Pressure Tank


Old 02-19-13, 12:41 PM
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Pressure Tank

I am thinking about buying a 50 gallon pressure tank (120 gallon equivalent), but am unsure if i should. Right now, i have a grundfos MQ-35. It turns on everytime i turn on the water....but i also get 65 psi throughout my house always.

Right now, the pressure is at 65psi stable. When i open a faucet, the pressure slowly drops until it gets belwo 30 psi, then the pump turns on and i get my 65 back again.

If i install a pressure tank, will i lose the 65 psi all the time? Does the tank pressure slowly go down until it reaches 30psi, (the level needed to make the pump turn on), or will the pressure stay 65psi even when the tank is losing water? i do not want the water pressure to slowly lose psi while the 50 gallon tank slowly uses water.

Am i thinking right or is that why this is a charged pressure tank system?
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Old 02-19-13, 01:14 PM
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Do you have a pressure tank now?

A 120 gallon tank with a tidal volume of 50 gallons is quite large for home use.

The pressure tank will have no direct affect on your water pressure. What it will do is smooth out the water pressure fluctuations and minimize pump cycling. If replacing or installing a new pressure tank I would also adjust your pressure switch or install a new one. 30psi cut in and 65psi cut out is a larger than normal range and probably quite noticeable when showering.
Old 02-19-13, 01:16 PM
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Why do you want a tank? That pump has an internal tank. I dont think that pump is designed for a pressure tank.

Pressure tanks you will get variable psi such as 40/60. This in turn turns the pump on and off.

Your pump states max psi as 51 psi. Possible you have the 45 model which states 65psi max.

Maybe you need to check the air charge in the pump. It says 22-25psi.
Old 02-19-13, 01:43 PM
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The air in the tank is what is putting out the pressure, however the actually amount of pressure is determined by your pump's cut in and cut out switch settings. The best way to visualize what is happening is to first think of the empty tank. You are supposed to set the air pressure in the tank at about 3 psi below the cut in pressure. So lets say you are using 30psi on and 60psi off for the pump switch. You would set the air pressure at 27 psi, inside the tank. So until the water pressure from the pump gets to 27 psi, it isn't even in the tank yet (or it would be pushed back out by the higher air pressure). So at 27 psi, it now starts to fill into the tank. If the air in the tank is going to cycle from 30psi to 60psi, then your science knowledge should tell you that the air in the tank must compress to 1/2 the volume (double pressure equal 1/2 volume and vise versa). So, what that means is when the water fills the tank's volume to 1/2 full, the original air that was in the entire tank, when it was empty, has now been compressed to 1/2 the volume of the tank and that lost air volume is taken up by water volume. So, your water draw down will start at 1/2 the tank volume and when it flows out to almost empty, the pressure will go from 60psi to 30psi, the pump will switch on and re-fill the tank 1/2 full, where the pump will switch off again, since the pressure will be 60psi. This goes on and on. My numbers are rough estimates for illustration purposes, there are some small variations with temperature and few other minor things.

Now in your case, you most likely have a defective bladder in your tank or all your air has escaped another way. If in the above example some of the air is lost (dissolves into the water when the bladder is broken or escaped out the nipple,etc.) then your draw down (amount of water between cycles) will be reduced. When the pump cycles off, if you have 10 gallons of air in the tank, you will get about 10 gallons of water between cycles. If the air has now been reduced to 1 cup of air, then you will only get 1 cup of water between cycles (since if the air volume doubles the pressure is cut in half and when the pressure that was 60psi is cut in half, that's 30 psi and the pump comes on). It sounds to me that you have less then a cubic inch of air in your tank and therefore the pump will cycle on as soon as a tap is opened up and 1 cu inch of water comes out. Basically you don't even have a tank. This is very hard on your pump.

You can fix it by emptying the tank. Setting the air pressure inside the tank to 3 psi below your cut in pressure and then refilling the tank with water. You should note, however, that this will either fix your problem or temporarily fix your problem. If your problem is lost air and you put the air back in and do not have a further air leak, your problem will be fixed. If the problem is a bad tank bladder, your problem will be fixed at first but overtime (a couple weeks at most) your air will dissolve into the water again and the problem of your pump cycling on too often, will be back.

You most likely need a new tank.
Old 02-20-13, 03:57 PM
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wow, thanks for the input.

My situation: I just installed a new Grundfos MQ-35. It works well. My output pressure is 65psi and I am happy. My issue is that it always comes on, not matter if I just run the sink for a second. To try and not make it run everytime I run the water, I thought that maybe a pressure tank would help me prolong the life of the new pump by not making the pump turn on everytime I use the water. I also thought that it might keep the pressure at 65psi always without turning the pump on, but I was worried that I would lose a small amount of pressure until the pressure got low enough to turn on the pump.

Sorry for the confusion fellas.
Old 02-20-13, 05:34 PM
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If you don't have a tank or your tank is defective, as I discussed above (no air in the tank for whatever reason), then of course your pump will come on, everytime you open a tap. If it didn't come on, you would not get any water, because you would not have any pressure, since you have no pressure from a tank. This type of operation (pump coming on everytime you turn on the water) will reduce the life of your pump ... SIGNIFICANTLY.

The better design is to have a pressure tank, where the pressure is now provided by the compressed air, that is trapped in the tank. Unfortuneately, by it's design, the pressure that is put on the water will vary between the cut in (pump turns on) and cut out (pump turns off) pressures. That being said, a pressure range between 65psi and around 30psi will not be all that noticeable and your pump will thank you. You could increase your cut-in pressure to 40psi, however, this will provide less water drawdown and therefore your pump will need to cycle on more often. You decide.

If you do get a new pressure tank, look to getting the biggest one your space will accomodate. Again, this will give you the most water between pump cycles (drawdown).

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