I have a 2 yr old Laars Max Boiler 75, 90K Btu/hr.Beckett AFII 150 burner. Never noticed until now, white smoke coming out of chimney. Smokes the entire time the burner is on. Is this just water in the tank...?

 

Chances are that it's 'steam', but not from water in the tank... when hydrocarbons burn, the hydrogen and carbon chains are broken, the hydrogen then bonds with the oxygen in the air and forms... H2O ! water... but it's superheated steam until it exits the chimney and cools down... and then it becomes visible steam, and then it evaporates...

Unless there is a HUGE white plume, it's probably nothing to be concerned about...

 

Look at that plume closely - with binoculars, if necessary. Ideally, the first few inches above the chimney outlet will be clear - proving that there is no condensation inside the chimney. This may be difficult to discern if you have a large, screened chimney cap. The visible plume appears after it mixes with cold, ambient air up to the saturation point shown on a psychrometic graph.

You can compare the length of the clear portion of your plume to that of your neighbors'. Those with the longer clear lengths have exit temperatures that are hotter - and lower combustion efficiences.

Check this out under various ambient conditions of temp and humidity. But, to be valid, observe after the burner has been firing a few minutes (after the flue has warmed up). Initially, right after the burner kicks on, there will likely be no clear portion of the plume. Much like a car's tailpipe exhaust, just after starting out on a cold morning.

 

Hi, Actually, i do have a large screened chimney cap. I had problems with condensing in the chimney last year and I had a SS chimney liner put in. Solved the problem. But there is a large cap.
Unfortunately most of neighbors have electric or woodburners. But I will try to check w/binoculars anyway. Thanks.
I have observed when burner is on... always smoke. BTW, the burner on;y comes on for 30 seconds to 2 minutes and then shuts off. No long continuous burn...

 

What is outside temperature and humidity?

 

I would just check the boiler for leaks. Shut off the make-up water and watch the pressure for a few days. Do it on a weekend or when you are off. Check often at first. If the pressure holds it is a combustion issue and may not be a problem.