Coal is tested and given an ash value but that is in an ideal laboratory environment. In real life you can probably expect 20 pounds for every 100 lbs of coal but to tell you the truth I really never measure it... Roughly about 1/3 the volume. Both of these are lower for hand fired stoves because they do a much better job of completely burning the fuel.
As for getting rid of there is few threads regarding that, locally here the borough picks them up. some will mix with salt for cindering the roads. You can use it for fill, in your driveway etc. If you live in town contact your local garbage delivery person. The ash is considered clean fill so that is not an issue.
I don't know if the alaska channing can be used as DHW heater out of the box but if Alasak doesn't offer that option there are add-ons available from third parties.
Others have even made them...
Check this article for one idea on how to set it up: http://nepacrossroads.com/kb/Hot_Water_Coil