Mystery vent in floor above crawl space

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Alleycat

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I posted this thread in the HVAC forum too, but thought someone here may have ideas too-

We have a new-to-us log cabin in central Maine. It is 20 years old, built on a crawl space which has a plywood skirt around it, with blue foam insulation inside the crawl walls. No insulation under the floor. The crawl is about 3 feet deep. All plumbing is in the right corner of the house: full bath downstairs that also houses the hot water heater, kitchen sink in adjacent room, 1/2 bath upstairs right above the downstairs bath. Heat is propane by 3 rinnai heaters: 2 in main downstairs living space and one small one that was recently installed in the downstairs bathroom.

We plan to spend some weekends here in winter and thus leave the heat at about 60 while we're not here. We shut the water off when we're gone and have an automatic generator.

Plumbing consists of a well line coming in to the plumbed corner of the house, via the crawl space. The crawl space is accessed through a trap door in the center of the bathroom floor. About a foot away from the trap door is a vent covering another hole in the flloor, about 4 x 6 inches, and that hole sits about 6 inches outslde of the hot water heater closet. The previous owner, who lived here year-round, had installed a little fan in the hole under that vent. We thought this might have been done to draw heat into the space during cold snaps but his widow now tells us she thinks it was to avoid moisture down there during one humid summer. Neither theory makes much sense to us.

Also, we are discovering that the Rinnai in that room cycles over and over, even at 60, and it seems like the heater may be drawing cold air up from that flloor vent as it runs. The heater is only about 2 feet from the vent

Any ideas what this vent is there to accomplish and whether it should be blocked off for the winter?

Many thanks for any input.
 

Kcodyjr

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Short term, I'd try turning that fan on, if indeed it blows downward, and see if the heater stops cycling.

Handwaving theories, and assuming it does indeed blow downward:

It could be intended to create a positive pressure in the crawlspace, so drafts have a harder time getting in through the outer cracks, or so any upward drafts back into the house are less cold and dank.

It could be intended to create a negative pressure in the room, for example to hide pipe smoke, or to cause air movement between rooms.

It could be intended simply to exchange the air, for example to suppress moisture buildup, remediate a radon problem on the cheap, or perhaps to grow plants in the crawlspace.

There could have once been a duct of some kind attached to the bottom of it.

He could have been trying to mess with the next owner's head.

He could have been flat out senile.

EDIT: more theories.

He could have been working down there, and wanted a supply of fresh air.

He could have suspected an issue with the heater, and wanted heavier gases removed from the room.

EDIT another idea...

That fan might be the only thing in the house causing air circulation. It's a poor man's approach if that's the case. If the heater does stop cycling in response, this would be the theory to pursue, and doubly so if the fan is blowing upward.

Note that all of these theories address the question of why the previous owner installed it. I've taken the assumption that it might or might not actually do something, which might or might not be what it was intended to do. It's yet another question whether its intended purpose was a good idea, and still another whether its actual functioning is a good or bad thing.
 
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hj

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The first "red flag" would be the statement that the water heater is in the bathroom, which is NEVER allowed, unless it is a sealed combustion unit, which the Rinnai could be, but then it would not be affected by anything that happens inside the building. There is no way for us to tell why that fan was installed, so all we could do would be to make guesses.
 
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