UV water purifier size calculator: GPM at NSF/ANSI 55 Class A dose from bathrooms

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Short answer

A UV water purifier is sized by peak flow in gallons per minute (gpm) at a stated UV dose, with the incoming water inside the manufacturer’s pretreatment limits. Peak flow is bathrooms x 2.5 gpm (the 10 CFR 430.32 showerhead maximum) plus 2.2 gpm for one open faucet: a 2-bathroom home pulls 2 x 2.5 + 2.2 = 7.2 gpm. The dose sets the class: NSF/ANSI 55 Class A is 40 mJ/cm2, Class B is 16 mJ/cm2, and VIQUA’s own Standard row is 30 mJ/cm2. A chamber passes less water as dose rises: VIQUA’s spec sheet rates the VH410 at 18 gpm at 30 mJ/cm2 but 14 gpm at 40 mJ/cm2, at 95% UV transmittance. The 7.2 gpm house sized to Class A needs a unit rated at least 7.2 gpm at 40 mJ/cm2: the VIQUA D4 (9 gpm) covers it. Water entering must also be under 0.3 mg/L iron, 0.05 mg/L manganese, 7 gpg hardness and 1.0 NTU turbidity, the Connecticut DPH UV guideline limits.

The GPM on the box is VIQUA’s 30 mJ/cm2 figure: at the 40 mJ/cm2 NSF/ANSI 55 Class A dose the same VH410 chamber passes 14 gpm, not 18, and that is the number to size against.

Peak gpm, the UV unit that meets it at your dose, and the pretreatment flags

2 bathrooms x 2.5 gpm + 2.2 gpm = 7.2 gpm peak. At NSF/ANSI 55 Class A (40 mJ/cm2), 95% UVT: smallest published match VIQUA D4 (9 gpm); next up VIQUA VH410 (14 gpm); VIQUA VH200 (7 gpm) misses. Pretreatment: no flags at the numbers entered.

Peak gpm: bathrooms x 2.5 gpm (10 CFR 430.32 showerhead maximum, one shower per bathroom) + 2.2 gpm for one faucet. Flow at each dose is VIQUA's published curve at 95% UVT (LIT520329R for VH150/VH200/VH410, LIT-520326-R for the D4 family). Pretreatment limits: iron 0.3 mg/L, manganese 0.05 mg/L, hardness 7 gpg (120 mg/L), turbidity 1.0 NTU (Connecticut DPH UV guideline, VIQUA and HQUA manuals). No source publishes a UVT correction factor, so the calculator flags low UVT instead of scaling the flow.

What is the formula?

Peak gpm = bathrooms x 2.5 + 2.2. The unit qualifies when its published flow at the chosen dose, measured at 95% UV transmittance, is at least that peak, and the water entering it is inside the pretreatment limits.

Variable	What it is	Where the figure comes from
Bathrooms	one shower running per bathroom	you
2.5 gpm per shower	federal showerhead maximum at 80 psi	10 CFR 430.32
2.2 gpm faucet	federal faucet maximum at 60 psi, one open	10 CFR 430.32
Dose	energy the chamber delivers to the water, in mJ/cm2	NSF/ANSI 55 Class A 40, Class B 16; VIQUA Standard 30
Flow at dose	the manufacturer’s curve, one gpm per dose row, at 95% UVT	VIQUA spec sheets LIT520329R and LIT-520326-R
Pretreatment limits	iron 0.3 mg/L, manganese 0.05 mg/L, hardness 120 mg/L, turbidity 1.0 NTU	Connecticut DPH UV guideline; VIQUA; HQUA manual

The bathroom method is the fixture-maximum method from the whole-house filter flow rate calculator, one shower per bathroom. WQA’s fixture-unit method on that page lands lower (6.8 gpm for the same 2-bathroom house), and a well delivers only what its pump and pressure tank allow, so a measured flow can replace the arithmetic.

Which dose class do I size to?

