The Ultimate Guide to Air Source Heat Pumps for Hot Tubs

HomeThe Ultimate Guide to Air Source Heat Pumps for Hot Tubs

The Ultimate Guide to Air Source Heat Pumps for Hot Tubs

Air Source Heat Pumps for Hot Tubs are a great alternative to the traditional gas or electric heating system. They require no installation of new pipework and can be placed virtually anywhere in your yard. The drastic reduction of energy costs plus fast heat up times makes this type of heating source very desirable for the DIY Hot Tub Builder. In this blog post we are going to look at everything you need to know about Air Source Heat Pumps for your DIY Hot Tub.

What is an Air Source Heat Pump?

Air Source Heat Pump for Hot Tubs

Air Source Heat Pump for Hot Tubs

In terms of Air Source Heat Pumps for Hot Tubs (ASHP) is a device that transfers heat either to or from the water in your tub. Under the principles of vapor compression refrigeration, the ASHP uses a compressor and condenser to absorb heat either from the air if we are heating the hot tub, or from the water, if we are chilling the Hot Tub (to use as a plunge pool in the summer) and releases it in the opposite direction. So, if we are heating the Hot Tub, then heat absorbed from the air outside is transferred to the water inside of the tub. Conversely, if we are using the ASHP in the summer to chill the hot tub into a plunge pool, then heat is absorbed from the water and released into the air.

How does it work?

An air source heat pump works by pumping a refrigerant through the heat pump’s copper heat exchanger coils. The compressor and condenser are used to change states of the refrigerant between cooler liquid and warmer gases. In heating mode, the liquid refrigerant boils as it passes through the heat exchanger coil.

Heat from the air that has previously been absorbed and stored in the refrigerant, is retained and carried within the refrigerant as it evaporates into a gas. The gas is then compressed using the electric pump, which increases the temperature of the gas through compression.

The gas passes through a pressure valve into the coil and the refrigerant gas is condensed back to a liquid and transfers the heat into the heating element which in turn, heats the water in your Hot Tub as it is pumped through the unit.

What are the Advantages of Air Source Heat Pumps?

Air Source Heat Pumps for Hot Tubs are one of the most efficient heating systems available. They can provide up to three times more heat than standard heating options, and they’re incredibly energy-efficient. This means that they are cheaper to run on your Hot Tub.

  • Low carbon footprint
  • Save money on energy bills
  • Can be used for both heating and cooling
  • Can work even in lower temperatures
  • High Seasonal Coefficient of Performance (SCOP)
  • Easy installation process
  • Low maintenance
  • Long lifespan
  • No fuel storage needed

What are the disadvantages are Air Source Heat Pumps?

One of the main disadvantages is the initial outlay – they are not cheap pieces of kit. However, factor in the money that they are going to save you and within 18months to 2 years they will have easily paid for themselves.

  • Lower heat supply than propane or natural gas
  • Lower efficiency below 0°C / 32F
  • Electricity is needed to run an ASHP
  • ASHPs can be noisy

Why are not all Air Source Heat Pumps are the same?

Air Source Heat Pump for Hot Tubs

Air Source Heat Pump for Hot Tubs

Loving a car analogy here at, two 6 litre V8 cars parked side by side are not necessarily the same are they? Yes, they are both cars, but you can pretty much bet they have different features, different performance and different price points. The same goes for Air Source Heat Pumps.

Just because you have two ASHPs that are showing the same heating capacity does not mean that they are the same. Let’s take a look why and also what you should be looking for.

For regular DIY Hot Tub builds, which fall under 3000 litres or 650 gallons, you are going to look for an ASHP that is going to output roughly 12kW of heating capacity which is going to be around 40 000 BTU.

Due to the nature of how an ASHP works, they have different efficiencies at different temperatures. Therefore, you need to pay clear attention to their performance in the temperature range of your environment.

For example, many units are not rated for working under zero degrees Celsius or 32 Fahrenheit. If you are planning to use your tub in the winter, then this is not going to work for you.

Also, you will find that at the cheaper end of the ASHP range, they will consume a huge amount of power at lower air temperatures but also have a lower heating output. Basically, they don’t work well in the cold.

The key is to look for the consumed power at a specific outside temperature and you can then look at the heating capacity at this temperature too. If you compare a cheap unit side by side with a more expensive unit, you will see that the power consumption on the more expensive unit is lower and the heating capacity is also greater. This is because the unit is more efficient.

Warranty. Do check the warranty of Air Source Heat Pumps for Hot Tubs. The cheaper units tend to have only a single year warranty which if you are spending over a grand on a piece of kit, seems a little off to me.

What about the running noise. Pay close attention to the decibel output level of the unit. The cheaper models tend to be a little on the noisy side which if you have neighbours close by, could be a problem.

Now it is time for the clever bits. These are the things that really set aside the differences between a cheap ASHP and a good quality one.

Inverter on the Motor

This sounds complicated but let me explain it in its most basic form. If a motor is inverted, this means that it has “technology” that can control how fast or slow it runs. Why is this important? Well, a fully inverted motor can run at anywhere between 1% and 100% of speed.

