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How to size your rain harvesting system

Jonathan Meier • Feb 07, 2021

Wanting to supply your whole house with rainwater? We've learned a few things about sizing systems over the years, and here's a quick guide to help in your system design.

 

Please note: This guide is based on average annual precipitation of 36", spread out relatively evenly between all months, as is the case for Ohio, Michigan, Indiana, Kentucky, West Virginia, Western Pennsylvania, Illinois, and much of the Midwest. If your rainfall amounts vary drastically per month, you may want to consider taking your roof sq footage, multiplying by 0.6, and then multiplying that number by the highest monthly precipitation amount (for instance, if May is your wettest month and you get 2.5 inches of rain in May, multiply by 2.5).

 

Second note: This guide assumes that you are not also using this water to irrigate (with the exception of occasional vegetable garden watering or watering plants around the house). Supplying an irrigation system like a lawn irrigation system uses a lot of water, and the system will need to be drastically upsized to accommodate.

 

CABINS WITH OCCASIONAL USE

If using a plastic cistern, install between 1200-1700 gallons.

If using a concrete cistern, install between 1000-2500 gallons.

Note: Ohio regulations require a minimum capacity of 2500-gallons for all rain cistern systems that supply a building/residence.

 

1-2 PERSON HOUSEHOLD

2500-3000-gallon capacity recommended

 

3-5 PERSON HOUSEHOLD

5500-gallon capacity recommended

 

6+ PERSON HOUSEHOLD

8,000-11,000-gallon capacity recommended. For most households, unless you have several outbuildings or a really big house, it does not make sense to install more than an 11,000-gallon system at your residence (again, assuming you are in the Midwest U.S. climate)

