I installed a new oil fired boiler. Once the unit kicks on, within a minute, all the water is out of the boiler. The unit shuts down (low water cut off) and the water returns within 1-2 minutes and the cycle starts over. The water is not all turning into steam. It's being pushed out of the boiler.
The hartford loop is correct. The other piping seems to be correct. What the heck is going on?

Check for blockage in system, possably the pressure valve or check valve is deffective.
As the water is heated it should pressurise the system the water is forced back out into the main without a check valve Your system may have a pump for pumping fresh water in(overcoming the pressure differential)it could be deffective.

You could be right about a faulty pressure valve. I am checking those (1 on each return line, 2 zones). The water is either being pushin up into the radiator piping through the feed or return line (2 pipe system).
Is a check valve required on the return lines?

You need to check to see if you have a pressure reducing valve on the water supply to the boiler. The pressure should not exceed 12 to 18 psig. If there is city water pressure, then the boiler pressure relief valkve will open. Remenber, as the temperature increases so does the system pressure.

This is a steam boiler. The water supply is completely shut off except when calling for make-up water. The water is pushed out of the boiler while the water is off.