Keep in mind when your talking about heating water there are two styles of heating water depending on your purpose.
*When your using a boiler for heating your house, or pool, your boiler may vary the amount of BTU's based on the incoming water temp; so if there is a 5°f difference between the water entering the boiler and the water leaving the boiler, it may fire at half power. Now if say there is a 15 or20°f difference that same boiler may fire at full power.
*when your using a Navian for heating drinking and bathing water your system will regulate based on the amount of water flow, not incoming water temp. So if your running 1 faucet it may run at 20% power but if you have 2 showers and the laundry running it may fire at 80% or 100% power. It's based on GPM flow rate.
So even if your preheating the water your Navian measures how much water it's heating not what the incoming temp of the water is. Most heating boilers are not setup for straight domestic water, normally you need an heat exchanger in between to separate the drinking water from the heating water. Keep in mind in your using a wood boiler outside you may have to have an antifreeze chemical in the water incase the power goes out or has other issues.