LsFarm wrote:The next plumbing method is a little more complicated, but will give you constant hot water.. You plumb the coal boiler to the oil boiler with it's own supply and return lines.. Hopefully you can fine unused ports on the oil boiler, otherwise adding a 'TEE' to the existing supply and return will be needed. In the loop that is created between the coal and oil boiler, you install a small circulator, like a Taco 007. I would run the circulator 24/7, they burn .78 amps.. costs about $10/month in electricity.. But you can add an aquastat to control this pump if you wish. The aquastat on the oil boiler should be set about 20-30* lower than the coal boiler's aquastat. Or some folks just put a switch on the oil burner to turn it off..
I'm trying to understand what you mean with this setup. I understand about having the two boilers constantly circulating between the two and possibly using a aquastat so it doesn't need to run all the time. But the t - setup has me confused if I also want to keep the oil boiler usable for house heat. The way I'm picturing it I would not have to T the hot supply line from coal to oil boiler. But after the oil boiler I would need to T it to the house radiator system with a circ pump and and the other part of the T would be to bypass the house radiators when only keeping the oil boiler hot for domestic water. It would need to be wired so that when house heat is called for it's circ pump would turn on but the other circ pump (the one that is a constant circ for DHW) must turn off at the same time or a lot of potential heating will be just going straight back to the boiler. Also, when/if the oil boiler starts the circ pump for the DHW bypass should also be off automatically. My house is old/large but only only has one zone. I'll try and post a diagram if this doesn't make sense. BG