Thermal Relief Valve 3/8" MPT 150Degree F

Thermal Relief Valve 3/8" MPT -- Pump Protection Against Bypass Heat

A thermal relief valve is the cheapest insurance you can put on a pressure washer pump. It costs under $10 and takes five minutes to install, but it can save you from a $400+ pump rebuild. This 3/8" MPT thermal relief valve opens automatically when water temperature inside the pump head hits 150 degrees F, dumping the overheated water and letting cool supply water flow in to protect the seals.

If you run a pressure washer commercially -- or even just use one regularly at home -- you need a thermal relief valve on your pump. Period. It is the single most overlooked wear item on a pressure washing rig, and skipping it is how contractors end up replacing pump seals and packings way too often.

Why Your Pressure Washer Pump Overheats

Every time you release the trigger on your spray gun, the unloader valve kicks in and water starts recirculating through the pump head in a closed loop. The pump is still running, still compressing water, and all that energy turns into heat. On a hot summer day with the machine running in direct sunlight, the water temperature inside the pump can climb past 140 degrees F in under two minutes of bypass.

Most pressure washer pump seals are rated for temperatures between 140 and 160 degrees F. Once the water gets that hot, the rubber seals start to soften, swell, and break down. You won't notice it right away -- the pump still runs fine. But after weeks of repeated heat cycling, the seals fail and you start seeing water leaking from the pump head or a gradual loss of pressure.

The thermal relief valve prevents this by venting the hot water before it reaches the temperature that damages seals. It is a passive, automatic safety device. You install it once, forget about it, and it quietly protects your pump every time you let go of the trigger.

How a Thermal Relief Valve Works

Inside the valve body is a wax element that expands when it heats up. When the water surrounding the element reaches 150 degrees F, the wax pushes a piston that opens the valve. Hot water flows out through the discharge port, and the pressure drop pulls cool water in from your supply line.

Once the water temperature drops below the trip point, the wax contracts and the valve closes on its own. No manual reset required. No electronics, no batteries, no adjustments. It is a purely mechanical device that works every time without any input from you.

Where to Install a Thermal Relief Valve

The valve threads into any open 3/8" FPT port on your pump head. Most triplex plunger pumps have a dedicated port for a thermal relief valve -- check your pump's manual or look for a plugged port on the pump head.

If your pump doesn't have an open port, you can add one using a tee fitting in the bypass line between the unloader and the pump inlet. The key is getting the valve close to the pump head where the hot water accumulates during bypass.

When the valve opens, it dumps water. Attach a short piece of hose to the barb fitting on the outlet side and route it to the ground so the hot water discharge doesn't spray onto your equipment or anyone nearby.

Installation Steps

Apply Teflon tape to the 3/8" MPT threads. Thread the valve into the pump port by hand and snug it with a wrench -- do not overtighten. Attach a short hose to the barb outlet and route it down and away from the machine. Run the machine and verify the valve opens when you hold the gun closed for a few minutes. You should see a small stream of water exiting the drain hose once the pump heats up.

Signs Your Thermal Relief Valve Needs Replacement

Constant dripping even with a cold pump. If the valve leaks when the water is not hot, the internal wax element or seal has failed. Replace the valve.

No water discharge during extended bypass. If you hold the gun closed for several minutes on a hot day and the valve never opens, the element may be stuck. This means your pump is not protected. Replace the valve.

Visible corrosion or mineral buildup. Hard water deposits can clog the valve opening over time. If you see white scale buildup on the valve, replace it rather than trying to clean it out.

Thermal relief valves are cheap enough that most contractors replace them once a year as preventive maintenance, or whenever they rebuild the pump. At under $10, there is no reason to run a questionable valve.

Thermal Relief Valve Specifications

Which Trip Temperature Do You Need?

Thermal relief valves come in different temperature ratings: 140, 145, 150, and 190 degrees F are the most common. The right choice depends on your setup:

This valve is rated at 150 degrees F, which is the right choice for most cold water pressure washers used in commercial and residential work.

Frequently Asked Questions

Do I really need a thermal relief valve?

Yes. If your pressure washer has a bypass-style unloader (most do), the water heats up every time you release the trigger. Without a thermal relief valve, that heat damages your pump seals over time. The valve costs less than $10. A pump rebuild costs $200 to $500. The math is simple.

My pump already has one. Do I need to replace it?

Check it once a year. Hold the trigger gun closed for 2 to 3 minutes on a warm day. If you see water dripping from the valve's drain hose, it is working. If nothing comes out, the valve has failed and needs to be replaced.

Will this fit my pump?

It fits any pump with a 3/8" FPT port. Most General Pump, AR, Cat, and Comet triplex pumps have a thermal port in this size. Check your pump manual or look for a plugged 3/8" port on the pump head.

What happens if the valve trips too often?

Frequent tripping means the pump is spending too much time in bypass. This usually means the operator is holding the gun closed for long stretches between passes. Try to minimize dead time, or consider upgrading to a buffer tank setup that feeds the pump cooler water.

Can I install this myself?

Yes. It threads in like any pipe fitting. Apply Teflon tape, thread it in, snug it with a wrench, attach a drain hose to the barb, and you are done. Five minutes, no special tools.

Is this valve for hot water pressure washers?

This 150 degree F valve is designed for cold water machines. For hot water pressure washers, you want a 190 degree F rated valve so it doesn't trip during normal hot water operation. We carry those as well -- check our thermal relief valve collection.

Related Parts

• All Pressure Washer Repair Parts
• Pressure Washer Pumps
• Fittings, Adapters, and Quick Connects
• Pressure Washer Hoses
• Pressure Washing Chemicals

Pair it with a pump oil change and fresh packing seals for a complete pump maintenance kit.