Mixing valve flow range

Thermostatic Mixing Valve Minimum Flow: Check the Smallest Demand

Check the smallest demand as well as the largest. Many makers publish a minimum flow below which a thermostatic mixing valve may not hold its set temperature accurately, so a valve sized only for peak use can be too large when one outlet runs alone. Compare both cases with the exact model’s published range.

Published September 28, 2026Revised October 4, 2026

Quick reference

Before you accept a valve

Minimum flow: Set by maker, per model
Smallest demand: One outlet running alone
Peak demand: Record it separately
Chart top end: Not the maximum, says Caleffi
ASSE 1017: At the hot water source
Low-flow shower: Match head to valve

Illustration of a mixing valve feeding three shower heads, one of them running, above a range bar with a shaded working band and a marker near each end

Overview

Why the low end of the range matters

Caleffi writes in its article on flow rates that many thermostatic mixing valve makers, itself included, specify minimum flow rates to assure control accuracy at the low end.

Leonard Valve’s LV-296 submittal sheet says thermostatic mixing valves will not provide the desired accuracy outside their flow capacity range. Peak demand checks only one end of that range.

What this guide covers

What minimum flow means
Finding the smallest demand
Minimums that makers publish
Reading a pressure-loss chart
ASSE standards by valve type
High-low valves and recirculation
Shower valves and low-flow heads

Every figure on this page belongs to the named maker’s or association’s document. None is a general rule or a WNS Global specification.

From smallest demand to selection

What does minimum flow mean for a thermostatic mixing valve?

Leonard Valve’s FAQs define minimum flow as the lowest flow at which a valve will operate and still supply an outlet temperature within specification.

The figure belongs to one model under the maker’s stated conditions.

Term	What it means
Minimum flow	Leonard: lowest flow at which a valve still supplies an outlet temperature within specification.
Flow capacity range	Leonard’s LV-296 sheet: outside it, valves will not provide the desired accuracy.
Pressure-loss chart	A graph of flow rate against pressure drop through the valve.
Smallest demand	The lowest flow the valve sees in use.
Recirculated flow	Tempered water returned through the valve by a circulating loop.

How do you find the smallest real demand?

Diagram of one mixing valve with a single outlet running, the smallest flow, and with all outlets running, the peak flow, both checked against the model’s flow range

The smallest demand is the lowest flow that will pass through the valve, often one outlet running alone. Leonard’s FAQs give a single person bathing as the low-demand case. Six steps:

List every outlet and mode the valve serves.
Mark the outlet with the lowest flow that can run on its own.
Record that outlet’s rated flow from its own data sheet.
For a recirculating system, ask the designer for the return flow through the valve.
Compare the lowest figure with the model’s published minimum flow.
Record peak, simultaneous demand as a separate case.

The smallest outlet’s flow is not always the flow through the valve. Leonard’s FAQs say some makers rely on recirculated water to cover a higher minimum, and that temperature control suffers if that flow is below the valve’s actual minimum.

What minimum flows do makers publish?

Illustration of three mixing valves of increasing size, each above a bar of a different height

The table lists figures from four makers’ own documents; each applies only to the model named.

Maker and model	Published minimum	Maker’s context
Caleffi 521 series	1.0 gpm	0 gpm when recirculation flow is 1 gpm or more.
Leonard LV-296-10-LF	1.5 GPM	LV-296-20-LF: 1 GPM.
Leonard type TM-30, 50, 80, 125, 150	5.0 to 18.0 GPM	Bulletin G-2B, February 2015.
Bradley TMV25 to TMV200	2.0 to 20.0 gpm	Standard valves, for constant flow.
Bradley HL45 to HL2X2	1.5 to 10.0 gpm	High-low valves.
Bradley S59-4000(A)	0.35 gpm	Point-of-use valve.
Watts LFMMVM1	0.5 gpm	Minimum ASSE 1070 flow.

Apart from Caleffi’s 0 gpm with recirculation, the minimums in these documents run from 0.35 to 20 gpm. A figure copied from one model, or guessed from pipe size, says nothing about another.

Is the top of the pressure-loss chart the maximum flow?

