How I added an Air Source Heat Pump to an Existing Hot Tub

Adding an Air Source Heat Pump to a Hot Tub

Many of you may have read my guide on how to add an air source heat pump to a hot tub, you can find that article here. In this post, I am going to share with the steps that I took to add an air source heat pump to this hot tub.

Just for total disclosure, Paul from the UK is not a customer but a good friend of mine for going on 15 years now. A good enough friend to let me lose on his 6 week old “pride and joy” that is his new hot tub! And, I don’t do on site work!

Another point to make here is that you are more than likely to void any manufacturer’s warranty if you modify the plumbing and electrics in any way. Paul’s ok as I’ll take care of him if he has a problem with his tub, but you should bare this in mind if you are going to undertake such a project you are going to be on your own in terms of warranty.


As a result of the rising cost of energy prices, Paul was not liking the daily costs he was seeing on his smart meter – primarily from running the tub. So, he was looking for an alternative way to heat it. Having already installed air source heating for his home, it was a logical choice.

Paul decided to go with the Comfortline 7KW model for his tub. This decision was based on the specifications for the product and the price point too. We looked at lots of different options but this one came out top.

With lots of photographs and video calls being exchanged, Paul and I put a parts list together for what was needed for the job and put a date in the diary.

The Location

We decided that we would install the hot tub’s air source heat pump next to the existing house pump. Aesthetically it looks the best. Normally, you would expect to have more clearance and more air flow in front of the heat pump.

However, because the location of the pump is somewhat of a “wind tunnel” on Paul’s property so there is no shortage of air flow around the units meaning we could get away with closer proximity to the walls.

The heating engineer that installed his home heat pump didn’t have an issue with it so why should I?

The hot tub air source heat pump Location

Wall Mounting the Air Source Heat Pump

One of the first tasks was to wall mount the air source heat pump. Most are located on the ground but we wanted to match the unit that was already in place – this was the main reason for wall mounting the unit.

We had to drill some pretty large holes to get the wall fixings in place (14mm) as the wall mount is heavy duty. Last thing we wanted was the heat pump to fall off the wall!

The Location installed

Changing the breaker

Paul’s Hot Tub was originally installed onto a 32A circuit breaker like many of the the plastic shell hot tubs. With the additional load of the Heat Pump, this was not going to be enough to run all the kit at once.

We changed the RCBO out for a 40A breaker to give ourselves enough current to power both the tub and the air source heat pump off the main breaker.

Just as a side note, we did get all the electrics checked by a qualified electrician before we switched anything back on!

Changing the breaker

Planning the Plumbing

Paul’s Hot Tub is pretty new (6 weeks old and he wanted me to cut holes in it!) and comprises of 2 jet pumps and a circulation pump. It was the circulation pump that we were interested in and what we needed to do was to intercept the hot water return.

Instead of sending it from the electric heater back into the tub, we wanted to send it to the air source heat pump.

Having a circulation pump makes it a lot easier as you have to worry less about the impact the air source heat pump will have on the flow rates of your jets. If you don’t have a separate circulation pump, you really need to be careful with the flow rates and you will need to use the bypass valve on the heater to ensure that you don’t loose the flow on the jets.

Planning the plumbing

Separate Switch

We also planned to put a separate switch onto the existing Gecko electric heater so that we could switch the heating to just Air Source Heat Pump or add the 2KW electric heater too. Just as a side note, in a future modification, we are going to make the switch smart so that Paul does not need to go into the cabinet if he want too switch on the electric heater.

The idea here was if Paul needed a boost in the cold weather, then he could switch on the electric heater too. The rest of the time he plans to just run it on the air source.

The challenge of the plumbing is the space under a plastic shell hot tub – in short there is not a great deal.

The plan that we had was to move the heater over closer to the pump and then break into the circuit to add the air source heat pump.

Paul also wanted the pipes to come out of his cabinet at the side of the tub rather than the font which would look a little ugly. It would also cause access issues too as the main access panel is at the front.

I was using rigid pipe for the project. If I did it again, I would definitely use “flexible” pipe inside of the hot tub cabinet it would have made life a lot easier. There is no forgiveness in the rigid pipe whatsoever so made life a lot harder to get the pipe in the right place and at the right angle.

We did get there in the end.

Bypass Valves onto the Air Source Heat Pump

The next stage of the plumbing was to put a bypass and isolation valve onto the air source heat pump. This is so the flow can be altered if necessary but also so that the pump can be totally isolated and removed from the circuit if needed.

My tip here is to take you time on the plumbing and actually build everything in situ. That way, you know that everything is going to fit nicely.

With the bypass valves in place, it was time to dig up Paul’s recently-done landscaping so that we could bury the pipes as much as possible. They are a little too large to be totally buried and we didn’t want to cut he recently laid slabs at all so he will “make do” with the rising stones instead of being totally flat.

We were using rigid 1.5” pipe and we had full lengths so that there was no need for any joins.

My top tip on the pipes is keep everything at 90 degrees. It makes life so much easier. Easier said than done and I must admit, some of the angels are not 100% perfect but they are pretty much there.

