I've started assembling my Jetstream boiler, I cemented the tank to the combustion chamber, installed side panels and the control panel. Looking through the manual I was trying to spot any allowance for cold water protection in the plumbing, here's what I found: None. The reason why this works is the boiler holds about 100 gallons of water and is fitted with a high and low temp aquastat. The high temp (230 f on the 30 psi model) will kill the combustion blower to prevent boil over. The low temp prevents circulator operation until the tank reaches 170 f. The return water is connected to an area of the tank where mixing can occur. So, from my first guess, it would be hard to have sub 140 temps in the boiler without tripping the 170 f low temp shutting down the circulator. I know of one that has been in operation for almost 30 years, year round, and he hasn't had any problems yet so hopefully I'll be ok!