On a previous post, the home owner stated the heat load was 48K BTU/HR.  Now here is the question.   Do you size the loop for the unit or for the heat load?  If the unit, why?  If the heat load then why?

A 3.5 ton unit (about 52K BTU) should be right for the 48K load).  Of course, a 3 ton unit with some minimal use of back up heating might be optimal - especially if the cooling load tended to be really low - say 24K BTU/hour.

If you install a 3.5 ton unit would the loop be sized different than for a 4 ton unit?  If yes why since the larger unit just cools the house faster.  The water temperature might get a tad cooler due to faster heat extraction but on the other hand it might not since the compressor would be providing more heat.   If the heat load is 48K BTU / hour does it really matter if the 3.5 ton unit runs 55 minutes or if the 4 ton unit runs 48 minutes?   The extra 7 minutes of no heat transfer should let the heat disappate.

Thoughts?  Comments?   Observations?  Snide remarks :)

You size the unit for the heat load at a given ewt. The ewt that you selected for a given unit dictates the amount of loop that is installed to achieve a given ewt for heating. In my kneck of the woods we over loop to cover a multitude of errors along the way on the design and unit side of things. Over looping results in insanely low ewt's as compared to other areas where loop lengths are designed per system. Hope this helps.