I had the same problem with my Shaver 165. I came on here and asked the same question. I received many answers some right and some made me do a few changes and work through the issues. I heat my 2800 sqft house and my 1200 sqft shop. The house is forced air and the shop is all radiant heat. I have temp gauges in my out going and incoming lines. Before the water left at 140 degrees and returned 70 degrees when the floor was on. Now the water leaves at 165 degrees and returns at 140 degrees. The stove can recover that much with out a problem.
I did the following to solve the issue:
1. I installed a bypass loop before the floor controls.
2. I opened the mixing valve on the floor all the way open, allowing the most return water back in the floor. This did alot of things including making the floor cycle more.
3. Installed a Ranco Control. I set it at 165 off with a 10 degree dif.
4. I installed a 75 cfm fan with a solenoid damper.