DIY Hot Tub, 750 gallons, Questions and Build Updates

Dirrby

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Oct 26, 2020
57
Poway, CA
Pool Size
800
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Hi there, TFP community! I’m building a 750 g hot tub and will be using this thread to post updates as well as ask for advice along the way. At the moment, I could really use some feedback on the pump and filter I’ve purchased. I’m concerned the pump is oversized and the filter is undersized. This will be a single pump system with a cartridge filter installed downstream of the pump. Here are some of the high level details of the design.
  • 750 Gallons
  • Cinder block, above ground construction
  • 220 v, 50 A power
  • 2 speed pump: 14 A @ 3,350 rpm, 1.2 A @ 1,000 rpm (4 hp cd, 5 hp spl)
  • Pump, filter and controls within 2 ft. of tub
  • 2.5 inch suction, 2 inch return lines
  • 150 sq ft. filter cartridge
  • 6 jets (2 inch water, 1.5 inch air supply lines)
  • 2x 4 kw titanium heating elements (8 Kw total)
  • Topside air control valve, no blower
Below are the mfg. specs on the pump. If anyone has any thoughts or advice on whether this will be suitable for my application I’d appreciate any feedback. Thank you!

1619119443634.png

1619119457886.png
 

ajw22

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Jul 21, 2013
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Northern NJ
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mas985

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A proper spa design starts with the type of jet you are using. What is the orifice size of the jet? Make/Model #?

A moderately strong jet requires about 45 ft/sec out of the orifice while a very strong jet requires about 55 ft/sec. From that you can determine the flow rate per jet and the total flow rate required as well as the head loss of the jet. This will determine the pump size that is required as well as the pipe size you should be using.
 

Dirrby

Gold Supporter
Oct 26, 2020
57
Poway, CA
Pool Size
800
Surface
Plaster
Chlorine
Liquid Chlorine
A proper spa design starts with the type of jet you are using. What is the orifice size of the jet? Make/Model #?

A moderately strong jet requires about 45 ft/sec out of the orifice while a very strong jet requires about 55 ft/sec. From that you can determine the flow rate per jet and the total flow rate required as well as the head loss of the jet. This will determine the pump size that is required as well as the pipe size you should be using.
Thank you for the reply, mas985. Here is the jet body that I've purchased. I still need to purchase the jet internal, but I was considering 3/8 inch orifices. So at 55 ft/sec through 3/8 inch orifice I would need 19 gpm per jet (114 gallons per minute total)? As for calculating head loss of the jet, I may need to do some more digging. I've found calculators for pipe runs and fittings, but not for the actual jet orifices. If someone could point me in the right direction, that be great too!
1619139680337.png
 

Dirrby

Gold Supporter
Oct 26, 2020
57
Poway, CA
Pool Size
800
Surface
Plaster
Chlorine
Liquid Chlorine
Sounds like a fun build, how bout some pics?
Yeah, I'm really excited about this project. I plan to start setting blocks this weekend so I'll post some pics on Monday of the progress.
 
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mas985

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There is a spreadsheet in my signature that can be used to do a full head loss calculation for the plumbing. The issue though is that I do not have those pumps you listed in the model. However, it really isn't necessary since those pumps are not really appropriate for spa jets since they are medium head pumps. Anyway, you will probably need to use 3" pipe for that flow rate and a fairly large pump. If I use the Intelliflo XF pump with your plumbing setup with 3" pipe, even at full speed, the pump will deliver only 109 GPM @ 81' of head which is short of the operating point you would need for that kind of flow rate. But keep in mind that 19 GPM for 3/8" jets are very powerful and could actually hurt your back. Moderate strength at 46 ft/sec or 16 GPM/jet could be supported by the WFE-6 @ 61' of head which is more reasonable and could possibly be supported by the pump you suggested.
 

RDspaguy

In The Industry
Mar 21, 2020
1,382
Cabool, Mo
But keep in mind that 19 GPM for 3/8" jets are very powerful and https://www.troublefreepool.com/att...84&hash=d42c3fb55feba0993f5c480695b55df9could actually hurt your back.
The smartest thing I've heard in this thread.
may have an interest in this.
:rolleyes:

will note that any pump or electrical wiring or controls within 6 feet of the water does not meet the National Electrical Code (NEC).
It must be gfci protected.
Ok @Dirrby. Are you a bricklayer? Pool builder? Dam engineer? Do you know if your cinder block wall is going to hold back all that water? What are you building it on? Can it support 3 tons of water without cracking?
What is controlling these pumps and heaters?
Are you using a liner, paint, plaster, or leaving the cinder block bare?
Have you calculated your expected costs and compared to used spas in your area?
As always, I strongly recommend you consider purchasing a used spa.
Good luck.
 

