Hello caper, so you don't have any automatic control of the air to the fire?? It is only manual?? If this is so, then it is going to be difficult to control your fire and the resulting water temperature.
Coal goes through several distinct stages.. With bituminous they are more noticable than with anthracite. First there is the fire from fresh coal,, it has volitiles burning off, with Bituminous, this is the gray/yellow smoke and heavy yellow flames and sooty fire.. then the fire burns down to a hot red mass of coals.. With bituminous, sometimes, depending on the coal, this stage of burning has the coal swelling and sticking together, making a big sheet of coal with the fire underneath. You have to break up this 'bridge' of coal to let air through so the upper layer of coal can burn... Then as the coal burns down, the mature stage of the coal is characterised by less heat output, the fire settling and the ash blocking some of the air flow through the grate..
All through the above stages the amount of air needed to maintain the fire varies. and the heat output varies.. so unless you have a way to adjust the air to the fire, it will be difficult to get an even water temperature..
What you may want to do is to just load your fire up very deep, add as much coal as you can,, and once the fire is established, cut back the air to a minimum, and see if using just the air inlet you can keep the fire burning at a low heat.. then just keep adding coal,, shake the ashes down, then add more coal.. many coal burners start just one fire for the year,, and keep it burning all winter long..
Hope this helps..