Pump doesn't havethe same horsepower as previous pump

[robert1]

Well basically we needed to replace the pump in our well. The pump that was previously in our well was 3/4 horsepower and we looked for the same horsepower but all the ranch store had was 1/2 horsepower. This is the only place in the area to get a submersible pump for water wells,that we know of,so we didn't have much of a choice. We install the pump and turn it on, now the waterpressure goes gets high enough but the pressure only last about 5 minutes and it goes down(20psi) when running the water. Then the pump does short cycles to bring the pressure back up. But when we don't run the water it maintains good pressure (60psi) according to the pressure tank.
We haven't replaced the water pressure tank and it looks pretty bad off so that might need to be replaced ,also we haven't put air into the waterpressure tank since the system was turned off 1 day ago.
So the question is, is the difference in horsepower causing the low water pressure or could it be that we need to charge the waterpressure tank with air or replace the water pressure tank altogether?
Anyway sorry for the long post, I just wanted to get you guys all the relevent info I could ,we are new to this whole well thing so we are learning as we go.

 

[Bob NH]

The 1/2 HP pump won't pump as much water at the same pressure, and won't provide the same pressure when it is pumping as much water. In general you can expect that the 1/2 HP pump will provide only about 2/3 as much water as the 3/4 HP pump.

There is a wide range of 1/2 HP pumps from many manufacturers, ranging from 5 GPM rating to 15 GPM rating. The high-GPM rated pumps won't deliver as much pressure and the low-GPM rated pumps won't deliver as much flow.

Five minutes of running water before the pump comes on is not surprising if you are running more than about 3 gallons per minute. Not being able to deliver more than 20 psi when you are running water suggests that you have a seriously undersize pump. How many gallons per minute are you getting when the pump is delivering only 20 psi? Do a timed test for by measuring how many seconds it takes to fill a 5-gallon bucket.

Post the make and model number of both pumps so we can get some idea of the characteristics.

Also, post the model number and size of the tank. Is it a bladder tank (with an air connection on the top) or is it a non-bladder tank?

In the meantime, I suggest setting your pressure switch at 30 psi start and 50 psi stop. Then, with all of the water out of the tank and the power off, set the air pressure in the top of the tank at 28 psi. That will give you maximum available drawdown.

 

[Sponsor]

 

[Valveman]

A bad pressure tank will cause your pump to cycle on and off but, will not cause the low pressure problem. The pump is what builds the pressure and the more water you are using, the less pressure it can build. It can probably only build 60 PSI when you are using 1 GPM or the faucets are turned off. When you try to use 5 GPM or so, the pump can't build as much pressure. If you can tell me if that is a 10 GPM or maybe a 7 GPM pump, and how deep your well is, I can give you the pressures at different flow rates.

 

[hj]

pump

The tank and its air charge determines the high and low pressures and the amount of water available before the pump starts. The fact that the pump comes on, builds up to the cutoff point and then turns off indicates that it has more than enough capacity. The short cycling is caused by a tank with too much, or too little, air in its bellows.

 

[Valveman]

"But when we don't run the water it maintains good pressure (60psi) according to the pressure tank." robert1

But when you use water, the small pump cannot deliver 60 PSI.


"now the water pressure goes gets high enough but the pressure only last about 5 minutes and it goes down(20psi) when running the water." robert1

You still may have a tank problem but, the low pressure is caused by too small of a pump.

 

[hj]

pump

If it shuts off at 60psi it can deliver that amount. And the fact that it short cycles shows that it also can deliver enough water to maintain the flow. It shuts off because it does not need to pump any more water at that point. The short cycling is because there is too small volume in the tank because of too much or too little air, to maintain a long flow between the on/off points.

 

[Valveman]

The pump can only build 60 PSI when no water is being used. I think they meant the pump doesn't take long to build to 60 when the faucets are shut off. I don't believe the pump could be short cycling when there is only 20 PSI while using water. The fact that it takes 5 minutes for the pressure to go down, means that the tank is delivering water for 5 minutes. Then when the tank is empty, the pump is too small to keep up.