Dave probably nothing you want to hear. But here goes.
We used to have a saying we can do it cheap, we can do it fast, we can do it right pick one.
A heating system requires the most field engineering of any system in your home. The heating system is comprised of individual components and those components must be assembled into a system, if all of the components are sized and fit together correctly you have a system that keeps you comfortable and runs efficiently.
When you try to replace only a part of the system you might be able to compare that to attempting to put a modern engine in a Model T without changing the transmission, rear end or anything else. Could you do it? Probably Could you ever consider it a well-designed efficient trouble free automobile?
With a two zone system such as yours you need to know how many BTU you need to replace the heat losses that escape from your house (the heating envelope) during the worst expected weather conditions to size the heating zone. The next thing you need to know is how many BTU are required to heat the domestic hot water in your indirect loop so that you can size the indirect zone. It takes 8.34 BTUs to raise the temperature of 1 gallon of water by 1 degree, in my area we calculate the incoming supply water at 55°F so if you want to raise the temperature of 40 gallon of water from 55°F to 120°F in one hour would require 21,648 BTUs/Hr at 100% efficiency.
Some people will use a rule of thumb based on your area and the construction of your house they will say that you have X square foot of either “tight” or “loose” house an use a questimate of “X” BTU per Square foot. Usually people using this method are guessing and often oversize the equipment. If you are doing cookie cutter houses in a development where are the same except for exposure direction you might get away with it but again you might get burned on some houses that have radically different orientations, or wind exposure etc.
The correct way to do it is to use Manual J program to calculate your heat losses and then to add in the BTU required for the indirect domestic water heating, factoring in the efficiency of your indirect loop.
Now in the rooms being heated you need to size the radiators or floor loop to replace the BTU’s lost from the room’s heat losses at the design temperature for the lowest average temperature for your area.
Let’s disregard the people skills and lay it out like it is. Dave you are an intelligent man and you realize you have a Kludged system, you have made it work but you suspect the present boiler is oversized. What leads you to that conclusion, does the boiler short cycle on cold days when you are using domestic water? If you are going to spend around 5K$ why not spend a little more and have an engineered system that will give you the most bang for your buck in terms of comfort, efficiency and longevity. I think you can change just the boiler and Kludge around and make it work but it will most likely never be the best system you could possibly have, it very well may end up in the category of making do.
Can someone take the BTU capabilities of your registers and floor loops plus your hot water demand and guestimate a boiler size? Probably but at best that is a questimate based on unknown factors, are your registers and floor loop size correctly sized in the present system?
Dave I can suggest a link where the wet heads hang out(Heating Help {the Wall}), these guys really know their stuff on Hydronics and Steam and they can answer specific questions but you need to do your homework. If you give them specific facts and ask a question you will get an answer if you ask vague questions with no data they won’t treat you kindly., so you need to be prepared to answer their questions to get any help from them.
http://www.heatinghelp.com/forum-category/76/THE-MAIN-WALL
