I have a 3/4'' main that comes into the house and then reduces to 1/2''. Then all my fixtures run off that in series. My thought is to i make it 3/4'' copper all the way and then run 1/2'' tees to each fixture. For the hot, the pipes coming out of the water heater and boiler are 1/2". Can i leave them 1/2'' but then increase to 3/4'' to run to the bathrooms? Then 1/2'' tee to the fixtures? Is there any problems or issues that with me doing it this way?
If you are looking to increase the water flow to the fixtures in your house but do not want to re-pipe the whole house, you could increase the supply pipe from the 3/4" inlet to the water heater. Tee off any of the individual supplies from the new 3/4" supply with reducers and the final pipe into the hot water tank could stay 1/2". This way you would double the supply water available to all the fixtures since the hot and the cold will have individual supplies instead of a 1/2" supply supplying both the hot and cold. Also, remember that almost all sinks and toilets use a 3/8" supply and only the tub and/or shower and the laundry tub use a full size pipe. Doing it your way would be fine.