Any physicist out there? - PeachParts Mercedes-Benz Forum

speednjay · #1 10-10-2020, 05:16 PM

I’m building a Frankenstein hot tub. External Soft tub motor To a regular hot tub. Motor is in a box About 10 feet away with piping dug underground. I’m going to supplement heat through convection heat absorbed through the fire pit. The soft tub motor is only 1 hp and uses heat from the motor itself to heat. I’ve had soft tubs for years and it works great 106 degrees all day. Why would I do this? I’m cheap it’s this is all free stuff except the piping and misc.

So my question is

can a 1 hp pump have the justice to push water the extra distance?

Abs has a Max temp rating Of 140f ., if I run a heat exchanger from a fire pit will I exceed the rating? I may run a separate line for that or could run a larger abs pipe for the water to absorb the heat more evenly. 1 1/2 to 3”.

I think I’m good on the specs but I won’t know till I fire it up.

t walgamuth · #2 10-10-2020, 10:52 PM

one hp is a lot of power for pushing water through a pipe. What diameter is the pipe?

Tony H · #3 10-11-2020, 01:15 PM

At our last house I remote mounted the pump on our spa so I would not have to hear it. I used 2" PVC. Did not make any difference in intensity. I insulated the pipe-helps with sound transmission and the obvious heat loss. I had envisioned a wood fired heater but we moved before I could do it. I had a buddy that lived up in the mountains and he built a wood fired box that was in the spa(a large wood tub) with a electric trolling motor to circulate the water. It would go from ice on top to 100 deg in a few hrs.

speednjay · #4 10-11-2020, 06:34 PM

Quote:

Originally Posted by t walgamuth

one hp is a lot of power for pushing water through a pipe. What diameter is the pipe?

1 1/2” it should work without bogging the motor down too much.

Angel · #5 10-12-2020, 02:30 PM

As I recall, my sump pumps are 1.5hp, and will lift water 13+ Ft and then throw it.
The question is not how far you need the water to go, but how far upward it needs to go. Im guessing that you don't have more than 3 or 4ft of vertical lift needed, so 1hp should be more than plenty.
...yes, there is a lot more than this that goes into the equation, but if you just need water to move, I expect it'll be fine.

I'm more concerned about the heat source. Basic wood fires exceed 400F (maybe 600-700F at the hottest part, hot enough to soften Iron but not hot enough to melt it away...those classes were a long time ago...)
- Do not let the water get stationary in those pipes.
...better yet, adjust heights to ensure 'natural circulation'

taking this thought another direction: Assuming you start the pump before the fire (I guess that should be obvious) you may consider a valve somewhere inline that lets you throttle water flow through the fire - this allow you to get the water hotter before it goes into the hot tub. Dicey game to play, but good installs will have isolation valves for pumps and heat exchangers anyway.

-Not a mechanical engineer....anymore

t walgamuth · #6 10-12-2020, 10:58 PM

Quote:

Originally Posted by speednjay

1 1/2” it should work without bogging the motor down too much.

I'd just take a stab at it but have done pipe calculations before and a 2" will flow nearly double what the 1.5 will. Might be worth an upgrade.

INSIDIOUS · #7 10-13-2020, 10:35 AM

plumbing --- chemical engineer, process engineer, wet civil ....

75Sv1 · #8 10-13-2020, 12:10 PM

My well in WV is roughly 200 ft deep. I think I have a 1.5 hp pump. So, the distances you are talking about, I'd think a 1 hp would work.

speednjay · #9 10-13-2020, 05:19 PM

Quote:

Originally Posted by t walgamuth

I'd just take a stab at it but have done pipe calculations before and a 2" will flow nearly double what the 1.5 will. Might be worth an upgrade.

The flanges on the pump are only 1.5”. Maybe next year I’ll redo it with 2”. I already have all the plumbing parts for the 1.5. Or maybe just the return line in 2” to prevent suction build up. It’s gotta push water out about 8 jets.

Ok gonna work on the fire pit heat exchanger later today. Currently have a propane pit but it’s been not reliable. I’m either gonna go to natural gas from the house or back to a charcoal/wood fire. It’s nice flicking on the fire on and off with a switch without all the drama of smoke.

I’m going to take have a y pipe tee off down to maybe a 3/4 steel pipe with a snake like thing that sits in the fire pit then tees back into the main line.

speednjay · #10 10-18-2020, 05:24 PM

Almost got everything together. It’s costing a lot more than anticipated. Just gotta plumb the last few feet, fill it up, check for leaks then click on the pump. Hopefully it all works

speednjay · #11 10-29-2020, 04:56 PM

It’s alive

t walgamuth · #12 11-04-2020, 09:41 AM

Quote:

Originally Posted by speednjay

The flanges on the pump are only 1.5”. Maybe next year I’ll redo it with 2”. I already have all the plumbing parts for the 1.5. Or maybe just the return line in 2” to prevent suction build up. It’s gotta push water out about 8 jets.

Ok gonna work on the fire pit heat exchanger later today. Currently have a propane pit but it’s been not reliable. I’m either gonna go to natural gas from the house or back to a charcoal/wood fire. It’s nice flicking on the fire on and off with a switch without all the drama of smoke.

I’m going to take have a y pipe tee off down to maybe a 3/4 steel pipe with a snake like thing that sits in the fire pit then tees back into the main line.

If you have 1.5" flange that should be fine for sure.

speednjay · #13 11-05-2020, 12:05 AM

Works beautifully. I gotta get the temp dialed in. For not having a heater it’s way too hot.

speednjay · #14 11-05-2020, 12:12 AM

Works beautifully. I gotta get the temp dialed in. For not having a heater it’s way too hot.

barry12345 · #15 11-05-2020, 01:00 AM

As a rule of thumb the output size on the pump is what you use for pipe size other than if flow restriction with friction is involved on long pipe runs.

You do have a ground fault breaker on that motor feed circuit? It is not optional in those situations.