Mod #83: DIY Water Accumulator

Mod #83: DIY Water Accumulator

Submitted on: 06/10/09

     Category: plumbing
Mod Rating: 12345

(49 ratings)

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Mod Description:

This mod comes to us courtesy of senior member professor95, otherwise known as Randy Agee, and is copyright with all rights reserved.

It is 3:00 o’clock in the morning. You and your significant other are out camping in your RV when you have the need to get out of bed and shed some water from your bladder. You manage to make it to the potty room with out awakening anyone and in a few moments you feel tremendous relief as the pressure on your bladder subsides. Just like your Mother taught you, flushing the potty is a desired practice after usage. You press your foot on the flush pedal and watch as the valve in the bottom of the bowl opens and a swirl of clean water comes in. Then, it happens!

The water pump on the RV turns on. It just so happens that the pump is under the bed where your significant other is sound asleep. There is a loud banging from the pump as it builds pressure. The plastic water lines vibrate against each other and the flimsy inside walls of the RV. Your significant other is no longer peacefully sleeping. In fact, she (or he) is wide awake and extremely unhappy. If any of this sounds familiar, you most likely do not have an accumulator as part of your RV’s self contained water system.

Mod Difficulty:

When hooked to city water there is little need for an accumulator. But, when you are drawing water from your RV’s storage tank and using the onboard 12 volt pump an accumulator can make camping a lot more pleasurable – perhaps even save a relationship.

Accumulators connect to the water line after the pump. They act as a water storage unit that can significantly reduce the frequency of pump cycling and smooth out water flow. Accumulators accomplish this by having a sealed cylinder that fills with water until the air pressure above the water level is equal to the maximum water pressure from the pump. When you do something, like flush the potty in the middle of the night, there is usually more than enough water and pressure stored in the accumulator so that the flush can be accomplished without the pump cutting on.

After market accumulators made specifically for RV’s are readily available. Their cost ranges from around $40 to well over a $100 depending on the size and construction features selected. The best accumulators have a rubber bladder inside that expands against an air charge. A simpler accumulator that does not have a rubber bladder but simply compresses air with water is almost as effective and can cost a lot less. In fact, you can easily make this type of accumulator yourself from easily found materials.

You may be wondering why anyone would want to make an accumulator when they can be purchased ready made so cheaply. The answer is each RV is different and finding the space for an accumulator can be a challenge. By making your own, you can custom select the size that fits your available space.

Photo #1 shows all of the needed parts to build your own accumulator. This particular accumulator is made from 3” schedule 40 PVC pipe that is 12” long. I could have just as easily selected a piece of 2” pipe or even 4”. The length is determined by available space to mount the accumulator vertically. Typically, large accumulators work better, but even small accumulators can make a big difference in water flow.

“A” is a 3” PVC pipe cap.
“B” a 12” long section of 3” schedule 40 PVC pipe.
“C” is a slip reducer that changes 2” to 1”.
“D” is a 1” slip reducer that has a ½” pipe thread in the center.
“E” is the cleaner and cement to join PVC pipe.
“F” is a brass ½” threaded pipe to ½” PEX adapter.
“G” is a ½” brass PEX Tee connector.
“H” is a short piece of ½” PEX pipe
“I” shows several ½” PEX compression ferrules
“J” is a tool for compressing the PEX ferrules
and “K” is a tubeless tire metal valve that has a threaded shaft, nut and seal.

All of these parts were purchased at Lowe’s, with the exception of the tire valve which came from a local Advance Auto supply store.

Photos #2 and #3 are close ups of a few pieces showing part numbers and sizes in detail.

In photos #4 and #5 you can see how a hole the same size as the tubeless tire valve shaft was drilled into the center of “A” for attachment of the valve “K”.

Photos #6 and #7 are of a dry fit for all the parts. Note than in photo #7 a section of ½” PEX has been attached between the adapter and the T.

Most all modern RVs are plumbed with a plastic pipe called PEX. It is extremely strong, easy to install and durable. The only “problem” with PEX is that you cannot make up connections with just a gear type hose clamp. Special tools and ferrules are needed to properly make a leak proof connection if not opting for the more expensive and cumbersome push-in connectors.

