[buffalobillpatrick]

No heat transfer plates.

This is the 4th house that I have done this way.
Works fine in climate zones 4 through 7.

Heavy (thick) heat transfer plates do work well & would lower the required water temperature by about 25*F
They are expensive to buy and time consuming to install.

I’m using a cast Iron Burnham ESC3 high altitude boiler that will deliver about 35,000 BTU’s at my 8,750’ altitude.

This type of boiler needs to run hot to boil off condensation from the exhaust gasses.

The boilers only task is to heat the 79 gallon Viessman dual coil indirect water heater via the bottom coil.

This tank provides Domestic Hot Water (DHW) to the fixtures
AND
acts as a buffer tank for the heating system, the boiler and the heating system are asynchronous with each other.

System heat is drawn out of the tank via the top coil.

The boiler will have only long semi-efficient cycles of at least 30 minutes, even longer in heating season.
The aquastat on the tank controls the boiler firing and will keep the tank 130-160* range.
(Kills Legionella)

This application (heating an indirect) would require a mod/con to run at 100% high fire at about 86% efficiency, about 1% better than my cast Iron boiler.