hvac guy suggested 300 cfm vent a hood to avoid mua--thoughts?

needinfo1April 18, 2013

We are struggling with what to do and have probably looked at every hood on the market. We think VAH is overpriced (come on around $600 for just a stainless steel chimney to fit on the already pricy unit). I've read complaints about them being noisy. I believe they used to be considered hard to clean, but that has been upgraded in newer models.

We live in an area with strict guidelines for MUA. When he was out today, here are the choices the HVAC guy said we would have.

1. We could buy a 300 cfm VAH because it is really more effective than the rated 300cfm. He and all of the guys who work for his company have them and like them. If we do this we'd be in compliance and have a pretty effective hood.

2. We could install heated MUA, something he said we really don't want to do because of the cost. He is coming back with the estimate for this. We know we don't want to spend the money on this, and now that the person who installs this for a living is recommending against us doing this, we are doubly sure.

3.We could put a gas insert in our wood-burning fireplace (something we have been talking about doing for years anyway). If we do this, we could go up to 600 cfm without any MUA.

We have a 30" AG Wolf with the 4 16K burners, but we aren't people who cook with a wok or do any real frying. We also can't go any wider than 30" for a hood. And, we've had the Wolf in for 2 months now with no venting other than the window and for the most part haven't missed having venting.

What are our choices if we decide not to go with the VAH? Are there any effective brands where we'd be able to stay under either the 300 cfm (option 1) or the 600 cfm (option 3) limits? We've decided that option 2 is completely out.

Or, should we just bite the bullet and go with the VAH? The appliance salesman (who also sells other brands and gave us a variety of options to choose from) also told us VAH.


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FWIW, I have a 600cfm VAH and don't find it particularly noisy. Mind you, all I've ever had to compare it to were cheesy builder grade hoods which are WAYYYYYY noisier (as well as much less effective). I had to go the heated MUA route, and it definitely is expensive, but in my case, worth it. In my old kitchen, I regularly set off the smoke alarms, and now that very rarely happens.

Is 300 cfm enough over a 30" Wolf? I would say it is touching the absolute low end of acceptability. But, if it is compatible with your cooking styles, and meets code, go for it. Many of us are in the position of having to make real-life compromises due to space and/or budget concerns.

    Bookmark   April 18, 2013 at 9:52PM
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Funny you mention removing your wood burning fireplace and going with a gas insert, as that is exactly what I did to avoid MUA. This got me into the 600cfm which is lower than I should use as well.
Typically 10cfm per BTU? so you have 64K BTU so I'd think 600 cfm is where you need to go. And truthfully, it will be safer and better on the environment than a wood burning stove. Of course that adds the cost of the fireplace and the cost of a bigger fan in your hood.
I would not count on the higher efficiency of the VAH models to really makeup for undersizing your cfm. The upgraded numbers assume minimal vent runs and it is sort of trickier to do this. I still chose VAH because I think they make good products and are simple to maintain.
Are you doing this work with a permit? If so, then you must follow these rules, if not there are factors about how and when you use them you may want to consider. For instance, the wood burner is only an issue, if you are actually burning wood and using the hood at the same time. How often does that really happen? Same goes with a dryer, and other devices that exhaust air from your house. Code dictates for worse possible scenarios. In my case, I have the option to make my hood 1200cfm, and I might reserve that much pull when I can safely open a window, but the code will never trust a homeowner to do such things.

    Bookmark   April 18, 2013 at 10:05PM
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I got a Broan E60E36SS, 36"W x 18" H x 24" deep hood, rooftop mounted 600CFM blower, duct silencer and connecting ducts (15 feet) and elbows for about $1000. I don't really know the cost of installation separate from other work, but was maybe 4 hours or so. Access was good, and I did the leg work.

Incredibly quiet. I'll measure the sound level some time soon. My wife is very sound sensitive, and she will not use her 435cfm Viking hood unless something in really smoking.

I haven't moved in yet to use it, but I never had any issues with my 40 year old Nutone, and I doubt that it was even 200CFM (but I don't fry, sear, cook much meat, etc.). So I don't think I will find 600cfm too small.

    Bookmark   April 19, 2013 at 1:22AM
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My contractor told me that 300cfm is what people use to vent their bathrooms so no way would that be enough for a kitchen.

I have a 1200 cfm hood. It's pretty good. Gets most of the smoke. I didn't even know a 2500 cfm residential fan existed until recently. But if I could do it again I'd get the bigger hood

    Bookmark   April 19, 2013 at 7:22AM
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Wow! What a range of experiences and opinions.

