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Can You Fill a Hot Tub with Hot Water? – The Simple Answer

If you’re like many hot tub owners, you may wonder if it’s possible to fill your hot tub with hot water? The answer is a tricky one and it’s generally not recommended to do so but yes, you can fill your hot tub with warm (not hot) water. 

However, it can be complicated and there are a few things you should know.

In this blog post, we’ll discuss, can you fill a hot tub with hot water and what you need to do to make sure your hot tub gets the right mix of hot and cold water and how to troubleshoot any problems that may arise. So read on for all the details!

Can You Fill Inflatable Hot Tub With Warm Water?

We know that when you’ve just set up your new hot tub, it can be a pain to wait for the water heater to kick in, but generally speaking, we don’t advise that you fill your hot tub directly with hot water. 

A spas shell shape is designed to withstand a maximum temperature of 40 degrees Celsius. Water any hotter than this can cause cracks and warping that may invalidate your warranty and cause a lot of hassle.

Household hot water supplies also often include water softeners that can corrode the internal components. Most hot tubs feature a temperature cut-out that will trip if you introduce water of an excessive temperature.

Why You Shouldn’t Put Boiling Water in a Tub

a tap with the letter H on it

Not convinced that hot water is a bad idea for filling your hot tub? Let’s go into the risks involved in a little more detail.

1. It may damage the spa

The most obvious reason to avoid boiling water in your hot tub is the potential for the sheer heat to damage your spa. 

The material lining most hot tubs is designed to stand up to water no hotter than 40 degrees Celsius. Boiling water is obviously much hotter than this, so you’ll run the risk of melting or cracking the hot tub shell, which hot tub warranties aren’t going to cover.

Also, any boiling water that gets pumped through the filtration system can damage the internal components of your tub, affecting the water circulation and potentially rendering your spa useless!  

2. Soft water issues

Most household hot water systems include a water softening filter designed to remove the minerals found in hard water. This is great for drinking water but can cause corrosion in the filtration system of your hot tub.

Many people may be unaware that their outside tap is unlikely to draw water through the same system, so it’s better suited for safe use in your hot tub despite your garden hose temperatures being lower than household water.

3. Could trip the heater

Modern hot tubs are pretty high-tech products, with several built-in safety features to protect both you and the spa itself. 

One of these features is a temperature cut out that will trip if it detects a water temperature hotter than 40 degrees Celsius. 

Tripping this system by introducing boiling water to your hot tub can result in permanent damage to the internal electronics, leaving you with a defective heater, which of course, won’t be covered by your warranty.

4. It could be dangerous for your health

Aside from the opportunities for hot water to damage the mechanisms of your hot tub, attempting to fill your spa with and sitting in hot water can result in serious injury.

Sitting in water higher than the recommended temperature of 40 degrees Celsius for long periods can cause heart problems, dehydration, dizziness and nausea, all issues we think you’d rather avoid! 

Also, assuming you don’t have a direct line from your combination boiler to your hot tub, you’ll likely be ferrying multiple vessels of boiling water into your spa, which can easily result in nasty burns.

5. System capacity

In a similar vein to our previous point, your household water supply’s limited hot water system capacity will likely result in any attempt to fill your hot tub with boiling water being a very long process. 

Your storage tank will probably hold around 180 litres of water, meaning filling even a small hot tub will result in several trips and unreasonably high energy costs.

Given the increased risk of injury, avoidable energy costs and time-consuming heating process, we think you’ll agree that just waiting for the hot tub heater to do its job is a much simpler option.

6. Bacteria

an image of bacteria floating

Your hot tub’s filtration system is specifically designed to kill harmful bacteria and remove any unwanted minerals that may make the hot tub water foamy and other bits of scum form in the water. 

Bypassing this filter and potentially damaging it with boiling water can result in these harmful additives sticking around.

Even if the introduction of boiling water doesn’t render the filter useless, it can easily cause the system to use extra energy that can increase your running costs and mean you’ll have to engage in more spa maintenance, including Jacuzzi filter cleaning and changing.

7. You can use a filter on your garden hose

All water supplies have trace amounts of metals and debris present from the pipes and the storage tank itself; these contaminants can cause all manner of problems, from water discolouration to issues with your water chemistry.

Once introduced, it can be challenging to remove these additives using only your hot tub’s filtration system, which is why it’s a good idea to use a filter on your hose when you initially fill your hot tub.

Hot tub filters can be used with mains pressure cold water systems, but a hot water system will likely not have enough pressure for the filter to work correctly, running the risk of overworking your pump and damaging your hot tub! 