As with any form of hot tub plumbing, a joint that is well made, is a joint that will not leak. If in doubt, cut it out and redo it. It will only leak later on.

Bypass Valve

Making your pipe Joints

Just a quick note on how to make a good joint. Firstly, you should be cleaning and roughing up both the pipe and the fitting with some glass paper before you make the joint.

Then, you should mark the ideal depth you want to get the pipe into the fixing. It is easy to lose track on how deep a pipe has gone into a fitting so give yourself a goal line you want to hit.

You need to work quickly once you have applied the pipe cement and you should apply it to both the pipe and the fixing. When you push in the pipe, you want to be turning it to avoid what is called “channelling” this can lead to leaks.

I also like to put a mark for where the fitting should line up with the pipe. Again, when you are twisting it in, it is easy to forget the angle you need to finish at. Have a mark to line up to and it makes life a lot easier.

Hold the join together for 10 seconds or so, then you can release – it will be at strength after a couple of minutes and full strength after about 4 hours. Wipe off any excess pipe cement and you have a great joint made.

Wrapping the Pipes for Additional Insulation

On Paul’s installation, there are a few meters of pipe run to the air source heat pump. To minimise the heat loss, we decided that we would wrap the pipes in a “foil pipe wrap” we did this before we cemented them in place as it was going to be very difficult once they were cemented into position.

Wrapped Pipes

Wiring the Air Source Heat Pump

As Paul already had runs of 10mm3 cable for his hot tub, it made sense that we could use that for the Air Source Heat Pump too.

We simply connected the air source onto the spa pack main block (just using it as a connector block, there is no control from the spa pack directly) – this was the simplest and cleanest way of doing it.

Adding a Switch for the Heater

As I mentioned earlier, we wanted to add a switch so we could change how the tub was heated. Loving a bit of DIY, this is very much the DIY option.

I broke into the live cable on the heater, and put a simple single pole switch on it. Nothing complicated but it certainly does the job. (This is a clear warranty violation for sure!)

Time for some Testing

At this point, it was time to see how the plumbing fared up. I had my suspicions that there was a gasket missing from one of the joints inside of the tub that used to be attached to the heater and this was confirmed as soon as I released the ball valve, water poured out of the connection.

Nothing that a 50p (70cents) gasket would not fix.

We also had one corner joint that was not made at 90 degrees that had the tiniest of leaks. Being the perfectionist that I now am when it comes to plumbing, we cut that out and remade the joint. Now it is perfect!

Hot Tub with Air Source Heat Pump

How Does It Work?

What we are essentially doing is controlling the on/off of the heat pump with the flow of the water from the circulation pump. Therefore, we have set the temperature on the air source to 40C / 102F and then we can set the temperature on the spa pack to our desired temperature. It can match of course.

What happens is when the spa pack registers the temperature is correct, it shuts down the circulation pump. The water flow stops and as there is no flow, the flow sensors in the Air Source Heat Pump tell it to stop heating – you actually get an error code on the model Paul has. This is not a problem as when the flow resumes, the heat kicks back in 3 minutes later. Voila – we are indirectly controlling the air source heat pump from the Spa Pack.

Things I would Change if I did another one.

The first thing, definitely for a plastic shell hot tub would be to use flexible pipe in the cabinet. There is so little room and rigid is unforgiving so I would for sure be using the flexible version. It would have saved a couple of hours work and stress of trying to get joints and angles to meet.

This is really the only thing that I would change. The short project went very much to plan without much of a hitch. Couple of small leaks needed addressing but this is somewhat normal I guess.

The Initial Results

The initial results of Pauls 7KW Air Source Heat Pump was that in 2.5 hours it had increased the temperature by 7 degrees – far quicker than the 2KW Electric would. He also did some tests on his Smart Meter for us. I will be doing a full post and a video on the results but the initial results speak for themselves.

Today, with an ambient temperature of 10C or 50F, his Air Source Heat Pumps is costing £0.33p ($0.41) an hour to heat the hot tub. Remember, we are heating from cold so it is not indicative of a daily running cost at this stage.

What is interesting is that at the same ambient temperature, the electric heater when turned on is costing an additional $0.57p ($0.72) an hour to run. That is almost double the cost of the Air Source Heat Pump.

Again, these are the specific findings for Paul’s tub when heating from cold today, but I will post a full update and video on the impact it has had on his running costs.

The electric heater is essentially costing him best part of twice as much as the Air Source Heat Pump. What he has also commented on this evening is that he thinks the heat up time is 2-3x faster too.

Can I help you with your Air Source Heat Pump addition to your tub?

As I mentioned in the article, I don’t do on site work as this was just to help out my friend Paul. However, I can help you with knowledge and guidance as well as the kit that you need and of course the Air Source Heat Pumps themselves.

I have the Air Source and the Connection kits in my store. If I can help you or you have any questions, drop me an email below if I can help you out at all.

Thanks and 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 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.

Today, I've helped over 1000 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

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