I have a 1/2" water main entry, and if I open water in 2-3 places in the house, the water pressure is very low.
How can I fix this?
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1/2" is a very small "main" - the only truly functional approach would be to upgrade it to a larger main.
If your multiple-water use scenarios are of short duration, you could get some benefit from placing a pressure tank (as used in well pump systems) in the house to provide a few gallons of buffering, but after a few gallons (variable with size of tank and variation in pressure) the stored water runs out and you are back to what the main can supply.
A pressure tank system would largely resolve the issue, if correctly sized, and if the rest of the plumbing in the house was larger than 1/2".
Unfortunately if the main is only 1/2, then it's likely that the rest of your house is also plumbed at 1/2", which means that even with creative placement of the pressure tank you'll still likely have the problem.
So you're looking at upgrading your interior plumbing at least a little bit, convert some of the 1/2" piping that serves as the main trunk to the other pipes to 3/4", and either using a pressure tank or upgrading your service feed to 3/4".
If you choose to use a pressure tank, and your home only has one shower, then you'll probably get reasonable volume using a 40 gallon pressure tank. This will give you enough water to shower or bathe with 20+ gallons, while others use a few gallons from each of your other faucets. Again, this won't work if your interior plumbing is also just 1/2". If you have two or more showers that might run simultaneously and for long periods of time, you'll want a larger pressure tank.
Just upgrading the piping without also upgrading the service or adding a pressure tank won't fix the problem, nor will upgrading the service or getting a pressure tank without upgrading your piping. You'll have to upgrade the piping either way, and then figure out how to increase volume.
If you're only concerned about the faucets, though, and not about the showers then you can put smaller pressure tanks under the faucets for the cold water line. A 2-3 gallon pressure tank at each affected faucet would go a long way to increasing apparent pressure and providing the desired volume. Spreading pressure tanks around like this removes the need to upgrade the plumbing, which could reduce the cost.
Another option is to replace your faucets with low-flow variety for the bathroom faucets. These can fulfill the needs of the bathroom with less water volume and conserve the water pressure. Older faucets, particularly those without aerators, use a large volume of water, but you don't need that much for hand washing and other hygiene needs in the bathroom.
It's not a pressure issue, but a volume issue. If you're on city water, then a water service from the meter to the house in 1"or 3/4" depending on the distance is in order. The main line under the house should be 3/4 and the branches will be 1/2. If it's on a community system, a pressure tank won't help, a jet pump would be the answer. But until you solve the volume issue with bigger lines, you won't fix anything.