Building an vanishing edge spa

[nickjohnson]

I am in the planning stages of building a vanishing edge hot tub using block construction over a poured pad and was looking for some help and suggestions in the plumbing and overall design. I have extensively researched the matter and have come up with the following:

900 gallon main rectangular spa, 9’-4” long weir with an inward facing edge 2’-8” above the catch basin (to avoid shooting the water overflow out past the wall face). Catch basin is designed to catch 275 gallons of swimmer surge (25 gallons per person, 11 people) above the minimum operating level of 16” (100 gallons). Looking at pushing 4 gallons/minute/ ft over the edge for a total of 37 gal/min (about 1/8”). Not looking for a sheet over the wier, just enough to get the vanishing look from spa side. Will have an auto leveler at the minimum operating level in the catch basin. Three pumps: jets, filter/heat, and edge. I am separating the filter/heat from the edge so I retain as much heat as possible while hot and don’t want the edge. Problem is the initial cold shot when I turn on the edge. This filter/heat only option also forces me into adding a skimmer to the main spa.

Questions: Should I add a skimmer to the catch basin? Add a filter to the edge circuit? Are my assumptions accurate for flow and catch basin capacity? Is it OK to have the edge pump 2’-4” above the minimum operating level of the catch basin to keep it centralized with the other pumps?

Any assistance would be greatly appreciated! Thank you in advance

 

[JasonLion]

Welcome to TFP!

I am assuming that this is a stand alone system. Things get more complex if you are sharing equipment with a pool.

The basin needs to hold a minimum amount of water so the pump doesn't get starved when running, surge from people in the spa, plus the water needed to raise the spa water level by the operating height of the negative edge, pus the water in the air falling off the edge into the basin. I believe you forgot the last two factors: 1/8" times spa surface area of spa + 1/8" times surface area of a sheet of water falling from the edge to the basin (it won't be a sheet in real life, but that is a good enough estimate).

Adding a skimmer or filter to the basin circuit is problematic. For a larger basin you might sometimes have a whole separate basin circulation and filter system, but there seems little point for a basin this small. Still, you do need to have some plan for removing debris. Often this is very simple, just leave room for a leaf rake to clear leaves from the basin, as long as you have enough access to the basin area to be able to run that leaf rake through.

A draw of over 2 feet for the edge pump is possible, but could cause minor issues. I would put a check valve down below basin water level that keeps water in the edge pump feed pipe at all times. That will greatly simplify priming.

You will want to run the edge during the final portion of the spa heat up. You also need to run the edge for at least 15 minutes each day to keep the water mixed.

 

[nickjohnson]

Thank you! The spa is indeed a stand alone system. I did not include the 1/8" times the spa surface or the water falling off as it only equals around 7 gallons due to the small surface area. The basin is 2' wide so it can be raked for leaves. I am trying to figure out a cover design that would cover the catch basin as well as the spa to both keep it cleaner and maintain more heat.

I like the check valve below the basin level idea, and I would put the siphon break on the spa side of the pump above the spa waterline (if I add this feature at all). Then in case the check valve malfunctions, the only water that will drain back into the basin would be in the pipe.