As pointed out above, if you cool wood fire exhaust too much, you get lots of condensation of creosote type stuff that is flamable, which increases the odds of having a chimney fire all else equal. With high efficiency natural gas, this is used without issue because you don't generally have much condensing other than the water formed in the cumbustion process and that tends to flush out the chimney and exchanger to some extent. With a forced air fire box with gas you need pressure switches or some other cutoff to prevent the flow of gas and flame if the fan pulling it into the box should fail. For wood, I would think you would need to ensure that your design could not back vent if the fan failed you in the middle of the night, for example. For the case of bubbling the exhaust gases through water for example, it would require a reasonable pressure difference to accomplish this which would be dependent upon the depth of the water through which it was to be forced, as well as having all of those condensates accumulate in your water.