Basement Bathroom HVAC Question

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MikeBPrice

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Hi all. Longtime lurker, first time poster. This forum is immensely helpful and I appreciate any and all input.

My scenario:

New construction house (2013). Unfinished basement being partially finished (by me) to add a great room and a bathroom. Bathroom is plumbed/framed/electrical finished. 50 CFM ventilator over the shower. Up until this point I have not considered adding an HVAC register to the bathroom. However, it seems like it would be very easy to do since there is a round duct running parallel to the length of the bathroom between the joists above the bathroom. The duct services one register in the dining room above.

My question:

The easy fix seems to be a saddle tap into the duct and add a register to the bathroom ceiling. What are the pros and cons of doing this? I propose to add a 90 degree tap instead of a wye which seems (in my untrained HVAC eye) like it would bring a bit of air to the bathroom while allowing the bulk of the air to bypass the tap with minimal reduction in the air servicing the register to the dining room above.

Does this sound reasonable/doable? Recipe for disaster? Your input is appreciated.
 
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Stuff

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Leave the dining room feed alone and address this as part of the ductwork installation for the great room.

Most building codes require heat for bathrooms. If you don't provide a vent then you could do an electric heater.
 

MikeBPrice

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Leave the dining room feed alone and address this as part of the ductwork installation for the great room.

Most building codes require heat for bathrooms. If you don't provide a vent then you could do an electric heater.

Thank you for your reply!

Interestingly enough,when the house was built the unfinished basement actually had ductwork with HVAC registers installed in a few places around the future great room area. Not sure if that was a mistake and they just went with it, or what. Nonetheless, saved me some work for this project. But I wasn't so lucky with the bathroom. So, to your advice, I am sort of back to square one on what to do with putting a register in the bathroom.

If you or anyone have any additional advice I'll certainly take it!
 

MikeBPrice

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A duct in a basement bathroom is of little use. An electric heater or heat lamp over head will do it. The fan and the light should be switch together.

If I understand you correctly, it sounds like the only consideration really is heating? The bathroom does share one exterior wall (behind the shower) but we've never experienced a "cold basement" in the winter (insulated walls). Honestly, I doubt the bathroom will ever seem cold either. But, in any case, my general thought process is that a register will provide heat and AC.....if for nothing else to move and condition the air in the bathroom to keep it from getting stagnant (in addition to minimal heating/cooling)

So, I guess my main question is - will I completely disrupt the balance of the dining room register above (or the whole system in general) by tapping into the duct that services that register? I'm ok with minimal siphonage, but if something like a takeoff from a single duct will kill the airflow to the dining room (or destroy the whole system), I don't want to do that. Any input?
 

MikeBPrice

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The bath fan is all you need. As stuff suggests, leave the duct work alone, or hire a professional to help.
Ok, so the main takeaway so far seems to be no need to put an HVAC register in the bathroom. Or get professional assistance in doing so.

From what i've read, the code doesn't necessarily consider a bathroom a "habitable" space, therefore an HVAC may not be required. The other 3 bathrooms in my house each have registers, including the powder room (interior to the house) and the separate toilet room in the master bath.

So, if I may posit the question this way: Assume the register will be installed by takeoff from the round duct above. What effect, generally, can I expect to take place on the dining room register above and/or the HVAC system as a whole? Anything? Lots? Nothing?

Sorry to be particular. Just want to get this right, but don't necessarily see it as a project requiring an HVAC company to do it for me unless we're talking a potentially detrimental effect to the house HVAC system.

Thanks!
 

Dana

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Whatever else you're doing, if the foundation isn't insulated, make sure you insulate it to at least IRC 2012 code minimums before closing it all in.

And don't just slap up a studwall stuff with fluff or you risk getting into the mold-farming biz. Some amount of air-impermeable insulation (read "rigid foam) between a studwall and foundation is necessary to control both groundwater and summertime/wintertime air condensation issues, which can't be done with interior vapor retarders the way above-grade walls can. In MD (zone 4A) a skinny inch of EPS and a 2x4 studwall with unfaced (or kraft faced, but not foil faced) batts will be sufficient- even R11s would do, given the R4-ish foam. At 1" 1.5lb density "type II" EPS runs a bit less than 3 perms for vapor permeance which would be sufficient to keep groundwater moisture from reaching mold inducing levels in a latex painted wall. You could use cheaper Type-I EPS if it has a facer to bring the vapor permeance down. Type-I EPS runs ~R3.8-R3.9 @ 1", Type -II runs ~R4.2-ish @ 1". both are pretty cheap, maybe 45-50 cents per square foot at box store retail, cheaper through distributors.