CuckooChris

Well-known member
Sep 14, 2016
84
Coplay, PA
Watch your power requirements. 2 x 4kW heating elements = about 36A + your pump on high speed is 14A, that's 50A. You said you were using a 50A breaker, so you may want to check with an electrician. I thought the rule was you shouldn't go above 80% of the breakers capacity.
 

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ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
28,518
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Watch your power requirements. 2 x 4kW heating elements = about 36A + your pump on high speed is 14A, that's 50A. You said you were using a 50A breaker, so you may want to check with an electrician. I thought the rule was you shouldn't go above 80% of the breakers capacity.

Max load on a 50 amp breaker allowed by the NEC is 40 amps (80%).
 
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Dirrby

Gold Supporter
Oct 26, 2020
57
Poway, CA
Pool Size
800
Surface
Plaster
Chlorine
Liquid Chlorine
Watch your power requirements. 2 x 4kW heating elements = about 36A + your pump on high speed is 14A, that's 50A. You said you were using a 50A breaker, so you may want to check with an electrician. I thought the rule was you shouldn't go above 80% of the breakers capacity.
Good point! I should have noted that the spa controller will be configured to only power one of the 4 kw heaters when the pump is on high speed and both elements on low speed. So I should be good to go at 32 A.
 

Dirrby

Gold Supporter
Oct 26, 2020
57
Poway, CA
Pool Size
800
Surface
Plaster
Chlorine
Liquid Chlorine
The smartest thing I've heard in this thread.

:rolleyes:


It must be gfci protected.
Ok @Dirrby. Are you a bricklayer? Pool builder? Dam engineer? Do you know if your cinder block wall is going to hold back all that water? What are you building it on? Can it support 3 tons of water without cracking?
What is controlling these pumps and heaters?
Are you using a liner, paint, plaster, or leaving the cinder block bare?
Have you calculated your expected costs and compared to used spas in your area?
As always, I strongly recommend you consider purchasing a used spa.
Good luck.
No, I am none of those things. Just an avid DIY'er hoping to build something that works, is safe, that I will enjoy and be proud of. The tub will be built on an existing pad that was installed by the previous home owner to park a large motorhome. I will measure the thickness and report back but I'm guessing it's at least 6 inches thick.

As for the construction of the spa I'm following plans readily available online. I'm hesitant to give credit to the web site because I don't want to break forum rules for new members. The lowest 16 inches of water column will be enclosed with 3 rows of cinder block with a total wall thickness of 24 inches. The inside 16 inches will serve as 16" high 16" wide bench seating. Above that, the walls will only be one row of cinder blocks, but the water depth at that point will be a maximum of 20 inches, or about 90 lbs of outward force on each cinder block by my calculation.

For water proofing I will use BaseCrete and then pool/spa paint.

I did look into purchasing both new and used spas but those options didn't appeal to me as someone who likes to DIY.

The control system is off the shelf and was purchased as a package with the heaters, pump, spa light and will be pre-configured. Apparently the company supplies spa controllers for many different hot tub manufacturer's and I was able to talk with their engineer who helped me spec out the system.
 

RDspaguy

In The Industry
Mar 21, 2020
1,382
Cabool, Mo
No, I am none of those things. Just an avid DIY'er hoping to build something that works, is safe, that I will enjoy and be proud of. The tub will be built on an existing pad that was installed by the previous home owner to park a large motorhome. I will measure the thickness and report back but I'm guessing it's at least 6 inches thick.

As for the construction of the spa I'm following plans readily available online. I'm hesitant to give credit to the web site because I don't want to break forum rules for new members. The lowest 16 inches of water column will be enclosed with 3 rows of cinder block with a total wall thickness of 24 inches. The inside 16 inches will serve as 16" high 16" wide bench seating. Above that, the walls will only be one row of cinder blocks, but the water depth at that point will be a maximum of 20 inches, or about 90 lbs of outward force on each cinder block by my calculation.

