Does 3/4” pex supply enough water for a house
Not a calculated method, but if that home is yours and you're planning to spend a good few years in it, I would put in the 1&quot; main PEX to water heater and as you go to edge water outlets of the house, convert that 1&quot; to 3/4&quot;. The cost of 1&quot; vs 3/4&quot; isn't too drastic and having the extra capacity, especially to the water heater will help with pressure issues. 3/4&quot; post water heater is good, unless you have 8+ hot drops.
If that home is a builder home and used for investment or you're trying to minimize the cost, do the calculation on needed flow and capacity. Most of the time 3/4&quot; is good for smaller homes.
Regarding manifolds, all builders that I've seen do not use main manifolds and just use tee's and split off the main pex. It appears, labor wise, its less work and less cost since the manifold adds a few hundred to the project and creates further runs but you lose the ability to shut off each drop.