Tape the seams with housewrap tape, and use can-foam to seal it to the foundation sill.

With an IRC 2012 code-min insulated foundation the heat load of a basement bathroom with a small-ish code min window might be self-heating, when occupied by a 250-350 BTU/hr human.
 

Stuff

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The problem you have is to get it right you can't just look at the one vent. With balancing a home's HVAC you need to look at everything to do a good job. Tapping a supply can work but lots of times causes other issues as most times it was undersized to start with. Since you are finishing off a great room as well you should get that reviewed at the same time.

Otherwise everyone is guessing without knowing details. What size and length is the duct to the dining room? Where does the duct originate? How many elbows, type, and location does it have? Solid or flex duct? If flex is it sagging anywhere? How well is it sealed? Is there a damper and what position is it in? Does the dining room have a return grill? Can you upsize the ductwork between the bathroom and the furnace/air handler?
 

MikeBPrice

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Whatever else you're doing, if the foundation isn't insulated, make sure you insulate it to at least IRC 2012 code minimums before closing it all in.

And don't just slap up a studwall stuff with fluff or you risk getting into the mold-farming biz. Some amount of air-impermeable insulation (read "rigid foam) between a studwall and foundation is necessary to control both groundwater and summertime/wintertime air condensation issues, which can't be done with interior vapor retarders the way above-grade walls can. In MD (zone 4A) a skinny inch of EPS and a 2x4 studwall with unfaced (or kraft faced, but not foil faced) batts will be sufficient- even R11s would do, given the R4-ish foam. At 1" 1.5lb density "type II" EPS runs a bit less than 3 perms for vapor permeance which would be sufficient to keep groundwater moisture from reaching mold inducing levels in a latex painted wall. You could use cheaper Type-I EPS if it has a facer to bring the vapor permeance down. Type-I EPS runs ~R3.8-R3.9 @ 1", Type -II runs ~R4.2-ish @ 1". both are pretty cheap, maybe 45-50 cents per square foot at box store retail, cheaper through distributors.

Tape the seams with housewrap tape, and use can-foam to seal it to the foundation sill.

With an IRC 2012 code-min insulated foundation the heat load of a basement bathroom with a small-ish code min window might be self-heating, when occupied by a 250-350 BTU/hr human.

Thank you very much. Very informative information to have!
 

MikeBPrice

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The problem you have is to get it right you can't just look at the one vent. With balancing a home's HVAC you need to look at everything to do a good job. Tapping a supply can work but lots of times causes other issues as most times it was undersized to start with. Since you are finishing off a great room as well you should get that reviewed at the same time.

Otherwise everyone is guessing without knowing details. What size and length is the duct to the dining room? Where does the duct originate? How many elbows, type, and location does it have? Solid or flex duct? If flex is it sagging anywhere? How well is it sealed? Is there a damper and what position is it in? Does the dining room have a return grill? Can you upsize the ductwork between the bathroom and the furnace/air handler?

Thanks again Stuff. I appreciate your input. I haven't measured the dining room duct yet, but eyeball measurement looks to be in the 8-10 inch range. The duct originates off the main air supply plenum and extends approximately 12 feet to the dining room register (two elbows, solid duct throughout). Not sure about the presence of a damper. There is a return supply grill located in the dining room.

I acknowledge that there are lots of variables and thus many limitations in getting a good professional opinion in an online forum like this. I really do appreciate all of your perspectives.
 

Dana

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The heat loss characteristics of basements are very different from above grade floors. The load doesn't change nearly as much with outdoor temperature as above grade floors, since the heat loss rates below grade to the soil change only slowly, seasonally,not with outdoor temperature.

Basement heating loads are also comparatively low, (especially if the foundation insulation is up to IRC 2012 or 2015 levels or better). It really want's to be controlled as a separate zone, not tied onto the first floor zone. If combined with the first floor zone the temperature balance between the upstairs and the basement will be changing constantly, not just seasonally.

Getting to the right solution starts with calculating the actual load. Without the load numbers hacking on the ducts and hoping unbalances the existing duct design (which may or may not be up to Manual-D standards) only to deliver a sub-optimal solution for the basement.

To that end, how many square feet of the exterior wall to the room(s) in question is above grade, what are the insulation levels of that wall, and how many square feet of what type of window (if any)?
 

Leon82

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I have a panisonic hv1100 (I think) no light

It heats the 4x8 bathroom as a supliment well. I'm not a fan of the lights in them but it heats well
 
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