Sizing a residential water system involves several considerations. You will need to know the amount of water required to meet the family’s needs inside and outside the house. You’ll need to know the vertical distance the water must be lifted from the source to its destination. You’ll need to know how much pressure is needed to overcome friction in the piping system. And finally, you will need to know how much pressure is required once the water gets to where it is needed – for instance, to supply a shower head.
Once you know this information, you can determine which type of pump best serves your application – a straight centrifugal, a shallow-well jet pump, a deep-well jet pump or a submersible pump. If the water level from which you are pumping – be it a lake, storage tank or well – is 15 feet or less, you can use a straight centrifugal pump. If it is 20 feet or less, you can use a shallow-well jet pump. If it is no more than 100 feet, a deep-well jet can be used. When you have more lift than 100 feet, a submersible pump generally is the best choice, and many pump installers prefer to use submersibles for every pumping level.
Once you have determined which type of pump to use, you need to size the pump. Pumps are rated based on flow rate in terms of gallons per minute (GPM), and pressure in terms of pounds per square inch (PSI) or feet of head. Knowing these factors will allow you to size a pump to meet the peak demand of your water system.
Determining the Flow RequirementsIn order to design the complete water system, you will need to know the water usage patterns of your customer in terms of gallons per minute and the production capacity of your well. There are several ways to estimate the water usage of a residential application, which we will detail in a moment. The GPM production capacity of a well is available from the well driller if it is a new well, or from experience if it is an older well for which you have no well test data. If you find that a 30-gpm pump is required to meet the peak flow demand, but the well only produces 25 gpm on a sustained basis, some storage capacity must be added. We’ll talk more about storage systems in the coming months.
Peak FlowIn a residential application, peak flow usually occurs when there are showers running or cooking is going on. This would likely be in the morning or around suppertime. Table 1 shows that two showers, a washing machine and a sink, all going at the same time, would be using from 12 gpm to 20 gpm, depending on the age of the fixtures. Note: Instead of sizing the pumping system to old fixtures, you would do well to suggest to your customers that they install water-conserving fixtures like low-flow showerheads and water-conserving toilets. Some water companies even offer rebates to encourage the use of low-flow devices.
It normally is not necessary to include the usage requirements for lawn-sprinkling and yard-watering because their usage can be timed around the household requirements. However, if they exceed the demand of the household, size the system to the yard-watering requirements.
An alternative method of determining flow requirements, suggested by the Water Systems Council, is to simply count the number of fixtures and faucets in the home. The total number of fixtures gives you the GPM required by the pump. For instance, a two-bathroom home might add up like this; each bathroom – three (one each for the bath tub/shower, sink and toilet), two for the kitchen (sink and dishwasher), one for the washing machine, one for a water softener (uses water during backwash). Total: 10 fixtures equal 10 gpm. Again, if the requirements of the outside watering system exceed these numbers, size the system the outside requirements.
The bottom line: Whichever system you use, be conservative in your sizing. You will never hear a customer complain, “My system provides too much water.”
Pressure TermsWe will talk about pressure in detail next month, but I wanted to introduce the terms now that we will be using to give us a running start next month. As mentioned above, pressure is measured in terms of PSI (pounds per square inch) or feet of head. To help visualize the relationship between PSI and feet of head, picture this – an empty piece of pipe, any diameter, about 3 feet long, with the bottom plugged and a pressure gage at the bottom. Stand it up straight, and fill the pipe until the pressure gage reads 1 psi. This is a hypothetical exercise, but if you had an accurate-enough pressure gage, you would see that 2.3 feet of water standing in the pipe produces 1 psi on the gage. That is the relationship between PSI and feet of head – 1 psi = 2.3 feet of head. If you lowered the water level to 1 foot, you would see 0.433 psi on the gauge. Both terms are used in the pump industry, and you can convert from one to the other by using the factors – 1 psi = 2.3 feet of head, or conversely 1 foot of head = 0.433 psi.
Another good way to remember this relationship is to picture a water tower. If you have a 100-foot-high water tower in your town, what would be the pressure in terms of PSI at ground level? Answer, 43 psi, about the pressure you’d like to have coming into your house. Plant that picture in your brain, and you will never forget how to convert from feet of head to PSI – a 100-foot water tower equals 43 psi. The feet of head number is larger than the PSI number.
With this brief introduction to the terms used in working with pressure, next month, we will continue our series on residential water system sizing with a look at how to determine the pressure requirements. ’Till then….