Drainback system - air-over-water or diaphragm pressure tank?

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magicdog

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I'm a long time reader, this forum has been a great source of information. I've been researching a redesign of the well water setup at my cottage and have come to a decision point.

I have a shallow well by the water which I'm switching from a jet pump to a submersible pump. The cottage is up a fairly steep hill, a run of about 65' from well to cottage. It's too rocky and steep to bury the 1 1/4" polyethylene pipe that runs from the pump to the crawlspace of the cottage.

Because the power cuts out often in winter, I've decided to use a drainback system. There are two setups I'm considering based off advice on this forum:

1. Submersible pump with check valve > Drainback valve in well > Two port check valve with snifter valve in crawlspace > Air-over-water pressure tank with AVC > Pressure switch > Cottage

2. Submersible pump with check valve > Drainback valve in well > Automatic air vent in crawlspace > Check valve > Diaphragm tank > Pressure switch > Cottage

I could also combine the automatic air vent and check valve with a two port check valve.

I would like to go with the second option as it means I can get a more affordable and available diaphragm tank. The fiberglass air-over-water tank with AVC I'm looking at is $1K (CAD) for a 40 gallon tank. A diaphragm tank also avoids waterlogging, which is a possibility as the cottage is unused for weeks at a time.

My concern is that the automatic air vent which has to vent up to 80' or so of air in the 1 1/4" pipe when the pump cycles. I've seen SCFM ratings on air vents but I'm not sure what my system needs. Others on this forum have had the pressure tank in their drainback system replaced with a diaphragm tank and it causes water sputtering as air goes through the plumbing. I want to make sure that doesn't happen.

How do I go about sizing an automatic air vent? Can I install a 1/4" vent on a two port check valve? 1/8"? Will that be enough?

Any help is much appreciated. Thanks!
 

LLigetfa

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I would worry about either system venting the volume of air fast enough. I have two WellMate fiberglass air-over-water tanks in series and my experience is that they cannot remove the air fast enough. I modified one of the two tanks by raising the AVC 12 inches so that it holds 12 inches more water to allow more time for the air to be bled off.

You could try using more than one external automatic air vent if you find one to be too slow. Waterlogging should not be an issue with so much air, unless the drainback fails. A Cycle Sensor Pump Monitor would detect such a condition.
 

LLigetfa

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One condition that could cause a rapid drawdown of water with air-over-water tanks is a power failure during water use such as a softener or filter backwashing. When that happens the air & water mixture causes the backwash to be violent and can drive the media into the head's valves and down the drain. Another condition is where the draw is much more than the Pump GPM can supply.
 

Reach4

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You could reduce the amount of air, if using a submersible pump, by reducing the run to 3/4 inch pipe. That would be plenty big to carry the water from a submersible pump to a house.

Using the nominal size in the calculation, 1.25 inch pipe will bring in 2.78% as much air as 3/4 would.

You know to not have any bellies in the pipe run.
 

magicdog

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You could reduce the amount of air, if using a submersible pump, by reducing the run to 3/4 inch pipe. That would be plenty big to carry the water from a submersible pump to a house.

Using the nominal size in the calculation, 1.25 inch pipe will bring in 2.78% as much air as 3/4 would.

You know to not have any bellies in the pipe run.

I do, the run from the well to the cottage is quite steep and there are no high spots.

I would worry about either system venting the volume of air fast enough. I have two WellMate fiberglass air-over-water tanks in series and my experience is that they cannot remove the air fast enough. I modified one of the two tanks by raising the AVC 12 inches so that it holds 12 inches more water to allow more time for the air to be bled off.

You could try using more than one external automatic air vent if you find one to be too slow. Waterlogging should not be an issue with so much air, unless the drainback fails. A Cycle Sensor Pump Monitor would detect such a condition.

You think the AVC in option #1 would struggle to let the air out fast enough? It's a fairly large tank at 40 gallons. Wouldn't it be a non-issue if the AVC air vent is at the top of the tank?

Best I've found so far is the Caleffi 501502A MaxCal Automatic Air Vent 3/4" which can vent at 9 SCFM max. I could install that inline before the check valve before the pressure tank.
 

