Many of people ignore the instructions the came with their water heater to install a pressure reducing valve and a expansion tank on the domestic water supply. When cold water comes into a water heater and is heated it has to expand, without the expansion tank the water has no place to expand and will build up excessive pressure. Water heaters come with a pressure relief valve set at 150 pounds. This means that the pressure will build up to 150 pounds before it is relieved. This will damage the plumbing system in your house partially your facets, the heating manifolds and valves in your heating system.

Also the lack of an expansion tank will cause a water hammer every time you close a faucet, again this will cause damage to your heating manifolds, valves and your house hold plumbing.

These problems are common to both wells and city water. 

Any time a water heater is used for both heating and domestic water the domestic water must have a anti-scald valve installed to protect people from getting burned.

This whole house mixing valve allows you to run the combo heater at any temperature to heat your house and allows you to adjust the domestic hot water to a safe operating temperature, it also increases the amount of hot water available.
This tank does not require dielectric unions. Do not use steel or galvanized pipe. Use copper only. Do not sweat fittings while on mixing valve or with rubber gaskets installed. Mixing valve needs to be installed below top of tank.