I am heating my house with a Traeger TPB-150 boiler.   This boiler is built to primarily burn corn or cherry pits but will also work just fine to burn pellets.  It differs from the standard pellet burning boiler by not having electronic ignition.

The boiler is tied into the radiator system similar to the diagram below, which is from the Traeger installlation manual.

My setup is slightly different than this. I have two zones in my house with both of my pumps in the Option 1 spot. I also don't have a domestic warm water system, so I don't have the tempering valve setup. But I do have the domestic hot water setup. I use the pellet boiler as a pre-heater to my electric hot water heater.

All of this is tied in series with my conventional, natural gas burning, traditional boiler. The natural gas boiler is controled with an aquastat. The idea is, if the house is calling for heat, and the pipes are cold, maybe the pellet fire is out and then natural gas boiler kicks in. In practice, it doesn't really work very well. I wouldn't build it this way again. It is hard to get the aquastat set right, and it seems like in a system without a buffer tank, the pipe temperature drops too much and the gas boiler was kicking in too often.

A much better system is to have two thermostats wired in your house. The first one, set to some comfortable temperature. Around here, that is 72. Wire this thermostat to your pellet burner. The other thermostat, set to some minimum temperature reading you won't want your house to fall below. Say, 50 degrees. Then, if you are gone for the weekend and the pellet fire goes out for whatever reason an hour after you leave home, eventually the gas boiler will kick in and at least keep the house from freezing up.