Valve Tips

Here is a valve tip that might save you a little time. Comment or post one of your own.

Manually bleeding a valve: Use the solenoid or use the external bleed screw? Why is there a manual bleed screw?

I find that most do not fully understand the differences between the function of the external bleed screw and the internal bleed using the solenoid. Of course, the obvious answer is that the external bleed leaks water off the top of the diaphragm to atmosphere. The internal bleed leaks water off the top of the diaphragm through a port to the outlet side of the valve. You won't have any water leaking in the valve box if you use the internal bleed.

So, why use the external bleed screw at all? First, let's talk a little about how the diaphragm responds when you bleed a valve. The diaphragm moves (open or closed) in response to a difference in force on each side of the diaphragm. If the force is equal, the diaphragm won't move. If the force is greater on one side, it will move away from that force until the forces become equal or it encounters a physical barrier.

Let's consider the internal bleed. When the solenoid is twisted to bleed the valve, it relieves pressure from the top side of the diaphragm and the diaphragm opens. However, at low flows, it does not have to move very far before the pressure equalizes and the forces are equal. This is due to the pressure drop across the valve seat and the back pressure due to the water in the pipe connected to the outlet of the valve. At very low flows, the diaphragm may lift off the seat so little that debris can become stuck between the diaphragm and the seat. When the valve closes, the debris causes a leak path and we have a 'weeping' failure of the valve. The first reaction of many after bleeding the valve with the solenoid several times is to take the bonnet off and clean the seat.

But next time, let's consider something else. Use the external bleed screw. When you open the external bleed screw, the top side of the bonnet is opened to atmosphere. How much back pressure is there on the top side of the diaphragm? NONE. ZERO. (One atmosphere, actually, but give me a break!) So, when does the diaphragm stop moving? It does not stop until it encounters a physical barrier, the top of the bonnet. This happens regardless of how low the flow is. This is very different from the condition caused when you bleed the valve internally. Maybe that piece of debris will get washed out because the diaphragm is wide open!

Try this next time you have a weeping failure. You might get lucky and not have to take the bonnet off to clear that debris.What other uses do you have for the external manual bleed screw? I have a few more, but let's see what you have to say.


LawnSite Gold Member
Redmond, WA
Good info on how valves work. But,

So when I tell the customer you don't have any leaks I bled the valve and the meter shows it's tight. When they call me 2 days later saying the valve is fully stuck open or weeping again, I can't get out there for days and or weeks unless I want to arrive there at 8pm, I'll be sure to tell them the rain bird reps said it's ' a okay! '

I'll stick with flushing the line and replacing the valves
Here is something I learned from the analysis we do on returned valves.

Slip x slip models sometimes come back with 'no-open' 'right out of the box' as the reason for the return. We most often find that the solenoid dump port is blocked with PVC cement. I had always thought this was caused by too much cement and the bead of cement that builds up on the leading edge of the pipe as it is pushed into the valve. However, when we cut them apart to get a good look at the source of the cement, we have never seen that bead of cement reach the port. The port is always 'painted' over. The cement dauber has been pushed beyond the socket of the outlet and deeper into the valve where the dump port is located 'painting' the outlet closed. People swear that they are careful to not do this, but it is obvious from looking at the glue pattern on the dump port that this is what has happened.

I am sure everyone who posts here is aware of the tip that follows, but I will give it anyway for those who might be new.

If a newly installed slip x slip valve will not come on electrically or when bleeding with the solenoid right after install, try the external manual bleed. If the valve comes on with the external bleed, that means the problem is related to the solenoid water path. Turn off the water, remove the solenoid and stick a paper clip down the hole in the center of the solenoid bowl. (I am told that on the Rain Bird DV a marker flag is the perfect size for this.) Be sure to push it all the way through the cement that is blocking the outlet of the port. It is pretty hard to harm anything doing this. Don't damage the top lip that surrounds this hole. Re-assemble the valve and see if it works now.
Somewhere on Lawnsite I saw comments that suggested that one was surprised that the resistance value of a solenoid changes depending on how long it operates. This is why that happens.

The longer the solenoid operates, the hotter it gets, to a point. They eventually reach a temperature at which the heat gain from electricity running through it and the natural heat loss of the solenoid are equal. The temperature stops rising. For example, the Rain Bird HV solenoid takes about an hour and a half of continuous operation before it reaches its maximum temperature. This is without water, just sitting on the test bench. When installed with water and operating, the pilot flow through the solenoid acts to cool the solenoid. (Consider this if for some reason you want to run valves without water for a long time. They will operate hotter with no water and may wear out sooner.)

As the temperature of the coil rises, so does its resistance. (An electrical engineer on here can tell us what causes this resistance to change in response to heat.)

So, in our literature we give a range of resistance readings that you should expect to see when measuring it in the field. Some of this is due to a little variation in the manufacturing of the solenoid, but most of it is due to the operating temperature change over time.

So, when you see resistance readings that are 10% below the range shown in our literature, you are possibly seeing a solenoid that has started failing by shorting some of the coils. Time to replace it. Those shorts cause an increase in current which generates more heat which causes more shorts and so on until the coil is burned out.
From time to time I get asked if Rain Bird Valves like DV or HV can be installed in anything other than normal, horizontal, bonnet up position. Some want to install it vertically so the bonnet would be on the side and the inlets/outlets on the top or bottom.

When I ask a few questions, I usually find that these are not irrigation applications, but something else. First, I personally do not recommend Rain Bird valves for anything other than irrigation applications. Those are the conditions for which they are tested. Other applications have different conditions and we have not tested for those.

However, sometimes it is an irrigation installation for the vertical installation. I do not recommend this installation. The plunger in the solenoid relies on a combination of forces to return to the seat after operation and close the valve; the spring and gravity. Being on its side greatly reduces or eliminates the force of gravity. We test them horizontally, not vertically. So, we cannot say how reliable they are in that orientation. They may start out working fine, but fail later.


LawnSite Platinum Member
Indianapolis, IN
There have been times with nursery applications that valves can be installed vertically but usually it's a short term because the place is constantly changing.


LawnSite Fanatic
metro NYC
Some of the old-old brass valves had solenoids with no spring assisting the much-heavier-than-today plunger, so they were horizontal solenoid up or else


LawnSite Fanatic
metro NYC
In California, why not? The sight of sprinkler plumbing doesn't bother people out there, probably because they've been lawn sprinkling decades longer than most of the rest of the nation.

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