring-loaded flapper valve

The key to making this work is a 2in one-way, spring-loaded valve (often called a flapper or check valve) fitted in the bypass line between the pipe to the heat pump and the pipe back from it.

• Low speed (heating and filtering): the pressure isn’t high enough to open the spring, so the valve stays closed and all the water passes through the heat pump. You get the full benefit of the heat pump.
• High speed (jets): the pressure builds, the flapper opens and extra water goes straight past the heat pump to the jets. The heat pump doesn’t see more flow and pressure than it likes, and you don’t lose as much jet power.

The best bit is that it’s all automatic. There’s no bypass valve to fiddle with each time you want the jets on. Some water still flows through the heat pump on high speed, so it can keep heating while you’re in the tub.

If you’d rather not source the parts one by one, we sell an air source connection kit with a flapper valve for single-pump systems on parts4tubs.co.uk. It’s the 2in kit with the reducers for a 1.5in heat pump connection, pipe, elbows, couplers, a bypass valve setup, cement and the flapper valve.

An in-cabinet alternative

If you have room inside the tub cabinet, the bypass can be built in there instead. The arrangement of the tees and the bypass valve below lets more water through to the jets when the pump is on high speed.

Check the low-speed flow first

Every heat pump has a minimum flow rate, and it won’t heat without it. On a single-pump tub, the heat pump has to work from the pump’s low speed, which moves a lot less water than high speed. So before you buy, compare the heat pump’s flow requirement with what your pump delivers on low speed.

As an example, the Comfortline 7kW wants 2–4m³ an hour (roughly 9–18 US gallons a minute), while the Comfortline 20kW wants 8–10m³ an hour. A big heat pump on a small low-speed flow will throw flow errors or refuse to start. If the numbers are close, keep the pipe run short, use gentle bends and keep the filter clean. If you’re not sure how to read your pump’s figures, see How to correctly size a hot tub pump.

The downsides

The main downside is that you’ll almost certainly reduce the flow to the jets a little. Physics says that adding more pipe, bends, valves and the heat pump’s heat exchanger to the circuit adds friction. Whether you notice it depends on how many jets you have and how the tub is plumbed, and unfortunately it’s not something you can check in advance. In my experience, nine times out of ten it isn’t noticeable.

If it is, you have a few options:

• Let the flapper do its job. Check it’s fitted the right way round and that the spring isn’t sticking.
• Open the bypass a little if you’ve used a manual valve. You’ll lose some heating efficiency, as bypassed water isn’t heated.
• Automate it. Experienced DIYers can fit normally closed motorised valves powered from the pump’s low-speed output, so the water goes through the heat pump on low and straight to the jets on high. It’s mains voltage work, so only if you’re competent, and get it checked. I describe it in my heat pump install guide.
• Add a circulation pump. This is the best long-term fix. The small circ pump feeds the heater and heat pump, and the main pump is left to do the jets. See How to add a circulation pump to a hot tub.

Setting the thermostats

The heat pump has its own thermostat, separate from your topside. On a single-pump tub, the pump runs on low for filter cycles and on high whenever you use the jets. If the heat pump is set higher than the topside, it will heat every time it sees flow, including filter cycles and jet sessions, and your tub can creep above the temperature you set. Keep the heat pump and the topside at the same temperature, and change both together.

There’s a nice side effect on 13A tubs. The spa pack’s own heater is locked out while the jets run on high, so the water cools during a long soak. A heat pump on its own supply keeps heating as long as water is flowing through it, so the tub holds its temperature much better in use.

Electrical requirements

This is the bit that gets a little tricky. In general, if you have a dual speed pump, you’re on a 13A tub. I’m generalising, and it won’t be the case for everyone, but that’s the usual rule.

A 13A supply is not enough to run the pump on high speed and a heat pump at the same time. That’s the same reason a 13A tub’s own heater can’t run while the jets are on. So the heat pump needs its own electrical supply. Either it goes on its own dedicated circuit, or the tub and the heat pump share a bigger, properly designed hard-wired supply (typically 32A) with the right protection for each. It should never be run off the tub’s 13A plug.

You could power the heat pump from the heater connection on the spa pack, but I don’t advise it, even though it appears to work. That output is switched, so every time the tub stops calling for heat, it kills the power to the heat pump mid-cycle and it can’t go through its proper shutdown procedure. Depending on the model, that can shorten its life. Let the unit “do its thing”. Give it a permanent supply and let the flow switch turn it on and off.

Check the heat pump’s spec sheet for the breaker it needs. The Comfortline 7kW, for example, recommends an 8A breaker with a 30mA RCD, and the 20kW recommends 23.5A. In the UK, a new outdoor circuit is notifiable under Part P, so use a registered electrician. In the USA, follow NEC Article 680 and your local permit rules. There’s more in A guide to hot tub electrics and Hot tub wiring explained.

Common mistakes

• Buying the biggest heat pump without checking the low-speed flow. If your pump can’t feed it on low, it won’t heat.
• Using 1.5in pipe all the way. Keep the runs at 2in to protect your jet flow.
• Fitting the flapper valve the wrong way round. Check the flow arrow on the valve body.
• Plugging the heat pump into the tub’s 13A supply or heater terminals. It’ll trip, or cut the heat pump off mid-cycle.
• Mismatched thermostats. Set the heat pump and the topside to the same temperature.

If it’s still not heating once it’s in, work through Help! My air source heat pump is not heating my hot tub.

Is a heat pump worth it on a small tub?

On my own big DIY tub, the heat pump cut my running costs by 66%. You can see the numbers in How I cut my hot tub running costs by 66%. A small 13A tub uses less energy to start with, so the saving in pounds is smaller and the payback takes longer. But a heat pump still gives you several kWh of heat for each kWh of electricity, where the spa pack element gives you one. At the October–December 2026 UK price cap of 26.32p per kWh, that’s the difference between about 26p for every kWh of heat from the element and well under half that from a heat pump. For help choosing, see How to choose the right air source heat pump for your hot tub or pool and the ultimate guide to air source heat pumps for hot tubs.

You’ll find the full range of heat pumps and connection kits on parts4tubs.co.uk. In the USA, parts4tubs.com has the pumps, valves and fittings.

Planning your own build?

If you haven’t built your tub yet, you can avoid the whole single-pump compromise by planning a circulation pump and a heat pump loop from day one. My DIY hot tub plans include the plumbing layout, control room and parts list to do exactly that.

FAQs

Can I add an air source heat pump to a 13A hot tub?

Yes. Plumb it in after the spa pack heater with a bypass and a flapper valve, and give it its own electrical supply. It can’t share the tub’s 13A supply.

Will a heat pump make my jets weaker?

Possibly a little, because you’re adding pipe, bends and a heat exchanger to the circuit. With 2in pipe and a spring-loaded flapper valve in the bypass, most people don’t notice a difference.

What does the flapper valve do?

It’s a spring-loaded one-way valve in the bypass. On low speed it stays shut so all the water goes through the heat pump. On high speed the extra pressure opens it, so more water gets to the jets.

Will the heat pump work on the pump’s low speed?

Usually, but check. The heat pump’s flow switch needs a minimum flow, and low speed moves much less water than high speed. Compare your pump’s low-speed flow with the heat pump’s spec sheet before you buy.

Can I power the heat pump from the spa pack heater terminals?

I wouldn’t. That output switches off whenever the tub stops calling for heat, which cuts the heat pump mid-cycle. Give it its own permanent, protected supply.

Got a single-pump tub and a question about adding a heat pump? 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