Venting an over the range microwave.

[Irving]

I am going to install an over the range microwave here. We've remodeled our kitchen, so this cutout was originally for a fridge and there is no vent.



Here is the other side of the wall, taken with the cabinet open to give a height reference.




I am going to be finishing the remodel on the laundry room as well, so at some point those holes will be patched and the water heater will be walled in from floor to ceiling. Eventually it won't be ugly, but for now, I can tear things up a bit to do them correctly if necessary. The range is gas. It has not been vented outside in the past at all. Does it matter that it is a gas range?

My questions are:
- I'd like to vent outside instead of recirculating. Is venting to the attic "good enough?" If not, should I try to tie in to the water heater vent?
- If there is a stud right where I need to cut into the wall for the vent, can I cut the stud and install a header, base, and side plates, assuming that this is a load bearing wall?
- How do I tell if the wall is load bearing?
- Any tips?

[Dr_Fwd]

Standard size of over the range microwave is 30", if that cabinet used to be above the fridge - it's more than likely 36"...

[Irving]

Yeah it's 39" actually. I may have to build little end caps to make it all look even. Maybe little shelves or something. I'll mount the microwave, then figure out that part.

[Tim K]

Venting to the attic is fine. Do NOT tie in to the exhaust for the hot water heater. You would be adding back pressure to that system, potentially causing the exhaust to flow the wrong direction.

[flogger]

If the roof is built with trusses, the wall may not be load bearing. You would need to check in the attic to see if it's stick framed or not.

Most new microwaves have an exhaust vent that can go out the back of the unit or the top. , this would allow you to stay in the cabinet and go straight up to the attic. You lose some storage room in the cabinet.

If you do vent to the attic I would recommend extending the pipe to a roof or gable vent, you can pump a lot of moist air up there and that could be an issue.

[flogger]

Is that a fan on the water heater exhaust pipe?

[hghclsswhitetrsh]

Is that a fan on the water heater exhaust pipe?

Vent damper.

hey Irving is that 3" pipe just kind of stuck up into that 4" pipe? Or is there a 3" pipe inside of the 4" pipe going all the way outside?

[Lurch]

Dont vent to the attic. The moisture and grease will cause a lot of damage up there. You maybe able to retro fit a roof vent but it has to vent outside. Also it wouldn't pass a home inspection when you go to sell.

[Tim K]

Baths are vented to attics all the time and they are much higher producers of moisture. There is nothing in the building code (last time I looked was 10 years ago) that specifies a vent must go outside. In fact, there is no requirement for a vent at all. I'll agree it's better to go all the way, but roof penetrations are not always simple. I've seen homeowners do it themselves and end up with a leaky roof.

Where are you planning to plug it in? I can't remember if a MW requires a dedicated circuit like a fridge, but I don't think so. They are big draws, however, and if you run one on a circuit that runs other high power devices you'll likely blow breakers.

[earplug]

It will be a bitch to clean the microwave over the oven.
I installed a kitchen vent and lived. I'd put one over your stove and vent to the outside. Don't forget a light over the stove.