Locate Return Recirculating Line
My 10 year old house was built with a dedicated hot water return line for a hot water recirculating system. The timered Grundfos pump is at the 75 gallon water heater. Since the water heater is just about ready to give up the ghost I am replacing it with a tankless. Because of the associated problems with tankless and recirculating, I am abandoning (removing) my my Grundfos and installing a D'mand pump at the master sink (that's the only place the recirc goes). I would like to use the return line instead of the cold line to plumb the D'mand. Where do I look to find the return line to splice into it. I have a slab construction and the copper tubing is in the slab. Would it have been common practice to stub up the two supply lines and the return line all together? In other words if I start tearing out sheetrock under the bathroom sink, is it likely that the hot and return are tee'd right there. I'm not asking for clairvoyance, just your opinion on where to start cutting.