Correctly sizing a hot tub pump isn’t just a case of plucking a number from thin air. Nor is it the case of just going “bigger is better” there is a little science, well, physics involved.
In this article, I will try and explain in the simplest terms possible how to select the correct size of pump for your hot tub without too many techie bits!
I’m replacing a pump for my Hot Tub, what size do I need?
As I am sure you know, on this blog and my YouTube Channel, I focus a lot on building your own hot tubs from scratch. However, a lot of my material is also watched and read by hot tub owners servicing their own tubs.
Therefore, I want to start with the replacement pump question. In short, you need to be replacing like for like. Look for a part number on the top of the pump. Look for a model number and do what most of us do, plug it into Google.
There are a lot of pumps on the market that are advertised as being direct replacements for certain models. Google is a wonderful resource and 9 times out of 10 you can find the model online you need. There will also be a matching store that will deliver it to you.
When you are replacing a pump, get your self some new “O Rings” for the pump unions. This is the rubber seal that you will find inside of the O ring that stops the pump from leaking through the union. Well worth replacing if you are taking the pump off the system.
What if I don’t know the model of the pump?
It can be a little trickier if you don’t know the model you need to replace, but usually not too hard to find then replacement pump you need. You should be able to find some power ratings on the label, even if there is not a model number. Looking for how much current is drawn is a good indication of the horsepower of the unit.
Take a look inside the control box (with the breaker off) if you have 3 wires going into the pump it is a single speed, 4 wires and it is a dual speed.
Check the Frame Size
There are pre-drilled holes in the housing around your pump where it will be bolted into place inside your Hot Tub. Depending on the size of the frame, the holes are further apart or closer together, so you would not be able to connect it correctly if you select the wrong size frame.
Either 48 or 56 are Hot Tub pump frame sizes. You should be able to locate one of those numbers on the sticker on your pump. If not, you can easily measure the distance between the bolts. It’s a 48 frame when the bolts are closer than 4 inches. It’s a 56 frame if the gap is over 4 inches.
So as you can see, we should be able to build up a set of credentials of your pump and then find you a replacement. If you are getting stuck, get in touch and I can try and help.
OK, so back to sizing a pump correctly on a new built or newly designed hot tub.
Misconceptions of Correctly Sizing a Hot Tub Pump
I want to cover a couple of the misconceptions that surround sizing a pump correctly.
Putting in a bigger pump solves your flow problems.
This is not necessarily the case. Flow is a combination of the pump, the pipes and fixtures and fittings, so we need to look carefully at these in combination. Increasing the pump’s size in terms of the Horsepower output without increasing the size of the pipes, can have little to no effect in some cases.
Increasing the distance to the Control Room can be solved by increasing the size of the pump.
Same as above really, you can increase the distance of your tub from the control room to the hot tub, but you can’t just increase the pump size. The size of pipe is important too as if the pipe is too narrow, then the water friction inside will be too much. Even if you increase the power of the pump, that will not be enough to overcome the friction so the end result will not change.
I can add lots of jets and just use a bigger pump
Again, I am sure you are beginning to see the pattern here. This is like the above. Each jet has a required flow rate. A pump has a maximum flow rate. Flow rates are decreased by distance, pipe size and the amount of fixtures and fittings used.
Therefore, just because you add a bigger pump, doesn’t necessarily mean you can overcome all the restrictions on the flow rate to achieve what you need. It needs to be calculated.
Pipe Size is Important when Correctly Sizing a Hot Tub Pump
The size of the pipe that you are using on your hot tub is important. This is because different sizes of pipe have different flow rates attached to them. This is really helpful when we are planning the pipes to and from the control room for example.
By using a larger pipe, we can overcome the water friction and loss of flow. In most cases meaning we can use a lower powered pump to achieve the outcome we need.
Lets take a look at some examples. In the example below, I am going to look at the flow rate on pipes with a constant 100GPM discharge on 100’ (50’ there and 50’ back) of pipe at different sizes with a few fixtures and bends added – again these are all constant. All we are changing the pipe size.
We’ll see how the Total Dynamic Head (in simple terms the amount of friction restricting the water flow) or TDH actually changes and all we are doing is changing the pipe size.
Data taken from the Cornell Pump Toolkit
Pipe Size Total Dynamic Head
1.5” Pipe 76.46’
2” Pipe 26.86’
2.5” Pipe 14.39’
3” Pipe 8.35’
You can see here just how much the TDH actually drops when you use larger pipes. Why is this important?
Why is Total Dynamic Head Important When Correctly Sizing a Hot Tub Pump?
Take a look at the chart above which gives some generic estimates of flow rates for pumps. If I know that I have a hot tub with 16 jets which is pretty standard in my deisgns and builds. I know that each jet needs 10GPM of flow so I need 160GPM of flow to power all my jets.
If I take the examples above on 1.5” pipe, even with a 5HP pump which is huge, I cant get to 160 GPM with almost 80’ of head – there would be next to now flow! This is due to pipe friction known as Total Dynamic Head.
Conversely, if I am on 3” pipe, then a 1HP pump isn’t going to be enough as it would max out at about 140 GPM which means not all my jets would work. However, a 2HP would just about get me there at 180 GPM.
I would be comfortable on a 3HP though to allow for a little extra loss and a 4HP and 5HP in this example would be an overkill and too much power and flow.
What Happens when I don’t have enough flow?
Apart from the obvious that not all your jets work, not having enough flow can also cause problems with the blower. If there is not enough flow then there is no venturi effect for the blower.
This means that the air is not being drawn into the jets. What actually happens is that the water pump starts to work against the blower and force water down the air lines as water always takes the easiest route. In turn, this extra pressure on the blower will eventually burn it out. You can see this in the tub in that the blower works until you turn on the jet pump then the “activity” in the tub gets less and less as the more water works against the blower.
So, not only will you not have a hot tub that works, you will also burn out some of your kit.
Food for thought I am sure!
In order to size a pump correctly for a hot tub, you need to know the pump’s maximum output capacity in gallons per minute. You also need to know what total flow rate you need to power all your jets.
From there, you need to calculate what the total dynamic head is for your setup which is the distance runs of pipe to and from your control room along with height changes and all of the fixtures and fittings you plan to use. There are lots of tools you can use online to help with this.
Then, armed with that information, you take the total dynamic head and compare that to the pump’s output range chart to see if it will work in your setup.
Easy right? It is if you do it day in and day out! However, if you need some help, then please do get in touch
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 900 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