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By Jonathan Meier 17 Feb, 2021
Back when our company was first formed, we installed a lot of rain cisterns that were used to irrigate lawns/turf. The experience, though, left us questioning the practice. On the one hand, rainwater is extremely beneficial for irrigation, and plants and grass respond well to it (especially in comparison to say, municipal water, which is chlorinated, or well water, which is generally iron-heavy and hard). And, on the surface, it looks as if rainwater harvesting for lawn irrigation would be the "green" thing to do. However, if we peel back the layers a bit, I believe we are asking the wrong questions when we're talking about "green" ways of watering lawn. I would offer that the first question should be, "Can I install a lawn that doesn't need to be watered?" Lawn irrigation uses a lot of water. If you irrigate to recommended amounts, then you want to add at least 1" of precipitation to your lawn per week. 1" of water on an acre of grass equates to 27,000 gallons! PER WEEK! Even the largest of residential roofs cannot collect that much water, which means that a rainwater harvesting cistern will only be able to supplement irrigation water usage. For this reason, we encourage our customers to explore low-maintenance grass seed -- Pearl's Premium being one example -- that require little to no irrigation. Even if you have to re-seed your lawn, doing so is a lot less expensive than putting in a large underground cistern to meet watering needs. With a lower maintenance lawn in place, rainwater harvesting then becomes much more feasible for lawn irrigation, allowing the customer to downsize their harvesting capacity while quickening their Return On Investment by potentially eliminating the need for municipal water back-up. Should we, then, use rainwater harvesting systems to irrigate our lawns? I believe we should, but only after re-examining the lawn itself. As always, thanks for reading!
By RainBrother 16 Feb, 2021
A common misconception about rain cisterns is that it's bad water. I've received countless calls over the years from folks who just bought a house that is supplied by a rain harvesting cistern who are worried sick about their new water supply. But is there cause for concern? Let's deconstruct this fear a bit. If you're used to city or county water, then, most of the time, that means you're getting water that is coming from an open reservoir (though some are sourced from groundwater). While reservoirs are very protected, their feed springs/creeks/waterways aren't as highly guarded, which means that hydrocarbons from oil leaks in cars/trucks, litter, road salts, and any number of human-made pollutants can and do find their way into reservoirs. Additionally, you often have traces of pharmaceuticals found in our waterways, as well as the "forever chemicals" (aka PFAs) leftover from industrial pollution. And then there's micro-plastics and decaying organic material (including but not limited to fecal matter and dead animals). And this is all before the actual conveyance, at which time lead and other heavy metals leach in from antiquated piping. While it's true that municipal water supplies are heavily regulated to protect the end user from some of these pollutants (most notably, bacteria), there is still a lot that doesn't get filtered out on a municipal level that must further be addressed on a house-by-house basis (side note: reverse osmosis is, in my professional opinion, the best way to add extra security for municipal water). By comparison, groundwater supplies are equally susceptible to contamination. Not only do folks on wells have to guard against agricultural run-off and unsafe levels of various organic compounds, but a groundwater supply is only as good as those who draw from it. If you draw from the same aquifer as someone who does not maintain their well, or as someone who has abandoned their well without care or regard, or as an industrial/mining operation that introduces pollutants, your well suffers. If we start by looking at the larger picture in regards to water security, it quickly becomes apparent that a localized, isolated water supply is the most ideal. And that's where rainwater harvesting comes in. Is rainwater pure? Unfortunately, no -- not in our industrial age. While rain is the result of solar distillation (which naturally sheds pollutants and contaminants), falling rain picks dust and contaminants back up, both from the air and, occasionally, from the roof. However, the concentration of said contaminants is often very low -- especially compared to an open reservoir -- and a properly designed rainwater filtration system can eliminate any water quality concerns and can produce some of the purest water available. ( See also our article on filtering rainwater for drinking .) But while straight, unfiltered rainwater may not be pure and potable as is, what one gains by using rain as their water supply is a reduction of risk: You aren't tying your water supply in with those around you and entrusting them to care for the water as much as you do; you aren't susceptible to PFAs and forever chemicals; you aren't getting pharmaceutical traces in your water; you're not running your water through out-dated underground lead piping (at least, hopefully you're cistern supply line is not lead). Moreover, rainwater filtration is predictable. The same rain filtration system can be put in in Kentucky as in Kenya. With groundwater/wells, this couldn't be further from the truth, and groundwater filtration is completely reliant on a detailed water test, often times meaning that your neighbor might not even need the same treatment/filtration as you. At the start of this article, I mentioned the number of calls we get from people concerned about their new home having a cistern. I would be remiss if I did not also mention that many of those same people call back months later to say how glad they are that they have a cistern for their water supply. One customer, in particular, was not only terrified of the prospect of using a cistern for his water (and called me at least three times before moving in to ask questions), but, after having lived in the house for three years, told me that he was going to have a cistern installed at any future home he lived in if he ever had to move. Not only are rain cisterns viable, but also, given the many environmental factors at play in our world today, they may indeed prove to be the wise water choice.
