We got a call for radiant heat not working on a new house. The plumber who installed it was nowhere to be found (and for good reason). We still are not sure exactly what he did but it appears he did all the radiant on the first and second floors in one loop right into the boiler, no manifold, no mixing valve, nothing. Who knows he might have teed off the 3/4" with 1/2" or something else wacky. We are talking about half a mile or better of radiant tubing (it's a huge house) in one loop.