There are two types of clogs that occur in galvanized water pipe. One type is the build up of scale (rust, calcium, etc.) on the inside of the pipe which slows the flow of water along a length of pipe. It happens slowly over a period of many years or decades, and with each new year the flow of water is just so slightly slower that you may not even notice it. That situation and various ways to fix it is covered in Cleaning out galvanized pipe.
But there is another type of clog that usually happens suddenly and represents a clog of loose debris at one location that can often be easily broken up by flushing or pounding on the pipe. That is what this article is all about. Even though the clog happens suddenly it can remain in the pipe for a long time and one might mistakenly think that it happened slowly and that it is the first type of clog. Of course, slow flow in a pipe can be due to both types simultaneously and what one thinks is a "one location clog" might be in a couple (or more) locations. Two ways of clearing this 2nd type of clog are 1. Pounding on the pipe with a hammer at the location of the clog with the water running. 2. Flushing the pipe by running water thru it as fast as possible, including reverse flushing in the direction opposite to normal flow.
By noting the plumbing fixtures where the water runs fast or slow, one may attempt to estimate the clog's location. However, the water at a certain fixture (toilet, faucet, shower, etc.) may run slow because the fixture itself is clogged (or because it's designed to run slow). If the fixture has a handle underneath it for shutting off the water for that fixture then the clog might be at the stop valve (angle valve) operated by that handle. Try opening it by turning the handle. If it's stuck and won't turn (try turning it in both directions) then you can gain leverage by using an adjustable wrench or pliers to turn it provided the handle is strong. Otherwise you could try pounding on the valve with a hammer while trying to turn it with the water flowing.
A clog is most likely to be located where the inside surface of the pipe has a "step" in it, like at couplings, tees, elbows, etc.
This is a quick way of breaking up the clog, provided you pound on the right place and the pipe is easy to get access to. Pound with the water running and if feasible, have someone watch the water flowing as it pours out of the water system and note the color of the water (it may look rusty). When the water runs perfectly clear as you are still pounding, you are perhaps finished pounding at that location.
One may flush out a pipe by running water thru it at full flow in either the normal direction of flow or the reverse direction of flow (back-flushing). Back-flushing is harder to do but is more likely to fix the problem. One might first try normal (forward) flushing and if that doesn't work try back-flushing. In tough situations alternating forward and back flushing may clear the clog.
This is easy. Just open a faucet all the way (including any stop valves that may be restricting the flow). Also remove any aerators, shower heads, etc. that may be restricting the flow.
You may want to also remove the faucet or stop valve if it's restricting flow and then turn the house water back on. Of course, you turn off the house water first before removing faucets or stop valves. Insure that when water is turned on to flush the pipe, that the water coming out of the pipe doesn't flood the floor. You might need to use a hose or large bucket for this purpose and it's a good idea to have someone watch the water coming out of the pipe and take action if needed to prevent flooding. Plan what actions that person will take beforehand.
The idea behind forward flushing is that the clog might break free if the water is made to flow faster, but the dilemma here is that if the clog is very restrictive, it will itself keep the water running so slow that the clog will not clear. Thus forward flushing isn't likely to clear bad clogs but may be useful to maintain pipe and reduce the formation of scale.
You run water thru the pipe in the region of the clog in the opposite direction of normal flow. This can release clogs where debris is stuck at ridges or "steps" inside the pipe. After finding the suspected restrictive section of pipe, you first turn off the water for your house and then connect a garden hose after the suspected section of pipe. Of course, use the largest size hose you have, hopefully 3/4 inch. You then open faucet(s) before the suspect section so that the water supplied by the hose will empty out of theses faucet(s). Then you turn on the water to the hose to backflush the pipe.
Major problem: If the water to your house is turned off, how do you supply water to the hose? One way is to use your neighbors faucet. Another way is: if there are two shutoff valves for your house, one at the water meter and one at the house (or similarly), then to backflush, you turn off the valve at the house, leave the water meter valve open and connect the hose to the pressurized section of water supply line that exists between the water meter and the house. Hopefully you have a faucet in this section of pipe. But if you don't, you can either install one or perhaps get water from your neighbor.
There are brass adapter-couplings one can buy at some hardware stores for connecting garden hose to galvanized pipe. To connect a garden hose male end to a male outdoor hose faucet you need a special female to female hose adaptor or you can make one out of a couple of hose-to-pipe adapters provided each such adapter has a female hose connector on it. You may need to also use galvanized pipe couplings to make the hose-to-pipe hook-up.