By Jonathan Meier 15 Feb, 2021
Have an old cistern? Suspecting that it may be leaking somewhere? While there are no easy methods to finding leaks, here are some tricks we've learned over the years: 1) Start by ensuring that the leak is coming from the cistern and not, say, your plumbing or an underground water line. First, if you have a pressure gauge inside your house on your pressure tank, watch it. Without any water running inside your home, is your pressure gauge dropping? If so, that indicates that there's a leak in your water line. In order to narrow in on where the leak is coming from, close off your whole house shut-off valve (usually found directly after your pressure tank). This will isolate your system so that you're reading the pressure between your cistern and the shut-off valve. Is the pressure gauge still dropping? If so, your leak is between the cistern and the shut off valve (and, most likely, it is a leak in the water line inside your cistern... perhaps a faulty foot valve, if you're using a jet pump). If the pressure is not still dropping, then the leak is after(or downstream from) your shut off valve (most likely, you have a leaky toilet). 2) If your pressure gauge does not drop when you look at it, then, before you go to bed, shut off the power to your pump and take a measurement of your cistern water level. Try to be as exact as possible. Write down the measurement. Then, in the morning when you wake up, take a measurement again in your cistern and see if it dropped. If the measurement dropped, you likely have a leak in your cistern tank. There are a few different ways to try to track down a leak. If you have a concrete tank, then chances are that you're due to coat the tank with a potable-grade waterproofing cement, like MasterSeal 581 (PLEASE take precautions when coating or parging the tank yourself to ensure proper ventilation and respiratory protection). But, you may be able to "patch" the cistern with hydraulic cement by tracing the leak one of three ways: 1) The waiting game: Don't use the water for several days and see where it stops dropping down. That would be the location of your leak. 2) Food coloring: Get inside the tank when it has water in it, and inject a brightly-colored food coloring via a syringe into the water along the perimeter of the tank. With a flashlight and a lotof patience, see if you can tell where the food coloring moves. If it sucks into a wall, that's an indication of a leak. 3) Pump down: My favorite method is to pump down the tank all the way and, while it's pumping down, get into the cistern. A lot of times (especially with cisterns made of concrete block), the leak points will send water back into the tank once you've pumped it out. In other words, you'll see a stream a water or a steadily running drip where you have compromises in your tank wall, because water that previously had been flowing out into the ground will now come back into the tank once you remove the water, since the empty void of the tank will now be the path of least resistance. This method works quite well, but the downside is that you have to drain out your precious water supply. I hope this helps provide some guidance in what can be a frustrating and trying process. Remember that, when in doubt, re-coat/parge the tank or install a cistern liner!
By Jonathan Meier 14 Feb, 2021
For several years, we carried the Grundfos SBA 3-45-AW automatic cistern pump . We've used many different pumps made by Grundfos over the years, and they are known to make quality, innovative products, so we jumped at the opportunity to carry an easy-to-install, pressure-tank-free alternative to traditional pumping systems. However, we recently became troubled by the number of failures that were being reported from the SBA model, and have since decided to stop selling these units. Automatic cistern pumps (i.e., pumps that do not require an external pressure switch to operate, or a large pressure tank) certainly have their place: We get thousands of requests per year from homeowners who are building cabins or have limited utility space and just can't spare 4-6 sq. ft. of floor space for a pressure tank. Also, when using a rain cistern or spring tank for irrigation, automatic pumps cut out the need for pump start relays. In our experience, though, the Grundfos SBA is too much of a gamble in the realm of automatic pumps. While Grundfos has done a great job honoring warranties, the number of warranty claims that we submitted on the SBA model far exceeds any other product that we've ever carried. In fact, if we get more than a 3% failure rate within warranty periods, we will drop a product. Because the SBA was new to the US market as of a couple years ago, we decided to trust the Grundfos brand since they are an international company that has been making pumps for 50+ years. However, after about 18-22 months of use, we were seeing far too many pump failures. Additionally, in the last month, we had as much as 30% of these SBA pumps be defective right out of the box. Mistakes in manufacturing happen. We get that. But, to add insult to injury, the staff of Grundfos has left us completely in the dark on why these failures were occurring, what was causing them, and what (if anything) they were doing to address and remedy these issues. On the flip side, we have been carrying another brand of automatic cistern pump -- the Strom BSP -- for nearly as long as we've carried the Grundfos SBAs , and have seen less than 0.5% failure rate. As a result, we are urging customers who really need that automatic cistern pump for their application to consider the Strom BSP . Please note, though, that we almost exclusively recommend that customers go with the 3/4HP model... not the 1HP. The 3/4HP Strom BSP is ideal for residential water supply to a house/barn/etc. Customers should only use the 1HP model if they are planning on running irrigation systems with the pump with flow rates of over 15GPM @ 50PSI, or if you pumping uphill more than 80 ft. Also, it is recommended that a 2-gallon pressure tank be installed with the Strom BSP somewhere in your plumbing supply (ideally either in the cistern or immediately where the water supply line enters the house/structure from the cistern). The Strom BSP comes with built-in run-dry protection, and an optional floating intake sleeve is available for the unit. #Grundfos #pressuretank #waterpump #automaticpump #cisternpump #irrigation