The small tool with the yellow handles (“J”) shown in photo #8 and #8a is one of the less expensive crimping tools available. A closer view giving more detail is shown in photo #8b. It works well with all squeeze PEX ferrules ranging in size from ¼” to 1”. The ferrules are a little more expensive, but the ranges of sizes that can be connected with this simple tool make it worthwhile to own. The red handled tool at the bottom is for the solid copper ferrules and is considerably more expensive. It also only compresses one size of ferrule necessitating owning several sizes if pipe diameters are mixed.

Photo #9 shows the difference in the squeeze ferrule and the solid copper ferrule. You must have the right tool for each type. If you do not want to invest in these tools, you can rent them (Home Depot) or perhaps even borrow them from a friend.

You may also find that none of these tools will be needed. Some RV water pumps are attached with conventional ½” white water hose that can be tapped with barbed fittings and gear clamps.

The accumulator must be mounted vertical so the air valve is at the highest point. You do NOT have to mount the accumulator next to the pump. It will work quite well even when mounted some distance away, perhaps in a kitchen cabinet where you have more room for a larger accumulator.

Draining all of the water from the system naturally rests the pressure inside the accumulator.

The accumulator shown in the last photo is in my 5th wheel’s basement area close to the water pump. Despite what advertising literature may tell you, all pumps – even the variable speed models – perform much better with an accumulator.

ModMyRV recommends these parts for this mod:
All-Purpose Pipe Cleaner
PVC Glue - 12 pt.
PEX Pocket Crimper - Combination 3/4, 1/2, 3/8
Milton 470 Metal Valve

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27 Comments For This Mod

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  1. jwangelin

    This seems to be a great mod. A couple questions though. After installing it, do I allow it to fill with water before pumping to the required P.S.I.? I believe that I am supposed to keep a water fixture open during the procedure. Wouldn’t adding pressure to an open system simply push all the water out of the lines?

  2. ModMyRV

    Allow the water lines and accumulator to completely fill, then pressurize the accumulator. There is already air inside the accumulator when the system is empty so there will be somewhat of an air cushion between the water and the top end of the accumulator even before pressurinzing.

  3. Bob Vaughn

    This mod interests me…so I went to my pump compartment ….I thought I would start by removing my pump. I became befuddled because of the pex connectors….but first I put water in the supply tank and turned on the pump…It ran and ran but no water ever came out of the faucets. I have never used the pump because we always have hook ups…So how do I test my pump before I start this mod???

  4. tokie856

    I did this mod with a 4″ pvc pipe 26″ long. I still can not flush without the pump activating. It does not make sense to me. If the pump provides 30 psi at shut off and I add 20 psi more, I have 50 psi until I open a faucet. At that point the pressure goes back down to 30 psi (the pump provided pressure). I just don’t see how I’ve accomplished anything that the pump wasn’t already doing. Why am I adding pressure only to have it decrease to the pump’s pressure after one use?
    Have I done something wrong. I tie in downstream of the pump.

  5. Professor95

    To: tokie856

    I think there may some misunderstanding on “adding pressure”. The air valve at the top does not need an “extra charge”. With the system dry let the pump cut on. Water will flow into the accumulator until the system pressure reaches the pump cut off pressure - in your case 30 psi. There will be an air bubble at the top of the accumulator with the same pressure as the upper limit of the RV pump. When you turn a faucet on the air at the top of the accumulator will force water out into the lines until the overall pressure drops to the cut-on point for your pump. Since you have a higher volume of stored water under pressure than you had in just the water lines, your pump should cut on less frequently and water flow should be smoother. One factor that influences the operation of this device is the pump MUST be able to flow MORE water than the faucet turned on requires. If the pump is too small for the flow and constantly runs wide open with a faucet turned on, the accumulator will not perform properly. On the other hand, if flow is less than the pump capacity you will not get the “burp-burp-burp…” typical without an accumulator.

    Another benefit regardless of pump pressure or capacity is IF THE PUMP IS OFF you still have a quart or more of water, depending on your accumulator capacity, under pressure to grab a drink or make a flush without turning the pump on and possibly waking the dead with pump noise! Does this help?

    - Professor95

  6. tokie856

    Thanks Professor 95.
    I thought some more about it and realized it might work properly by creating a larger air pocket. When I first installed the accumulator I filled all lines and it fully, then pressurized the accumulator. As state above one twist of the faucet and the extra pressure was gone. I realized I needed to remove some water from the accumulator to give it the air bubble and the compressive quality of the air bubble. Now it works much better.
    Thanks for the help though.

