Creating a Pool Filter Backwash Pipe.. and Fixing My Failure

Backwash Hose to Pool Filter Backwash Pipe

In this post, I replace my pool filter backwash hose with a permanent pipe-based system… and almost completely screw it up! Think of this in three parts; I first build it, then I break it, then I fix it.

The Old Way

We have a DE (Diatomaceous Earth) filter for our pool. It does a fantastic job of filtering out particles but does need to be backwashed or flushed on a somewhat regular basis. We’ve tended to put off that chore since it was enough of a pain to discourage us from doing it until past the point of absolutely needing it. Performing a backwash required multiple tools and multiple steps.

The filter is supposed to have a valve that closes off the backwash pipe when it’s off but ours leaks like a sieve. To combat this, we installed a screw-on pipe fitting:

The cap isn’t water-tight so it needs to be tightened down with a wrench in order to show any semblance of keeping in the water. That also does mean that the wrench is needed to loosen it, too. Specifically, a wide-mouth pipe wrench is needed:

Note how chewed up the cap is from being tightened and loosened so many times.

The next step is to fetch our hose from storage and unfurl it…. okay, I’m kidding. Properly drying out the hose; curling it up; and then putting it away is far too time consuming to be worth doing. As a result, we just keep the hose out in the open in between backwash sessions. This absolutely destroys the hose in the AZ sun, so we have to replace it roughly once a year (or two, if we’re lucky).

We slide the hose onto the end of the pipe:

This picture should also show the next piece, but I lost it just before undergoing this project. It’s a hose clamp that we tighten around the hose to keep it from flying off when the backwash pressure gets going. You can see the tiny holes forming in the hose from the clamp. We need to trim off a couple of inches every so often when they get too bad.

The hose clamp is tightened and loosened with a bolt and that takes enough turns that doing it manually isn’t worth the effort. So we get out our drill with the appropriate sized hex driver and use that. That’s the second required tool (three, if you count the specific sized bit).

Everything needs to be reversed after the backwash is done.

A Better Way

There has to be a better way and after some thought, this is what I came up with — a permanent pool filter backwash pipe. I put a ball valve in place of that old cap which will do a decent job of sealing off the water but allow for it to be turned on with the turn of a lever. Then the hose is replaced with permanent PVC pipe, which will always be in place and should last much longer.

Here’s the video for the motion picture folks:

Stay tuned to the end to see how I completely screwed up as well as how I scrambled to fix my blunder.

Dry Fit

The first step is to remove the old cap and fitting. It’s easy enough to cut PVC with a hacksaw and so I just turned mine upside down and went to town:

With that off, I could do a dry fit of all of the pieces. It’s important to do a dry fit since you only get once chance at gluing together PVC pipe.

I started each piece by doing a rough measurement in-place:

It’s important to make accurate measurements in some cases, but not in this one. So I just held a piece roughly in place and marked off where I wanted to cut it with a permanent marker.

I could cut it with my hacksaw but it’s quicker and easier to just use a miter saw:

I went through each piece that way until I had enough for my dry fit:

Digression: When is 45 not 45?

As you can see from the dry fit, I have one section that goes off at a 45 degree angle. This requires two 45 degree PVC elbows to permit the pipe to continue on parallel to the first length. I picked out two 1-1/2″ 45 degree PVC adapters at Lowes and used them during the dry fit… only to discover that they each worked notably different, even though they are both ostensibly 45 degree elbows.

Here’s what they look like:

But let’s see what happens when you hook them together into what should be a 90 degree angle:

That’s clearly off by a visible amount. What happens if you spin one of them around to give you a 180 degree angle with a bend in the middle:

Yep, it’s clearly not right. One of those must not actually be 45 degrees.

I decided to use the one with the wider sweep (the bottom one) since it gave me more working room and that necessitated a quick trip to Lowes to pick up a matching mate. Who knew?

Digression II: When is PVC not PVC?

It turns out that all of the elbow fittings (the 90 degree one and both 45 degree variants) are the wrong type of PVC! There are actually three main types of PVC – foam core, solid, and CPVC.