Dose	Name	What it is for	Source
40 mJ/cm2	NSF/ANSI 55 Class A; VIQUA’s “NSF/EPA” row	minimum dose for microbiological reduction claims, including Cryptosporidium and Giardia	ANSI summary of NSF/ANSI 55-2024; Connecticut DPH
30 mJ/cm2	VIQUA Standard	the manufacturer’s own rating row, the gpm printed on the box	VIQUA spec sheets
16 mJ/cm2	NSF/ANSI 55 Class B; VIQUA’s “US Public Health” row	supplemental bactericidal treatment of water a local health agency already deems acceptable	ANSI summary of NSF/ANSI 55-2024

Connecticut DPH requires Class A or an approved equal for UV as primary disinfection of groundwater, with a solenoid that shuts off flow below 40 mJ/cm2. Wisconsin’s SPS 382.50 guidance (written for healthcare hot water, not homes) uses the same 40 mJ/cm2 minimum, alarming at 38.

A well with a positive coliform result is water of unknown microbial quality, the case Class B is not for. Size to 40 mJ/cm2.

What does a worked example look like?

Take 2 bathrooms, sizing to Class A, water tested at 95% UVT, 0.1 mg/L iron and 5 gpg hardness.

Peak flow: 2 x 2.5 + 2.2 = 7.2 gpm.
Dose row: 40 mJ/cm2.
Read the 40 mJ/cm2 column below: VH200 7 gpm misses, D4 9 gpm covers, VH410 14 gpm covers with margin.
Pretreatment: 0.1 mg/L iron is under 0.3, 5 gpg is under 7, no flags.
Answer: a D4-class chamber or larger, behind a 5 micron pre-filter.

How much water does each unit pass at each dose?

VIQUA figures are the manufacturer’s published curves at 95% UVT.

Unit	16 mJ/cm2	30 mJ/cm2	40 mJ/cm2	Ports	Source
VIQUA VH150	12 gpm	5 gpm	3.5 gpm	(see sheet)	LIT520329R, Feb 2025
VIQUA VH200	16 gpm	9 gpm	7 gpm	(see sheet)	LIT520329R, Feb 2025
VIQUA D4, D4 Premium, D4+	23 gpm	12 gpm	9 gpm	3/4 in MNPT	LIT-520326-R, 2017
VIQUA VH410 , VH410M	34 gpm	18 gpm	14 gpm	3/4 in FNPT / 1 in MNPT combo	LIT520329R, Feb 2025
HQUA-OWS-12	no curve published	12 gpm rated; 30 mJ/cm2 is the listing’s claim	no curve published	3/4 in MNPT	HQUA manual 20180101

The three D4 variants share one 2017 sheet and one curve. The HQUA manual gives pretreatment limits, a 9,000-hour lamp and a 12 GPM rating, but no dose-versus-flow table and no NSF/ANSI 55 listing, so its row cannot be sized to a dose.

What has to be true of the water first?

UV removes nothing; it needs water clear enough to pass light. Every document here sets nearly the same inlet limits:

Parameter	Limit entering the UV unit	Sources
Iron	under 0.3 mg/L	Connecticut DPH; Wisconsin DSPS; VIQUA; HQUA manual
Manganese	under 0.05 mg/L	Connecticut DPH; Wisconsin DSPS; HQUA manual
Hardness	under 120 mg/L as CaCO3, about 7 gpg	Connecticut DPH; Wisconsin DSPS; VIQUA; HQUA manual
Turbidity	under 1.0 NTU (Wisconsin allows 5.0, prefers 1.0)	Connecticut DPH; Wisconsin DSPS; HQUA manual
Tannins	under 0.1 mg/L	VIQUA; HQUA manual
Colour	under 15 colour units	Connecticut DPH; Wisconsin DSPS
Hydrogen sulfide	non-detectable	Connecticut DPH; Wisconsin DSPS
Pre-filter	5 micron or finer on the supply line	Connecticut DPH

The iron and manganese figures match EPA’s secondary standards of 0.3 mg/L and 0.05 mg/L; the turbidity figure comes from the UV guidance, not EPA, whose secondary table has no turbidity row. Above a limit, the fix goes ahead of the chamber: an iron filter, the manganese guide, the water softener size calculator above 7 gpg, a sediment filter above 1.0 NTU, the rotten egg smell guide for hydrogen sulfide. The well water test panel lists what to order.

