Water volume problem

With water running in a shower and a second fixture is used water flow is greatly reduced. I have done the following; checked the water pressure at multiple fixtures and they all read about 65psi, verified that the main water shut off was fully opened, had the water company check the pressure at the service meter and it was fine. The service line to the house is 3/4" blue poly pipe and the inside pipe is all copper. Would replacing the service line with 1" or 1 1/4" minimize this problem? Or is there another solution. The house has 3.5 baths if that matters. Thanks for any assistance!

Wayne

You leave out too much information to even wager a guess.

We'd have to know things like the actual distance between the shower and second fixture along the developed length of pipe both vertically and horizontally to understand how the operation of one might affect the other...

The size of supply lines that served each branch and each fixture...

Whether or not there were any flow restricting devices in the system...

And a host of other things.

Welcome Wayne:
Just as a basic rule of thumb; the service line should be 3/4" and on to the water heater, then to each group of 3 or more fixtures. After reaching the groups a 1/2" line should serve each fixture.
We could get into flow, friction from distance, friction from Ells and Tees and fixture units used in the house but the basic will work for almost any residential system.
You might check for a secondary valve on each fixture group, then make sure the valve comming out of the wall at each fixture is wide open, then make sure your 3/8" supply lines are not over 36".
Please let us know how you come out with it and if we can help further.
Glenn

Thanks for your response Glenn. I have checked all of the shut-off valves to the house and fixtures. They appear to be completely open. What's the chance that the main shut off is partially blocked?

All of the copper stubs (that I can see) out of the wall are 1/2". And none of the 3/8" supplies are over 15". The pipe to the water heater is 3/4". Since the house is two stories I really can't tell how many 1/2" pipes run off of any individual 3/4" runs.

Pretty much any 2 fixtures (shower, toilet, washing machine or dishwasher) will impact the water flow. The sink faucets do not seem to have as much effect. The problem is not limited to fixtures at opposite ends of the house or both in the same area. If you turn on 2 showers at once there is enough water for both. The problem is when one shower is being used and a toilet gets flushed or the second shower is turn on after the first or heaven forbid, a third fixture is used, the poor soul in the first shower gets hot real quick.


I also did the 5 gallon bucket. I am getting approximately 16gpm from a 1/2" outside faucet. Generically speaking, what would the effect be of increase the main supply line? I am not totally sure I understand the pressure verus flow. Would understanding that help me? Thanks again for your help!

Wayne

Hello Wayne:
Your problem is not the pressure but the flow. You have suffficient pressure to use even the upatairs fixtures but there is a flow restriction.
Think of your garden hose with a nozzle on it. If you open the nozzle all the way you have full flow; however, if you only partially open the nozzle you have the same pressure but less flow. Pressure comes from the height of the municipal water supply and is equal in all directions throughout the system (provided the distribution system is all on one level). Flow comes from the size of the supply line. A fire truck could not get enough water from your supply line to pump on a fire; but a 4" fireplug supplies plenty of flow.
I'm betting you have a restriction in your line somewhere that can be caused by several things: 1. A kink in the supply line (likely in a wall) 2. Using too much solder on a fitting feeds droplets of solder into the line which will accumulate and cause a blockage 3. Other trash of unknown origin in the line can accumulate and cause a reduction of flow.
If I open my mouth wide and blow out all my air it will not make much of a noise; however, if I pucker my lips or restrict the air with my tounge it causes a whistle. The same applies to your water supply systme. I suspect the restriction is at the main valve on top of the water heater. You can listen to that valve closely while others turn the water on in different places. If you hear a whistling or screaming from that valve, it may have the sloder droplets on the top of it. There should be a union nearby that you can open and check for the solder. But then, you said flushing a commode causes hot water at the shower, which means the cold water is obstructed at a point beyond the water heater; I hope you have access to that part of the line so you can check for solder.
A stethescope will help to pinpoint the blockage, or you can put a long screwdriver on the pipe, put your thumb on top of the screwdriver and your ear on the thumb. Let me know how you come out and I will work with you some more if necessary.
Glenn