A domestic heating and hot water system contains somewhere between eight and twenty valves, and almost none of them are interchangeable. They look similar, several share the same thread sizes, and two of the most important ones share an acronym while doing opposite jobs.
That is why the wrong valve usually gets fitted without anybody noticing. It works. Water flows, the system heats, nothing leaks. The consequence only arrives on the day the valve was supposed to do its actual job, which for a safety valve might be years later.
This page sorts the heating system valve types by the job they do rather than by what they look like, and points to the detail on each one. If you are trying to work out which valve you need, start with the job.
Four Jobs, Not One Category
Every valve in a heating or hot water system is doing one of four things:
- Isolating or regulating flow. Shutting a circuit off, or setting how much water goes down it.
- Controlling temperature. Blending hot and cold to hit a target, or opening and closing on a temperature signal.
- Protecting against pressure or temperature. Discharging when a safe limit is exceeded, or holding supply pressure down.
- Protecting against freezing or backflow. Preventing damage or contamination.
Identify the job first and the shortlist is usually two or three valves rather than twenty.
Isolation and Flow Regulation
Ball valve. A quarter-turn full-bore shutoff. Open or closed, nothing in between. Using one to throttle flow damages the seat and gives no useful control, because almost all of the flow change happens in the last few degrees of travel. Specify these for isolation only.
Gate valve. The older alternative, still found on legacy installations. They seize in one position after years of sitting untouched, and a seized isolation valve is worse than none because it gives false confidence. Not worth specifying on new work.
Balancing valve. This is the one that regulates rather than isolates. A static balancing valve has a measurable, settable position so a known flow rate passes at a known pressure drop. A dynamic balancing valve goes further and holds its set flow rate as system pressure varies, which matters on multi-circuit installations where other zones opening and closing changes the pressure available.
Balancing is the difference between a system that heats evenly and one where the far rooms never get there. On a manifold the equivalent control is the flow meter on each circuit, and the principle is identical: the longest, most resistant loop stays fully open and everything else is throttled back to its calculated figure.
Thermal bypass valve. Opens a bypass path on a temperature signal, commonly to get a circuit up to temperature faster or to protect a heat source from cold return water. We cover it in thermal bypass valves in fast heating systems.
Differential pressure bypass. A spring-loaded valve across flow and return that opens above a set pressure. It exists to give a circulator somewhere to push water when every zone has closed.
Temperature Control
This is the largest group and the one where substitution errors do the most damage, because several of these valves will physically fit where another was intended.
Thermostatic mixing valve, or TMV. Blends hot and cold to a set outlet temperature on potable water, for scald protection at baths, showers and basins. The defining feature is the fail-safe: if the cold supply fails, the valve shuts down rather than passing full-temperature water. That is why TMVs are mandatory in healthcare, aged care, childcare and schools. Standards include AS 4032.1 in Australia and New Zealand, and EN 1111 and EN 15092 in Europe. See why we need thermostatic mixing valves and thermostatic mixing valve installation.
Tempering valve. Does the same blending job on a residential hot water system to a wider tolerance, typically around plus or minus 3 degrees, covered by AS 4032.3. It is the standard fitting on a domestic storage heater and it is not a substitute for a TMV in the high-risk applications above. Our page on the tempering valve on a hot water system covers the duty.
Anti-scald or scald protection valve. A related family focused specifically on preventing sudden temperature spikes at the outlet. See anti-scald valves and how scald protection works.
Blending valve for a heating circuit. Looks like a TMV, does the same arithmetic, and must not be confused with one. It drops primary water to 35 °C to 45 °C for an underfloor heating circuit, it is sized for continuous circulation rather than intermittent draw-off, and it carries no potable water approvals because the water in a heating circuit is never drunk. See underfloor heating mixing valves.
Solar-specific mixing valve. A TMV rated for much higher and more variable inlet temperature, because a solar store can exceed 80 °C in summer while a standard valve may only be rated to 65 °C or 70 °C. See solar water heater mixing valve.
Zone valve with thermal actuator. On a manifold, each circuit has a valve insert opened and closed by a thermal actuator driven by a room thermostat. These are on or off devices, not proportional throttles, and the wax element inside takes three to five minutes to travel.
Thermostatic radiator valve, or TRV. Self-contained room temperature control at each radiator, with no wiring. See thermostatic radiator valves.
