Possible to oversize a pump for a water system?

[adam759]

Hello everyone,

I've been doing loads of research but have not come to any meaningful conclusions so I thought asking directly might help! I am looking at upgrading our water system pump at our horse farm. Some background first, the building is 300 long and 100 foot wide with 4 aisles, each 100ft long with 1 1/2" PVC running down each. Each aisle has a slop sink and about 4 hose connections. There are also two horse showers. As you can imagine this system can get to be pretty demanding. Our well company put in a tiny Grundfos JP07S-CI 3/4HP 15GPM shallow well pump a while ago and it just isn't cutting it anymore - it trips out on low limit almost every day in the summer, not to mention all the hoses trickling during watering time. The system pressure is set at 55PSI. We have 3 storage tanks, x1 500 gallon and x2 275 gallon tanks all connected together and fed by another well pump that is underground outside. These tanks feed the system pump directly, it is on the floor next to them.

My question is is it possible to put too big of a pump in? I am looking at a new Dayton 5WXU1 3HP 138GPM (at 15ft head) centrifugal pump. I know someone who is selling it fairly cheap. My main concern is, if the water system demand is low to moderate about 70% of the time, will this cause the pump to cut on and off a lot since it is such a high flow rate? We do have an 81 gallon pressure tank on the system as well. We could push the PSI up to around 60 or 70 as we have all PVC, but even then, will the pump just cycle like crazy damaging it? I know it would obviously handle the high demand perfectly during the day when everyone is watering and showering their horses, I just don't want to put it in and then have it either burst lines because it rose the pressure too quickly or have it die in a month from cycling. I just don't know what will happen.

I would be very appreciative of any suggestions or input! Thanks in advance!

 

[Geofd]

I would give grundfos your numbers or engineer, and if you are drawing from a well you would need numbers on that also

 

[adam759]

That is the thing, we are not drawing from a well on the pressure side. The well pump is completely separate and fills the storage tanks. The tanks then passively feed the Grundfos. Also what do you mean by numbers? Like PSI, Head ft, GPM, pumping distance?

 

[Valveman]

All pumps are over-sized most of the time. Pumps need to be sized large enough to handle peak demands when everything and everyone is using water at the same time. However, that rarely happens. Water is used as needed and only one person or one faucet maybe being used at one time. So, the pump is over-sized and cycles on and off way too much.

We used to install larger pressure tanks to slow or reduce the number of cycles. But that becomes impractical as an 80 gallon size pressure tank only holds 20 gallons of water. To get a pump that produces 138 GPM to run for a minute filling the pressure tank, it would take about 7 of those big tanks ganged into a manifold. Even then you only get the minimum 1 minute of run time.

With a Cycle Stop Valve controlling the pump, it can produce as much or as little water as needed without cycling the pump on/off. A 2" CSV would let use 138 GPM if needed, but would also make that pump supply a single 3 GPM shower by itself when needed. All of this would work with one of those 80 gallon tanks that has 20 gallons water draw.

However, the pump you mention will not work. First it is 3 phase, and you most likely do not have 3 phase commercial power at your house? Next that pump has a max head of 81', which is only 35 PSI, and will not work with even a 20/40 pressure switch setting.

Get as large a pump as you think you need, just make sure it has a max head of over 160' (70 PSI) so you can use a 40/60 pressure switch. Then use the appropriate size Cycle Stop Valve to make it do any water demand you need while making the pump last longer and use only 1 pressure tank.

 

[Geofd]

That is the thing, we are not drawing from a well on the pressure side. The well pump is completely separate and fills the storage tanks. The tanks then passively feed the Grundfos. Also what do you mean by numbers? Like PSI, Head ft, GPM, pumping distance?

That is the thing, we are not drawing from a well on the pressure side. The well pump is completely separate and fills the storage tanks. The tanks then passively feed the Grundfos. Also what do you mean by numbers? Like PSI, Head ft, GPM, pumping distance?

Yes tell the grunions rep what you mentioned in your post

 

[Valveman]

If you talk to the Grundfos rep or any pump company rep, they will sell you what they make the most money on, which is the worst thing for you. Pump companies are in business to sell pumps and get as much of your money as they can doing it. They will suggest you get a VFD or Variable Frequency Drive, as that is their cash cow scam of the day. They make you think varying the pump speed will save energy, which is not true. But if you fall for it you won't mind paying much more for the pump or care that it doesn't last very long before you need another expensive pump.

BTW I started selling and even had a factory authorized assembly shop for Grundfos in 1973. I can assure you there is no one at Grunfos who will tell you the truth about this. Like the techs at any pump manufacturer, they only know what they are told to promote, which is their most profitable product and the opposite of what is best for the end user.

 

[adam759]

We do actually have 3 phase power. This is at a horse farm with about 140 horses. We have a big grain auger that uses 3 phase. Other than that the service is unused. We also have a standard single phase service for lighting, fans, etc. I didn't consider the max head being an issue at first, but now that makes sense. Would 35psi even with the high flow rate be a deal breaker for a system like ours? I found this pump for $500 locally brand new, it is usually over $1000 from what I can tell looking up the pump on distributor sites. Our water is quite rusty and the stainless steel aspect of it was attractive as well (easy to clean, resistant to mineral buildup / corrosion, etc).

The well company we used years ago is no longer in business. They put in the initial system with the 500 gal tank and the small pump - I don't think they realized how much demand we have especially in the summer. At peak demand we have 4+ hoses running along with a horse shower, as well as the sinks / toilets in our office area. I have not bought the Dayton yet, but if it absolutely won't work would a pump with a higher max head and similar flow rate be suitable with a cycle sensor? Could you recommend any brands / models? Thank you so much for all the info so far.

 

[Valveman]

With a storage tank and booster pump you can have as much pressure and flow as you want. That is as long as the well pump can keep up with the demand? Do you know how many GPM the well or well pump can supply? You can also start adding up the number of hoses and things you would like to run all at the same time. Figure out your max demand, then you can size the booster pump. Figure out how much water the well can make, and you can determine how long you can run the booster pump and any given time.

But no, that pump will not work. With 35 PSI max you cannot even use a 20/40 pressure switch. Since you have both, single phase or three phase won't matter, but max head does. Find a pump that will produce the max GPM needed, and has at least 70 PSI max head so you can use a normal 40/60 pressure switch. Be happy to help with the pump once you give me a max GPM you need?