Rick Stanley said:
Now, in considering other options, I looked at what I already have in place and that is this: There's an oil boiler with a dhw coil in it. Sitting 2 ft away is a 40 gallon tank, piped into the coil, with a circulator in the loop. I look at that and think that there must be a simple way to use a second smaller dhw flat plate hx to replace to coil in the boiler and then I could let the oil boiler go cold when I'm burning wood and make use of the tank and circulator that's already in place.
Does any of this make sense?
OK - From your description above, it sounds like you have an indirect tank already. The circ is pumping water in a closed loop between the coil in the boiler (where it is being heated) and the coil in the 40 gallon tank. If I am wrong, then please straighten me out. There should be some means for expansion on that circuit as well. I have not seen a setup like that before.
Now, how are you going to keep water from passing through your furnace? If you are putting an HX on the return leg of the zone manifold before it gets to the boiler, then are you adding a bypass circuit to send the heated water to the supply leg of the zone manifold? If so, then the boiler will go cold, and you cannot heat the DHW via the coil in the boiler.
However, rather than buying another small HX, why not just pipe the circ you are already using on the DHW loop to circulate the heated water from the supply leg and then back to the return leg (before the HX)? The extra piping is going to be cheaper than the HX, and the amount of work is probably the same or less than adding another HX.
BTW - if you completely bypass the oil boiler as above, how are you going to handel backup situations? Unless you are using motorized isolation valves, you will have to manually open the circuits to allow flow through the oil furnace again. You will also have to shut down the oil furnace electrically, too.