Foam core PVC is essentially extruded cellular foam with a thin layer of solid PVC covering it. It’s perfectly fine for drainage situations, but isn’t strong enough for use in cases where the water will be under pressure.

Solid PVC is, as the name suggests, made from solid PVC. It’s rated for water under pressure and is great for any liquid under 140 degrees F.

CPVC is like PVC only it’s only used for hot and cold water and can handle temperatures above 140F (up to roughly 200F).

Okay, why does this matter? Well, all of the elbows I had were made with foam core PVC which I thought was fine because I equated a backwash pipe with a drainage pipe. Nope. A backwash pipe is considered to be under a pretty decent amount of pressure and so it actually requires solid PVC. There is a chance that the elbows on my backwash pipe may well blow out some day. Oops.

Where does CPVC come into play? I found out after the fact that the filter’s valve housing was made of CPVC. This could matter because technically speaking you aren’t supposed to be able to glue PVC and CPVC together (different chemical compounds). In fact, it’s forbidden for any plumbing system that needs to be inspected. Pool filter plumbing doesn’t need to be inspected and so it doesn’t matter from that perspective. I also have a PVC glue that can supposedly handle both. Plus, it might not matter — I’ll talk more about that later.

Permanent

Gluing up PVC pipe is very straightforward and is one of the simplest things a DIY enthusiast can do. I started by scuffing up the mating surfaces with sandpaper, but I’m not showing that because it’s not technically speaking a required step. I just happened to have the sandpaper on hand so why not.

The real first step is to apply a coat of PVC primer to both of the mating surfaces:

The primer and glue are so often used together that you can typically buy both in a combo pack. That’s where my primer and glue came from, this time around. The primer is used to soften the PVC in preparation for the glue (hence no hard requirement to sand the surfaces) and is typically purple. I’ve also seen clear primer, but have never used it. I believe that the purple primer is required by code in certain cases, to make it easy for an inspector to see that the primer was properly used.

Okay, technically speaking the primer wasn’t strictly required in my case, either. That’s because the glue I used is rated for use without a primer in non-pressure systems up to 6″ and pressure systems below 4″. My backwash system is a non-pressure system and is only 1-1/2″.

The next step is to apply the glue to both surfaces:

PVC glue comes in various colors, depending on the manufacturer as well as what properties the glue has. They all can have different “bodies” and different “set times”. Those don’t matter 99% of the time. I see that the glue I used was “medium body” and “very fast set”. That’s fine. Any PVC glue from a reputable company would be fine.

I don’t put a prescribed amount of glue on the surfaces — just a thin layer on both the pipe and the fitting.

The final step has to be done very quickly. I push the pipe into the fitting and give it a slight twist, to help evenly distribute the glue. I then hold it in place for at least 10 seconds and then ensure that it doesn’t move at all for 30 seconds.

The glue actually melts the PVC when making the bond, which makes it a super secure connection. After that initial hardening session it’s impossible to remove without cutting it off and redoing the connections entirely.

I systematically went though all of my connections following this same procedure.

End Result

Here’s the completed business end starting from the filter housing:

That ball valve is so far working perfectly. When I installed it, I had the valve open and could hear the filter hissing and gurgling from the influx of air from the existing leak. That noise instantly stopped as soon as I turned the lever to “off”, though. That was very gratifying, being able to do that very simply by hand without needing to put significant force on it with a wrench.

The pipe then has a relatively long run alongside our fence:

I’m currently supporting the pipe with some blocks I had laying around. I’ll very likely attach the pipe to the fence with some pipe hangers in the near future.

The pipe finishes off with a hard right into our outer back yard in an area that we don’t care if the grass grows or not. We have a salt water pool and it tends to kill grass if we backwash frequently.

And done… or so we thought!

Snap!

Most of this build article was written prior to actually using the system. We tried it out for the first time a few days later with April manning the valve and me watching the output from a distance. She turned it on… and within seconds, the entire system was going haywire!

If you look at the above pictures and read my descriptions, you might note that there’s no references at all to securing the pipe to the fence with clips. Indeed, I didn’t think they were necessary and so I just rested the pipe on some spare blocks I had on hand on the assumption that I’d add clips at some later date.

