Heating Help

Two Stage Burner

I have a Hydro Therm Hi85 boiler, it has a two stage burner.
My question is what makes my boiler switch down from High(85,000btu) to the lower(50,000btu) burn? Suppose to come on at High if needed (85,000btu) and switch to Low burn (50,000btu) after catch up. Will come on in Low if just warming water. 
Also is it more economical to run on auto or switch to Low? Low has a hard time keep up when weather is cold, will run often and long time.

Burners:

It's always been my understanding that the perfect "sweet spot" in heating is when the outside is cold, at design temperature, the circulator should never stop, the burner should never stop, and the structure temperature should hold at the setting.
Under that supposition, your system is running at close to maximum efficiency when on low fire. If you switch to high fire and the burner is cycling, you are wasting fuel. I'd leave it on low fire until the burner can't keep up.
But, that's my opinion.

If you

are asking what control makes the circuit, it's hard to read the diagram (too small) but generally speaking there is a 2 stage aqua stat.

The way it generally works would be more than say 20 degrees from set point, both stages run, but as the temperature nears set point, say within 10 degrees, highfire will drop out.

The set points and differentials vary, and on most models are adjustable.

Hard to read the

diagram, FAX me a copy at 401-270-1893 and I will take a look at it and see if we can put your mind at ease.