8. Chemicals evaporate quicker

To maintain healthy and safe water quality in your hot tub, it’s important to keep an eye on your chemical levels. 

The best spa chemicals like chlorine are essential for safe hot tub use and to stop it from becoming a cloud hot tub as open water that sits outside, and has plenty of people dipping in, is a prime breeding ground for nasty bacteria and viruses. 

You should regularly check your chemical levels no matter how often you’re using your hot tub, but if you do so after using excessively hot water to fill the spa, you’ll notice that the chemical levels drop much faster.  

This is due to the boiling water causing them to evaporate before they have the chance to do their job! 

Quicker Heating Without Hot Water

a woman sat in a hot tub with her eyes shut

Rather than risking damage or injury by shifting boiling water between your hot water tank and your spa, there are a few simple ways to heat your hot tub water faster. 

A little patience and a bit of perseverance go a long way, so although keeping on top of these tasks can be a little tricky at first, making them part of your hot tub routine is more than worthwhile! 

Use the hot tub cover

One of the simplest ways of retaining hot tub heat is to make good use of the insulating cover. Storing your hot tub with the cover sealed will do a great job of preventing heat loss and, in turn, ensure that the water heats faster when you want to use the tub.

If your hot tub has a frost protection system, making use of the cover will help to minimise the amount of work it needs to put in, but even hot tubs without this feature can retain their warm water overnight so long as you remember to include attaching the cover into your daily routine! 

Add insulation

Most modern hot tub panels will have some degree of heat insulation built-in, but if you notice that the cold weather makes it hard for you to heat water consistently, you can always improve this by adding more.

Placing a thermal blanket on top of the water and under the cover is perhaps the simplest way to trap a little extra heat, but if your hot tub has inadequate insulation, you might want to look into using spray-on foam under the hot tub shell.

You can also try installing fibreglass boards around the back of the panels, as this proper insulation material will measurably improve how long your hot tub takes to heat up.

Turn on the jets

Another simple way to ensure your hot tub water heats faster is to turn the jets on as the water heaters kick in. Switching on the hot tub jets allows the system to circulate hot water evenly around the spa, speeding up the heating process considerably. 

This works best if you pop the hot tub cover back on for a short while whilst the jets get up to speed. This way, you’re making use of both the circulation of hot water and the insulating properties of the cover. 

Good maintenance

To give your hot tub the best chance of heating consistently, you should always try to keep on top of your maintenance. In our experience, many of the issues people have with poor heating times can be traced back to poorly maintained components.

Change and regularly clean your filter cartridges with warm water and filter cleaners to allow for better water circulation, and make sure to run your hot tub’s filter cycles to keep the system active. 

You should also drain and clean your hot tub every 3-4 months, as this will give you the chance to use a chemical cleaner to clean out any gunk trapped in the internal pipes.

How to Use Hot Water in a Hot Tub

Buy a second hose to connect to your hot tap

Suppose you are dead set on using hot water to fill a hot tub. In that case, we recommend that you at least look into purchasing a second hose to attach to the hot tap, as this will allow you to use hot water simultaneously with cold water to lessen the likelihood of damaging your hot tub.

Use a filter

Your hot water tanks are more likely to cause sediment and trace levels of metals to be present in the water.  If you are using this supply to fill a hot tub, you should consider using a water filter to prevent these additives from damaging your hot tub’s water circulation system.

Top up with buckets

a green bucket of water being poured

As far as we’re concerned, the safest way to use hot water to fill a hot tub is to top up the much safer cold water with buckets of the hot stuff. This will protect the hot tub shell from damage and keep the water temperature low enough to prevent the temperature cut out from tripping.


Is it cheaper to fill a hot tub with hot water?


Filling a hot tub with hot water may seem like the quickest and simplest solution, but it’s actually not recommended. The high temperatures can damage the spa, trip the heater and introduce bacteria into the system.

There are ways to increase your hot tub’s heating capacity without resorting to this method. By insulating your spa properly and positioning it in an area that receives plenty of sunlight, you can ensure your spa experience is enjoyable. 

Have you tried any of these methods? Let us know how you got on? If you need further help, be sure to check out our hot tub tutorials where we discuss many of the questions that stump spa owners.

John Devlin

Hello, Welcome to my website. I’m John, and I created the InflatableHotTubGuide while researching these spas nearly a decade ago. Since its creation, the site has become the leading UK resource for many models. As a passionate hot tub user, I love to test and explore all the latest machines this industry keeps creating. I hope you find our content helpful.