I have a customer that did not like the heat loss he was getting from his HX. So we directly connected his Woodmaster 5500 to his cast iron oil boiler. Everything is working fine but I was wondering if there was any problems that we might not be thinking of. Like rusting out house oil boiler block or ruining components of either boiler. His OWB is above his house so it is giving the house oil boiler approx. 15 PSI of water pressure. If this is not a big deal I am going do do the same with my Central 5630 OWB. My OWB water level is about half way up my house and the OWB sitting pressure in my basement is 10PSI(Yes I have a pressure gauge). My house oil boiler is a Weil Mclain WGO4 cast iron boiler. Will doing this ruin anything? Also why are OWB manufacturers recommending HX's? Is it so you can adapt to any system or conditions that way?