I find several things extremely frustrating about this entire experience. As will2kz says, codes are written for worst case scenarios. We've probably had 3 or 4 fires in our fireplace this past winter, yet having this fireplace is a major reason we need to do something if we want to go over 300 cfm. OTOH, we know our fireplace liner needs major work, and it will probably be just as much money to just put in the gas insert with its stainless steel liner as to do the repairs, and this would kill two birds with one stone.

As far as our ducting, we are limited to 6" ducts. We'd go to the ceiling between the first and second floors of our 2 1/2 story house and make a right turn with a run of 5 or 6 feet to the outside. I don't know how much a run like that would compromise the efficiency of any system.

attofarad--I've been leery of Broan because I had read that many of them are quite noisy, so I am interested to hear that you have a quiet set up, but you also have the outside blower and silencer. We can't do that.

Vent-a-Hood is a big question though. Are their claims a bunch of hooey? Or, will we really get a more effective hood than we'd get with a comparable cfm hood from another manufacturer? And, does anyone here have a low, 300 cfm VAH over anything similar to my range? If so, what is your experience?

    Bookmark   April 19, 2013 at 8:14AM
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I think your HVAC person is lazy. And wary of pricing himself out of your job if he were to suggest actually doing things the correct way. Find another HVAC contractor. One who has actual positive experiences with a HRV. And who knows how to do this successfully.

300 CFM would be like trying to run a performance sports car on 84 octane fuel.

For the benefit of others who are in the buying stage, if you don't do a lot of high heat cooking or searing or stir frying, then you probably don't need a pro grade range. It will cause you nothing but headaches to try to fit it into your home and the cost/benefit ratio would deem it absurd to even try.

On the other hand, if you are someone who does all of those things, the pleasure of being able to do them in your home without asphyxiating yourself IS worth the cost of doing it correctly.

Remember that even if you don't cook a lot now, that your habits may change in the future when you actually have the capability to do what your equipment won't allow you to do now.

    Bookmark   April 19, 2013 at 9:29AM
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Thanks. Interesting comments. The HVAC company is one of the most experienced and highly recommended in our area. The bid won't be here until next week, so we don't actually know what the dollar cost will be.

I did just look at the VAH brochure, and with our range, 600 cfm is the recommended. So, if we converted the fireplace to gas, we could go this high.

I'll occasionally stir-fry and I do sear things sometimes. That is why we decided to go with the Wolf so we would have the power if and when we want it. But, the reality for us is that a very small proportion of our cooking is that super high heat, smoke and smells and grease-producing type of cooking. That is why we are actually contemplating using the lower cfm venting with a VAH. (I'd still like to know though if VAH really pulls 50% more than a comparably rated cfm in a different brand.) Plus, if on a rare occasion we needed to, we could always open a window. It's not optimal, but it is reality for some people.

Once again, for us, I think our main objective is to have a hood that will be powerful enough to remove the heat the Wolf puts out. The heat is nice during our long winters, but we know when it is hot our, we won't like it.

    Bookmark   April 19, 2013 at 12:56PM
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From what I've read, the VAH stuff is mostly marketing hype. I personally like hoods with baffles. Is the MUA requirement for 300 CFMs or more or 400 CFMs or more which I've seen referenced in some posts. I have a 600 CFM hood, stir fry and deep fry often and rarely put my hood on high. Mostly I run it on low and occasionally on medium. You could most likely make due with 300 or 400 CFMs if makeup air is cost prohibitive. We all make compromises. By the way 6" ducting will limit your choice of hoods although Kobe does make some hoods that work with 6" ducts.

    Bookmark   April 19, 2013 at 1:05PM
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Get a variable speed hood at the higher end of CFM.

You do not have to use it on full blast all the time.

Wile it may make some baffles a little less effective, when you use a remote blower it works very well.

There is a reason the blowers are on the roof of commercial kitchens.

    Bookmark   April 19, 2013 at 1:15PM
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Thanks for all of the suggestions... Compromises and limitations are what this is all about for us. I won't even get into the point that I really dislike the looks of hoods...That is a completely different discussion topic.

Our area is 300 cfm as an upper end limit unless some other things are done. If we'd convert our fireplace to gas, we could go 600. If we do full mua, sky is pretty much the limit. But, even if we wanted to spend the money (which we don't) on mua, it seems to me that in our situation this is an awfully big expenditure for an awfully small stove. If I had a monster size, 48" range with lots of high burners, there would be no question about mua. But, all of that money for one 30" range seems pretty silly, especially because there is one thing I forgot to mention that I believe is key to our situation; we have a 100-year-old, leaky house. You know the kind I mean where you can sit by the window and feel a definite draft.

Roof blower isn't possible on our 2 1/2 story house.

