Can I add an Air Source Heat Pump to my Hot Tub?

Air Source Heat Pump-2

Yes, you can add an Air Source heat pump to your existing hot tub. However, there are a few things that you need to consider. In this article, I will explain the steps you have to take and explain how to add an air source heat pump to your existing hot tub.

With the increase in energy prices all over the World, hot tub users are looking at ways in which they can economise the cost of using and heating their tubs. An Air Source Heat Pump is a great way of doing this.

Many of the larger companies have installation teams that will come out and fit an Air Source Heat pump to your hot tub. However, at I like to focus on the DIY elements so this article will explain exactly what you need to do.

How big of an Air Source Heat Pump do I need?

Like anything, choosing the size of your ASHP is very subjective. However, I like to make things easy so here is my quick guide. Firstly, you want to go for the largest Air Source Heat Pump that fits into your budget.

In my opinion, there is no point in adding a 5KW ASHP to your existing Hot Tub. Whilst some may disagree, I don’t believe that the gains are worth the expense. As a bare minimum, again, in my opinion, you should be looking at a 9KW or above for a 4-6 person tub. Any tub larger that this, you should be looking for 12KW as a minimum.

Air Source Heat Pumps go up to very large sizes so what is the upper limit I should be thinking about? Again, this is a subjective matter, but in my opinion, you don’t need any more than a 24KW Air Source Heat Pump on your hot tub.

The bigger the pump, the quicker it will heat up. Also, the bigger the pump, the less the heat up time will be affected in cooler weather when the output drops. This is the reason that I don’t believe a 5KW heat pump is useful as in the cooler months, your output might be dropping down to 2 or 3KW.

In this article, I am not going to look at the benefits of Air Source Heat Pumps, as I want to focus on the way in which you connect up your pump to your existing hot tub.

Select your Location for your Air Source Heat Pump

The first thing you need to do is decide on a location for the air source heat pump. You need to choose a location where there is good air flow. You need to have space all around the air source hear pump, ideally 30cm / 12” from a wall.

You need to make sure that there is nothing in front of the fan. For example, you cant have the Air Source Heat Pump boxed in or inside of a shed. They don’t work like that. You should always check the manufacturer’s installation guidelines but as a general rule, you must have good air flow right around the unit and they most not be covered or restricted in any way.

How much pipe do you need?

Next, you need to measure how much pipe you are going to need to get to and from your hot tub. Remember, the water needs to flow into the air source heat pump, be heated, then flow back into the hot tub. Measure the distance to and from the hot tub to your proposed location for your Air Source heat pump, then add 30% extra. This is how much pipe you need.

It is also a good idea to think about insulating the pipes if they are above ground. In this way you can minimise the heat loss as the water travels to and from the tub.

What size pipe do I need?

In general, on Hot Tubs, water lines or pipes are 2”. Therefore, I would recommend that the water lines are 2” to and from the air source heat pump. This is to make sure that there is enough flow available.

How do I connect the Air Source Heat Pump to my hot tub?

This is where the investigation begins. Firstly, you have to identify if you have more than one pump in your hot tub. If you use more than one button to operate the jets then you probably do. If you open the service cover, you should be able to see exactly what you have.

If you have more than one pump, then the chances are you have a circulation pump as well as a jet pump or at least a pump that does the circulation too.

In general, a circulation pump will be the smaller of the two. You may have more than one pump as some larger hot tubs have three or even four pumps.

What you need to do is identify which is the circulation pump or if it is a dual speed pump, which pump is doing the water circulation.

This should be a case of turning on your hot tub and turning up the heat. There will only be one pump running at this point and this is the one that we will need to use to get water to your air source heat pump.

Drain the tub

Now that we have identified which pump is used by the hot tub to heat the water, we now need to drain the tub.

Once we have emptied the hot tub, we are going to need to cut into the water lines of the hot tub so we can attach the Air Source Heat Pump.

As a general rule, you should be looking to cut into the water pipe just after your spa pack. If you are not sure what the spa pack is, this is the square box that has all the pumps, blowers and lights connected to it.

Here is an  example of a Spa Pack.

