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OneFlow® Anti-Scale System For Tankless Water Heaters

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OneFlow Anti-Scale System For Tankless Water Heaters for up to 5 GPM (18.92 lpm)


This OneFlow(R) anti-scale system conditions cold tap water and provides scale protection on internal plumbing surfaces to extend the life of equipment by transforming dissolved hardness minerals into harmless, inactive microscopic particles. Used for single residential tankless water heater units, the specially designed components are installed before a water heater to protect against the effects of hard water. The OneFlow anti-scale system is not a water softener and does not add chemicals or remove minerals, but provides environmentally friendly technology to prevent scale buildup. It's a salt-free water conditioner for tankless water heaters that you can feel good about.

Tankless water heaters can be a breeding ground for scale. Heat increases the tendency for limescale deposits to form; the higher the heat the greater the buildup. Tankless units are designed to operate at high efficiency. Hard water can work against that efficiency by forming scale on critical components resulting in malfunctions and service calls.

Now there's a solution - OneFlow(R). The same anti-scale technology that's been used successfully in commercial plumbing is now designed to protect residential tankless water heaters in your home. OneFlow Model OFTWH-C for single residential tankless water heaters is a cost effective and proven technology to ensure scale-free operation for your tankless water heater.


  • Compatible with all major tankless water heater brands
  • Maintains manufacturer's stated water heater efficiency
  • Works to prevent heater malfunction due to scale build-up
  • Diminishes existing scale problems and costs associated with de-scaling
  • Environmentally friendly alternative to water softeners



  • Compact design for easy installation
  • Designed for tankless water heaters up to 5 gallons per minute
  • Virtually maintenance free - no salt or chemicals to add or maintain
  • No electricity or backwashing required
  • Simple sizing & installation with standard 3/4 IN connections and included mounting bracket
  • Inlet ball valve and filter wrench for easy shutoff and filter changes
  • Long lasting OneFlow cartridge ' change once every two years

Click here to download yourOneFlow Anti-Scale System Manual



Inlet / Outlet Connections: 3/4 IN FNPT (19.05 mm)
Nominal Flow Rate: Up to 5 gpm (18.92 lpm)
Maximum Pressure: 90psi / 6.2 bar
Maximum Temperature: 100 °F / 38 °C
Minimum Temperature: 40 °F / 5 °C
Capacity: OFTWH-C-RM cartridge does not have a grain removal capacity, however, other contaminants present in the water will gradually degrade the effectiveness of this cartridge. Change the OFTWH-C-RM cartridge at least once every two years.



  • OneFlow(R) Anti-Scale System, Model: OFTWH-C
  • Full Flow Mounting Bracket and screws
  • Ball Valve PVC, 3/4" F x M
  • OneFlow Cartridge
  • Full Flow Housing Wrench
  • Manual

Product Dimensions

  • Mounted including Ball Valve (10" W x 18.5" H x 8.625" D)
  • Note: Requires a 3" Minimum Removal Clearance under housing.

(The overall height and the height of the inlet fitting varies due to material variations and assembly tolerances. Please allow additional clearance above the filter for making connections.)

Feed Water Chemistry Requirements
pH: 6.5 - 8.5
Hardness (maximum): 30 grains (513 ppm CaCO3) *
Water Pressure: 15 psi to 90 psi (1.03 bar to 6.2 bar)
Temperature: 40 ° F to 100 ° F (5 °C to 38 °C)
Free Chlorine: <2 ppm
Iron (maximum): 0.3 ppm **
Manganese (maximum): 0.05 ppm **
Copper: 1.3 ppm ***
Oil & H2S: Must be Removed Prior to OneFlow
Total Phosphates: < 3.0 ppm
Silica (maximum): 20 ppm
TDS: 1500 mg/l € 


* Systems using OneFlow technology are effective at controlling lime-scale formation inside the plumbing system at influent hardness levels up to 30 grains per gallon (513 ppm CaCO3) of calcium carbonate. Due to variances in water chemistry, 30 grains per gallon is a recommended hardness maximum due to potential aesthetic issues related to soft scale residue formation outside of the plumbing system. Testing should be performed to determine proper application where hardness levels exceed 30 grains per gallon.

**Just as with conventional water softening media, OneFlow media needs to be protected from excess levels of certain metals that can easily coat the active surface, reducing its effectiveness over time. Public water supplies rarely, if ever, present a problem, but if the water supply is from a private well, confirm that the levels of iron (Fe) and manganese (Mn) are less than 0.3 mg/L and 0.05 mg/L, respectively.