    Bookmark   April 19, 2013 at 1:35PM
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Hi, needinfo. We had the same constraints (also in MN) and went through the same thought process. I did get the 300cfm VAH. Mine is 42" wide, with a 6" duct, over a 36" 5-burner Bertazzoni.

For most of my everyday cooking, it's adequate, or very nearly adequate. How's that for a ringing endorsement? :-) On occasion when I sear meats, it can have trouble keeping up. The other night I deep fried fish, and despite the cold weather had to crack open the back door. When I finished cooking, I closed the door, left the fan on low for a couple hours, and nothing lingered after that. So it works, just not at anywhere near the level of the 1200cfm machines that GWers seem to favor.

I do find it somewhat noisy. Some people would say unacceptably noisy. We all have different tolerances. It's not so loud that it interferes with conversation in the kitchen. I do, however, turn it to low or off when we sit down to dinner in the dining room (through an open doorway, 20-some feet away) because of the volume.

My only other concern is that it isn't deep enough to quite cover the front burners--21", maybe?--so really smoky, steamy, greasy things are best cooked on the back burners. I don't find this a problem, but others might.

This post was edited by mnerg on Fri, Apr 19, 13 at 14:48

    Bookmark   April 19, 2013 at 2:46PM
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Funny, we are all in MN. I suppose the rest of the country is out enjoying spring, while we sit with our fresh foot of snow in late April pondering websites....

I think you should go 600 cfm, ditch the old fireplace. Make sure your HVAC guy is on the right table. Old houses and new houses are held to different standards. Your 100 year old house would clearly let in more than my 13 year old house. The thing that saved me was square footage...
Some hoods, like the ventahood, may come in multiple blower configurations for the same hood. Of course and external blower would allow this too. You could build it one way, and if you need more juice, just add blowers, or change out the external blower. In your case you are stuck with 6 inches of ductspace, which more or less leaves you to sub 600 cfm anyway. But other lurkers take heed.
My hood is actually a 1200cfm hood. I've promised to disconnect the two 300cfm fans for the inspector to get everything legal and safe. BUT, I could always add MUA in the future and reconnect if I wanted the other space. Some homeowners might consider reconnecting after inspection and using the other 600cfm with a door open... but that's not at all what I'm condoning. Of course homeowners will do, what homeowners will do.

    Bookmark   April 19, 2013 at 11:21PM
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Yes, only people in lovely MN with a never-ending winter would be sitting at the computer. At least, since we are doing a lot of this kitchen minor re-vamp ourselves, there hasn't been any compelling reason to want to go outside!

mnerg--So your VAH is adequate for your cooking but noisy. I'm interested to hear this because our kitchen is long and narrow with my table perhaps 20 feet away from the stove. Although we have a dining room, we primarily eat in the kitchen

will2kz--I think the guy was using the correct table to do the calculations. We just figured out the correct square footage of our house, and we could probably eek out 20 or more cfms with the correct figure there.

We got the bid back and unheated mua is only $1,000--definitely as bad as we had expected. But, it is unheated, not heated. Something else that my husband had discussed with the HVAC guy was our water heater--old and traditional. Now we are thinking perhaps we might spend the equivalent money that mua would cost to replace our 15-20 year old water heater with a current model that has a vent fan. I guess if we did this we could also move up to the 600 cfm limit because the water heater wouldn't be counted against us as much.

What a hassle! Whoever thought, before we jumped into the intricacies of this, that just getting a new range and hood would be such a hassle.

    Bookmark   April 20, 2013 at 11:20AM
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I had the same issue when we did our kitchen 8 years ago. I have a 36 inch Wolf cooktop. I decided to go with 300 CFM VAH and have been completely satisfied with my decision. In retrospect I think I obsessed way too much about this decision. I do use a wok and some higher heat searing of fish but no other frying or greasy cooking. My VAH sucks everything up and out no problem. My VAH is on the outside wall and ductwork is very short. I think that impacts the effectiveness. In general the noise of kitchen exhaust fans drives me nuts so I only use mine when I think it's absolutely necessary. So glad I didn't spend a small fortune on MUA system.

    Bookmark   April 21, 2013 at 10:43AM
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It's a lovely yet chilly spring day here, and no, you're not alone pondering mua and venting.

Reading this thread I see MN must show homeowners how to comply with their code by closing up fireplaces etc...Perhaps too your local code people talk with you on how to comply, such as not using one of two internal 300 blowers.

