Hot Water & Pressure Valve Buyer's Guide: Which Valve Do You Need?
Hot Water & Pressure Valve Buyer's Guide: Which Valve Do You Need?
Every hot water install comes down to a handful of valve decisions — and getting the wrong one specified is one of the most common (and costly) callbacks in the trade. This guide breaks down the valves you'll come across on a standard hot water job, what each one actually does, and which one fits which scenario, so you can spec the right part the first time.
Quick Answer: Which Valve Does What?
| Valve Type | What It Does | When You Need It |
|---|---|---|
| Tempering Valve | Blends hot and cold water to a fixed safe delivery temperature | Standard residential hot water outlets, per AS4032 |
| TMV (Thermostatic Mixing Valve) | Actively maintains a set delivery temperature even when supply pressure or temperature fluctuates | Aged care, healthcare, childcare, and other high-risk occupancies under AS4032.1/AS4032.3 |
| PRV (Pressure Reduction Valve) | Reduces mains pressure down to a safe working pressure for the system | Properties on high-pressure mains, or where fixture/appliance pressure limits apply |
| Pressure Limiting Valve | Caps peak pressure at a selectable maximum | Hot water unit inlet protection, particularly on mains-pressure systems |
| Expansion Control Valve | Relieves pressure build-up caused by water expanding as it heats | Any sealed (non-vented) hot water system |
| PTR Valve (Pressure & Temperature Relief) | Safety valve that discharges if pressure or temperature exceeds safe limits | Mandatory on every hot water storage unit |
| Duo Valve | Combines an isolation valve with a non-return (check) valve in one fitting | Cold water inlet to a hot water unit, replacing older stop-and-check arrangements |
| Frost Protection Valve | Opens automatically to drain water before it can freeze and split a pipe | Exposed pipework in frost-prone areas |
Tempering Valve vs TMV: The Mix-up That Causes Callbacks
This is the one that trips people up most often. Both blend hot and cold water down to a safer delivery temperature but they're not interchangeable, and a compliance audit will catch the difference.
A tempering valve, like the Reflex Standard Tempering Valve Red Cap 15mm or the AVG Tempering Valve Yellow Cap 20mm, is a mechanical, thermostatic blend valve suited to standard residential outlets. It's the default choice for most homes under AS4032.
A TMV, such as the TMV Thermostatic Mixing Valve 15mm or the pre-assembled TMV 15mm Plumbed In Box, holds a tighter tolerance and responds faster to pressure or temperature fluctuation which is why it's specified (and often mandated) in aged care, healthcare, and childcare settings where scalding risk carries a higher duty of care. If the job is anything other than a standard home, check the compliance requirement before you default to a tempering valve—it's the wrong part for the job even though it looks like a cheaper, simpler fix.
Managing Pressure: PRV vs Pressure Limiting Valve
If a property sits on high mains pressure, or a hot water unit's inlet pressure rating is lower than what's coming in, you need something upstream of the unit to bring it down.
A PRV, like the AVG PRV R/A Pressure Reduction Valve 20mm Olive, reduces pressure to a fixed or preset level.
A selectable pressure limiting valve, such as the Tri-Set Selectable Pressure Limiting Valve 15mm FxF, lets you dial in a specific cap useful where the manufacturer's inlet pressure limit is known and you want to set to that exact figure rather than a generic reduction.
Either way, this is inlet-side protection for the hot water unit itself, separate from the tempering/TMV valve that protects the outlet.
Two Valves Every Sealed System Needs
A sealed (mains-pressure, non-vented) hot water system needs two safety devices that aren't optional extras—they're what stops the unit becoming a hazard:
- An expansion control valve, like the Expansion Control Valve 15mm, releases the pressure that builds as stored water heats and expands.
- A PTR valve, such as the PTR Valve PTR15 (HT55), is the last line of defence—it discharges if pressure or temperature runs away beyond safe limits. Every storage hot water unit needs one fitted, full stop.
On the Cold Inlet: Duo Valves
Rather than fitting a separate isolation valve and a separate check valve on the cold inlet to a hot water unit, a duo valve combines both in a single fitting. The Duo Valve F x F 20mm and Duo Valve Inverted Flare 15mm cover the two most common connection types—it's a simpler install than running two fittings and one less joint to leak.
Cold Climates: Frost Protection
If pipework runs through an exposed or unheated space in a frost-prone area, a Frost Protection Valve opens automatically at a set low temperature to drain water out of the line before it can freeze, expand, and split the pipe. It's a small, inexpensive part next to the cost of a burst-pipe callout in July.
Common Questions
What's the difference between a tempering valve and a TMV?
A tempering valve is a mechanical thermostatic blend valve suited to standard residential outlets under AS4032. A TMV holds a tighter temperature tolerance and reacts faster to pressure/temperature swings, and is required in higher-risk settings like aged care, healthcare, and childcare facilities.
Do I need both an expansion control valve and a PTR valve?
Yes, on a sealed system they do different jobs—the expansion control valve manages routine pressure build-up from thermal expansion, while the PTR valve is the emergency relief device that activates if pressure or temperature exceeds safe limits.
Can I use a PRV instead of a pressure limiting valve?
They both reduce pressure, but a PRV typically reduces to a fixed or preset level, while a selectable pressure limiting valve lets you set a specific cap to match a manufacturer's stated inlet limit. Check the hot water unit's specifications before choosing.
What size tempering valve do I need?
15mm covers most single-outlet residential installs; 20mm is common where a higher flow rate is needed across multiple outlets. Confirm against the manufacturer's flow rate specification for the fixtures being supplied.
Is a duo valve a legal requirement on a hot water unit?
Isolation and non-return protection on the cold inlet is standard practice for hot water unit installation—a duo valve is simply the combined-fitting way of achieving both in one part rather than two.
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