  7. Lloyd BC

    Personally, I would have left the ‘Accumulator’ on the fresh supply. You don’t need water to ‘flush’ RV toilets, so why even turn the ‘grey’ pump on at night? However, having a smoother flow on your ‘fresh’ is always nice (especially when showering)

  8. David Waters

    The air valve is the only thing I am vague on. When the water is drained from the system, the accumulator automatically drains and fills with air. When the lines are pressurized, the accumulator automatically fills with water with air at the top that has no where to go except to stay there and pressurize the accumulator and the lines. So, what is the function of the air valve? Thanks.

  9. Lloyd BC

    David Waters (September 23rd, 2009 at 7:47 pm)
    The air valve is the only thing I am vague on

    An accumulator without a ‘Bladder’ or ‘Floating Diaphram’ will slowly become ‘waterlogged’ as the air is absorbed by the water. They then lose effectiveness. The valve allows you to open a tap and re-charge the accumulator with air, restoring its effectiveness. Modern well-pump systems are set up to do this automatically or they use the bladder type pre-loaded tanks that don’t ‘waterlog’. However, RV manufacturers figure we are used to burping supplies and will accept them as they come.
    Why did you move the Accu. to your grey water flush system? Was it just because it happened to be on that section of the plumbing? As I said before, you really don’t need water to ‘flush’ an RV toilet, so why worry about having the pump on when others are sleeping? Plus, the accu is more advantageous on your fresh supply, ’specially when showering.

    BTW, 30 psig = 2 atmospheres, so IIRC, by pressurizing an empty Accu to 30PSIG, you will have 2/3 vol water, 1/3 vol air, at best. But it will definitely help with the burps.

  10. Lloyd BC

    You know, I just had a revelation concerning the faucets in my trailer. All the fixtures in the bathroom (probably originals) will run the pump full out, but the kitchen tap (Delta Laundry, which replaced the original Price Pfister), does the burp-burp-burp routine, when run full on. I guess the originals didn’t have flow restrictors, but the new one does, for water conservation. Never thought about it before, because the taps are either full on or off, never trickling. We’ll see what happens when I install our new ‘Oxygenics’ shower head, in the spring. I might just have to add an ‘Accu’ to smooth out the flow. If I do, I will make it as tall as I can fit under the kitchen counter and use a remote air charge valve I happen to have.

  11. Chr$

    Great idea. I have space large enough where my old tank style water heater was to put one. I plan to use an expansion tank from an under counter RO water system that I can get used for $5. I have a 77 GMC motorhome.

  12. Turd Herder

    It seems to me that the system will become waterlogged very frequently unless the airhead is positioned above the highest discharge point in the potable water system. In most cases this would be the showerhead.

  13. George

    My wife gets up sometimes and I go right back to sleep and there’s no need to leave the pump on at night. You can flush “P” without water. Follow the septic tank rule, “If it’s yellow let it mellow, if it’s brown flush it down.”

  14. tom anderson

    How about leaving out the air fitting and install an innertube? Get a fat beach cruiser tube and put a little air in it until you can just fold it in. A better way would be go to a jobsite and scroung a 8 or bigger cut off scrap pvc. Put an 8″ trailer tube with a small amount of pressure in and cap. You could make it taller and use two tubes. Lots o water.

  15. rex

    8 inches dia = 50 sq inches in area and times 30psi(assumed)= 1500lbs force on the ends.
    This is an explosive force because you are compessing air, and you are using plastic tube for this??
    Need I elaborate more.

  16. sean

    Rex, look at the pic. The white pvc Piping is rated at 260 PSI. 30 PSI is diddly for the components listed.

