When you are thinking about your DIY Hot Tub, the control room placement is kind of key. In this article, we will examine the variables that can effect the control room position and ultimately the functionality of your tub.
Distance from the Hot Tub
The distance you have your control room from the hot tub is important. In a perfect world, I always advise that they are as close as possible. I know however, in some instances, this is just not possible. Or, it is not aesthetically pleasing to do so. Therefore, you need to have a run of pipe to and from the Hot Tub.
Increasing the amount of pipe involved in the system, increases what we call “head”. In the most basic of terms, this is essentially the amount of water that the pump needs to move and circulate through the system. I want to keep this explanation as simple as possible so I am not going to cover the calculations that are done in order to calculate the amount of head. The purpose of this article is to allow you to understand the implications of control room position will have on “head”.
As a general rule of thumb, the bigger the “head” the larger the pump that is needed. There are a number of calculations that one can do to calculate the amount of “head” that is going to be in the system. If I am designing your system, then this is certainly something that I do for you.
In addition to the physical distance, one must also take into account any rises in height. This rise in height needs to be added to the “head” value as obviously, pushing water uphill is more difficult so there is more stress on the pump. I guess it is kind of logical.
Every twist, turn, bend, Tee joint, filter, valve etc etc all has an effect on head as they increase the friction the pipe has on the water as it flows through the pipes. When you add all of these together, you calculate what is called Total Dynamic Head. This is the linear equivalent of the friction loss of all of these components.
What can happen is if these calculations are not done correctly, you simply do not get the flow you need to power your jets correctly in your tub.
How Do We Combat Friction Loss?
There are a few things that we can do at design stage to combat this.
Firstly, you can increase the power of the pump. Bigger pump, more flow, so it can be situated further away from the Control Room. Pretty straight forward I guess.
Secondly, you can add pumps. What I mean by this is that you can run two pumps in parallel (please note running pumps in series has little impact on flow). By combining two pumps together in parallel, you can create a more powerful flow of water through your jets.
Thirdly, you can separate the jet pumps from the circulation system. If you are running the water through the filter, through a heater, this is all adding the the head and loss of flow. Separate this and use a separate smaller circulation pump for this aspect of the tub, and just have the jets running directly to the pump and back.
Lastly, if the distance is great, say 30′ or 10m away from the tub, you are going to want to use larger pipes on the run to and from the tub. Larger pipes have less friction so less impact on flow.
For example, the friction loss on 30′ or 10m to and from the tub of PVC schedule 40 pipe 2″ in diameter with an initial flow of 260 GMP is equivalent to 55ft of Total Dynamic Head. Increase those pipes to 3″ and that reduces to 12 ft of head. (source – Cornell)
This is a huge difference just by increasing the diameter of the pipes.
A mini conclusion here is that we need to make sure that the flow calculations are correct before you start you build. It is much harder to rectify once pipes and plumbing are in place.
Control Room Above or Below Ground?
Ideally, below ground.
If you think about most hot tubs, and by most hot tubs, I mean the plastic shell ones. You will always find that the pumps are located in the base. Again, I am stating the obvious here.
However, the position of the pump is important. Hot tub pumps are designed to force water out through the jets at high pressure. The are not designed to suck air. What do I mean here?
I’m trying to keep my explanation as simple as possible and not use technical jargon. Think of a hot tub jet pump as being able to work, eg force water out at high pressure through the jets, only if it has water in the first place.
A hot tub jet pump is not designed to suck air out of the pipes to draw in the water from the hot tub. In fact, they are really bad at sucking air. What that means, is that you need to “feed” water into your pump. How do we do this? Simple, gravity.
Water will aways fill a system to a level. The level, is the water height and we know that the water that is in the system will pretty much always (ignoring air locks here) will be at the same height in the entire system.
Therefore, we know that if our pump is below the water line, then it will be “fed” water by gravity and will not have to suck it from the tub. This is exactly what we are looking for if we are using Hot Tub Jet Pumps – the pump must be located below the water line so that gravity can do its thing and water can flow into the pump.
You may hear the term “prime a pump” being used. This is just that. Priming a pump is removing the air that is in a system so that it is completely full of water only. Remember, Hot Tub pumps are really bad at sucking air, so we need the pipes to be full of water in order for them to work.
If the hot tub pump is above the water line, water will not travel uphill by gravity, so water will not flow into the pump. The pump will not be able to suck water up hill as they are not designed to do that, and the system will fail, regardless of how big of a pump you have. The system will not “prime”
The rule here is simple. If you are using a hot tub jet pump, or a hot tub circulation pump, they must be located below the water line on the tub.
If you are still not sure about this, it’s simple. Ask yourself this question. If I filled my hot tub full of water and did not have a pump connected, would water naturally flow into my control room? This is what you are looking for. If the answer to this is “no” then you need to change the control room configuration.
Above Ground and Above the Water Level Control Rooms
If there is no way that you can locate your control room, in particular the pump to be below the water line, then don’t worry. There is a solution, we just need to change the type of pump that we will use.
As you will have already read in this article, hot tub pumps are not good a sucking air to get themselves to “prime” (be full of water). There are pumps that are specially called “self-priming pumps” that are used normally for in ground swimming pools. These pumps are designed to “suck” – not in a bad way of course!
What these pumps can do is basically draw the water into the pump themselves. So, rather than having gravity do it, they can do it for themselves. They can prime themselves, hence the technical name, self priming pumps.
There is a downside to these in that the flow rates of self priming pumps are lower. They are designed really for swimming pools and moving large bodies of water slowly through a system. They can however be used as jet pumps but the flow outputs must be checked. Unlike Hot Tub pumps, where a 3HP from one manufacturer will have a very similar output to a 3Hp from another – this is not the case with swimming pool self priming pumps. They differ immensely.
It is also the case in general that the what I call “swimming pool tax” comes into play. Anything that has swimming pool associated with it is more expensive and this is the case with the pumps too. They can be anything up to twice as much for the same output as their hot tub counterpart.
So, in conclusion, you can locate your control room pretty much wherever you like. However, you must take into account ehe distance and the impact this will have on head and and subsequent flow. You also need to make sure you know whether you control room, specifically the pump, is above or blow the water line of your hot tub. Above the waterline, you need a self priming pump. Below the waterline, you can use a regular hot tub jet pump.
Planning, planning, and more planning. Very important if you want your system to work.
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