For water proofing I will use BaseCrete and then pool/spa paint.

I did look into purchasing both new and used spas but those options didn't appeal to me as someone who likes to DIY.

The control system is off the shelf and was purchased as a package with the heaters, pump, spa light and will be pre-configured. Apparently the company supplies spa controllers for many different hot tub manufacturer's and I was able to talk with their engineer who helped me spec out the system.
Well, you've clearly done your homework. I am glad you are using a manufactured control system and not trying to diy that as well. Safety is always my first concern.
I hope it works out as you expect. (y)
 

Dirrby

Gold Supporter
Oct 26, 2020
57
Poway, CA
Pool Size
800
Surface
Plaster
Chlorine
Liquid Chlorine
Regarding jet sizing I found this cheat sheet in the how-to instructions I'm following. Once I have a better understanding of the dynamics of my system I will figure out which jets I need.

1619193916124.png
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
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Pleasanton, CA
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12 PSI is a bit on the low side. Normally you want at least 15 PSI for moderate jets. Here is a full table for jets based on pressure:

1619194848755.png
 

Dirrby

Gold Supporter
Oct 26, 2020
57
Poway, CA
Pool Size
800
Surface
Plaster
Chlorine
Liquid Chlorine
There is a spreadsheet in my signature that can be used to do a full head loss calculation for the plumbing. The issue though is that I do not have those pumps you listed in the model. However, it really isn't necessary since those pumps are not really appropriate for spa jets since they are medium head pumps. Anyway, you will probably need to use 3" pipe for that flow rate and a fairly large pump. If I use the Intelliflo XF pump with your plumbing setup with 3" pipe, even at full speed, the pump will deliver only 109 GPM @ 81' of head which is short of the operating point you would need for that kind of flow rate. But keep in mind that 19 GPM for 3/8" jets are very powerful and could actually hurt your back. Moderate strength at 46 ft/sec or 16 GPM/jet could be supported by the WFE-6 @ 61' of head which is more reasonable and could possibly be supported by the pump you suggested.

12 PSI is a bit on the low side. Normally you want at least 15 PSI for moderate jets. Here is a full table for jets based on pressure:

View attachment 325681
Thank you! I would definitely prefer to target something in the moderate range. Previously you mentioned that my medium head pump may not be suitable. Is there anything I can do to improve my chances of making it work? Smaller jets, larger filter, larger suction and return tubing, etc. Again, I am planning to use 2 inch return and 2.5 inch suction. I would guess the overall runs including bends, etc., won't exceed 15 ft. from pump to furthers drain or jet, but I know it's easy to under estimate things like that.
 

mas985

TFP Expert
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May 3, 2007
14,686
Pleasanton, CA
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Salt Water Generator
Smaller jets will make it work. With that pump, 15' runs, 2" return with 4x90s and 2.5" suction with 4x90s, here is an estimate by jet size:

3/8": 14.7 GPM/jet - <Moderate
5/16": 11.5 GPM/jet - >Moderate
1/4": 8.1 GPM/jet - Strong

These may be on the high side since I don't know what your heater will do to head loss and flow rate. This would be assuming you use a bypass so that the heater head loss is not an issue.
 

DanF

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Mar 17, 2019
412
Chandler, AZ
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At 750 gallons, that seems to be a massive tub. Mine's 330 and it feels big at times. Good luck and looking forward to pics!
 

Dirrby

Gold Supporter
Oct 26, 2020
57
Poway, CA
Pool Size
800
Surface
Plaster
Chlorine
Liquid Chlorine
120 blocks down, 68 to go. This was my first time laying block and i was having a really tough time getting the morter to stick on the sides. For the verticle gaps i will be able to back fill those, but the gaps on top of the bench seating that wont work as the morter will fall through. I think I'm going to have to fill those gaps with expanding foam and then pour an inch or inch and a half thick layer of self leveler, repair morter, or cement on top of the bench seats. I'm open to ideas.

I'm also going to build a pavillion over the hot tub (10 ft X 12 ft to the outside of the posts) using 8x8, 4x6, 4x4 and 2x6 rough redwood. The lumber is going to get delivered on tuesday so im excited to get started on that as well. The spa controller and pump have a 3 week lead time so i still have a few more weekends before those arrive. Still a lot to do.
 

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