There is a wide range of sizing parameters and issues available with regard to point-of-entry (POE) softening systems. Big-box retailers offer a limited number of choices when it comes to system sizes and capabilities, and many dealers try to simplify their business model by operating in the same manner.
Use this to your advantage by marketing custom-designed conditioning systems for each specific application. After all, customers have varying needs and water quality expectations. A household of eight requiring hardness and chlorine removal will need a significantly different system from the retired couple next door with a desire for a nonregenerating scale-reduction system. Use your expertise to allow the flexibility in system design that will give you an edge over the competition.
Sizing a residential softener system may seem simple but there are many factors to consider. A family of five that uses about 375 gal of softened water per day (75 gal per day per person) at an influent hardness of 20 grains per gallon results in a daily softened-water capacity need of about 7,500 grains. A 1-cu-ft 32,000-grain softener seems to fill the need perfectly because it will theoretically last four days between regenerations; however, the 32,000-grain rating is based on an inefficient salt setting of about 15 lb and nets an efficiency rating of only 2,133 grains removal per pound of salt (32,000 divided by 15).
Due to various economic, environmental and legal issues, it is now common to set softeners to regenerate with 6 lb of salt per cubic foot of resin. A well-designed 1-cu-ft softener regenerated with 6 lb of salt should provide a more environmentally friendly capacity of about 24,000 grains. This works out to an efficiency rating of 4,000 grains removal per pound of salt.
This lower salt setting results in a nearly twofold salt efficiency rating, provides many benefits and in most applications will have few, if any, drawbacks. Thus, from an efficiency standpoint, the 1-cu-ft softener is now capable of removing about 24,000 grains, or softening 1,200 gal (24,000 grains capacity divided by 20 grains influent hardness) before needing regeneration. Based on the 1,200-gal capacity divided by 375 gal used per day, the 1-cu-ft softener will need to regenerate about every three days.
Now let’s consider the type of regeneration initiation and required reserve capacities. Time-clock-initiated softeners are generally less efficient, especially in residential applications, and have become far less popular. For our purposes, we will focus on the more common demand-initiated systems.
Installing a single-tank meter- initiated softener in a residence requires a certain amount of reserve capacity. This is to account for the set delayed regeneration time, usually 2 a.m., and the amount of capacity that is needed to maintain soft water during the period between the call for and the actual start of regeneration. The softener, for example, may receive a need-to-regenerate prompt nearly 24 hours before it is able to actually perform the regeneration. This is because the set capacity may be reached at 4 a.m., but the delayed regeneration function will prevent regeneration until 2 a.m. the next morning.
Regeneration is delayed to a low-volume timeframe like 2 a.m. to lessen the use of hard water and to prevent hard water from filling the water heater during regeneration when soft water is not typically available. Reserve capacities are generally calculated as one full day of soft water use by multiplying the number of people in a household by the 75-gal average daily soft water use. Variable reserve functions available on most modern digital valves help to mitigate this issue by automatically adjusting the reserve capacity based on historical water-use data. Twin-alternating systems nearly eliminate the need for a reserve capacity by using an immediate regeneration function.
The reserve-capacity issue is exacerbated when applied to many commercial applications. Inconsistent water use in hotels and restaurants cry out for twin-alternating or demand-flow systems. Adding a full-day reserve to a single metered commercial softener may require such a large part of the available capacity that the meter becomes moot. The large reserve capacity may result in daily regenerations unless the system is grossly oversized.
Oversized softeners may sound like a good option, but this can lead to channeling and further efficiency issues. So now we must adjust the 1-cu-ft system’s capacity setting from 1,200 gal less the 375-gal reserve to 825 gal between regenerations. Now the regeneration schedule is trimmed to about every other day. Other factors such as TDS, iron or manganese can further lessen the available capacity to the point where the 1-cu-ft system will require daily regeneration.
The required reserve capacity of 375 gal remains unchanged whether we are talking about a 1- or 2-cu-ft system. For the 1-cu-ft system, more than 30% (375 gal reserve divided by 1,200 gal capacity) of the capacity is set aside, mostly wasted, for reserve. Compare this to a 2-cu-ft system in the same application. Only about 15% (375 gal reserve divided by 2,400 gal capacity) of the capacity is set aside for reserve. In this example, the 2-cu-ft system is far more efficient from a capacity standpoint.
If a 2-cu-ft system is better than a 1-cu-ft, how about using a 10-cu-ft system instead? Considering only efficiency, this might make sense because the reserve-capacity factor would be reduced to only about 3%. However, even negating the space, cost and other pertinent objections, large systems with low service flow demand will suffer from internal channeling. Making the most of available capacity is dependent on getting relatively equal flow through the entire resin bed. Hub and lateral style or distributor plates will help to provide a consistent flow through the resin bed.
To limit channeling, size the system to flow at least 3 gal per minute per sq ft of bed area. This would be about 2.3 gal per minute for a 12-in.-diameter mineral tank. A 10-cu-ft system using a 24-in.-diameter mineral tank would need to flow at a minimum of about 10 gal per minute to limit channeling issues. The typical residential application generally has a flow demand in the 2- to 5-gal-per-minute range. This is why the oversized residential system will suffer from efficiency issues due to channeling. A system that is channeling due to low flow rates will typically be difficult to diagnose hard water problems because it will run out of soft water prior to the calculated gallon figure.
Peak softener flow rates should not exceed about 15 gal per minute per sq ft of bed area. In a 9-in.-diameter tank, this is about 6.5 gal per minute and about 12 gal per minute for a 12-in.-diameter mineral tank. Excessive service flow rate results in increased pressure drop, lower capacity and can even damage the resin.
Undersized softeners are inefficient, work the components harder, increase pressure drop and will likely fail sooner. Oversized softeners are wasteful and inefficient. Properly sized softener systems, especially single tanks with adjustable reserve capability, twin-alternating or demand-flow systems, offer the best capacity and efficiency capabilities.
Twin-alternating softeners theoretically regenerate when the full capacity of one tank is exhausted, immediately bringing the standby tank into service. Twin-alternating units are great for most applications and especially those with high or fluctuating capacity demands. Temporary pressure drop due to regeneration water demand can result.
Demand-flow systems bring tanks online based on fluctuating water flow requirements. A bank of four units will typically bring one, two or even three units into service as flow needs dictate, with one tank left in regeneration or standby. These are beneficial for inconsistent flow applications such as hotels. Be sure to base sizing on the system capability, not on pipe size. A 2-in. pipe can generally flow at a much higher rate than a softener with 2-in. connections. It is not unusual, for instance, to use a softener system with a 3-in. controller for an application with a 2-in. pipe.
Brine tanks are typically sized for lower salt settings. Make sure the salt and brine capacities are adequate for the regeneration levels required. Many factors must be considered when sizing any kind of POE softener or filtration system. Seek guidance when needed, size and apply appropriately and you will gain the long-term rewards that come from customer referrals and a reputation for doing the job right.