I have a shallow well with a 1/2 hp pump on it.  I may eventually get a 3/4 hp pump, but the current flow is around 20 gpm.  I want to put a pressure tank on it so I can have water on demand rather than opening a hose bibb and then turning the pump on.  I can buy a NIB 14 gallon tank from work for probably half price because we don't need it.  My question is, what size tank do I need for irrigation?  My suspicion is that, once a hose bibb is on, the full flow will be going to the bibb and the tank really won't be building pressure unless the bibb(s) isn't fully open.  I don't want to short cycle the pump.  This will only be used for irrigation of a ~3 acre section of land.



Thanks.

Would recommend a call into Bee Cave Drilling  -  Talk to Greg...  That guy is very helpful even if it is answering these types of questions over the phone.  He can easily give you a couple of options to consider.

14 gallons sounds tiny, especially if you want to avoid short cycles, but I'm not an expert.

Try this: http://www.wellmate.com/en-US/support/calculator/

Big as possible. Check out Craigslist. I pickedup a hhugeppressure tank at auctionffor3 bucks. It will be pplumbedinto my iirrigation system, uf my pump
ever arrives...



ETA: Just to clean that up: Get as big as possible, as it will reduce start/stops with your pump (which is what is hard on the pump). Check out craigslist and/or farm auctions. I scored a HUGE pressure tank from a farm auction (was in the milk house). It is going to be plumbed into my new irrigation system, should my HF pump ever arrive.....

I would find a way to watch the power the pump is pulling and turn on the hose bib and pump.



I figure the pump is kicking on and running the whole time the hose bib is open.  Doubt it ever shuts off.  With this part said, if you don't know the specifics of the well for the amount of recovery it has then dropping in a bigger pump might move more water in a shorter time and run it dry til it recovers.



Anyway, I would think a little differently.  



The pressure tank does provide working pressure when the well pump is turned off and it is perfect for things like using water at a sink in a house.  



For irrigation I would almost look at using the well water to fill up a resevoir if possible and then let gravity get the water from the water tank to your irrigations lines.



If you are using the water pressure to spray water then yeah I guess filling a water tank up probably won't work unless you then set up another pump to get water from the resevoir to the irrigation method.



To let the pump shut off and not kick on really quickly I figure you would need a monster sized well pressure tank.



But if you use soaker hoses or something like that, some water tanks on stands might work.  I have never done it but have owned a place with a well and pulled a well pump by hand a couple times.



With I knew as much back then as I thought I did.

I use a 14 gal on my pump that is for irrigation as I am just trying to smooth things out.
Before I had the tank the pump would come on randomly for a second which I assume would be a random leak or my wife getting some water for the dogs etc.
I have the biggest I could find for the house.

If you are using it for irrigation then you shouldn't be using a pressure tank at all...



You should be using your well pump to fill a 1000+ gallon tank, and then another pump made for this purpose for the pressure you need for 'time of usage' pressure.



With that said we have a small garden (around 4 8' beds) around a dozen chickens and we currently only have our 'house' water (well) setup to rely on.  We have around 110G pressure tank but I think it has about 50G available.  Ideally sometime in the future I'm planning to add 3 more pressure tanks so when power is out we can still shower, and drink for days w/out running the well/generator.  We also plan to add 2x 2600G tanks, and add a 2nd pump to our "good winter" well. (It's a 2nd well 400' higher elevation that only gets usable if we get a good wet winter.)



14G pressure tank won't do anything for irrigation.

I do need a pressure tank.  Right now I am running the system as you recommend.  The trouble is as follows:



A. If I want a small amount of water to wash my hands I have to walk to the pump to turn it on.

B. If the hose is kinked or if I am not flowing as much water as the pump wants to push it will trip a thermal overload safety.

C. I have to have at least one hose on all the time.



If I have a tank, three possibilities exist, none of which require me to go turn the pump on:



1. I open one of more hose bibbs completely such that all of the pump's flow is pushed out the hoses and the tank does not fill.  In this case the pump will run perpetually.

2. I open one hose bib partially to water at slow speed.  The tank will drop to 40 psi, the pump will turn on, and then turn off at 60 psi.

3. I open a hose bib slightly to wash my hands.  The pump probably won't turn on.



A larger tank only serves the following purpose:



In case 2 above it will take longer for the tank to reach 40, and the pump will run longer to get it up to 60.  This would reduce pump cycling when not using much water.  

In case 3 above I can use more water to wash my hands without the pump having to turn on.



When irrigating, I will most likely be watering with one or more hoses nearly on full blast.



I bought the half price 14 gallon tank and I will test it out soon and report back with my findings.  I will try all three cases (1-3) above and monitor pump cycling rate.