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Sandy1 10-10-2011 06:10 PM

Microwave Installation
We are venting the microwave out the back. Does anyone know the distance from the top of the microwave to the vent on the back? How far down should the vent be from the upper cabinet in order to attach to the microwave's exhaust? Trying to locate the vent on the wall but don't have a new microwave yet. The vent has been placed one inch below the cabinet above. Is that correct or typical for its location?

Also, I see in the install instructions that dampers are required. I believe there is supposed to be one at the microwave and then one on the roof vent. Does the microwave itself have this damper? The vent we have there does not have one.

The contractor has installed a vent that shoots out horizontally from the back rather than vertically. Then attached a flexible aluminum vent that bends upwards to connect to the 6 inch conduit maybe 18 inches above. (The range was moved down a bit). I'm not comfortable with this setup. Do you think it will be alright?

gregzoll 10-10-2011 06:17 PM

The dampers are there to keep the cold air from coming in from outside, and the inside conditioned/heated air from the house escaping outside, along with keeping critters from coming into the house. As for the duct work, you will need to get the microwave/vent hood before doing that part, or download the instructions from the manufacturer website.

Sandy1 10-10-2011 06:35 PM

Looking at the instructions for the one I'm considering, which I was going to give to them, I don't see that dimension noted. They just refer to a template. The ductwork was done and closed up already although he said they might have to adjust it.

What do you think about the horizontal rather than vertical shoot and what would seem to be a pretty tight bend of metal flex? Code says smooth metal interior for venting. It seems some people (supply houses and contractors for instance) think the flex is alright for microwave exhaust anyway. Mfr also talks about not bending around too much -- obviously not an efficient pathway. What problems might arise from this?

So do we only need a damper at the roof? Not the back of the microwave also?

They are doing this without installation instructions. They are in building construction but also said they haven't installed a microwave before. I'm just not understanding this. I don't like people guessing at things with my house.

Sandy1 10-10-2011 07:00 PM

The install requires it be mounted to at least one wall stud. If they use 90 lb anchors instead (stud not available), do multiple anchors increase the total load that will be carried? I see that the instructions say support for 150 lbs is required.

oh'mike 10-10-2011 07:17 PM

Do you have the duct in the wall ? Is it inside the studs and heading up into the attic?

Typical location for a rear exit duct is 1/2" below the bottom of the cabinet.

Post a picture if it's already installed,if you can.---Mike---

hvac122 10-10-2011 07:25 PM

No they are not supposed to use any for flex for venting, screws either. If pipe is in a unconditioned space it needs to be insulated.

hardwareman 10-10-2011 08:03 PM

just get the microwave there for them to eliminate any problems. Anytime we sell a microwave for a new house or remodel the con tractor ALWAYS calls us to deliver before they start running venting. As far as mounting to studs, you must hit at least one stud. Over a 30" span there has to be one in there somwhere.

Msradell 10-10-2011 09:38 PM


Originally Posted by Sandy1 (Post 746069)
The install requires it be mounted to at least one wall stud. If they use 90 lb anchors instead (stud not available), do multiple anchors increase the total load that will be carried? I see that the instructions say support for 150 lbs is required.

You definitely need to be able to attach to studs to hold it up, if not it will be on the floor before long! I'm not sure why the wall it's being attached to doesn't have studs but if for some reason it doesn't the contractor will be required to put blocking in that wall to support the microwave.

Sandy1 10-10-2011 10:38 PM

I'm not ready to buy a microwave yet. I've found two potentials but haven't chosen and am not sure which finish at this point. I want to see my kitchen first. The online information doesn't seem to include exact information about the templates.

I think we'll be ok with the location then if 1/2" is typical. There's an inch and seems to be enough play to move it up a bit if necessary.

I don't know if they used screws. I bought the UL181 tape with the red writing. What they had was silver. Maybe tape is all that's used these days? I'm pretty sure I read somewhere the other day that the vents are both screwed and taped -- and to use the UL181 with red writing. So hopefully that isn't common practice to use screws.

