Makes and Models of Hot Tubs

Now that we have a brief introduction to typical hot tub filter types, how do we make a selection and what size do we need for a given job? The intended use of the hot tub and/or the local building and health codes will guide us in answering these questions.

Sizing and Selection

Given that there are at least three types of filtration, how do you select the one that's right for your hot tub? First, you need to know the required size; then you can apply several simple selection criteria.

Sizing

In many jurisdictions, the required turnover rate for a hot tub is once every 30 minutes. Using this information and knowing the total volume of your hot tub, you will know how many gallons or liters per minute the filter must be able to handle.

As an example, my hot tub is 1000 gallons (3785 liters). To turn that over every 6 hours, I need a filter that can handle:

1000 gallons (3785 liters) in 30 minutes
     = 1000 gallons ÷ 30 minutes (or 3785 liters ÷ 30 minutes)
     = 33.3 gallons per minute (or 126 liters per minute)

Many building or health codes also determine the maximum number of gallons or liters per minute of water flow permitted for every square foot or square meter of a filter's surface area. Just for the record, many codes also specify the minimum rate of flow during backwash. These may differ for residential and commercial pools, but many jurisdictions require the following:

Note that cartridge filters are rated at 1 gpm per square foot (3.8 liters per 1000 square centimeters) maximum flow rate on most residential applications and the more stringent 0.375 gpm per square foot on commercial installations. This is because cartridge filters are used mostly on hot tubs where many bathers sit in a relatively small amount of water, creating lots of bacteria, oil, and dirt for the filter to handle. The 0.375 rule simply means a commercial installation will have a larger filter than one at home where the bather load will probably be less. So with this information, if we chose a sand filter, we divide the required flow rate in gpm for our sample hot tub, 33, by the maximum flow rate of our local code for sand filters, 15. The next step is to divide 33 by 15, which equals 2.2 square feet. This hot tub therefore needs a sand filter of just over 2 square feet. Since sand filters are not made this small, a hot tub of 1000 gallons would not be serviced by a sand filter unless it were attached to a pool with a larger volume of water.

Using the same example, my 33 gpm rate with a DE filter is:

      33 / 2 = 4.1 square feet

So we need a DE filter of at least 4 square feet. There are no DE filters this small either, so again this filter medium would not be used for a 1000-gallon hot tub by itself. Finally, we have the cartridge filter in the same application:

      33 / 0.375 = 88 square feet

There are plenty of cartridge filters of this size. Just for comparison purposes, note that we would need an 88-square-foot cartridge filter to do the same job as the 2-square-foot sand filter or the 4-square-foot DE filter . Another sizing and selection criterion is dirt. How dirty does the body of water get that this filter must service? You want to oversize the filter a bit so you don't have to clean it as often. This time between filter cleanings is called the filter run. So you might as well get a filter 20%, 50%, or 100% larger than you actually need so it can hold more dirt and thereby leave more time between cleanings—right?

Well, not exactly. If the pump cannot deliver the flow rate in gpm for the square footage of this huge filter you have just installed, then only part of the tank will fill with water, effectively giving you a smalle filter anyway. Also, the pump won't backwash the filter completely if it can't match the gpm rating, which is why plumbing codes call for a minimum backwash gpm flow rate. Make sure your pump is rated for a flow rate at least the same as or greater than that of the filter.

Selection

Knowing the size filter you need for your hot tub is the first selection criterion. Price, size, and efficiency might be additional criteria to consider. Price is a factor of the current market and supplier availability in your area. Check with your hot tub supply store. The overall size of the units in the above example might be a deciding factor if the equipment area is small. Sand filters are large, even in the lowest square footage configura- tion. A cartridge filter is much more compact, but the filtration medium (the cartridge) can be expensive. A DE filter will be slightly larger than cartridge filter, but the cost of DE is less than that of sand or cartridges.

Finally the efficiency of the filter is worth considering. Filters are d based on the size of particle they can effectively remove from the water as it passes through the medium, expressed in microns. A micron unit of measurement equal to one-millionth of a meter, or 0.0000394 inch! To put it another way, the human eye can detect objects as small 35 microns, talcum powder granules are about 8 microns, and table t is about 100 microns. However, here's a rule of thumb:

• Sand filters strain particles down to about 60 microns

• Cartridge filters strain particles down to about 20 microns

• DE filters strain particles down to about 7 microns

The efficiency of DE filters is affected by the cleanliness of the medium. Remember that DE is composed of millions of tiny spongelike particles. As the DE becomes clogged with fine particles, it will lose the bility to strain the smallest impurities. Sand becomes less efficient as it ages. The sharp edges of the grains become round, allowing the finer dirt to pass by.

Because cartridges do not rely on organic material such as sand or DE, :they are not affected the same way—as long as the cartridge surface area remains clean, it will filter the same size particle. However, extremely old cartridges that have been acid-washed many times will stretch out somewhat, creating a mesh that is not so fine as when new. Dirt, however, does affect the cartridge filter, as it affects all filters, but with the cartridge it may actually have a positive effect!

Dirt makes the mesh of the polyester cartridge material even tighter, essentially acting as DE. I have known some service technicians who even add a small handful of DE to a cartridge filter after cleaning it, to start this process. Thus, with each successive pass of water through a cartridge filter, it will become more efficient as the mesh gets finer and finer from the retained particles. This is also true with sand filters up to a point. Of course at some point, dirt prevents water from passing through the filter and the unit ceases to function properly.