We are installing a Toyotomi OM128 to 3 Toyotomi Convector HC20 Units. The lines going to the units will be pex with glycol. But the fittings from the OM 128 to expansion tank and circulating pump and all fittings are black iron. I was told after installing half the fittings that I was supposed to use brass or copper fittings or else the fittings will rust. This is a closed loop heating system. Do I need to change fittings before I finish install?