I'm pretty amazed as such great discussion within your town code offices, as no matter how much I ask, our code office just cites the IRC code and says it's up to us to figure it out with out GC prior to inspection. So here I sit in vent hood hades still pondering what to do. The Minnesotans that I've meet are nice people. :)

    Bookmark   April 21, 2013 at 11:22AM
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gibby--I am afraid this is verging on obsessing for us (just as it was for you), and I am so glad to hear that your 300cfm VAH seems just fine for you.

sparklingwater--We were at a complete loss as to what to do and couldn't get any local government help about his other than--below 300cfm don't worry, want to go up to 600 might be possible without mua if you check with a HVAC person. Actually, it was when we had the HVAC guy out that we got these various options. Good luck. Perhaps if you get a HVAC person out, you too will get some ideas.

    Bookmark   April 21, 2013 at 2:23PM
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How are you guys getting ahold of the code ?
I'm in Toronto and I have no clue where to find what kind of code we have up here.

    Bookmark   April 21, 2013 at 5:38PM
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MN is not the only place with never-ending winter. Right now, where I live in MT, snow is falling at about 1" per hour. Springtime in the Rockies is, well, probably sometime in June.

Anyway, while I cannot help much with the HVAC guy and your local regulations, I can recommend that you go ahead with the new, power-vented water heater. Putting one in here (a cetnury old house) made a perceptible dent in the gas bill. Plus, you can easily add MUA for the water heater. I built my own MUA system for the utility room in the basement, but the maker of my water heater, Bradford White, offered (and probably still does) a MUA system integrated with the power-venting. I believe it was called "PDX." Might want to look into that if the MUA requirements remain confused.

FWIW, I'm running a 600 cfm hood over an NXR with four 15k-btu-hr burners. I mostly run the hood on the medium setting (probably around 350 cfm.)

    Bookmark   April 21, 2013 at 5:53PM
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"Roof blower isn't possible on our 2 1/2 story house."

Go out the wall then.

    Bookmark   April 22, 2013 at 11:52AM
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The HVAC 'guy' is a hack.

Please don't put in substandard components, and then leave a mess for the next owner to clean-up.

If you can't afford the hood, then you can't afford the project.

    Bookmark   April 22, 2013 at 1:43PM
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jwvideo--Thanks for the helpful information on the two different points.

brickeyee--So you are suggesting an external blower when we go out sideways at the level of the first floor ceiling.

How noisy are these blowers? We live in a typical 50 foot urban lot and wouldn't want to subject our neighbors to something like this ( the blower would have to be on the side of the house right next to their door and porch.

    Bookmark   April 22, 2013 at 1:52PM
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It's not a matter of not being able to afford the hood. My question rather is whether VAH is worth the premium price and would just installing a VAH solve all of our problems without going the MUA route since "supposedly" VAH is more effective than a comparable cfm hood of another manufacturer. Is it more effective? I don't know.

    Bookmark   April 22, 2013 at 1:57PM
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A 300 CFM hood is not "substandard". Hood CFMs are often oversold by manufacturers. The rule of thumb assumes that you are running all of your burners on maximum a lot of the time - rarely if ever happens. In fact, 300 CFMs would handle 2 of your burners on maximum at the same time - another rare occurrence - unless you are stir frying on two burners at once which actually is something I have done on occasion.

If it were me, I'd buy a 300 CFM hood with baffles and be done with it.

    Bookmark   April 22, 2013 at 2:11PM
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I'm in agreement with your assessment of how hood capacity is determined, and that is one of the reasons I started to ask these questions because I know I will never be running all 4 burners at full capacity.

Do you know of any 300 cfm hoods with baffles? When I was doing a search for baffles, I really could only find them in hoods that were 600 cfm or more. Or, maybe the VAH is the only option at 300 cfm with anything other than mesh filters, and their configuration is the closest I'll get to baffles in a low cfm hood.

    Bookmark   April 22, 2013 at 2:18PM
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DO not put in a remote blower. They are loud and ugly and from what you said about your installation you would be looking at this big blower on the side of your house. I do not think you can get a remote blower at 300 CFM because they are not effective at the lower CFM. It sounds like the 300 CFM VAH would be the easy answer for your situation so I would just go with it. It sounds like you know what you would be getting into so it comes down to making a choice where you want to make the trade off.

    Bookmark   April 22, 2013 at 3:23PM
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I just went and looked at the Kobe website - I found some hoods that max out at 290 CFMs but they have mesh filters.

I suggested baffles because they're easy to clean - VAH squirrel cages always sounded squirrely to me but I have no first hand experience with them.

This post was edited by weissman on Mon, Apr 22, 13 at 16:10

    Bookmark   April 22, 2013 at 4:08PM
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It is hard to know where to start here. The flow rate (cfm) is determined by the uprising velocity of the cooking plume effluent and the aperture area of the hood, and not to first order by the BTUs. I would start at 90 times the area in square feet.

