What is a compressor cold start valve?

What is a compressor cold start valve? Is it an unloader valve? Maybe yes, it’s the only unloader valve on a specific brand of compressor. Or maybe no, the compressor may not have a cold start valve, but only an unloader valve. The answer may depend entirely on your air compressor. There are some that have both, and others that have one or the other.

Whether it is a compressor cold start valve or a compressor unloader valve, both perform a similar task, and that is to allow any air trapped over the piston when the compressor stops to vent off.

How the unloader valve works is described on another page on this site.

Here’s how a cold start valve works.

When the air compressor starts, the cold start valve is open, and has allowed any trapped air from over the piston to escape when the last air compressing cycle was completed.

While air can escape out of the cold start valve on start up, it cannot escape (usually) as fast as pressure can be built by the pump. The compressor pumping action and even back pressure from the tank will enable the pressure over the piston to increase. When it reaches the setting inside the cold start valve, that valve will close.

Air flow from the cold start valve should stop at this point, and all air being built by the pump gets directed into the tank.

What’s inside an air compressor cold start valve?

While different manufacturers may use different material compositions in the cold start valve parts, typically all will contain a body (usually brass), a piston, a spring and an O-ring.

These parts will work together, responding to an increase in pressure in the chamber over the piston, or if the cold start valve is installed elsewhere in the line from the pump to the tank, to vent air as needed and to close in its normal cycle.

When a compressor cold start valve always bleeds air.

This is not all that unusual. While the compressor is running, air continues to bleed out of the unloader valve hole, even though tank pressure is rising. Then, when the cut off pressure is released, air should stop bleeding out of the cold start valve, when the air over the piston and in the line to the tank is exhausted.

If air continues to bleed out of the cold start valve when the compressor is off, that points to a leaking tank check valve. There are pages about this issue and how to fix this on this site.

If air continues to blow out of the cold start valve all the while the compressor is running, then something has gone awry with the cold start valve.

The usual cold start valve problem.

The ongoing venting while air is building in the tank is most common to compressors that are oil lubed.

A little oil wicks by the piston ring / seals all the time when an oil lubed compressor is running. If the seals are in pretty good shape, very little oil gets by. As the compressor ages, these piston rings / seals wear, and more oil gets by. So much so that not only is the oil getting into the air going into the tank (another reason to drain the tank often) but some of that oil will blow out the cold start valve as well.

If there is “crud” in the air / oil mix, that will gum up the cold start valve, and it may lodge open.

Pull the valve, rinse it in solvent, and see if that resolves the issue. If not, it may be necessary to replace the cold start valve.

Is the oil fill tube vent blocked?

Also, check to be sure the small vent holes in the oil fill cap are not blocked.

While oil can wick up into the compression chamber as the pump cycles, so, too, can air blow by the piston seals / rings and start to pressurize the oil sump. If the sump vent is blocked, that pressure may increase to the point that it’s forcing oil past piston seals / rings that aren’t badly worn, but now unable to seal, particularly if these seals are directional.

Clean the oil fill cap vent holes to ensure this doesn’t happen.

Question / cold start valve issue on a compressor?

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