I am a homeowner here in New Hampshire. I found this site very helpful during my recent installation of a condensing boiler, and I hope someone can offer some insight to a problem I'm having after commissioning. Bosch Greenstar combi 100 boiler, model ZWB28-3A. 3 zones with Taco IFC pumps on the supply side of the secondary plumbing, and regular thermostat/relay control. I followed the installation and applications manual very closely, but I'm having an issue when I open the Webstone tee to allow the primary/secondary flow to happen.
When I have the tee closed (see pic), and only have 1 zone open (remaining zones shut off via full ball valves), the boiler works awesome and heats either zone up quickly and maintains the temperature very well. When I open the tee valve to allow full primary/secondary functions with multiple zones, the flow seems to bypass the boiler and flow only in the secondary circuits. The temp reading on the boiler says 190 degrees +, but the pipes cool down and no heat is produced, and the flame icon goes out on the boiler. It's almost like the internal pump on the boiler is not running so the water takes the path of least resistance and bypasses it altogether. When I close the tee valve to force water through the boiler, it again works great. No fault codes are ever displayed.
Some background info: I bought the unit secondhand, but it was new in the box and never installed previously. I am not a licensed plumber, but in NH you are allowed to perform these types of functions on your personal house that you occupy, and I've worked as a plumbing helper for many years in my younger days so I do know the basics of plumbing.
I've gone through the manuals and searched online, but any issues with any similarities has to do with the plumbing done wrong and no primary/secondary tees installed. Since it is essentially a brand new unit, I would not think the internal boiler pump failed right out of the box, but I am not sure how to verify it's operating correctly. I wonder if there's some kind of special set up configuration, but all the manual says is there is a way to adjust the pump speed if needed.
Thanks in advance for any help before I have to call in a professional who is experienced with the Bosch boilers.