LLigetfa

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You think the AVC in option #1 would struggle to let the air out fast enough? It's a fairly large tank at 40 gallons. Wouldn't it be a non-issue if the AVC air vent is at the top of the tank?
Don't forget that the AVC is located low in the tank and tries to maintain the air level so it is 2/3rds air and 1/3rd water. See pic below but realize it is only an artist's concept and not a true representation. The AVC is the black item at the bottom of the grey tube so it cannot let out air that is above the black AVC. It does most of the air removal when the pressure is down to the kick-in switch setting. As the pressure raises, the air compresses and is out of the reach of the AVC.

Since when the pump shuts off and drains back, the tank will be about half full with the rest air. When it starts and pushes all of the air out of the line, it will add it to the air that is already in the tank and as I said, will be out of the reach of the AVC.

As I mentioned in my previous post, I modified mine by raising it 12 inches so that I have more water than air.
7568044_orig.jpg
 
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LLigetfa

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Here is a pic of my two tanks in series. The water level (at max pressure) on the first (blue) tank is at the top of the foil bubblewrap insulation. It is designed to be almost empty when the pump turns on (hence why it lets the air out so slowly) so If a very heavy draw drops the pressure below the start pressure, air will move forward to the second (grey) tank that has the shortened AVC dip tube. Since that tank has a foot more water, air will not move forward into the iron filter and softener.
04-13-53-38.png
 

magicdog

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Don't forget that the AVC is located low in the tank and tries to maintain the air level so it is 2/3rds air and 1/3rd water. See pic below but realize it is only an artist's concept and not a true representation. The AVC is the black item at the bottom of the grey tube so it cannot let out air that is above the black AVC. It does most of the air removal when the pressure is down to the kick-in switch setting. As the pressure raises, the air compresses and is out of the reach of the AVC.

Since when the pump shuts off and drains back, the tank will less than half full with the rest air. When it starts and pushes all of the air out of the line, it will add it to the air that is already in the tank and as I said, will be out of the reach of the AVC.

As I mentioned in my previous post, I modified mine by raising it 12 inches so that I have more water than air.
7568044_orig.jpg

You're right that would happen. I never realised that. That looks to be nearly the same tank I'm looking at - a Flexcon FLU40SQEZ. It uses the same type of AVC.

I'm now thinking of installing of installing a Netafim Guardian 1" air vent prior to the check valve. It was recommended in a couple of other threads.

Worst case, even if there is air making its way through I can add a second air valve after the pressure tank.
 

LLigetfa

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Worst case, even if there is air making its way through I can add a second air valve after the pressure tank.
Using inline air purge valves negates the need for a hydro-pneumatic tank so you could stick with a conventional diaphragm (captive air) tank. Just create a high loop of large diameter PVC onto which you install the air purge valve.
 

LLigetfa

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That looks to be nearly the same tank I'm looking at - a Flexcon FLU40SQEZ. It uses the same type of AVC.
The grey one is Flexcon. The blue one is WellMate. They are nearly identical and probably come off the same assembly line but Pentair (who bought WellMate) has a higher price.
 

Reach4

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Using inline air purge valves negates the need for a hydro-pneumatic tank so you could stick with a conventional diaphragm (captive air) tank. Just create a high loop of large diameter PVC onto which you install the air purge valve.
And by large diameter, I suspect you mean 4 inch pressure-rated PVC. Would that be upstream of the air over water tank, or what?
 
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LLigetfa

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And by large diameter, I suspect you mean 4 inch pressure-rated PVC. Would that be upstream of the air over water tank, or what?
Yes, 4 inch or bigger and of course pressure rated, not sewer pipe.

Of course it should be before an upright captive air tank since you don't want air to go up into the tank and steal the drawdown. The OP did consider a second unit after the tank JIC some air collects in an upright captive air tank. If the first one is grossly inadequate or fails that would cause short cycling since air collecting under the diaphragm would reduce drawdown which in turn would cause short cycling.

If there is a downflow softener after the pressure tank, it could accumulate some air to prevent air spurting out of a faucet and knocking a glass out of your hand. Accumulated air would be purged on backwash. If air does get past the softener, it needs to be removed with an air separator.
 