If you motor is not inverted, it will only have set speeds. Perhaps, slow, medium and fast. This means that a fully inverted motor is much more efficient as the technology can sense that the motor only needs to be running at 21% for example to reach the desired temperature. An un-inverted motor might have to run at medium for this which would be 29% faster using 29% more energy than our inverted motor. Therefore, there is a saving of running costs. An ASHP with a fully inverted motor will save you roughly a third on the running costs!

Soft Start Motors

Cheap Air Source Heat Pumps will not come with a soft start motor? Why is this important? When a traditional motor starts, it needs more power initially to get it to turn. To do this, it draws more current from your circuit for split second. This is known as a spike. A current spike if it is big enough can knock out your breaker killing the power to your whole hot tub system.

Soft start motors avoid this as they are specifically designed to be able to start to turn without the need of drawing excess power from the circuit thus avoiding the spike on your system. A good quality ASHP will have a soft start motor in it.

Heated Lines

Better quality ASHPs come with heater lines often referred to as a heated train. What this does is at lower temperatures, the unit can sense if the refrigerant lines are likely to freeze and it sends a warm flow of fluid down to defrost them. How clever is that.

So there we have it. In conclusion, we’ve learnt what an ASHP is, how it works and the different types of pumps available as well as what features to look for when you are shopping for your own.

Happy Tubbin’

Hydro Royal Air Source Heat Pumps

Looking for reduced hot tub energy or running costs? With the global increase in fuel costs, most notably, electricity costs, many of us hot tub owners are looking for ways to reduce the running costs of our tubs.

Personally, I am in the same boat. My tub was always expensive to run. However, with the April 2022 price increases it became astronomically expensive to run. I needed a cheaper way of doing things.


I built my hot tub 4.5 years ago now. At the time, my advisor John from the pool supply store told me that I was on the limit of the volume of water for a regular 3KW spa pack. I knew that but the alternatives at the time were custom made big electric heaters which were ridiculous in terms of the cost. Never mind the running costs.

Until the price increase in April 2022, my Hot Tub was costing £50 / $75 a week to run. I know this is expensive, but it is a luxury.

I also did not include any insulation in it when I built it so it is pretty inefficient in terms of retaining temperature too. I certainly do not recommend any of my customers do it this way now!

Fast Forward April 2022

If we fast forward to April 2022, electricity pricing has effectively doubled overnight. At £100 / $150 a week to run my hot tub running costs were getting out of hand. I needed a better way of doing things.

Enter the Air Source Heat Pump

I fitted an Air Source Heat Pump to a Plastic Shell Hot Tub

My good friend of 18 years Paul asked if I would help him fit an Air Source Heat Pump to his hot tub. Of course I said yes. It was pretty straight forward to do. You can read about this one here. You can also view the video on my YouTube Channel

Adding an Air Source Heat Pump to a Hot Tub

Adding an Air Source Heat Pump to a Hot Tub

What really caught my attention was the numbers that he was texting me the following day.

Firstly, he said that with his new 7KW Air Source, he was heating 2.5x faster than on his 2KW electric heater. That alone is pretty cool.

Then there were the running costs. When he was running on his electric heater, he was running at 93p ($1.16) an hour on his Smart Meter.

At the time he was texting he knew it was cheaper, but subsequently has sent me some more concrete figures.

He’s looked at the costs running on different power outputs (depending on the ambient temperature) and on the low end, it costs him 22p ($0.27) an hour to run 2/3 cheaper and on the high end 43p ($0.54) an hour to run which is over 50% cheaper.

Bottom line, Paul is running his hot tub on an air source heat pump on average at over half the cost of the electric heater. Plus, he benefits from 2.5x heat up speeds.

This is win win.

This was also why I wanted to add one to my hot tub. I needed some figures like this.

So, I have added an air source to my hot tub. If you want to learn how, you can read this article.

However, let’s look at some numbers as this post is about how much money I am saving.

Air Source Heat Pump Installed

Air Source Heat Pump Installed

Reduced Hot Tub Costs – How Much am I saving?

I’m going to split this section into three parts. Heat up costs, running costs and then regular “maintaining heat” costs, then I am going to draw a general (averaged) conclusion on the savings.

Let’s look first at the heat up costs.

Heat Up Cost Savings

Before I added the air source heat pump, I would turn my Hot Tub up from 33C to 39C (91.4F to 102.2F) on a Thursday evening. I would leave it heating up over night as it would take roughly 12 hours to heat up.

If we factor in the costs, this is roughly £0.90 ($1.11) an hour – multiplied by 12 hours which is £10.80 ($13.38) – ouch!

I know that everyone’s prices are different so turn that into KWH – that is 36KWH of electric to get up to temperature. Still an awful lot.

With my air source heat pump, I can now heat up from 33C to 39C (91.4F to 102.2F) in about 1hr 20mins at 15C (59F) ambient temperature outside. This is pulling roughly 8KWH in electric when the heat pump is in heat up mode. This is a 21KW Air Source so quite a big one.

