I'm trying to get my furnace to jive with the btu calculations to make sure I don't under or oversize the next boiler we put in. I want to add radiant heating to 6 rooms in my house also. I'm renovating the home and the look on the hvac guy's face when I ask them these questions is priceless, and no one can seem to give me a definitive answer. They'll say roughly 30k btu or 40k btu. I'd like to have it more dialed down, so I know I'm not going to have problems in the future. The HVAC guy was telling me that he's putting in this coil shw-2347 and a smaller one for a 2 ton system. I'm just not 100% sure how to calculate the amount of btu's it would need to run from my boiler. I know there's going to be some head pressure, but if the temperature drops. I want to make sure the pump and boiler can keep up. Any advice would be appreciated. Here is the spec on the coil itself http://www.builditsolar.com/Projects/SpaceHeating/SHW-2.2-4-6-09(1).pdf

The answer to your questions starts with a study of how much heat your structure looses to the outside environment thru the walls(total area applied to R value of the wall/floor/ceiling components), windows and doors(Total area applied to Rvalue/Ufactor) and thru air exchange(% of building air volume exchanged per hour) for your average environmental conditions when maintaining a desired interior temperature.

Untill you know this information, everything else is a WAG(Wild A&& Guess), which is what it sounds as if the HVAC guys you have consulted with are doing based on experience with homes near your size...

yes, I'm not talking about a heat loss calculation, I'm talking about the amount needed to heat the coil sufficiently so the blower will provide sufficient heat.
How much BTU this will take so I have enough to power the rest of the system, I'm looking for that specific calculation.

If you have a heating history on this place, it's possible to calculate an upper bound for the whole-house heat load based on how much fuel the old boiler used. To get there we would need the boiler's input BTU and DOE output BTU, your ZIP code, (for weather data and outside design temperature), and some mid to late winter fuel bills with EXACT meter-reading or fill-up dates.

The amount of heat you get out of the air handler coil is primarily a function of the water & air temperature, with air flow & water flow being secondary effects with the water flow less of an effect than the air flow. I have an air-handler coil rated for something like 80,000 BTU/hr with an EWT of 180F at it's lower speed, 100K at it's maximum speed. At the ~125F and blower speed I'm using (it's lowest) it's good for maybe 42-44,000 BTU/hr, which is still overkill- larger than my whole house heat load, and I have other radiation on other zones.

If you don't know how much heat your hydro-air zone needs, there's no telling what water temp you'll need, to deliver that amount. Entering water temps much below 115-120F can be uncomfortably tepid on the receiving end at the register, even if it's delivering the heat. A coil rated 40,000 BTU/hr @ 180F EWT (with the return air at 60- 65F) delivers about 20,000 BTU/hr @ 120F. A coil rated 30,000BTU/hr @ 180F EWT would deliver 15,000 BTU/hr @ 120F EWT. If the zone needs 15,000 BTU/hr at your 99% outside design bin, either one is going to do just fine, the only difference is that the 40K coil can deliver it at a lower water temp, which affects the efficiency of a condensing boiler.

So, to get it more "dialed down" you need to know...

... the heat load of that zone first...

...then the water temperature you're planning on running (which may be variable)...

...then the cfm of the air handler...

...and WAY down the list comes the water flow and how much pump it takes to deliver that with the distribution plumbing and coil you're using.

The tables on page 4 & 5 of the spec you pointed to should help you figure out what it takes to get the heat out of the coil and into the rooms, but you need to run a Manual-J or IBR load calc to figure out how much heat the zone really needs.