Vian Power Spa Pack

Trace your plumbing

If you trace the plumbing, you should see that water from the lower drains comes into the front of the pump. Then, from the pump it will go into a filter, from the filter into your spa pack and then from your spa pack, it will go back into the jets in the tub.

If you have multiple pumps on your Hot Tub, one of them will follow this plumbing layout and that is the one we want to use.

What we are going to do is cut into the water lines after the spa pack to add the additional heat source which in our case is going to be the air source heat pump.

You will need to remove a 10cm/4” section of the pipe. You can use pipe cutters or a hand saw to do this. Be careful you do not catch any of the other pipework and put holes in anything! Last thing we want is leaks.

With a section of pipe removed, you now need to glue in with PVC Pipe Cement a couple of 90 degree 2” bends to be able to take your pipework outside of the tub over to your air source heat pump.

The chances are that you are going to need to cut holes in the exterior of the tub to allow the new pipework into the system so be mindful of this too.

Manual Bypass on the Air Source Heat Pump

ASHP Bypass

You need to create a manual bypass for your Air Source Heat Pump with a combination of ball valves that look similar to the above. The reason for this is firstly, the Air Source Heat Pump can be isolated from the rest of the plumbing and disconnected if needed.

Secondly, we can use the central gate valve to regulate the flow. If there is too much pressure or there is too much flow, by opening the central ball valve you can allow more water to flow past the Air Source Heat Pump which reduces pressure inside it.

Please note, it will not be as efficient when heating as not all water will be passing over the heating element.

Reduce Down your 2” Pipe to fit the Air Source Heat Pump Unions

In order to minimise the risk of flow reduction, I would be running the pipes to and from the hot tub on 2″ piping. I would be doing this even if the Air Source Heat Pump itself has a different connection. Usually, they are either 1.5″ unions or 50mm connections.

Therefore, I would be converting this over to 2″ so as to increase the flow rates as much as possible in the pipe work.

To do this, you will need some 2″ reducing bushes and then some 1.5″ pipe (or 50mm) to make the connections.

Potential Flow Problems on Single Pump Systems

You have to be mindful that you are adding additional lenghs of pipe as well as bend and junctions within the Air Source Heat Pump. This all adds additional friction for the flow of your water and you may find it affects your jets.

For this reason, we have added into the system a manual bypass valve on your Air Source Heat Pump.

If your flow has been impacted, you can open the manual bypass a little to let more water though. Remember, in doing this is will be less efficient in terms of heating as not as much water will pass into the Air Source Heat Pump.

In the worst case scenario, if your flow is severely impacted, you may need to stop the flow of water all together into the air source heat pump before you use the tub. This would involve closing the inward and outward gate valves and just having the water on the manual bypass.

As I said, this is a worst case scenario and will not be needed in most instances.

Can you Automate the Valves?

If you are quite handy, then yes, it is possible to automate the valves. What you can do is use some normally closed 230V solenoid valves. Below shows a smaller 1″ valve, I would not recommend you deviate from the 2″ as the flow will be reduced dramatically.

Solenoid Setup
Solenoid Setup

If you have a dual speed pump, you can wire them off the pump’s low speed connectors. What this means is that when the pump was on low speed, they would be open. They would allow the flow of water through the air source heat pump to be heater.

When you switched to high speed, power would drop and they they would close and divert the water so it didn’t go through the ASHP thus improving the flow to the jets.

The principal here is quite simple. However, this should only be undertaken by a competent DIYer that is more than comfortable with working on electrics as you will be working on 220-240V. You have been warned!

What Supplies do I need to connect an Air Source Heat Pump?

Lengths of 2” Pipe

Short length of 1.5″ or 50mm (whatever size the unions are)

Pipe Cement

90 Degree bends 2”

2” straight couplers

3 x Ball Valves

2” to 1.5” or 50mm reducers (check what size unions your ASHP comes with)

Can I help you with your Air Source Heat Pump hot tub conversion?

If you need some help with your project, then please get in touch below. I can supply a range of Air Source Heat Pumps, all the supplier you need as well as provide help and guidance on how to fit them.

Happy Hot Tubbin’