Dual Spa Filters: Double the Filtration!

Here's a short story about how I doubled the filtration on a spa that we have on our vacation home. We rent out the mountain home and hot tub when we're not using it (which is most of the year), and we were having some trouble keeping the water clean between visitors, without changing the water (too much).

My husband actually gets credit for the idea - he said "why don't we just add another filter on the spa?". We took a look at the complete spa filters and found one that would fit inside the spa cabinet.

Here's how we added a second filter to our portable spa.

Adding a second spa filter

The filter that comes with most portable spas is a skimmer filter, located on the suction side of the spa pump. The second filter is added on the pressure side of the pump, or after the pump. These inline spa filters, as they are called, have an internal bypass feature that opens up when the pump is on high speed, so it won't affect the jet flow.

Inline spa filters are available in 25ft, 50, 75 and 100 sq ft sizes. The canister becomes very tall, as the square footage of cartridge increases, and in my case, we opted for a 50 sq ft spa filter that we could fit underneath the cabinet.

There are 3 main ways to install a secondary spa filter, under the cabinet, outside the cabinet, or top loading, through the cabinet corner.

UNDER THE CABINET

This is how we opted to do it, because our spa shell or cabinet doesn't have corners where we could cut out a 6-3/4" hole and use a top load filter. We of course had limited space under the cabinet in the equipment bay area, under the spa, but took some careful measurements and decided that we could use a 50 sf inline spa filter.

Adding a second filter required a little re-plumbing from the pump, in and out of the new filter, and then back to connect to the return pipe. To clean the filter, I just close the suction valve and unscrew the ring, and then pull the entire canister and cartridge out.

You can use inline filters with bottom connections (at ground level), but the top mount style where the pipes connect on top of the filter, is usually easier plumbing, and bottom mounts need vertical space to pull out the cartridge. My inline filter is a top mount, sits a few inches off the ground, so you can separate the canister from the filter top (after loosening the brown lock ring).

Wherever you locate a spa pressure filter, be sure it is after the pump and that you plumb in the filter in the direction of the flow indicator (printed on the manifold, where the pipes connect). It should also be easily accessible for cleaning, without contorting your body in awkward positions!

OUTSIDE THE CABINET

If there is no room inside your spa cabinet, you could go hot tub style, with the secondary spa filter located outside of the spa cabinet. Less attractive, but it makes the filter very accessible for cleaning and easy to monitor the filter pressure gauge. And, outside the cabinet gives you room to add a large second spa filter, like a 100 sq ft Waterway Extended Top Load spa filter. A Hayward Star Clear filter also makes a nice choice.

The usual way to install a secondary spa filter outside the spa cabinet is to use a 2" hole saw attached to a drill to cut through the cabinet - 2 holes, one for the pipe coming into the filter (from the pump) and one for the pipe coming out of the filter and running back to the return line. Be sure to plan out the complete pipe run before you cut the holes, and be sure there are no wires or studs running over the area where you are using the hole saw.

You can use 1.5" (or 2") schedule 40 flexible PVC pipe or rigid PVC pipe, and try to use as few 90's as possible to decrease the amount of added resistance. We installed our second spa filter using only two 90's - but when installing outside the cabinet you'll have to use 3 or 4, 90° fittings. Essentially, you are cutting the pipe after the pump, and diverting the water in and out of the filter, before reconnecting it to where the pipe was cut.

Use fresh PVC glue and primer, and only use deep socket couplings, aka pressure fittings (not drain fittings). You can find all the supplies you need at a good hardware/home store.

TOP LOAD FILTERS

Many spas with a square cabinet have molded circular areas in the corners of the spa. Top load spa filters are those that stick out of the top of the spa shell, but under the spa cover. They are called Top Load because you unscrew the top and pull out the cartridge, or load the cartridge from the top.

Most Top Load spa filters are sized to fit a 6-3/4" hole, which is cut into the corner of the spa shell, using a Jig Saw or Dremel tool. A lock nut holds the spa filter canister tightly against the spa shell. Top Load filters are not underwater, they sit above the water level. Spa Skimmers are another type of filter that can also be installed on some spa shells.

The plumbing on Top Load spa filters is on the bottom of the canister, with the connection point dependent upon the size of the filter. You can buy Top Load filters in 25, 50, 75 and 100 sq. ft. sizes, each one taller than the next. The plumbing connection can be either threaded, or slip for smooth pipe connection.

Use fresh PVC glue and primer, and only use deep socket couplings, aka pressure fittings (not drain fittings). You can find all the supplies you need at a good hardware/home store.

Soooo... if your spa can use extra filtration, to extend the water life (time between water changes), or to help reduce the amount of sanitizer or other needed spa chemicals - consider adding a second filter to your spa or hot tub.