  17. Gary Steward

    Hello all,
    I thought this would be a good idea for our cabin since I use a Shurflo RV water pump pumping from a 400 gal fresh water tank. The plumbing in the cabin is minimal (1/2″ pvc to a 30 gal water heater then to bath sink, bath shower, kitchen sink. (no toilet, we have a composting) I used 3″x12″ sch 40 pvc. The accumulator is located 24 inches from the pump vertically installed. After connecting I turned the pump on and the accum filled with water and the pressure at the valve was 50 pounds with water coming out of the valve at the top. For a short time things seemed to operate better ie less on/off, on/off, on/off and less noise from the pump. Within a day or so it was back to the same oh-same oh except for the noise. My question is: should I add more pressure to the accum or drain the system and charge the system before turning the pump on? Thanks for any response

  18. Rusty

    First, I need to say this is one of my favorite mods, ever. I built one and now my friends are copying.. Instead of using bushings (which are expensive and waste space), I capped both ends then drilled and tapped the PVC to screw in the fittings. @Gary: The water needs to enter and exit from a valve at the BOTTOM of the tank. If you’re drawing water from the TOP, then you’re removing the trapped air and your accumulator tank is nothing more than a fat pipe.

  19. Gary Steward

    Hey Rusty, thanks for the response. Sorry I’m just now replying back. I basically built mine the same way, i.e. capped it with PVC and screwed in the air valve fitting at the top. The water enters and exits at the bottom via a T, not a valve. Is it a valve or a T and if it’s a valve what kind? Like I said it quietened things down but the pump still cycles on/off, on/off, on/off and when you check the pressure at the valve water comes out. I’m not ready to give up on it because I do enjoy the quietness. Any ideas?

  20. Cho

    Having installed and maintained water well pressure barrels for over 50 years, I can assure you that you REALLY want a bladder (inner tube) to prevent waterlogging. All pressurized “open’ air systems, such as this, are perpetually waterlogging from the 1st moment of use. Use tom anderson’s idea, or a derivation.
    Personally, I used a Utilitech 2-Gallon Expansion Pressure Tank
    (Model LET-2) from Lowes. For just $39, you can have an accumulator that works, maintenance free, right out of the box.
    Just tee it into the fresh water line at any point you chose.
    Precharge the bladder to 1 psi LESS than your pump cut-on pressure;
    this causes the pump to start just slightly Before the bladder empties thus avoiding a flow stall.

  21. Gary Steward

    Thanks Cho for the good advice. I guess the PVC accumulator works good in some applications, just not mine. It became water logged as soon as I turned the water on. On top of that it cost me about $25 to build. I’ll definitely try the expansion pressure tank. Thanks

  22. Gary

    Where do I find the cut on pressure for my Shurflo 2088 pump. It says 45 psi on it but I assume this is the max or cut off pressure

  23. JohnS

    This is a great mod and many ways to accomplish having a pressurized reservoir of water to reduce pump starts and cycling. I was fortunate enough to find room for a 2 gallon pressure tank at the big orange box for about $40. It is similar to the tank you get with a RO system, but only 8″ in diameter. Works very well and moved it to my new RV. I have noticed that if the flow is high enough the pump may start anyway and run long enough to fully recharge the tank. Tapping it into the plumbing near the toilet helps to reduce this. I was lucky to have room in a corner of the bathroom vanity, right where the toilet line hooks to the plumbing. If you tap it in right at the pump it is more likely to kick in the pump because of flow and pressure loss when the high flow device is way down the line. Also having a bladder in the pressure vessel is worth a few $$. Remember to mount it with the inlet down so winterizing is more effective.

  24. clyde stamp

    Ok my tank . accumulator works.. But connected to city water, i opn faucet n water flows at normal rate but quickly tapets off to slow trickle.

  25. Jesse

    Just to confirm, you never need to hook up an air pump to the air valve on top and pressurize the accumulator? Is that right?

    What is the point of the air valve on top?

  26. Cho

    @Jesse…..Incorrect….you need to add air any/every time the accumulator “waterlogs”…the air precharge dissipates into the water …whereupon the pump cycles its if there were no accumulator..add air to reduce the cycling frequency….using a “factory” accumulator eliminates the need for ever adding air…see my comment from 2012…

  27. GH

    Rex, where did you get 8” from? I can’t think of where you would get 8” diameter pipe. The biggest diameter I saw on here was 4” and that does not compute. It would be 376 lbs of force at that size. At 50 psi you have 630 lbs of force. 4 in. PVC can take that much force. These accumulators really need to be at least a gallon or it doesn’t seem worth it. On a water well system it’s called a pressure tank. The accumulator is playing that role on a smaller scale. Using this will definitely increase the life of the pump.

1 Trackbacks For This Post

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