Question-and-Answer Resource for the Building Energy Modeling Community
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Certainly you would want to combine them all together. In terms of delivering the heat to the zone, it doesn't matter if they are a bunch of little radiators, or one big one - the heat output into the zone will be the same.
If its 100% necessary to account for lost heat through the pipes, I personally would do it as an external calculation, to increase the overall heating energy by a certain small percentage. I would not change the efficiency of the boiler. The argument that you could make is that the heat lost through the piping systems is still contributing to heating the building, by radiant heat from the piping. It's not really "lost" heat.
