[Greenbuilding] Question on tight house, carbon monoxide

[KTOT (g)]

I have a very tight home built last year in a winter climate. I'm totally 
off-grid with propane refrigerator and stove. Both those were brand new (and 
high end) a year ago. My heat is solar thermal with a backup on-demand 
Rinnai propane heater that pretty much never comes on and is located far 
away from the main floor anyway (and a CO alarm in the utility room never 
shows anything but zero).

Every time I make soup or jams (or other canning which involves sterilizing 
then heat processing the jars in boiling water), both of which involve 
boiling liquid for multiple hours, my digital CO alarms read above the safe 
levels so I end up opening doors, even on cold winter days and nights, to 
let the CO out. I do keep one alarm near the kitchen, but the one in the 
bedroom also gets very high readings. These are both brand-name, brand new 
CO alarms as well so I trust them (plus once when I forgot to check them, a 
regular CO alarm did go off in the same circumstances).

I do run the exhaust fan, which is the kind that has a filter but is not 
vented to the outside (due to the location of the stove and fan).

I can cook other items or use the oven for hours on end with no CO # showing 
on the alarms (which doesn't show a reading until CO hits 30 ppm at least). 
So I don't think the stove is leaking CO.

Is this normal? Why is water or soup liquid causing a CO alarm to have high 
readings? Any insights?