Reach4

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So from lake to softener:
submersible pump, check valve at pump, drain valve (or small hole), long pipe, snifter valve, check valve, air release inverted U, precharged pressure tank, house valve, softener. A hole is probably needed rather than a drain valve, since there will be considerable pressure when the pump turns off.

I would put an extra boiler drain valve at the bottom of the ascending part of the inverted U to allow draining sediment.
(snifter valve, check valve) would often be combined by having a tapped hole on the upstream side of the check valve housing.
 
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LLigetfa

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A hole is probably needed rather than a drain valve, since there will be considerable pressure when the pump turns off.
Are you talking about a hole in the side of the pipe or in the pump's check valve? In the side of the pipe will rob some flow/pressure and can enlarge over time. A hole drilled in the check valve or a slot cut in the valve or seat if too large, can cause the pump to spin backwards which can break the shaft if the pump starts while it is still spinning backwards. A very small hole or slot might be too slow to drain and a hole can easily plug whereas a slot in the seat is self-clearing. A drain back valve can be combined with a small slot in the seat where the slot ensures the drain back opens.

the Flowmatic Model 70 automatic drain valve will open at 10 PSI provided the 0.43 PSI per foot of elevation does not exceed that. For good measure, a second unit could be located near the top side check valve which would not be subjected to the .43 PSI per foot of elevation.
 

Reach4

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I was referring to an underwater and under-ice hole near the submersible pump.

If there is a drainback valve that is designed to open once there is less 10 psi in the drop pipe, that valve may never open. We don't yet know how much altitude change we are looking at.

I think the pipe would need to enter the lake below the frost line.

I think you would need a snifter valve on/at the topside check valve, and that should require a small vacuum to open, and I think you are not talking about the topside. So I think you are proposing a slot in the underwater check valve housing or seat.

On the slot in the seat idea, you are proposing to feed the water slipping by the seat slot though a pump that does not have an additional check valve? Likely I am misunderstanding.
 

LLigetfa

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If there is a drainback valve that is designed to open once there is less 10 psi in the drop pipe, that valve may never open. We don't yet know how much altitude change we are looking at.
Correct. 0.43 PSI per foot would limit the elevation change limit to around 23 feet. This is not factoring residual pressure on pump stop. That too has to be bled off.

Modifying the check valve built-into the pump so that it leaks would drop the pressure so that the drain back sees less than 10 PSI. There is no reason to have and actually there cannot be a second check valve at the pump.

The pipe at the water level would freeze if it is not buried below the frost level unless it is heat traced.

A snifter requires a small vacuum to open whereas a second topside drain back would open at 10 PSI and allow air into the pipe. The topside drain back would not be subjected to the 0.43 PSI per foot that the lower one is.
 

LLigetfa

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You could of course go with a turnkey system designed for lake water systems.

Drainback Done Right​

We’ve perfected drainback with our patented valves and systems. Improvised makeshift drainback systems used to result in air shooting through faucets or would drain through the pump and risk motor damage. Our patented systems have perfected drainback to eliminate these concerns.
 

LLigetfa

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I re-read the OP where he said the cottage is 65 feet up a very steep hill so the head pressure will be too high for the Flowmatic Model 70 automatic drain valve to open unless several are placed about 20 - 25 feet apart along the run. The Flowmatic Model 70 automatic drain valve is spring loaded and it takes a 5 GPM flow of water to overcome the spring and close the valve. That means that most of the air will get purged by the drain valves. It also means that water will spurt out the valves.
 

magicdog

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Reading the more recent posts I'm realizing the Flowmatic 70 valve I intended to get would probably be subjected to more than the 10 PSI required to open.

I'm not sure if it makes sense to install several of the Flowmatic valves along the run up the hill as they will be subjected to freezing conditions. The Flowmatic websites specifies installation below the frost line.

I think the simplest solution, as shown on the Cottage Water Supply website, is to install a simple wye fitting with reducer. Or even simpler drill a hole.

Seems less elegant and it would rob the system of some efficiency but it would likely still work. Even a 1/8" hole would be more than fine.
 
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