The flex is drywalled in now. I asked them not to use it if possible because it wasn't to code. Today he said the elbow I bought didn't create the right angle. I have a feeling he meant with the horizontal style transition adapter it wouldn't work. It would seem that if they'd tried the straight adapter along with a 90 rect elbow there would have been much less winding around to get to the 6" conduit above... even if they'd still have to use a bit of flex. That would have been just a small amount of the flex and a more direct route. If it would assemble. I don't know. I had to go out and didn't see whether they even tried it. Someone else had bought the horizontal piece and told them to use that so that's what they did, I think. Did as told, regardless. Does anyone know the advantage of the horizontal adaptor here? I think its intent was to be able to notch out the stud instead of cut it. But it was necessary to cut it out anyway.

You asked for it ... pictures. Here's what the last guy did. Smashed the conduit to fit behind the stud. Looks like masking tape. Venting separated. You wonder why I worry?

Second picture, that stud directly next to the conduit is where the new exhaust goes. The farthest one was already cut for the first hood. It has been cut to fit the vent. I'm very relieved that a stud finder shows my plea for stud support was heard. They've bridged the broken stud to the one on the left, below the vent, so it's good and strong for mounting support.

So the winding path and flex is my only concern now. I just don't understand why they would favor that route over a more direct one, possibly with smooth interior venting material for the bend. Other than being in a hurry to get it done at this point, to get the cabinets up asap without going out for parts and designing something else.

There's no way that will be changed because it's been drywalled in and the orders from above are to move forward. It's now something to worry about and keep me up at night at this point.

I hope they straightened out that bent conduit to make it air tight. I should have asked to see when it was completed before drywalled over. If only for peace of mind. They're very conscientious in many ways, I just don't understand why they're not more concerned about some things. I've had too many things go wrong around here to simply trust others or be comfortable with the idea of not following instructions.

This is why inspections are necessary! No one wants to go that route. I'd think having homeowners like me in their hair is probably worse than any old inspector coming around. I have to say it's also well worth the permit money to have a professional oversee these technical issues to make sure the codes in place for safety are truly met.

Sandy1 10-11-2011 07:05 AM

Can you add a damper flap to the vent? Maybe not the best idea to mess with ourselves as it might not actually work correctly and open properly. I'm hoping the microwaves have one on their back automatically. That would make sense to me that they handle it to some degree up front. The end piece that's in there doesn't have one. Dust & pink stuff from the attic falls down into that plumbing wall.

Is there supposed to be one damper in the system or two (back of micro plus roof)?

oh'mike 10-11-2011 07:20 AM

They most likely used a rectangular in wall metal duct---3 1/4" x 10"--

The old round duct should have been removed as it was wrong and is now old and wrong--

The new flat duct should be adapted to round up in the attic where you can see it---go look

The microwave comes with a damper ---also the outside vent should have another one built in--


Sandy1 10-11-2011 07:44 AM

Mike, I'm not sure why you are saying the old round duct is wrong. Documents show round duct being used. They put flex from that down to the adaptor piece which fits to the rectangular opening of the microwave.

Sandy1 10-11-2011 07:50 AM

On the support stud that's required. Exactly what is the story there. The two side studs are further out than the sides of the microwave. The stud that runs down the middle of the unit was cut for the vent opening, then secured to the stud on the left. That securing horizontal stud runs about 8 inches down from the vent.

Are there numerous anchoring holes on the mounting plate? I see a grid of holes on the drawing but it kind of looks like the there are end holes used for the mounting process. If the end holes are the only ones that can be used, with at least one attached to a stud -- there is no stud running at the end of the unit.

Are we alright mounting to the center stud or the horizontal bracing stud attached to it (about 8-10" from the bottom of the cabinet on the left side?

Exact placement is wrong but I drew the vent and flex in and added the image of the support bracing stud in here:

Sandy1 10-11-2011 08:33 AM

okay, talked to the guys and they said if they need to get back into the wall later for stud support they can. I was very concerned about having everything closed up by the time a micro arrived. They usually work commercial, so know a lot about heavy duty support etc. I'm going to stop worrying about that part since they can adjust if need be. He said he did mount his own microwave at home, just not the venting part, so should be plenty familiar with where the mounting plates etc fall. I thought he'd said he hadn't installed one before when they were scratching their heads about something. I couldn't get a micro in hand as they'd wanted, by the time all this was underway and they needed it. As often as these things need replacement, I would think it would be fairly standardized.

oh'mike 10-11-2011 06:44 PM

Hanging the thing safely will not be a problem---the duct is---that looks like a squashed 4" duct

Fine for the old unit maybe but not large enough for a microwave vent---

Size for pipe should be 6" or 7"

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