Let’s turn this into monetary terms, that is costing me £2.50 ($3.10) to heat up the tub and it is doing it with 10 KWH of electric.

This is a saving of 72% on the heat up costs – WOW!

Reduced Hot Tub Running Costs

It is a little more difficult to calculate the running costs as was never quite sure if the electric heater was on all the time – but there are some considerable benefits. I can also calculate in the conclusion what the cost savings are.

We’ve seen the heat up time increase – I do have a 21KW air source heat pump on my tub but I have nearly 3000 litres (660 gallons) of a water to heat. As you saw above, for my friend Paul’s tub, he was using a 7KW Air Source (1200 litres) and saw the benefit.

My heat pump when running, draws around 8KWH of electricity and costs roughly £2 ($2.48) to run per hour.

However, huge benefit for a hot tub owner here is that I am seeing zero heat loss from my tub when I used it for a couple of 90-120 minute sessions – again at around the 15C (59F) ambient temperature outside. This is massive for a hot tub user.

Previously, I would be dropping from 39C to 36C (102.2F to 96.8F) in that time and then have to get out as it was too cold.

Hot Tub Maintenance (keeping it warm) Costs

I’ve found over the years that holding the hot tub at a lower temperature during the week when I don’t use it is a cheaper way of running the tub. This is compared to letting it go cold.

Previously, I used to hold it with my electric heater at 33C (91.4F) as I have explained. I have dropped that now to 31C (87.8F) as I have faster heat up speeds now so don’t need to hold it quite as high.

So what did this cost to do on the electric heater?

It is difficult to say really as I did not monitor the smart meter as I have been doing to get the costs for the air source. However, if we assume on April 2022 price increases which doubled, this was costing me £100 ($124) a week to heat in total.

Let’s subtract the heat up cost which was £10.80 ($13.38) which is 36 KWH and assume that the tub was running on full power for 3 x 1.5 hours (Friday, Saturday and Sunday usage) which is 40.5KWH. If it was then dropping down to 36C before I got out, and needing 6 hours to heat back up each time, another 54 KWH of electric.

Heat up and running costs are about 130.5 KWH. However, but that does not allow for the “keeping the hot tub hot” during the weekend. If the tub is losing 0.5C an hour, but then takes 1 hour to heat back up – that is another 36KWH per day x 3 to add to this value. This totals 238.5 KHW – Current prices are around £0.28 ($0.35) a KWH so that is £66.78 ($82.71)

This means that if my tub was costing £100 a week to run, the maintaining at 33C was costing me £33.22 ($41.15)

Wow – lots of calculations here but we have our costs that we can revist in the conclusion.

With the air source heat pump, this is a little easier as I just took some screen grabs from my Smart Meter to help with the calculations.

The week before I connected the Air Source, I took some average daily usage reading from my Smart Meter

On average over 5 days, I was using 22.94KWH of electric in my home. You can see some of the screen shorts of my meter below.

Smart Meter

Smart Meter

What I wanted to find out is what it is costing me to maintain the hot tub at the 31C (87.8F) I have it set to now.

So, over the same period, with the Air Source Heat Pump holding my hot tub at 31C (87.8F), I am using 35.86 KWH, so 12.92KWH a day more. At current prices of £0.28 per KWH, that is costing me £3.62 a day to hold the temperature.

Smart Meter

Smart Meter

Conclusion on How much I am Saving on my Hot Tub Running Costs

From April 2022 when the price rises came into place on electric here are my findings on my reduced hot tub running costs.

On 3KW Electric Heater

Initial Heat Up Costs                           £11 ($14)

Weekend Usage Costs                        £56 ($69)

Weekly Maintenance Costs                £33 ($41)

Total Weekly Costs                             £100 ($124)

On Air Source Heat Pump

Initial Heat Up Costs                           £2.50     ($3.10)         (arguably I don’t need this as this number is in the figures below but I will leave it in anyway)

Weekend Usage Costs                        £16.50     ($20.48)      (3 days @£5.50 a day average / 20KWH)

Weekly Maintenance Costs                £14.48   ($17.97)         (4 days @£3.62 a day average / 13KWH)

Total Weekly Costs                             £33.48 ($41.54)

Total Saving of 66% on my hot tub running costs!

What about the ROI?

Taking the retail cost of my Air Source Heat Pump which is £2564 ($3181) and I am saving £266 ($330) a month, I will have an ROI (return on investment) in 10 months.

Not bad at all, especially when electric prices are due to rise again in October 2022.

Can I help you with an Air Source Heat Pump?

If you would like some help with the parts and how to fit them to your own hot tub, then please do get in touch below.

Hydro Royal Air Source Heat Pumps



By |2022-08-31T15:57:04+00:00March 11th, 2021|Build A DIY HOT TUB|Comments Off on The Ultimate Guide to Air Source Heat Pumps for Hot Tubs
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