***Pursuant to the EPA drinking water standards, the copper concentration permitted is up to 1.3 ppm. Typically originating from new copper plumbing, high levels of copper can foul OneFlo media. New Copper lines need to be passivated for a minimum of 4 weeks before placing unit into service. For applications with copper concentration greater than 1.3 ppm, please consult Watts Water Quality Technical Service. To further minimize any problem with excess copper, avoid applying excessive flux on the inner surfaces of the pipe and use a low-corrosivity water soluble flux listed under the ASTM B813 standard.

OneFlow media does not reduce silica scaling. While silica tends to have a less significant effect on scale formation than other minerals, it can act as a binder that makes water spots and scale residue outside the plumbing system difficult to remove. This 20 ppm limitation is for aesthetic purposes.

All other contaminants must meet the requirements of the USEPA Safe Drinking Water Act. Specific Mineral and Metal MCL's, identified in Watts published Feed Water Chemistry Requirements, supersedes the USEPA SDWA. Water known to have heavy loads of dirt and debris may require pre-filtration prior to OneFlow.


What is OneFlow?

OneFlow is an anti-scaling water treatment technology designed to protect plumbing components, pipes, fixtures, valves, fittings, hot tanks, etc. from the negative effects typically caused by water hardness. OneFlow is an environmentally friendly alternative to water softening or any other treatment device designed to address scale. OneFlow prevents scale by transforming dissolved hardness minerals into harmless, inactive microscopic crystal particles. These crystals stay suspended in the water and have a greatly reduced ability to react negatively and form scale compared to untreated hard water.

How does OneFlow differ from a Water Softener?

OneFlow uses a specially designed media to convert the dissolved primary hardness mineral (calcium carbonate) into a relatively insoluable microscopic crystal so that it cannot cause harm downstream. A water softener, on the other hand, uses the principles of "ion exchange" to remove calcium and magnesium ions from water while replacing them with sodium ions. A water softener requires a control valve, backwashing, drain line, regeneration and a brine tank to store salt while OneFlow requires nothing but an inlet and outlet connection.

Does OneFlow remove the important minerals in water?

No. OneFlow preserves the beneficial mineral content while preventing the possible negative scale formation associated with these minerals. By rendering the hardness minerals harmless while still keeping them present, you truly get the best of both worlds. Other treatment methods fall short. In a 2005 report on Nutrients in Drinking Water, the World Health Organization writes "We conclude that hard water is good because it contains nutrients valuable in themselves. To minimize heart disease risk, the ideal water should contain sufficient calcium and magnesium to be moderately hard."

What factors may effect OneFlow's system performance?

Water chemistry and peak fl ow rate are the guidelines used to assure optimal system performance. Most municipally treated (city) water falls well within our performance guidelines to ensure scale control results. However, there are chemistry limitations related to possible foulants and interference ions. Water known to contain visible dirt and debris must be strained using a wye-pattern strainer or equivalent (100 mesh) before OneFlow tank systems. More details are available on the product datasheets and IOMs. As with any water treatment technology, correct sizing of the system is important to ensure proper performance. However, OneFlow is more forgiving of flow variations and unplanned peaks than a conventional water softener.

How easy is it to install a OneFlow system?

For anyone who has ever installed a Water Pressure Reducing Valve or a Backflow Preventer, installation of a OneFlow system will be relatively easy.

What are the ongoing maintenance costs and considerations for OneFlow?

One of the great benefits of the OneFlow technology is the dramatically reduced maintenance requirements. Other than the planned media replacement, there is nothing else required. For tank systems, the media replacement is once every three years, and for filter housing systems, it's once every 2 years. That's it.

Why is OneFlow a "Green Technology"?

OneFlow  is truly a "Green product" because it saves water and energy in the following ways:

  1. Reduces Energy Consumption OneFlow uses no electricity and reduces energy consumption by keeping heat transfer surfaces free of energy-robbing scale formation.
  2. Reduces Chemical or Raw Material Consumption OneFlow requires no chemical or significant ongoing consumable (salt, as an example).
  3. Reduces Waste Water Discharge and Water Consumption Because OneFlowdoes not discharge or require backwashing it meets both objectives.
  4. Reduces Space Requirements OneFlow systems use approximately 75% less floor space than water softeners.
  5. Retains nature's minerals in the water. The beneficial minerals, calcium & magnesium, are completely retained.