Dropping Water Pressure

[DumberthanaPlumber]

I have 58 psi water pressure where it enters the water heater. When I turn on any faucet in the house, the pressure drops to 0 and trickles. There is a 3/4" supply line from the town, to 1/4" (about 12' long) to enter the house, then I have 3/4" water lines to each faucet, then switched to 1/2" to supply the faucet. Any ideas on what makes the pressure drop so fast? Sounds like the 1/4" line is the odd ball, but I'm reading decent pressure at the water heater, after the 1/4" line. Anyone?

[HayZee518]

the quarter inch line is your problem. any pipe in your house will give you 58 psi but the volume of water just isn't there. your feeder from the street is 3/4 to the meter, out of the meter should be 3/4 to your hot water heater and the distribution lines should be 3/4. the utilization lines to your fixtures can be dropped to 1/2 and 3/8 for the supply tubes. the quarter inch line is acting like a venturi offering pressure but throttling down the volume of water.

[LazyPup]

I am totally amazed that you are even getting the trickle.

Pressure is by far the most misunderstood concept in all of plumbing. Most people assume that if they have 50psig at the main they will then have 50psig at the faucet or showerhead but such is not the case.

When all the faucets and valves are closed and there is no flow the pressure in the system will be at equilibrium with the supply pressure from the municipal main or home well pressure tank. This is known as “Static Pressure” or “At Rest Pressure” and it will be equal at all points in the system regardless of line size or elevation.

Once a faucet or valve is opened and flow begins the pressure in the system is influenced by a number of factors. First of all, a portion of the pressure in the system is being relieved by the amount of water discharging from the faucet to standard atmospheric pressure.

If the water makes a vertical rise from the source to the point where it discharges the flow is opposed by the physical weight of the water in the riser. This opposition to flow is known as “Vertical Static Head” and it is equal to .434psi per foot of vertical elevation. By example, consider a house where the water line enters the basement at floor level. We then have:

8’ Floor to ceiling.
1’ for the floor joists and flooring on the first floor.
8’from the first floor to the ceiling.
1’ for floor joists and flooring on the second floor
6’ from the floor to the showerhead.

24’ Total vertical rise.

The physical weight of the water in that riser is then 24’ x .434psi = 10.41psi

If we had 50psig at the water main the resultant pressure at the shower head would first be reduced by the weight of water in the riser which opposes the flow therefore the resultant pressure would be 50psig minus 10.41psi = 39.59psig.
Next we must consider “Friction Head”. Friction head is the physical opposition to flow caused by friction between the water and the inner walls of the pipes or eddy currents caused by directional fittings such as Tee’s and Elbows. Friction head is generally defined by an equivalent amount of “Vertical Static Head” thus if a line was said to have 20 feet of friction head it would mean that the opposition to flow is equal to a 20’ vertical rise or 20’ x .434psi = 8.68psi. This pressure is also in opposition to flow so it would also be subtracted from the supply pressure thus if we had 20’ of friction head in our above example the resultant pressure at the showerhead would be;

X = Supply pressure minus (Vertical Static Head + Friction Head)
X= 50psig – (10.41psig vertical static head + 8.68psig Friction Head) =
X= 50psig – 19.09psig
X= 30.91psig

This resultant pressure is known as “Dynamic Head” or the working head pressure at a specific point in the system.

This can be seen in the plumbing codes by example, The International Residential Plumbing Code requires a minimum “Static Head Pressure” at the main of 40psig but when we examine the “Fixture Supply Dynamic Head Pressure” chart we find that a shower is only required to have a flow rate of 3 gallon per minute at 8psig dynamic head.

When sizing water lines per plumbing code specification we first make a list of all fixtures in the structure that will be connected to the water supply. We then consult a table in the code that assigns a “Fixture Unit” value to each fixture, from which we can compute the total number of “Fixture Units” required by the structure.

We then measure the distance from the source to the furthest fixture and consult the line -sizing table to determine the required size of line.

Per code the minimum permissible line size is ¾”

Now let us consider your ¼” line.

When comparing the volume or capacity of different sized lines we cannot use diameter but rather we must use a formula that compares cross sectional area thus the formula is:

LARGE DIAMETER SQUARED divided by small diameter squared.
Where:
¾” = .75”
¼” = .25”
N= (.75 x .75) / (,25 x .25)
N= 0.5625 / 0.0625
N= 9

This means that a ¾ line has the same capacity as 9 lines at ¼” each or conversely your ¼” line has 1/9 the capacity of the code minimum ¾” line which no doubt accounts for why you are getting a trickle.

In addition, as the diameter of a line is decreased the velocity of flow in the line must increase proportionally however as velocity of flow increases the Friction head will also increase proportional to the velocity thus while you have 58psi Static Head Pressure once flow begins you have nearly zero Dynamic Head Pressure.

[waterless]

Hi guys, need your help. My local council recently connected me to the mains that runs under the footpath outside my house. It is a 9 inch mains, the connection they made off that which is about 3 foot long is 1/2 inch. I wanted 3/4 inch but they connected 1/2 inch. It is my responsibility to make the connection from the boundary into the house as it is private property.The distance to where water goes into the instant gas boiler is 50 foot from the new connection. It then rises a floor to supply a bathroom and radiators. If i continue on with a 3/4 inch line from the 1/2 inch line will this provide me with a better pressure/flow rate, or should i stick with the 1/2 inch pipe? thanks

[HayZee518]

the 3/4 inch will act as sort of a manifold from the supply and will be under a constant pressure and volume. you should technically have the necessary volume when you downsize to your fixtures.

[waterless]

so im better off going with the 3/4 inch line, than sticking with the 1/2? also if i put up a tank on my flat roof, which is 7m high and put up a 240 gallon tank, would i get greater pressure from it, than from the mains outside (please note that the water from the tank would have to have the pressure to go down to ground level and then back up a floor. Any help greatly appreciated as the job is going to get underway by saturday and my family havent beeen able to take a shower in almost a year as there isnt even the pressure at the moment to raise the water up a floor, that is why we got the new connection which we have to hook up. The old line that came in from the back is over a 100 years old and is only 1/2 inch yet it serves a row of houses! In Ireland regulations stipulate you have to have a storage tank, at the moment we have a direct system (ie we rely on the pipe pressure to feed the whole system. I look forward to your replies

[HayZee518]

1/2" feeds a whole bunch of houses?? In your previous thread you said the mains are 9 inch and the houses tap off this line. an on roof storage tank will always be better, just if I'm reading this right will take a bit of time to fill up. how about a booster pump to get that water up there?

[waterless]

hi hayzee, yes the old pipe that ran at the back of the house is 1/2 an inch and around a 100 years old serving several houses! Up to 9 months ago the pressure from this pipe was low but acceptable, but now the water barely trickles out of the taps! Instead of replacing the old pipe, the council just connected us to the 9 inch one running outside the front of our house.