Rob R. wrote:If you want to continue using the tankless oil in the oil boiler, your proposed setup won't work...think about it, with your piping arrangement the only time heated water flows from the coal boiler to the oil boiler is when one of your zones is calling for heat. You need to circulate between the two boilers if you want to use the tankless in the oil boiler.
Exactly, you will need a separate pump to do that job. I would also recommend a stack damper on the oil burner to reduce thermal losses up the oil burner's chimney flue if you chose that route.
Ones I have seen when they want to use both, just use the coal boiler to keep the oil boiler hot. That is a circulator loop to oil unit, and then the thermostats still run circulators for house
Are the two of you suggesting the same thing? I'm not sure I fuelly understand. Could someone point me to a diagram of this setup? Would you wire the circ pump between the two boilers to run constantly? Or could/would you set it up so that when the t-stat on the oil boiler called for heat it would just start the circ pump that was between the two boilers?
Also, why could you not use a water to water heat exchanger?