Please help! Bathroom heat duct question.

[Allie]

Since this is a heat question, I will lead with we live in Alaska, where it gets pretty cold.  We have heat but no AC, so cooling isn't an issue.

We just started pulling apart our smallest half bath.  It sits off the main hallway, pretty close to our great room in a warmer area of the house.  When we removed the vanity, we found a heat duct opening.  It wasn't grated or rectangular like the other openings that are meant to heat the home, it is about an 8 inch circle with some mesh over it.  It went right under the vanity.  Any hot air would be going into the 6 inch tall enclosed base of the cabinet. 

Under the house, after some detective work, we discovered that the duct splits into two at the very end of the run.  One side goes into the master bathroom (which is always freezing) and the other turns into two smaller circular ducts that blow hot air under the half bath vanity and into the void around the jetted tub in the master bath.   

I can kinda sorta see the reason to heat the jetted tub area.  We are planning to redo the master bath as well, so that may come out anyway.  But, is there any reason the heat needs to go under the bathroom vanity?  We live in a very cold area, so I was thinking maybe it was to prevent pipe issues, but the heat isn't being directed towards the pipes, just into the empty, enclosed space under the vanity. 

Does anyone have any idea why the vent is there?  Do we need to keep it open?

I would love to close both of these off and try to get more heat into the actual master bath.  Any thoughts?

[Spork]

I'm not in construction and don't know jack about super cold climates...  But I'll go out on a limb and say your assumption is correct.  I bet its to keep pipes from freezing.  In our wimpy southern climate, we just open the cabinets when it freezes so the house air can get next to the wet wall.

[Bob W]

It is common practice to vent under vanities.  Typically there is a vent opening at the kick plate in front of vanity.  It helps to keep you warm when using the sink.

[Allie]

It is common practice to vent under vanities.  Typically there is a vent opening at the kick plate in front of vanity.  It helps to keep you warm when using the sink.

Maybe they just forgot to put in the vent in the vanity? 

So, since this is in a warm part of the house, a bathroom you wouldn't use naked, and was never vented before, we could close it off without too much issue.  I can't see the pipes freezing in his particular room without some serious issues with the heating system...

[frugalfighterpilot]

If it's in a central part of the house with no chance of freezing (i.e. not open to an unheated area in any way) I say close it off and get more hot air into the master bath.  If however, it is near an outside wall/open to a crawl space/etc. then the warm air on the pipes is probably a good idea.

I'm not a professional, so take my advice with a grain of salt...

[Greg]

Likely the vanity installer or trim carpenter forgot to cut the vent in the toe kick panel.