Illustration of a flow chart with a rising curve, a shaded working band, a ringed low-end point and a cross at the top end, beside a data sheet

Not according to Caleffi. Its article says a flow-against-pressure-drop graph is shown instead of a maximum, and that a reader may be tempted to treat the top end as the maximum: “don’t do that.”

Point	What the maker says
Why not the top end	Caleffi: excessive velocity, potential noise and erosion.
Velocity guide Caleffi cites	Copper Development Association, as quoted by Caleffi: maximum 4 to 5 ft/s in copper tube for hot water below 140 °F.
Caleffi 521 series	Maximum recommended differential 20 PSID, about 15 GPM on its graph.
Bradley Navigator	Select for a 5 to 15 psi drop; avoid drops above 20 psi.

Both makers express the upper limit as a pressure drop, not as the last point on the curve.

What do the ASSE standards say about flow?

Illustration of four stacked standard documents, a water heater with a mixing valve and a shower head with its own control plate

ASSE publishes separate standards by valve location and use, and makers describe a different test flow for each. The standard texts were not read; editions are from ASSE’s standards list. For ASSE 1016, CSA Group shows a later reaffirmation.

ASSE 1017-2023

Mixing valves installed at the hot water source. For the 2009 edition, Leonard’s chart shows ±3 °F for valves flowing 3.0 to 5.0 GPM at 10 PSI, ±5 °F from 5.0 to 40 GPM and ±7 °F over 40 GPM. Acorn Controls says the default test starts from the flow at a 10 PSI differential, then halves it.

ASSE 1016

Compensating valves at the point of use for showers; CSA Group lists the 2017 edition as reaffirmed in 2026, while ASSE’s list shows it as reaffirmed in 2021. Leonard’s chart gives the minimum flow for ASSE 1016-2017 as 2.5 GPM or the maker’s minimum rated flow, whichever is less.

ASSE 1070

Temperature limiting devices for sinks, bidets, lavatories and bathtubs (2020, reaffirmed 2025). Acorn Controls says there is no default flow rate; the maker shows the lab its advertised minimum.

ASSE 1069

Mixing valves for individual or multiple fixtures (2020, reaffirmed 2025). Acorn Controls says these are tested at the advertised flow rate if there is one, otherwise at 2.5 GPM.

Standard the valve is listed to
Edition named on the listing
Minimum flow for that listing
Where the valve is installed

What options do the documents show when demand varies widely?

Illustration of a small mixing valve above a large one, both fed from the same hot and cold supplies, beside a single valve with a dashed recirculation loop

The documents cited show four options where flow varies between low and high:

One high-low valve

Bradley’s Navigator brochure describes its high-low valves as a single valve design for both high and low demand, best used where usage fluctuates.

Two valves

Leonard’s FAQs give the small valve in its New Generation High/Low Systems a true minimum flow of 1.0 GPM.

Recirculated flow

Caleffi’s brochure gives its 521 series a minimum of 0 gpm when recirculation flow is 1 gpm or more. That is a condition for one model, not a general remedy.

Point-of-use valves

ASSE’s guidelines say ASSE 1017 valves are not intended for point of use. Bradley’s brochure lists point-of-use models with minimums of 0.35 and 0.5 gpm. Neither presents them as replacing a source valve.

Leonard’s LV-296 sheet tells the specifier to consult the flow capacity chart and not to oversize.

Does minimum flow matter for a shower valve?

Illustration of a mixing valve feeding a shower head with many spray lines and a flow dial reading high, beside the same valve feeding a head with few spray lines and the dial reading low

Yes, per ASSE’s guidelines: the showerhead flow rate must not be less than the valve maker’s published minimum flow rate. ASSE’s white paper defines a low-flow showerhead as one below 2.5 GPM.

Source	What it says
ASSE guidelines	Showerhead flow must not be less than the valve maker’s published minimum flow.
ASSE white paper, revised March 2012	Match the showerhead’s maximum flow to a compensating valve whose minimum flow is equal or lower.
Same white paper	If the valve’s minimum exceeds a new showerhead’s flow, replace the valve with one that matches.
Delta T17TH135	Cartridge minimum flow rating 1.2 gpm.
Watts LFMMVM1	For ASSE 1069 use: shower heads rated at 1.5 gpm or higher.

