First, these are two completely different systems. The boiler heats the house and may heat the hot water for the house, but the two systems are separate. Typically the hot water for the home is between 45 and 60 psi, though some people run it at 30 - 35, but that is low pressure. The boiler is always run at about 15 psi. The water in the boiler, never touches the water in the domestic bathing hot water. Any heat transferance is done through a coil either in the boiler or the water storage tank. Sometimes the coil is external. Most of the time the domestic hot water is heated by its own water heater.
If your water pressure is low, you can turn it up on the solenoid valve atached to the pressure tank. However, you shouldn't do it unless you know what your doing. In addition, you may need to check the pressure in your pressure tank. If the tank has lost its diaphram and the air and water are mixing, then you won't get good pressure, no matter what you set the pressure switch to. I typically install a 20 gallon pressure tank, most plumbers install a 15 gallon. I have found that they don't provide enough pressure for a family. The more storage you have in your pressure tank, the longer the shower you can take. Think about it this way, a shower has a 2.5 gallon head. If you run it for 10 minutes, you are out of water pressure (unless you have a variable speed pump in your well). Some of the newer homes have variable speed pumps in the home and only use the pressure tank as a buffer. That way you can use the faucet to get a glass of water without your pump kicking on. Without a buffer tank, you would have your pump turning on each and every time you use the water.
Raymond VinZant Plumbing Prof.