We are moving to a house with in-floor heating and it has been off all summer. We turned it on and we cannot get the return water in the pipe to heat up. All zone valves are open, I replaced the thermostat valve, boiler (munchkin) running fine, have flushed the lines for air bubbles. Etc . we can't figure it out, and snow is going to fly. Please help. Iam new to this system but have been doing my best until now, I could use some help. Thanks

You want to find a contractor that has a positive attitude about Munchkin boilers or condensing boilers in general and have them over to do a proper clean and check which will include flow checks and should include a combustion analysis. Most condensing boilers are installed without the benefit of the installation manual and nearly all without anyone filling in the installation check list.

This is not a DIY project as a condensing boiler starved of water flow will likely live a short life.

Definitely something you'll want a contractor to take a look at. It could be something as simple as a part coming loose or needing to be replaced. How old is the system?

Is there heat at the supply to the manifolds?