Hot Water Distribution
Get fresh hot water the moment you turn on a faucet.
It is hard to treat this subject alone without also considering its relationship with the Radiant Heating system because the pump for the radiant heating heat exchanger also provides the hot water circulation.
A 3/4" PEX pipe goes around the house past each room that needs hot water. In each room a reducing tee is used to feed water via a few inches of 1/2" PEX to a mixer valve under the sink that makes hot water of the right temperature for that room. The output from the mixer valve feeds to the 1/2" PEX shutoffs that feed out to the fixtures in the room that need hot water.
Having one mixer per room means that eg a kids bathroom can be set to a lower temperature than an adult's bathroom. It also means that the hot water circulation loop round the house can be kept at a high temperature (eg 135 deg F) so that there is no danger of Legionnaire's.
Potable water certified PEX pipe
Use 3/4" for the loop that goes around the house. For the spurs off use a small amount of 1/2".
PEX pipe insulation
The hot water house loop is done with 3/4" PEX and this needs to be well insulated to reduce heat loss.
PEX Reducing Tee (High Temp)
It is best to use fittings with the ability to handle up to 200 deg F so that there is a wide margin.
Temperature mixing valve
A mixing valve is used to allow the water boiler to operate at a higher temperature than comes out of the faucets. One is provided in each room that needs hot water, eg one in each bathroom. It is positioned under the sink together with the 1/2" PEX shutoffs for all the fixtures (both hot and cold shutoffs).
1/2" PEX shutoffs
These will be mounted in the bathroom/kitchen/utility cabinets under the sink. One is used for each fixture. (Separate ones are needed for the cold water shutoffs.)
The stems are just about long enough (1/4") such that the control can be mounted behind tile. Use a 1/2" electrical conduit clamp and PL-Premium adhesive to mount them securely to a 2-by lumber base. 1/2" PEX staples at each end also help. It must be secure because lots of force is needed to turn the handle.
A secondary hot water heater fitted with a lower power heating element is useful in a power outage situation and for making sure you never run out of hot water. If you have a secondary hot water heater then it is useful to be able to route the hot water to different hot water zones.
Need adapters from 1" NPT to 3/4" PEX.
The default case is that the low power water heater just feeds His Master Bathroom, and all other cases are fed from the high power water heater.