Well, our pool pump running at 3250rpm propels that water at a pretty rapid pace and it comes out at a surprisingly high pressure. The pressure is enough to essentially act like a rocket propulsion at the end of the pipe run. I also learned that a 20 foot long piece of PVC pipe can bend a lot!

So what happened is that the pipe started whipping back and forth at a pretty high rate of speed and spanning maybe 5 or 6 feet of distance. I shouted to April to turn it off but it took her a few seconds to get to the pump to deactivate it. In that amount of time, the vibrations and movement of the very flexible pipe proved to be too much for the very non-flexible valve assembly and a critical elbow on the assembly snapped right off!

The Problem

The part that snapped in half was an elbow that was specifically designed to fit inside of the valve housing assembly. That is, it had an “interference fit” (tight friction fit) that was suitable for glue to hold it in place permanently and water-tight. Since the non-snapped off part was still glued inside, I only had the interior diameter of that elbow to work with. It had an diameter of 2″.

The problem with this is that there are no standard PVC pipes or fittings that fit inside of a 2″ pipe. What I needed was to find some standard piece that fit as close as possible and try and work from there.

After lots of measuring of individual parts at Lowes, I discovered that the exterior diameter of a 1-1/4” coupler fit very very close. It’s not perfect, but it’s as close as I could find. That is, it’s not an interference fit, but it did look like it fit in there without any visible gaps.

With that one standard fitting in place, I was able to make a series of other iterations to get where I need to be. Next up is a 3” long 1-1/4” pipe that fit into the coupler. To that I attached a 1-1/4” to 1-1/12” bushing . From that I could attach a 3” long 1-1/2” pipe. To that I attached a 1-1/2” 90 degree elbow. At that point all I needed was a standard coupler and length of pipe and I could re-attach my original pipe. Whew.

Here’s where the CPVC possibly came into play, though. See, the manual for the housing assembly claims that it is CPVC! What’s not clear to me, though, is if the elbow was or not. If it was, then that meant I’d be gluing my PVC coupler to it with a potentially weak bond. Well… not that I had much choice. It was either that or replace the entire housing and that would cost far more than I’m comfortable with. So I glued it and am hoping for the best.

Five Years Late

I wasn’t convinced that my hacked solution would be able to hold up the pressurized water during normal operation and so I decided to see if it was possible to fix the leaky valve in the first place. Turns out… yes! It was trivially simple!

I simply unscrewed a top nut on the housing by hand and pulled out the valve slide. I could immediately see that it was completely missing two crucial O-rings that clearly needed to be there. I popped into my local pool supply store and they had a pack of O-rings specifically for that slide.

All it took to install them was to roll them on the slide and lube them up with silicone grease.

After that, I just slide the slide back into place and hand-tightened the top nut and voila it worked like a charm. No leaks at all.

What is blowing my mind is that it’s been like that for at least five years. We’ve had multiple pool pros out to look at various things. Not one has noticed the leaky valve. At no point did it ever occur to me to try and fix it, either. Instead, I just created that stupid workaround with the screw on cap. Gah.

Clipped

The final and crucial step in my rework was to securely attach the pipe to the fence with clips. I got galvanized clips that will hopefully stand up in the sun. The holes were drilled with my trusty rotary hammer drill and then I screwed them in with Tapcon masonry screws. Simple.

The end assembly got two clips — one to keep it from rocketing backwards and the other from oscillating up and down.

Conclusion

And it worked! See the video for the backwash system in action, but it absolutely works like it’s supposed to with nothing breaking.

My hacked insert to the housing assembly does leak a little bit while backwashing, but not enough to bother me and it doesn’t leak at all during normal operation.

Speaking of normal operation, it seems like the entire system is working far quieter and more efficiently than before. We both noticed that independently. My guess is because there’s no more air being introduced into the system like there was with the leaky valve. Dunno for sure.

The ball valve I attached is now completely useless since the fixed primary valve works like it’s supposed to. Ah well. I left it because it was easier to do that than to remove it.