Temperature Control for Hot Tub Project

[brendangillespie]

I'm building a hot tub from scratch; I was able to obtain a free 1999 Jacuzzi hot tub shell from Craigslist. I have two heaters and a couple of various pumps from a previous hot tub project that I built in the past with much success. The only difference now is that in the previous project I had a functional heater control box, so I didn't have to worry about temperature control as I do with my current project.

This time around, I have slightly less to work with in regards to the electronics, but on the up side the complexity (more jets) and overall condition of the shell is much better. I've already built the frame for the shell and checked out all the plumbing and hoses; what I need to do now is wire the heater(s) and pumps.

What I plan to do is build a separate box that will sit next to the tub, which will have the heaters and pumps along with all the corresponding wiring inside. This way access to the wiring and components will be much easier for operation and maintenance. I will be running intake and outtake plumbing from inside the tub to the box and back for suction and to the jets. I will have a circulation pump running all the time and a jets pump to be run only when using the tub. I have two heaters that I want to install; I figure that I might as well use both since I have two, and that even though the current draw will be greater, it will still heat up the tub faster and help to maintain temperature when it is cold outside (the tub will be on a back deck).

Here’s my dilemma: with no control box, I have no way of regulating temperature other than manually turning the heaters on and off via a switch. What I would like to do is have a simple control with which I can set the temperature, and have the heaters turn on when the temperature drops to a certain degree, and turn off when it reaches the desired setting.

Thanks in advance for any help you can provide. I learned a lot from the previous hot tub that I built, but having issues with this one since I don't have a control box, and won't buy a new one. That being said, please don't just tell me to go "buy a control box". I don't want to shell out a couple hundred bux for it, plus I want the satisfaction of building my own! I've done my share of Googling for research thus far into this project; there have been TONS of other guys that have built their own, just few with a good description that I can use.

Thanks!

Brendan Gillespie

[kervis]

Well then.

And I thought I was pretty cool fixing my water feature knob all by myself!  

Good luck in your search.  At first I thought your post might be for a Science Fair project of some type.  We are currently building a Mousetrap Car at my house.  I can't imagine building a hot tub.  

[Chas]

The simplest would be to buy a control system or 'spa pack.' That would also be the safest.

If you want to save money and if you have the skills needed, you could design and build a system, but you would want to be absolutely sure that you include protection against dry-firing the heaters, overheating the spa water, and leaking stray voltage into the water.

That would mean incorporating one or two high-limit switches, a control thermostat, a pressure or flow switch, current collectors and some form of GFI protection. The current collectors may already be built into the housings of your heaters - you did not specify what type of equipment you own.

 8-)

[Hillbilly Hot Tub]

How much would he actually save over purchasing an older style control pack with time clocks, with temp and hi limit probe?

Maybe talk to some local dealers that may have upgraded one of these older systems and have the old one laying around??? Just a thought. I know we just scrap metal them when we get them.

[brendangillespie]

I already have GFI protection and am going to use waterproof switches isolated from the hot tub in the external heater/pump/control box I'm going to build.  I'll have to get out of the hut tub to control things but I'm OK with that.

All I need is the control thermostat and the high limit switches.  Protection against dry firing the heaters is already taken care of - the heaters will never be on if the pumps are off.  I already have a temp sensor installed, I just need to figure out where to buy a good temperature control.  Any tips?

I really appreciate help from those that aren't quick to pass of my home-built project - it's not so much me trying to save money from buying a spa pack, it's more enjoying building my own.

I want to make it clear that I'm not going to buy a spa pack.  So much thanks to those that are willing to offer helpful tips instead of just telling me to buy a pack.

[zroger73]

I am an electrical engineer at an industrial process oven manufacturer, so I'm familiar with temperature and high limit controllers for that application, but I can't think of anything that would be cheap. There are many manufacturers of generic process controllers such as Yokogawa and Watlow that could be used. If this were *my* project, I would tend to go way overboard and use a PLC with a touch-screen interface with the appropriate I/O modules to connect an inline temperature sensor, pressure sensor, and switch inputs. Outputs would energize one or more motor contactors for pumps and relays for lights and ozone. With PLCs, the possibilities are limitless for a fully-automated hot tubbing experience complete with advanced diagnostics that could verbosely indicate maintenance needs or component failures. Right now, I've got a PMA KS 108 Advanced PLC/HMI combo sitting on my desk that programs with a free software package called CoDeSys. This type of setup would run about 10 times the price of an off-the-shelf control system, but would have about 100 times the "wow" factor. Even simple digital indicating temperature controllers and stand-alone high limits will exceed the cost of something pre-engineered such as Hot Spring's IQ 2020. Of course, a PLC would also afford the ability to monitor and control the spa over the Internet. In theory, you could also add chemical sensors and chemical dispensers to completely automate the maintenance process. Carry it further by adding solenoid "dump" and "fill" valves for automated water changes, too. Don't forget the water level sensor for automatic top-offs and the appropriate coding for the PLC to detect the presence and number of users.

[Chas]

If somebody turns on the pumps while the tub is empty, you can dry fire the heaters. Please don't rule out the possibility of this happening. Also, if the flow is reduced - from a dirty filter or a towel dropped into the tub or any of a hundred other things I have seen happen - the heaters can overtemp fast enough to do some damage before the hi limit switch responds. So - I would incorporate some sort of flow switch. This one is available from Robert's Hot Tubs and is rated at 20 amps, but they also have a low-amp model if you only plan on having it run the relay coils:

Here - http://www.rhtubs.com/store/plumbing.htm

Here is a page of thermostats, at a site where you can also buy contactors, high limits and other parts.

http://www.spadepot.com/shop/Thermostats-C702.aspx


 8-)