Ultraviolet
Ultraviolet dose calculator
Quick answer
Dose is inversely proportional to flow. A system rated 12 gallons per minute at the Class A dose delivers only about 32.0 mJ/cm2 at 15 GPM, below the NSF/ANSI 55 Class A figure of 40 mJ/cm2. An undersized ultraviolet system fails silently, because the lamp still lights.
Ultraviolet dose is the simplest arithmetic in this category and the most dangerous to get wrong. The faster water moves through the chamber, the less time each organism spends under the lamp, so dose falls in direct proportion as flow rises.
That is why an undersized system fails without any symptom at all. The lamp is lit, the indicator is green, nothing tastes different, and the delivered dose is a fraction of what the label implied because the house exceeds the rated flow every time two showers run.
Work out your own delivered dose
At 15 GPM this 12 GPM system delivers about 32.0 mJ/cm2, below the Class A figure of 40.
| Figure | Value |
|---|---|
| Delivered dose at your flow | 32.0 mJ/cm2 |
| Against Class A | 40 mJ/cm2 |
| Against Class B | 16 mJ/cm2 |
| Flow that holds Class A | 12.0 GPM |
| Verdict | Below Class A |
Shown with the example figures above. Change any field to run your own numbers.
The working
Published figuredose at a flow = rated dose x rated flow / actual flow NSF/ANSI 55 Class A = 40 mJ/cm2 water NOT known to be safe NSF/ANSI 55 Class B = 16 mJ/cm2 supplemental only
Dose is inversely proportional to flow because it is lamp intensity multiplied by exposure time, and exposure time is chamber volume divided by flow rate. The class doses are published in NSF/ANSI 55. A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
Ultraviolet systems and consumables
Size on peak flow rather than household size, and confirm the dose the manufacturer publishes AT that flow. Some systems quote a higher gallons per minute figure by rating it at a lower dose.
VIQUA VH410M UV Water Treatment System, 18 GPM
$1,310.00VIQUA is the residential ultraviolet brand most water treatment professionals specify, and the monitored version reports lamp output rather than only lamp hours.
Best for: A whole house on a private well, where flow can reach 18 gallons per minute
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VIQUA VH200-F10 UV Whole House Water Filter System, 9 GPM
$980.00A 9 GPM stainless chamber, which suits a smaller house where an 18 GPM unit is more than the plumbing will ever deliver.
Best for: A one or two bathroom house on a private well
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VIQUA VH150 UV Water Treatment System, 5 GPM
$600.00The smallest whole house VIQUA, for a cabin or a low flow installation.
Best for: A cabin, a small dwelling or a point of entry with genuinely low flow
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VIQUA S2Q-PA Stainless Steel Ultraviolet Water System, 5 GPM, 22 Watt
$460.00A long running 5 GPM unit at the lowest VIQUA price, with lamps and sleeves widely available years later.
Best for: A low flow installation from a brand with a long parts tail
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iSpring UVF55FS Whole House UV Water Filter, 12 GPM, 1 Inch Ports
$299.99 Published specsA 12 GPM system with a flow sensor so the lamp only runs when water moves, at well under half the price of the equivalent VIQUA.
Best for: A budget whole house ultraviolet installation
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HQUA TWS-12 Ultraviolet Water Purifier, 12 GPM
$259.99A widely sold 12 GPM unit with cheap and available replacement lamps and sleeves.
Best for: A whole house installation where consumable cost matters
Check price on AmazonEvery figure shown is the manufacturer own published specification. None of this is a recommendation for water you have not tested: the arithmetic above is only as good as the numbers you put into it, and on a private well those numbers come from a certified laboratory rather than from a chart.
The two classes are not interchangeable
Class A delivers 40 mJ/cm2 and is for water not known to be safe. Class B delivers 16 and is supplemental treatment for water already considered safe.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Class | Dose at end of lamp life | What it is for |
|---|---|---|
| NSF/ANSI 55 Class A | 40 mJ/cm2 | Disinfection of water that is not known to be safe |
| NSF/ANSI 55 Class B | 16 mJ/cm2 | Supplemental treatment of water already considered safe |
These look almost identical on a product listing and they are not equivalent. Buying Class B for a well that has tested positive for coliform is one of the more dangerous mistakes available in this category.
