If you're buying an old house, beware of old water supply pipes; specifically, galvanized steel or lead. These pipes can lead to poor water flow in homes, to the point where you can't even run water in two places at once.
Water Supply Pipes
VS. Water Distribution Pipes
For this post we will focus on water supply pipes. That's the pipe that brings water from the city main into your building. The rest of the water piping found inside your home is water distribution piping. This is an important distinction because the rules for these two materials aren't the same. Again, we will be talking about water supply pipes.
Galvanized Water Supply
The problem with galvanized water piping is that it corrodes on the inside, leading to smaller inside diameters. Think of clogged arteries. These lead to high blood pressure in people, but it will not lead to higher water pressure in a home. It doesn't lead to lower pressure either, despite the common misconception of homes having poor water pressure. It's actually quite unusual for a home to have low water pressure because the pressure will be whatever the city delivers. Clogged water mains lead to a massive reduction in water flow, however.
To know if you have galvanized steel water supply piping, look at your pipe where it comes through the floor. If this water piping has a threaded fitting, it's probably a galvanized supply pipe.
If the water piping coming into the home can't be found, access is blocked, or the main valve is located too close to the floor to determine the type of water supply piping, one option is to call the municipal waterworks department to find out what type of water supply piping the house has.
At times the water flow throughout the house was quite minimal, yet all of the water piping looked great, and there was a copper water supply entering the house. Apparently, the water supply piping didn't always get replaced entirely. In some cases, there would only be a partial replacement out to the street. The fix in these conditions is to have the yard and street dug up again and have the rest of the water supply replaced.
An easy way to determine a problem with the water supply pipe is to quickly turn on an exterior faucet. If water comes out with a burst but the flow drops noticeably after a fraction of a second, it's almost certainly a problem with the water supply for the house. In most cases, this indicates an old galvanized supply pipe, but could also indicate a problem with an old lead supply pipe.
When you see this, you probably have a major issue with the water supply piping coming into the building. If the water supply pipe to a house needs replacement, plan to spend several thousand dollars. You'll need to dig up the yard and replace the water supply pipe out to the city main, the homeowner pays for this. If the city water line is on the opposite side of the street, it's more expensive.
If the home has a lead supply pipe, water flow may be restricted because of a damaged pipe, but I've also seen homes with lead water supply pipes that still had acceptable water flow. The other obvious concern with a lead water supply is with lead leaching into the drinking water for the home. In these cases, the water can be tested for lead. Visit the EPA for more info on lead in water.
Testing Water Flow
To help determine if water flow is acceptable start running hot and cold water at the laundry sink faucet, then run upstairs to the highest plumbing fixture, preferably a shower, and verify there's still water flow. If these is no water flow or insufficient flow to take a shower, it's a major concern.
In most cases, no flow at the supper fixtures under these conditions indicates a problem with the water supply pipe or old galvanized water distribution pipes in need of replacement. A couple of other possibilities would be a main shutoff valve that is partially closed or a problem with a water softener that restricts water flow throughout the house.
There are too many variables for me to cover every possible scenario, but hopefully, these tips give enough info to conduct a basic test for water flow on old houses.