What pressure will I get at the spigot outlet?

[Pat N]

So my house has 3/4 in copper plumbing. I'm replacing the frostfree sillcocks (10 inches long with fancy-schmancy devices for those who can't remember to shut off their outside h2o in the winter), with simple standard hose bibbs. Here's my question: When replacing the sillcock, I can either use 3/4 in, or step it down to 1/2 in pipe for that last 10 inches. Because of the existing hole that was drilled for the sillcock, it's easier to step it down to 1/2 inch. The water outlet hole in the hose bibb, where the hose attaches will have the same size hole whether I attach it to 1/2 inch or 3/4 inch. Will stepping down to 1/2 inch cause me to have greater pressure behind the h2o coming out of that hose bibb? Will I find that if I turn on that bibb with no hose attached, water comes blasting out like a fire hose because I used the 1/2 inch leading up to it? Or will it just flow out much the same as it would out of the end of a hose?

 

[LLigetfa]

I'm replacing the frostfree sillcocks...

I must be missing something. If you have frost-free sillcocks, then why replace them and what exactly are you replacing them with?

 

[Sponsor]

 

[LLigetfa]

OK so I read it a few more times and deduced that you no longer want them to be frost-free. Using larger pipe can provide more volume which some equate to more pressure. Using smaller pipe will give you less volume which can result in more of a pressure drop.

 

[John Gayewski]

The pressure will be the same (only an unmeasurable difference would take place) the difference your thinking of would be velocity. Pressure comes from the pumps the city uses to send water to your house you can't increase it without a pump. You would get a velocity change similar to holding your finger over a garden hose.

A frost free silcock is not for people who forget to turn the water off. It's so you can use your hose in the winter. It also keeps water out of the pipe that penetrates the wall. From your post I'm gathering that your plan is to drain these pipes where they penetrate during the winter months.

 

[Jeff H Young]

I would expect an unnoticeable amount of volume drop from using a half inch pipe the last 10 inches. the 3/4 would actually be better not worse but that again you'd probably couldn't notice

 

[Pat N]

The pressure will be the same (only an unmeasurable difference would take place) the difference your thinking of would be velocity. Pressure comes from the pumps the city uses to send water to your house you can't increase it without a pump. You would get a velocity change similar to holding your finger over a garden hose.

A frost free silcock is not for people who forget to turn the water off. It's so you can use your hose in the winter. It also keeps water out of the pipe that penetrates the wall. From your post I'm gathering that your plan is to drain these pipes where they penetrate during the winter months.

Hi Jeff, thanks for your answer/info. You're right in that I'm concerned about the velocity. My thinking was that pressure at the exit equals velocity. I'm concerned that with the 1/2 inch pipe, when I use the spigot without a hose, for example to fill a bucket, that the spray will be unmanageable. Much like filling same bucket with a hose nozzle adjusted for a long distance stream. I would rather the effect be that of a hose end with no nozzle attached. A much slower flow.
Since both pipe sizes would ultimately exit the water through the same size hole, would there be any difference in velocity? ie: 1/2 inch pipe to spigot exit hole vs 3/4 inch pipe to spigot hole. Of course presuming the spigot exit hole is equal to or smaller than either pipe.

 

[Jadnashua]

For a short distance, a restriction acts like a nozzle, but at 10" or so, you will primarily get the added friction of the smaller pipe and the water trying to push through it, lowering the dynamic pressure.

FWIW, there are at least a few frost-free silcocks that claim they won't be damaged if a hose is left on in the winter. There are a couple of methods to do that:
- a separate drain that opens when you shut the water off similar to how a common diverter tub spout works
- separate spring loaded ball check valve that opens when the pressure rises to relieve the pressure.

Here's one of them Woodford Model 30 Freezeless Faucet (woodfordmfg.com)

A common frost-free valve will be damaged if you leave a hose connected and it freezes, as can happen if the thing is not installed properly with downward slope, so it can freely drain once the hose is removed.

 

[LLigetfa]

A common frost-free valve will be damaged if you leave a hose connected and it freezes, as can happen if the thing is not installed properly with downward slope, so it can freely drain once the hose is removed.

I just changed out a frost-free hose bib last week that had freeze damage and it did not have a hose left attached. It also was sloped to drain.

The problem was iron buildup inside both the tube and the long valve stem which reduced the annular space enhancing the capillary action to keep the water from draining. I have an old thread on this.
https://terrylove.com/forums/index.php?threads/frost-free-sillcock-freeze-damage.65614/

 

[Jadnashua]

Chrome plated steel won't last forever...I don't know if the Woodford guts are all brass, but the spec sheet says all structural parts are brass...that won't rust. Not the cheapest ones out there.

 

[Pat N]

My reason for swapping out the sillcock for traditional bibb, is that they seem problematic - if something goes wrong, they are way over engineered to easily trouble shoot/repair. I have 2 out of 3 originals - (all 20 yrs old, so I give them that. Tho I don't know when they went bad. I just bought the house 2 yrs ago.) that are leaking and impossible to fix. The only benefit I see from them is if I forget to shut off my h2o indoors. I'm in Ct., and not on a farm, so I don't see me watering a garden in Jan. If I did in fact need some water outdoors seems it would be just as easy to just fill a jug. Seems like overkill to me that just brings problems.

Like the nursery rhyme...when she was good she was very good, but when she was bad, she was horrid!

Oh, also, they all come with the 'twist forever' knob. At my previous home I swapped out every last one of that type in my basement for ball valves. Imagine you have a leak causing damage, and when you go to turn off the water further upstream, the valve stem breaks ....and it's your only cut off!