Fan coil valve. In commercial air conditioning, a fan coil valve regulates water through the coil. Two-way valves throttle flow; three-way valves divert or mix; four-port assemblies combine the control valve with isolation and balancing in one body. Sizing is by Kvs, and the usual rule is that the valve should take 30 to 50 percent of the circuit pressure drop at design flow. Too little and the valve has no control authority, meaning the first ten percent of travel does most of the work and the rest does nothing.
For the broader category, temperature control valves covers the family.
Pressure and Temperature Protection
Temperature and pressure relief valve, the T&P or TPR valve. A safety device on a hot water cylinder that discharges if either temperature or pressure exceeds its setting. It is normally shut and may never operate across the life of the vessel, which is exactly why it gets neglected. Three rules that are not optional: the discharge must be piped to a safe visible point, nothing may be fitted downstream of it that could restrict flow, and it must never be plugged. See seven facts about the T&P valve and temperature and pressure relief valves. Replacement cost and intervals are covered in conditions affecting T&P valve replacement cost.
Pressure relief valve. The same safety principle responding to pressure alone, used where temperature is not the risk. See pressure relief valve and the wider pressure valves overview.
Pressure reducing valve. A continuously operating control valve that takes incoming mains pressure and holds a lower, stable downstream pressure. It is not a safety device and it does not discharge. See pressure reducing valves in heating and plumbing.
Read those last two descriptions again, because they bring us to the single most common source of specification error in this whole field.
The Acronyms That Cause Real Mistakes
PRV means two different valves. Pressure Relief Valve and Pressure Reducing Valve. They do opposite things. A relief valve sits closed and discharges on overpressure to protect a vessel. A reducing valve works continuously to lower supply pressure. Order “a PRV” from a supplier and you have a fifty percent chance of receiving a valve that cannot do the job, and in the case of a reducing valve fitted where relief was required, the vessel has no overpressure protection at all. Always write the word out.
T&P, TPR and PTR are the same device. Different regional conventions for a temperature and pressure relief valve. No difference in function.
“Mixing valve” covers two incompatible products. A potable thermostatic mixing valve and a heating circuit blending valve. They look alike and thread alike. The potable one carries drinking water contact approvals; the heating one does not and must never be used on a hot water outlet.
Tempering valve and TMV are not synonyms. Different standards, different tolerance, and only one has the fail-safe cold shutdown. Substituting a tempering valve where a TMV was specified is a compliance failure in a healthcare or childcare setting.
Freeze and Backflow Protection
Freeze protection valve. Opens and drains a small volume of water when the temperature falls to its trigger point, so the pipe cannot fill with expanding ice. Two installation requirements decide whether it works: it must be mounted vertically with the outlet pointing down so the drain path is clear, and it must sit at the point most exposed to cold. Fitted horizontally or in a warm location it will do nothing on the night it is needed. See freeze protection valve facts and our 910078NT anti-freeze valve.
Check valve, or non-return valve. Allows flow one way only. On a hot water system its job is preventing crossflow between hot and cold, which otherwise shows up as warm water at the cold tap and unstable temperatures at mixed outlets. Required on both inlets of a mixing valve.
Material and Approvals: What Actually Changes the Part Number
Two valves with the same function and the same thread can still be the wrong and right part, and material is usually why.
Brass grades. CW617N, also written CuZn40Pb2, is the standard hot-forging grade. CW602N, CuZn36Pb2As, is the arsenic-inhibited dezincification resistant grade. Several markets require DR or DZR brass for potable applications, and a CW617N body will not comply there however well it performs. Specify by grade, not by “brass”.
Stainless grades. AISI 304 (1.4301) suits closed heating circuits. AISI 316 (1.4401) adds molybdenum for chloride resistance, specified for coastal installations or higher chloride water.
Potable water approvals are market specific and there is no global equivalent: WRAS in the UK, DVGW with KTW-BWGL assessment in Germany, ACS in France, NSF/ANSI 61 plus NSF/ANSI 372 for lead content in North America. Check the approval against the specific part number and its size, because an approval covering one body does not automatically extend to the next size up.
Heating circuit valves carry none of these, which is correct for their duty and disqualifying for potable use. Our note on HVAC valve materials goes further.
A Short Selection Routine
Six questions, in order. They narrow almost any valve requirement to one or two candidates.