By Rain Brothers 13 Feb, 2021
We carry several high quality water pumps that can be used in a variety of applications. Check out our guide to determine which pump will best suit your needs. IF YOU HAVE AN UNDERGROUND WATER TANK, AND YOU… Have a cabin, mobile home, tiny home, or building with limited or no indoor utility space: Strom BSP (3/4HP) Want to supply an irrigation system where each zone is designed to expel at least 5 gallons per minute, or GPM, (but not more than 8 GPM) at ~ 50 PSI: Strom BSP (3/4HP) , Franklin C1 with pressure tank Want to supply an irrigation system where each zone is designed to expel between 8-14 GPM at ~ 50 PSI: Strom BSP (3/4HP) Want to supply an irrigation system where each zone is designed to expel between 12-17 GPM at ~50PSI: Strom BSP (1HP) Want to supply an irrigation system where each zone is designed to expel less than 5GPM at less than 50PSI (for instance, small scale drip irrigation): Franklin C1 with minimum 20-gallon pressure tank , Strom BSP 3/4HP WITH relief line feeding back to tank (ask us for drawing that illustrates this) Want to supply a house or building with pressurized water, and the home/building has at least 4 sq ft of floor space in a utility room: Franklin C1 with 20-gallon pressure tank Have an existing cistern with a water line to the tank but no power supply at the tank: Franklin VersaJet 3/4HP Jet Pump with pressure tank If energy efficiency is a concern (i.e., if you are in an off-grid home): Franklin C1 with the largest pressure tank you can afford (usually best to source the larger pressure tanks locally to save on freight but we do carry tanks up to 119-gallons in capacity) IF YOU HAVE AN ABOVE-GROUND WATER TANK, AND YOU… Have a cabin, mobile home, tiny home, or building with limited or no indoor utility space: Strom BSP (3/4HP) . PLEASE NOTE: Winterization of pumps will be required if you live in an area where temps get below freezing. Want to supply a house or building with pressurized water, and the home/building has at least 4 sq ft of floor space in a utility room: Franklin C1 with 20-gallon pressure tank , Franklin VersaJet 3/4HP pump with pressure tank . PLEASE NOTE: Winterization of pumps will be required if you live in an area where temps get below freezing. #waterpump #cisternpump #jetpump #Grundfos #tinyhome #rainwaterharvesting #pressuretank
By Jonathan Meier 12 Feb, 2021
Considering a bladder tank for your rainwater harvesting project? May I suggest going back to the drawing board? Yes, you've probably seen them advertised as the perfect option for harvesting rainwater underneath your deck or in that unused crawl space. What better way to implement a cost-effective rainwater harvesting solution! But rain bladders (or "rain pillows" as they're sometimes known as) are fraught with issues. And, here, I'm speaking as a professional rainwater system installer with well over a decade of field experience who has tried -- and failed -- at capturing rainwater in bladders. Here are my concerns with this type of system: 1) Flexibility. They are flexible. And, in being flexible, they are unpredictable. They shift, they expand, and they move. Three things that you don't want in a water storage scenario. And because they move, you have to map out all the ways in which they can move as you're planning your install. No thanks. 2) Expandability. If you press the supplier of your rain bladder, they'll likely be forced to admit that the fabric that is used to retain the water (which, by the way, is undoubtedly quality fabric!) is one that cannot stop expanding. If more water comes, the bladder will get bigger. And bigger. And bigger. And while it may have to get extremely large before it ever reaches a tear point, the problem with the expansion is not in the fabric -- it's in the surrounding structure(s). For instance, if it's in a crawl space, an ever-expanding bladder can serve as a hydraulic lift on the floor joists above. Same is true with a deck. Bad idea. 3) Do you need a third reason at this point? Ok, I'll keep going. Bladders cannot be cleaned. This means that your pre-filtration has to be extremely refined, which is difficult to accomplish if you want to harvest large quantities of water. In my experience, you want a rain catchment system that is cleanable to ensure that you have great water quality. 4) Still here, eh? Alright, I got another for you... Bladders are difficult to plumb. Because they flex and move and shift and expand, running a plumbing connection to them (either from your downspouts or to a pump) is difficult to accomplish. You have to really know your way around plumbing fittings and different types of pipe to be able to plan for flexibility while ensuring water-tight connection points. And if you don't plan well, then you've got all your drainage going to exactly the spot where you won't want it -- inside your house (if you're looking to install in a crawl space) or under your deck. And you probably won't know there's an issue with your plumbing connection until you're three inches into a heavy rain event. 