Heat a hot tub from boiler in your home? Absolutely! As energy prices continue to rise around the world, people are looking at more economical ways to head their Hot Tubs. For regular readers of the blog and those who subscribe to my YouTube Channel, you will have seen that I often refer to Air Source Heat Pumps as being the most economical way of heating a hot tub. That said, they are not without their drawbacks.
In this article, we are going to look at how you can heat your Hot Tub with your home heating system.
Why would you want to heat your Hot Tub with your home heating system?
Financial! Economies of heating is probably the best reason. It’s cheaper! If you are upgrading your home heating system or are looking to specify something for a new build, then it can be much cheaper to buy a slightly larger setup that can cope with the demands of your hot tub too. With the right sized boiler or water storage unit for Air Source Heat Pumps, you can comfortably heat a hot tub as well as your home.
How do I heat my home and my hot tub from the same system?
This is the clever bit. With a separate heat exchanger unit, you can generate enough heat to heat both. However, I must preface this comment with you have to have the system sized correctly for both your domestic needs as well as the hot tub. We’ll look at that later in this article.
If you have a system already in place, then you need to check whether there is the extra capacity to heat the hot tub too.
How does the heat exchanger work?
The heat exchanger may sound like a complicated piece of kit, but it is actually quite simple. Hot water from your boiler unit is circulated around the outside of the heat exchanger, “heating up” the unit.
The water for the hot tub, which remains totally isolated from the hot water from the boiler, passes through the exchanger unit and as it does, through contact, the heat is “exchanged” into the water thus raising the temperature.
Quite a simple procedure, but it is effective.
How does this save money on heating my hot tub?
Again, this one is easy to answer.
If you are already heating you home and you are using the boiler for your hot water, your central heating or your underfloor heating, then you are already paying for the energy to heat the water in the first place.
What the heat exchanger allows you to do is tap into this heat source that you are already paying for and use it to heat your hot tub. This means you don’t have to pay for it twice.
It’s a little too simplistic of an explanation as I am essentially saying this is 100% efficient and there is not any extra cost which is untrue, but this is pretty much it in a nutshell. There is certainly a cost saving in using this method.
What do I need to heat my hot tub with my home’s heating system?
A Boiler or a Hot Water Storage Unit
Firstly, you are going to need a boiler or a hot water storage unit of some description. This method relies on hot water being circulated to heat up the heat exchanger, just like we circulate water to heat up a radiator in a home. Therefore, there has to be a source of hot water somewhere.
You also need a pump to pump the water round the system. You would normally have one of these with your radiators or under floor heating.
A domestic boiler is usually around 30KW in power. What you need to know before you can buy the right heat exchanger is what the temperature of the water that is leaving the boiler is going to be. 82C or 179F is at the high end of a domestic boiler but that temperature might be more like 60C or 140F depends on the model and the system.
This temperature is called the input temperature and you need to know this as different heat exchangers will perform differently and output different amounts of heat depending on what the input temperature is.
Know Your Home’s Energy Requirements
Next, you are going to need to know what your home’s energy heating requirements is. For example, if you are planning on using your under-floor heating system, you need to know how much energy it needs to work – this is linked to the flow temperature of the water from the boiler in a wet system.
For example, if you are only outputting water at 40C / 104F from your boiler to your underfloor heating and you have a 30KW boiler, you have plenty of capacity to increase the water temperature to send to your heat exchanger on your hot tub.
Conversely, if you are maxing out your boiler and it is running all the time to keep your underfloor or regular heating running at 40C / 104F , you are not going to have the additional capacity to heat the water for your heat exchanger.
They need to be compatible and a qualified heating engineer will be able to assist you further on this and the calculations needed.
Direct Connection to the Boiler
In general, you are going to need a direct connection to the boiler from your heat exchanger for your hot tub. Running the water through the underfloor heating first for example, then to the heat exchanger is not going to be anywhere near as efficient as getting the hot water directly from the boiler.
The Zone Valve is a fancy name for a divertor valve which will turn off the supply of water to the heat exchanger when the hot tub is up to temperature. If this were not in the system, then the hot tub would continue to heat all the time that the pump from the boiler was running.
Obviously, this is not what we want so we need to be able to control this independently from the home’s heating. We should be able to heat the water and send it to the heat exchanger even when the home’s heating system is not running.
A qualified heating engineer will be able to set this up correctly for you.
Digital Thermostat for your Heat Exchanger
You need to have a Digital Thermostat which is connected to the heat exchanger so you can tell what the temperature of the water is. This should be set to the desired hot tub water temperature and should be independent from the spa pack.
You will have a connection from the thermostat to the zone valve to turn off the zone from heating once it is up to temperature.
Digital Thermostat Pocket for your Heat Exchanger
This is a simple piece of kit that allows you to put the sensor probe into the hot tub water flowing through the heat exchanger without any water escaping. Sounds much more complicated than it actually is.
How do I Choose the Correct Size of Heat Exchanger for my Hot Tub?
There are a couple of considerations we need to think about here before we do the calculations. Firstly, the bigger the exchanger, the more potential heat that is on offer. I say potential here as the boiler needs to be able to deliver the input temperature at the correct level to get the most out of the system.
We also need to work on the basis that systems will not be 100% efficient. Take it as a given that there will be some heat loss along the way from the water outputted at the boiler and what gets to the heat exchanger.
Volume & Surface Area
Next, we need to know the volume of the hot tub and we need to know the surface are of the hot tub.
I’m currently working on a project which is a big hot tub, 11 200 litres or 2463 gallons. To calculate the minimal BTU that is required you take the hot tub’s surface area in feet (4.5m x 2.5m) is 15’ x 8’ – 120sqft.
Multiply the result by 1000 and that gives is 120 000 BTU which is a good indication of the amount of heat needed. I would normally increase this by 20% to allow for inefficiencies, temperature fluctuations etc so something around 144K BTU would be ok for this size of hot tub.
This is the same calculation you would do to size up a heat pump, propane or gas heater too.
From here, I need to take that 144K BTU number and look for a heat exchanger that is going to give me something in the region of this figure. In this case, I am looking at a model that will output 167K BTU which is 49KW. Th next model down is only 102K BTU at 30KW which is not going to be enough.
You can of course over specify things and have a too larger heat exchanger so its not really worth going too big – aim to get your calculations correct.
I then need to look at the input temperature chart for this model of heat exchanger. In this case, at 15C or 59F of input water temperature from the boiler, I am going to get 16KW of heat from the heat exchanger. This will not be enough and will take a very long time for the tub to get to temperature if it ever will. This is why the input temperature is important.
However, if I know that my input water temperature from my boiler is going to be 50C or 122F then I can get 52KW from this model which is more than I need. More heat equals quicker heat up times. If I am sending the input water temperature at more than this, I know I am going to be in good shape for heating the hot tub to temperature. In short, this 49KW model will be ideal on my particular setup as a mid level input temperature from the boiler will get me the desired KW output.
Don’t forget the bypass Valve!
As many heat exchangers are based around the flow rates for swimming pools, they are going to be a lot lower than on your hot tubs. Especially, if you only have one dual speed pump and all the water is being forced around the system at high pressure.
When you plumb in a heat exchanger, don’t forget the bypass valve so you can manually adjust the flow rates going through the exchanger and let any “excess” pass by.
Can I help with your hot tub project?
From system design to parts supply, I can certainly help you. Please drop me a line below to see how I can help you with your own hot tub project.
Happy Hot Tubbin’
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Thanks - Andi
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.
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