How should a buyer specify a thermostatic mixing valve?

Illustration of a clipboard with seven ticked lines beside a mixing valve feeding three shower heads

These steps turn the facts above into a selection record. They are procurement guidance, not a plumbing design.

State where the valve sits. At the hot water source or at a fixture.
Record the smallest demand. The lowest flow and the outlet that causes it.
Record peak demand. Simultaneous use, as a separate line.
Get the model’s range. Minimum flow and recommended upper limit.
Check the pressure drop. At peak flow, against the maker’s limit.
Record supply conditions. Hot and cold temperatures and pressures.
Record the listing. Standard, edition and certifier.

Keep each scenario visible

A larger connection or a higher chart endpoint is not evidence of better temperature control.

What should you send and ask before a quotation?

Illustration of a document, a message bubble and two flow dials reading low and high

Have the supplier or plumbing designer resolve any low-demand mismatch before ordering.

Question	Why it matters
What is the model’s minimum flow?	Leonard: valves will not provide the desired accuracy outside the flow capacity range.
Under what conditions?	Some minimums assume recirculation.
What upper flow is recommended?	Caleffi: the chart’s top end is not the maximum.
Which standard is it listed to?	Source and point-of-use valves differ.
Is a high-low option offered?	Bradley describes these for fluctuating use.
What is the showerhead’s rated flow?	ASSE: it must not be less than the valve’s published minimum.

Include with your request

Outlets served and their rated flows
Smallest and peak demand, separately
Recirculation return flow, if any
Supply temperatures and pressures
Project location and required standard

This guide sets no temperature or pressure limit and makes no scald-protection promise; confirm requirements with the local authority. WNS Global confirms any product and terms it offers separately in writing. This guide does not state that WNS Global products have the features, ratings or listings named above.

Illustration of a mixing valve between two question marks

FAQ

Mixing valve minimum flow: questions buyers ask

Short answers tied to the guide’s evidence.

What is the minimum flow of a thermostatic mixing valve?

Leonard Valve defines minimum flow as the lowest flow at which a valve still supplies an outlet temperature within specification.

Is there one minimum flow for all thermostatic mixing valves?

No. Apart from a 0 gpm figure Caleffi gives with recirculation, the documents cited here give minimums from 0.35 gpm for a Bradley point-of-use model to 20 gpm for a Bradley TMV200.

Can peak flow alone settle the choice of thermostatic mixing valve?

Not according to Leonard’s LV-296 sheet, which says thermostatic mixing valves will not give the desired accuracy outside their flow capacity range.

Is the top of the pressure-loss chart the maximum flow?

Caleffi says not to assume so; for its 521 series it sets a maximum recommended differential pressure.

Does a larger thermostatic mixing valve control temperature better?

The sources used here do not say so. Leonard’s LV-296 sheet tells specifiers not to oversize a thermostatic mixing valve.

Can recirculation flow cover a mixing valve’s minimum flow?

For one model, yes: Caleffi lists a 0 gpm minimum for its 521 series when recirculation flow is 1 gpm or more.

What is a high-low thermostatic mixing valve?

Bradley’s Navigator brochure describes its high-low valves as a single valve design for both high and low demand.

Can an ASSE 1017 mixing valve be used at a fixture?

ASSE’s guidelines say ASSE 1017 valves are not intended for point-of-use applications.

Does a low-flow showerhead affect a shower valve?

ASSE’s guidelines say the showerhead flow rate must not be less than the shower valve maker’s published minimum flow rate.

Does checking the flow range guarantee scald protection?

No. A flow check is one selection step, and this guide makes no scald-protection promise. ASSE’s guidelines say ASSE 1017 devices used alone do not provide adequate scald protection.

Illustration of a mixing valve below two flow dials reading low and high, with a tick

Summary

Check both ends of the range

Select a thermostatic mixing valve against two demands: the smallest flow it will see and the peak. Take both limits from the exact model’s documents.