No doubt this is "dumb" question, but where does the design delta T of 10 degrees come from for radiant floor? And what does it mean exactly? I assume that it means that there will be a 10 degree drop in temperature between the water leaving the GARN unit and the water returning. If this is the case, then how can you always use a delta 10 when the return temperature would vary depending on length of run and efficiency of the house overall would it not? Given how this number is important in establishing flow rates and hence pipe sizing, I am just trying to get my head around this number. Any help would be appreciated. Thanks.

  Normal 0 MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman";} I applied the delta T to the supply temperature entering the supply on each radiant floor zone and the return on that same zone at the manifold. I have DHW and a coil drawing heat from the supply before returning to the boiler plus heat loss from the pipes on the loop which would create false readings.

Hi Sawyer: I appreciate your response but unfortunately am still unclear as to where delta T comes from and how the rule of thumb of using a delta T for radiant floor came from. Are you suggesting that a delta T of 10 is for each loop (zone)?