Ultraviolet light has to reach the organism
Ultraviolet needs clear water: iron below 0.3 mg/L, manganese below 0.05 mg/L and hardness below 7 grains, or the sleeve scales and blocks the light while the lamp still reads as working.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Parameter | Limit |
|---|---|
| Ultraviolet transmittance | 75% or higher |
| Turbidity | Below 1 NTU |
| Iron | Below 0.3 mg/L |
| Manganese | Below 0.05 mg/L |
| Hardness | Below 7 gpg, or the sleeve scales |
| Tannins | Below 0.1 mg/L |
Published by essentially every manufacturer. A system installed outside these limits does not deliver its rated dose no matter what the lamp is doing, which is why ultraviolet goes LAST in a treatment train, after sediment, iron and softening.
What ultraviolet does not do
Ultraviolet removes nothing. It inactivates organisms so they cannot reproduce. Everything that was in the water is still in the water afterwards, and every dissolved contaminant passes through untouched: nitrate, arsenic, lead, hardness, iron and taste are all unaffected.
It also provides no residual. Water is protected at the moment it passes the lamp and not afterwards, so contamination entering downstream of the unit is untreated.
Frequently asked questions
What size UV system do I need?
Size it on your PEAK flow rather than your household size, then check the delivered dose at that flow. A system rated 12 GPM at the Class A dose drops to about 32.0 mJ/cm2 at 15 GPM, which is below the 40 mJ/cm2 Class A figure. Work out peak demand from fixture count first.
What is the difference between Class A and Class B?
Class A delivers 40 mJ/cm2 and is intended for water that is not known to be safe. Class B delivers only 16 mJ/cm2 and is supplemental treatment for water already considered safe. They look nearly identical on a listing. A Class B unit is not a treatment for a coliform positive well and must never be used as one.
Why does my UV system need filtration in front of it?
Because the light has to physically reach each organism. Turbidity shades them, and iron, manganese and hardness scale the quartz sleeve so the light never leaves the lamp assembly. Manufacturers publish limits of roughly 1 NTU turbidity, 0.3 mg/L iron, 0.05 mg/L manganese and 7 grains hardness, which is why ultraviolet goes last in a train.
How often does the lamp need replacing?
Annually, on a date, whether or not it still lights. Output at the germicidal wavelength falls steadily while the visible glow barely changes, so there is no usable warning. Replace or clean the quartz sleeve at the same time, because a scaled sleeve blocks the light while every indicator still reports normal operation.
Does UV make my water safe to drink?
Nothing on this site certifies water as safe to drink, because that is a laboratory result rather than a product property. Ultraviolet at an adequate dose inactivates bacteria, viruses and protozoa in water clear enough for the light to work. It does nothing about nitrate, arsenic, lead or any dissolved contaminant, and it does not address why a well became contaminated.
Can I just run the water slower?
In practice no, because flow is set by whatever fixtures are open rather than by choice. That is exactly why the system has to be sized for peak demand. Some units include a flow restrictor to guarantee the rated dose, which protects the disinfection at the cost of capping how much water the house can draw at once.
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Researched, not professional advice. This page is compiled from published standards, regulatory limits, manufacturer specifications and owner-review consensus, not hands-on testing. Nothing here diagnoses a water problem or says any product makes water safe to drink. Figures described as a rule of thumb are water treatment trade convention rather than published standards, and they are labeled that way wherever they appear. Test your water before you buy equipment: on city water start with the Consumer Confidence Report your utility publishes each year, and on a private well only a certified laboratory test tells you what you have. Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, they are invisible and tasteless, and they need a laboratory and usually a licensed professional rather than a product. Remember that a certification covers a specific claim under a specific standard, so "NSF certified" with no number attached tells you very little.