If the boiler is gas fired (as opposed to electric) it still requires combustion air and has a chimney. Negative pressure in your home (from too many or too large of bath fans, dryers, kitchen fans etc) will suck air that is trying to be vented from your boiler into your home. That air, once burned 2X creates carbon monoxide, deadly.

you made reference to a sump/equipment room in your first post.

anyways, portable dehumidifiers are somewhat small perhaps one could fit in the enclosure or another strategic location.

just another angle to consider, strip the moisture from the air instead of exhausting the air.

Would house air vents solve this?

Thats why i thought i would clarify the "room" more of a large closet space and will be lots of space under tank. I dont think a dehumidifier can handle the volume?

I understand. The boiler is on the other side of the house and I will not be able to tee into its vent. If there are house vents or more added to the general area where the tank fans are sucking out, this should solve the pressure issue?

I was actually looking at getting one myself and for $200 you can get a portable dehumidifier that is rated for a 2000+ sf home: http://www.sears.ca/catalog/dehumidifiers/12996

I think that rating is based on a regular humid house, not one with a 425g tank? I have read that dehumidifiers have not worked for people with large systems due to the amount of evaporation?

Just came across the following;

I can have access to two other vents coming from a gas fireplace, I imagine one is an intake and the other is out?

This has been mentioned earlier, but...
If you have a bathroom fan on 24/7 you are exiting warm air, and cold air is coming in from outside to replace that air. You have to heat all that cold air. Not very cost effective method.
Even if you put in a vent in the closet that replaces that exhausted air, your chimney from your boiler might flow backward and you could get carbon monoxide poisoning. You should talk to a heating specialist.
You want to have + pressure in a house, not negative.

Once you get the pressures figured out, there are humidity control units that will only start the fan at a preset humidity level.

Just took mine out of comission, doesn't cut it in cold weather with large tanks.