5) Good luck pumping from them. You have two options when it comes to pumping: submersible or external. Submersible pumps are not an option for rain bladders because there's isn't enough height, and you run the risk of burning a hole in the fabric. So that leaves an external pump, such as a jet pump. In order for a jet pump to work, the water from the bladder has to gravity drain into a check valve and then into the pump (you can't install a foot valve in a bladder so gravity draining is the only option). In order for the water to gravity drain to your suction end of a jet pump, the pipe has to leave the bladder at the bottom of the bladder. However, putting a pipe on the bottom of a bladder can create the opportunity for the weight of the water inside the bladder to shift onto that pipe, thus crushing it or breaking it. So then you're option is to put it on the side of the bladder toward the bottom. But because bladder tanks don't get very tall (unless they overfill), it is difficult to get the water inside the bladder to drain toward the pump, so much of the bladder will prove to be unusable. In all my years putting in rain systems, I installed exactly three bladder systems. I am a careful planner and really wanted the idea to work. One never worked (and not for lack of trying on my end). One worked well, but for a short period (but this one was installed outdoors and not under any deck, so movement/shifting/drainage was not a concern). And the third required constant vigilance and innumerable sleepless nights on my end (not to mention a ton of creative engineering) in order to work, and, even that one proved to be too much work for so little pay-off. Being a contractor in an uncommon field like rainwater harvesting means that I have had to learn a lot of lessons on my own. And I made a lot of mistakes early on and had to pay for every one of them. Rainwater harvesting into bladders is one mistake I will never make again.
By Jonathan Meier 11 Feb, 2021
One of our most frequently asked questions is, “Can I use your cistern pumps with my drip irrigation system?” The answer to this question can get a bit tricky because drip irrigation itself is a confusing subject. Three types of irrigation systems get lumped into the “drip irrigation”category, so to answer this question, first we need to know what type of drip system is being used. DRIP TAPE First, there is drip tape -- an inexpensive (and usually disposable) flat plastic tape that is designed to work at 10 PSI maximum. Often times, a 10 PSI pressure regulator is required to run drip tape, or else the entire irrigation system is liable to blow apart. 10 PSI is low pressure. In fact, it is much lower than the majority of pumps are able to handle. Every pump has an operating range associated with it —also known as a pump performance curve. It’s a curve because flow (measured in gallons per minute) and pressure (measured in pounds per square inch) are directly related in a pump’s performance. As you increase your demand for flow, you decrease the pump’s ability to build pressure. Likewise, the reverse is true: As you demand an increase in pressure, you decrease your flow rate. For example, our Grundfos SBA 3-45-AW will operate at 52 PSI when the flow being demanded is 5 gallons per minute, and it will operate at 38 PSI when we call for 10 GPM. When we follow the pump’s curve, we see that at 10 PSI (which, again, is the operating pressure for drip tape), the Grundfos SBA will produce a flow rate of 23 gallons per minute! It takes a LOT of drip tape to get to that kind of flow rate. So what are the options? Can you use a pump with drip tape systems? Yes and no. If you stick a Grundfos SBA or a Strom BSP automatic pump in a tank and run it to your drip system, chances are good that you’re going to cause that pump to short cycle constantly…it will detect a flow being demanded by your irrigation system, but, without sufficient flow rates (in this case, 23 gallons per minute), it will be able to build up so quickly to the 10PSI pressure that it will shut off immediately. That’s a quick way to burn up a pump. There is a solution, though. A traditional drip irrigation system will be set up, in order of flow, as Pump —> Valve (automatic or manual) —> 10 PSI Pressure Regulator —> Drip Tape. If you know you aren’t going to have enough flow being demanded from your drip tape system, the simple solution is to install a “T”fitting after your valve but before your pressure regulator and run a 1/2”or 3/4”or 1”return line back to your water tank. In so doing, you will create a relief port for excess pressure and excess flow generated by the pump, causing it to run less frequently. And, if you bring your return line right into the top of your water tank, you have the added benefit of circulating the water and reducing the likelihood of algae growth. DRIP EMITTERS The second type of drip irrigation system is one that uses drip emitters. Like drip tape, drip emitters typically require low pressure, and they always drastically reduce flow. Unless you are using hundreds of emitters at once, it is not recommended to use a pump to provide water to drip emitters. Instead, we recommend using zero-pressure emitters, such as Flag Emitters, where the pressure created by gravity on an above ground tank is sufficient to provide water to the emitters. DRIP TUBING The third and final type of drip irrigation system is drip tubing. Drip tubing is round and generally has drip emitters embedded every 8-24”. Unlike drip tape, drip tubing can work under pressure…generally as high as 50-60 PSI. Drip tubing works well with our cistern pumps, though, again, you will want to reference the pump’s performance curve to see how much flow you should have in your irrigation system at a given pressure. For instance, if you want to run the Grundfos SBA pump at 50PSI, you need to make sure that you zone out your irrigation system so that each zone is operating at 8 GPM. Each drip tubing roll will specify the spacing of the emitters and the flow rate of each emitter, so if you are using tubing that has emitters every 12”, and the emitters have a flow rate of 0.5 gallon per hour (tubing is usually measured in gallons per hour, not gallons per minute) per emitter, then you would 960 feet of drip tubing to get a flow rate of 8 gallons per minute and to stay within the operating capacity of the SBA pump. If you don’t need that much drip tubing, then, again, we recommend installing a return line back to your rain tank that can let water freely flow into your tank to bleed off excess pressure/flow. #dripirrigation #automaticpump #cisternpump #driptape #rainwaterharvesting #irrigation
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