Why does UV transmittance change the answer?

UV transmittance (UVT) is the share of 254 nm light that passes through one centimetre of the water. VIQUA prints every flow figure at 95% UVT; the HQUA-OWS-12 manual sets a floor of 75%.

Neither publishes a factor for water between those points, so the calculator flags a low or untested UVT and sends you to the manufacturer for a derated flow rather than inventing a multiplier. Tea-coloured tannin water is the usual reason a well misses the 95% basis, which is why VIQUA caps tannins at 0.1 mg/L.

Only after the calculator has a number and a clean panel

The 7.2 gpm Class A house above is covered by the VIQUA VH410, 14 gpm at 40 mJ/cm2, combo 3/4 in FNPT and 1 in MNPT ports , with room for a third bathroom (9.7 gpm). The smaller D4, which the sheet rates at 9 gpm, was listed as unavailable on Amazon when this page was checked, so it is named here and not linked.

The HQUA-OWS-12, 12 GPM, 3/4 in MNPT is the cheaper chamber and carries no NSF/ANSI 55 listing and no published dose curve; its 30 mJ/cm2 figure is listing copy. Skip it for a coliform-positive well. Skip both until iron, manganese, hardness and turbidity are under the limits above.

Where do the numbers come from?

Peak flow: 10 CFR 430.32; WQA Residential Water Treatment Design, Tables Five to Seven.
Dose classes: ANSI’s summary of NSF/ANSI 55-2024; Connecticut DPH; Wisconsin DSPS.
Flow at each dose: VIQUA spec sheets LIT520329R and LIT-520326-R.
Pretreatment limits: Connecticut DPH Table 1; Wisconsin DSPS; VIQUA sheets; HQUA-OWS-12 manual section 2.2; EPA secondary standards.

Frequently asked questions

What size UV water purifier do I need for a 3-bathroom house?

Three showers at the 10 CFR 430.32 maximum of 2.5 gpm plus one 2.2 gpm faucet is 9.7 gpm. At the NSF/ANSI 55 Class A dose of 40 mJ/cm2, VIQUA's spec sheets rate the D4 at 9 gpm, which misses, and the VH410 at 14 gpm, which covers it. At VIQUA's 30 mJ/cm2 Standard rating the D4's 12 gpm is enough.

What is the difference between NSF/ANSI 55 Class A and Class B?

Class A systems deliver a minimum UV dose of 40 mJ/cm2 and carry microbiological reduction claims, including Cryptosporidium and Giardia. Class B systems deliver 16 mJ/cm2 and are meant for supplemental bactericidal treatment of water a local health agency has already found acceptable. Connecticut DPH requires Class A, or an approved equal, for UV as primary disinfection of groundwater.

Why is the VIQUA VH410 sold as 18 GPM but only 14 gpm at Class A?

Because a UV chamber delivers a fixed amount of energy, so dose falls as flow rises. VIQUA's spec sheet LIT520329R prints one dose curve at 95% UV transmittance: 34 gpm at 16 mJ/cm2, 18 gpm at the 30 mJ/cm2 VIQUA Standard, and 14 gpm at the 40 mJ/cm2 NSF/EPA dose. The 18 on the box is the middle row.

Does hard water matter for a UV purifier?

Yes. VIQUA's VH and D4 spec sheets and the HQUA-OWS-12 manual all set the inlet limit at under 7 grains per gallon (120 mg/L), and Connecticut DPH's UV guideline uses the same 120 mg/L as CaCO3. Above that, HQUA's manual says the water should be softened first, which is the water softener size calculator's job.

What is UV transmittance and how do I find mine?

UV transmittance (UVT) is the share of 254 nm ultraviolet light that passes through one centimetre of the water. VIQUA rates every flow figure at 95% UVT, and the HQUA-OWS-12 manual sets a minimum of 75%. Neither publishes a correction for lower values, so ask the lab that runs your well panel to add a UVT reading before you size.