How to size a UV system for a well

A UV system's dose is a function of how long water spends inside the lamp chamber, and that dwell time is set entirely by flow rate: a chamber rated to deliver a certain dose at a stated gallons per minute delivers proportionally less dose the moment flow rises above that figure, regardless of how many people live in the house. Two households of the same size, one with a single bathroom and one with three bathrooms and a walk-in shower running simultaneously, have very different peak flow demands even though they have identical daily water use, and daily use is the wrong number to size a UV system against entirely.

Peak simultaneous demand is normally worked out from fixture count, the same convention used for whole house filter sizing, since two or three fixtures running together is a routine event in most households, not an edge case.

NSF/ANSI 55 defines two classes by the dose delivered at the end of rated lamp life, and the difference between them is the difference between a system meant to disinfect an unknown water supply and one meant only to add a small margin to water already considered safe.

A unit rated to deliver its stated dose at a specific gallons per minute delivers less dose, in direct proportion, the moment actual flow exceeds that rating. A Class A system rated for 40 mJ/cm2 at 10 gpm delivers roughly half that dose, around 20 mJ/cm2, if the actual peak flow through it reaches 20 gpm, which is well below the 40 mJ/cm2 the classification exists to guarantee. This is exactly why sizing on genuine peak flow rather than a comfortable guess matters: a system installed for a house's typical flow but overwhelmed occasionally by peak demand fails silently at precisely the moments demand is highest.

The lamp itself gives no indication that this is happening. It lights normally regardless of whether the dose reaching the water is 40 mJ/cm2 or 15, which is the core reason UV sizing has to be done correctly the first time rather than corrected after a symptom appears, because there is no symptom.

Ultraviolet light has to physically reach the organism, so several water quality prerequisites are published by essentially every UV manufacturer, and installing a system outside these limits means it will not deliver its rated dose no matter how correctly it was sized on flow alone.

Ultraviolet light inactivates organisms so they cannot reproduce; it does not filter, remove or leave behind any residual disinfectant in the water. Everything that was present before the lamp is still physically present after it, in the sense that UV changes nothing about the water's chemistry or particulate content. This makes it fundamentally different from a filter or a softener, and it is never a substitute for finding out why a well tested positive for coliform in the first place. A UV system disinfects the water passing through it going forward; it does nothing about whatever is causing bacteria to enter the well, which needs its own investigation.

Replace the UV lamp annually on a fixed date rather than waiting for a symptom, because output falls steadily over the lamp's life and a lamp that still glows can be delivering well under its rated dose long before it visibly dims or fails to light. Clean or replace the quartz sleeve on the same schedule or sooner, since a scaled or clouded sleeve blocks the light reaching the water even from a fresh lamp behind it.

A positive coliform test is a public health matter first and an equipment question second. Contact the local health department, get a certified laboratory retest, and investigate why bacteria is entering the well, such as a compromised well cap, a cracked casing, or surface water intrusion, rather than treating a UV system as the complete answer to a well that has tested positive. UV disinfects water going forward; it does not fix whatever is letting bacteria into the well, and installing Class B equipment on a coliform-positive well specifically, rather than Class A, is dangerous because Class B was never certified to disinfect water in that condition.