Baffles will not separate grease from the air at really slow air speeds, so if one intends to go as low as possible, then a mesh that is routinely cleaned is probably better. However, mesh hoods typically have undersized apertures, so capture is degraded at the hood periphery. In other words, the hood is smaller than listed. Baffles will at any speed provide fire blocking, their other purpose.

All fans have fan curves, including those made from magic lungs. The fan curve plots flow rate versus pressure drop across the fan, which results from duct friction, duct transition flow disruptions, mesh or baffle restriction, lack of MUA, etc. Typical fan curves are slightly convex, with cfm on the abscissa and pressure on the ordinate. When the pressure drop reaches some maximum, such as an inch or two of water column, the flow reaches zero. At zero pressure drop, the flow is (should be) the rated flow. The pressure drop is never zero in situ.

VAH may be counting fan and hood, which can also be the rating used by some others at some times. It depends on whether the rating is for the hood with fan or for the fan only. The VAH rated flow certainly does not include the losses from the ducting and duct transition to the cap at the outside. Unfortunately, unless susceptible to a calibrated measurement, code enforcers will look at the fan rating and not actual flow for enforcing MUA rules. Ideally, they would test for negative house pressure vs. what combustion appliances present are not allowed to exceed without risking back-drafting.

The relative loudness and ugliness of outside fans has to be compared to the relative social ugliness of loud inside fans. YMMV. I would not, however, duct to my neighbor's door. Some other path should be adopted.


    Bookmark   April 22, 2013 at 7:48PM
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>> I would not, however, duct to my neighbor's door. Some other path should be adopted.

Depends on what you think of your neighbor :-)

    Bookmark   April 22, 2013 at 8:14PM
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Depends on whether I want to sleep in an alerted state of awareness. :D

    Bookmark   April 23, 2013 at 11:00AM
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"It's not a matter of not being able to afford the hood. My question rather is whether VAH is worth the premium price and would just installing a VAH solve all of our problems without going the MUA route since "supposedly" VAH is more effective than a comparable cfm hood of another manufacturer. Is it more effective?"

As an engineer (not HVAC) there is no magic bullet, there is no best. However I have an opinion as a person who cooks and cleans the oven/hood.

Those mesh filters are a mess. The restrict the air flow, and IMO, useless, uncleanable, should never be sold.

Then there is VAH. My wife and I looked at that 20 years ago. Their pitch is that the fans run 'so fast' that the centrifigual force extracts the grease from the air - then you have to clean that mess? Clean squirel cage fans? It's a fan that is 'pushing air', and they are noisey.

Baffles are by far the easiest to clean - put them in the dish washer / soak in bathtub.

Outside exhausters are the most quiet, and effective. Here you are 'pulling air' out of the house.

I would use a hood with baffles, and an outside exhauster with a speed control.

This link shows both external and inline fans, which I think are much better than a fan motor in the hood.

Here is a link that might be useful: Exterior Fan

    Bookmark   April 23, 2013 at 11:59AM
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"How noisy are these blowers?"

the ones I have had are not noisy outside at all.

"DO not put in a remote blower. They are loud and ugly..."

It just looks like a tiered aluminum mushroom about 18 inches tall.

I had one over the end of my driveway about 15 feet up at one house (no one looks up) with my out door seating area just around the corner of the house.
Never noticed the fan running much except on a very quiet day on maximum.
And then the noise was mostly air rush from a 1,20 CFT/min blower.

Another house had the blower over a small flat roof area connecting the house to the garage with the central AC unit sitting on the roof.

Even the noise in the bedroom (the duct went through a corner of the closet to get up above the flat roof) was fine.

The huge patio on the back of the house was fine all the time.

I remember when a small branch from a tree was hitting the fan blade.

THAT was noisy and readily apparent.

The little leaflet fell like a parachute, branch end down then poked trough the coarse screening on the blower between the aluminum sections.

These are axial blowers and are not nearly as noisy as a centrifugal blower.
They use larger blade area and lower speed to move high volumes of air.

It is not an 'in hood' blower moved to another location.

Next time you go to any restaurant (they almost universally have very large axial blowers on the roof under the 'mushrooms') see how much you can hear from outside.

Yes, rooftop is better for directing noise away, but usually the most noticeable thing is the small, not any sound.

I am sure there is a cheap unbalanced axial blower with no motor vibration isolation somewhere that sounds like a jet engine.

Sometime you DO get what you pay for.

This post was edited by brickeyee on Tue, Apr 23, 13 at 12:59

    Bookmark   April 23, 2013 at 12:54PM
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