The first question that needs to be answered is... Is the current boiler powerful enough to supply heat to both the home AND the addition? Was a heat loss calculation performed at the time the changes were made? People I knew some years ago added on to their house. The husband was very handy and did all the work himself. Passed inspections with flying colors. No problems, right? Wrong. The boiler was barely big enough to provide heat to the existing house. When they added four rooms and close to 1,000 square feet it totally overwhelmed the boiler's ability to heat the home. Luckily they were planning on a boiler replacement within a couple of years, so in the spring, after a rather uncomfortable winter, they had a properly sized boiler installed. If the boiler is undersized, there's not a lot you're going to be able to do short of adding more insulation, shutting off rooms that aren't used, and investigating supplemental forms of heating.

Don't want to wire all three to match. It would require pulling new wiring two two zones and would require finding thermostats that would do it. An easier solution would be to remove the burner control wire and cap it off. It would do essentially the same thing, I think. The main zone and the front zone were added at the same time when the house was converted to forced hot water in the 1950s. The back zone, for the kitchen only, was added in 2003 when Mom and Dad redid the kitchen. I don't know exactly why my Grandfather (Dad and Grandpa installed the boiler in 1954ish and as far as I know did the wiring) wired the front zone differently from the main zone, but I suspect it might have something do do with the fact that the front zone controls rooms that he and Grandma really never used unless there was company. What I'm finding to be odd is that no one I've talked to in the HVAC community has ever seen an EL terminal on an R845A controller. I'm going to try calling Honeywell and see what they have to say.

This is very common in boiler country. It is called an indirect water heater. There are many manuf of these tanks. I would not increase the size of the boiler for this. You pay a penalty in oversizing the boiler and you can achieve the same thing with a "domestic priority control". Essentially, when there is a call for hot water the heating zones close and the boiler capacity is converted to dhw recovery/output. I should ask, how big is the house, heat loss calc. Number of baths and any high flow fixtures, etc.

Wow, 4-8 degrees is a lot and way too much. Was the sunken room added later? This room has three exterior walls, I bet. Zoning can work, but running a single room, completely separate small zone with a big boiler can lead to awful short cycling. If an efficient boiler is going to be installed eventually, a low mass model, it won't work well at all. What kind of fuel is used for the boiler? Natural gas, I hope. The best approach might be to improve the insulation and air sealing in the room. If that is not enough, adding more emitter need not be very expensive. In 1961, most probably Al fin-tube convector was probably used. You can add length by lining more wall with added convectors. You can put in higher capacity emitter of the same length. It will cost more, but you can add panel radiators or take out the baseboard and install all panel radiators. It is a sunken room so maybe you bring the floor level up a little and put in radiant floor heat, but this will be the most costly. It will, however, be the most comfortable heat you ever had if done right. If you add radiators and keep the existing zones, thermostatic radiator valves (TRV) can add some control for the temp in the individual room -- not too costly probably. No matter what you do to add hydronic heat to this room, you should hire someone that is knowledgable. It might be simple, but if not and it is goofed up, you might cause other problems and be worse off. A good hydronic installer might come in and quickly determine that your system is designed with a very large heat drop from boiler outlet to boiler inlet on the other end of the loop. If the bedroom in question is the last room in the zone, the simple solution might be to move that room to the boiler end of the loop meaning that it gets hotter water (maybe 20 degrees hotter) and more heat. Calculations will immediately show if that will work or it won't. Back to the beginning, insulate and improve air sealing first for the biggest bang for the buck, especially if you have fuel oil or propane heat. Here is an article that I just ran across. It is more about other issues than adding heat to a single room, but there is a section about added emitters including several approaches illustrating the questions involved. It might help you and others. don't get too bogged down in the calculation detail if you don't want. The diagrams and descriptive prose are pretty easily grasped. http://spacepak.com/modules/news/upload/%7B7C28944F-EA07-4E17-B21B-79B37040FA07%7D_water_works.pdf If that link does not work, search "Authored by John Siegenthaler Lowering Water Temperature in Hydronic Heating Systems" Sieganthaler has written many good articles about hydronic heat systems that are pretty easily grasped by people that are not engineers.