- Which of the four jobs is it doing? Isolation, temperature control, protection, or freeze and backflow.
- Potable water or closed circuit? This decides whether market approvals apply, and it is not negotiable.
- Which market? The approval set and often the required brass grade follow from the destination, not from the specification sheet.
- What temperature and pressure will it actually see? Including the worst case, such as a solar store in August or a primary circuit before any blending.
- What flow rate, and what pressure drop is available? For a control valve, size on Kvs and check it will take 30 to 50 percent of the circuit pressure drop.
- Is it a safety device? If yes, confirm the discharge arrangement and that nothing downstream can restrict it.
Work through those and the part number follows. Skip the second and third and you will discover the problem at the destination port.
Legom Valve Range
Jiaxing Legom Technology Co., Ltd. manufactures heating and hot water control valves from our facility in Jiaxing, Zhejiang, supplying more than 90 countries.
Across the four jobs above, our HVAC valve range covers thermostatic mixing and tempering valves in standard, horizontal and electronic versions, solar-rated mixing valves, temperature and pressure relief valves, pressure reducing and relief valves, freeze protection valves, thermal bypass valves, dynamic flow balancing valves, and fan coil valves in two-way, three-way and four-port configurations.
We manufacture our own wax thermostatic elements rather than buying them in. That is what lets us hold response consistent across a production run, and it means we can develop an element to a specific temperature curve when a customer needs one rather than selecting from a catalogue.
Alongside the valves we make the rest of the hydronic system: underfloor heating manifolds, thermal actuators, oxygen barrier pipe and hot water system components.
For distributors and brands specifying to their own range, our OEM and ODM service covers body configuration, material grade, setpoint range, element specification, branded packaging and the documentation pack for your market. Our note on valve suppliers in your supply chain sets out how we work with distributors, and innovation trends in valve design covers where the products are heading.
Tell us the job, the water type and the destination market, and we will come back with the approval position before we come back with a price.
Frequently Asked Questions
What is the difference between a pressure relief valve and a pressure reducing valve?
They do opposite jobs and unfortunately share the acronym PRV. A pressure relief valve sits closed and discharges when pressure exceeds a safe limit, protecting a vessel. A pressure reducing valve works continuously to lower incoming supply pressure to a stable downstream figure, and is not a safety device. Always write the word out when ordering.
Is a tempering valve the same as a thermostatic mixing valve?
No. A tempering valve holds a wider tolerance and suits residential hot water duty, typically 50 °C. A thermostatic mixing valve holds a tighter tolerance and shuts down if the cold supply fails, which is why it is required in healthcare, aged care, childcare and schools. One is not a substitute for the other.
Can I use an underfloor heating mixing valve on my hot water outlet?
No. A heating circuit blending valve carries no potable water contact approvals and is sized for continuous circulation rather than intermittent draw-off. It will physically fit, which is precisely the problem. Use a valve approved for potable water in your market.
Which valve sets the flow rate to each circuit?
A balancing valve, or on a manifold the flow meter on each circuit. Never use a ball valve to throttle flow: it damages the seat and gives almost no usable control, because nearly all the flow change happens in the last few degrees of travel.
Why does my freeze protection valve not seem to work?
Usually installation. It must be mounted vertically with the outlet pointing down so the drain path is clear, and it must be located at the most cold-exposed point. Fitted horizontally, or somewhere warm, it will stay shut on the night it was meant to open.
What brass grade should I specify?
CW617N for general duty. CW602N, the arsenic-inhibited dezincification resistant grade, where water chemistry is aggressive or the destination market requires DR brass for potable applications. Specifying only “brass” is how a non-compliant shipment happens.
How do I size a control valve?
By Kvs against your design flow rate, then check control authority: the valve should take roughly 30 to 50 percent of the circuit pressure drop at design flow. Below that it has little authority, meaning a small part of its travel does most of the work and the rest achieves nothing.
Reviewed by Maggie Shen, Director at Legom, on October 9, 2026. This article was reviewed for technical accuracy, including the AS 4032.1 and AS 4032.3 distinction between thermostatic mixing and tempering valves, the PRV acronym collision, T&P valve discharge requirements, freeze protection valve installation orientation, the CW617N and CW602N brass grade difference, market-specific potable water approvals, and control valve sizing by Kvs and control authority.