Water intake expansion tank size?

[MHM]

I've got a water intake from a lake for 2 outside spigots that are used primarily for watering the lawn and garden. When installed last year, the plumber put in a 1/2HP pump and only a 2 gallon expansion tank with a 38 PSI charge. The lift from the lake to the pump is less then 10' and the run is around 50'. It was a problem from the beginning with the pump constantly cycling off and on. My complaints about that fell on deaf ears. Now, after less then a year the pump relay contacts burned out and I have his attention. He will now replace the pump and put in a larger expansion tank. The question is how large a tank? The local plumbing supply house recommended a 39 gal tank through my contractor but I don't know what they based it on. The plumber says a 10 to 20 gal tank would be fine. He says anything bigger would result in a slow draw down which I would see as a slow reduction of the pressure and a marked difference in the sprinkler coverage from the pump cutoff to when it kicks back on. From what I can find a typical sprinkler will use around 10 to 15 gals per minute, so running 2 sprinklers will draw down a 20 gal tank (~7 gal drawdown?) in well under a minute. That doesn't seem adequate and even a 39 gallon tank seems marginal at best. What size tank should I use and will I really notice a big difference in the sprinkler coverage between a small and large tank? (I would think that the coverage should be the same at the turn on and turn off points of the pump independent of the tank size but he's saying that with the small tank it's barely noticeable since it happens so quick. That's the part I don't buy.)

Thanks,

 

[MHM]

One correction to my post. I believe the sprinkler gpm rate I found is way too high for me. I believe mine would be closer to around 3 gpm

 

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[RHinNorCal]

I think the sprinkler rate you were imagining (10-15gpm) refers to residential fire sprinklers, which are required to deliver a minimum of 13gpm. But back to the real question: the average person uses somewhere between 50 and 80 gallons of water a day; considering that, 39 gallons doesn't seem like overkill to me. I have a 40 gallon tank and it's fine for a family of 4. I would also expect you to hear from valveman shortly, who will explain how you can use a cycle stop valve in conjunction with your 39 gallon tank to deliver relatively constant pressure regardless of demand.

 

[Jadnashua]

IF the water is only used for non-potable uses like watering the lawn, and that essentially runs for a long period of time, then a small tank would be fine since the pump would run almost constantly anyway. If the existing system was turning on/off frequently, it sounds like the check valve may have been leaking.

But, if it's being used for 'normal' things in a house, unless you use something like a cycle stop valve, the tank is too small. The goal is to try to keep the on/off cycles to a minimum and have the pump run for at least a few minutes at a time, and with a 2g tank, even flushing a toilet would cause it to turn on for a very short time.

 

[MHM]

IF the water is only used for non-potable uses like watering the lawn, and that essentially runs for a long period of time, then a small tank would be fine since the pump would run almost constantly anyway. If the existing system was turning on/off frequently, it sounds like the check valve may have been leaking.

But, if it's being used for 'normal' things in a house, unless you use something like a cycle stop valve, the tank is too small. The goal is to try to keep the on/off cycles to a minimum and have the pump run for at least a few minutes at a time, and with a 2g tank, even flushing a toilet would cause it to turn on for a very short time.

Thanks,
It’s strictly for non-potable water. The old pump wasn’t coming on when the water was off so I think the check valve is fine. It was going on and off constantly while running sprinklers. Sometimes multiple times within a minute. The plumber says a 39 gal tank will result in a very noticeable reduction in pressure for longer periods of time over a smaller tank (10 to 20 gal) Is that true? I understand a larger tank would be better on the pump but I don’t want a slow and very noticeable reduction in pressure and output until the pump kicks back on.

 

[Reach4]

Increase the sprinkler head GPM, or restrict the pump to what the sprinklers take. A CSV is the easy way to do that.