CFM needed and in line "booster"

I bough an over the range microwave with a built in 300 cfm stove vent.

300 does not seem to do the job and was told I should install an inline 'booster fan' that is 400 cfm.

The pipe run to the vent exit is long.

6" galvanized
4x 90 degree elbows
5 ft horizontal
18 ft vertical
1 wall mounted vent that has a louver

Can I use an inline "booster"?
Or do I have to start all over with a bigger unit?

 

Have you verified the louver vents are opening when the unit is on and they are not stuck or sticky? What is the make and model of your OTR microwave?

 

I'd be trying to figure how to reroute to make it shorter and get rid of all those elbows.

 

The 90's are killing you. Each one can add something like 10 to 15 feet equivalent length. If any of those can be replaced with 45° bends and if any of the duct can go up to a larger size all the better.

Although hoods at 400 and up are supposed to come with a warning about make-up air, a 300 is still a concern, if you have a naturally drafted furnace and/or water heater. That's another topic if you are concerned.

Bud

 

300 cfm is not actually that much even before the restrictions of the elbows. You might need to tame your expectations a bit.

Also placement above the stove and capture area are very important but often not well calculated.

As far as inline boosters go. I'm always a fan of doing an end of run extractor instead of middle of run if you can manage it in a way that doesn't bother you aesthetically. You can run a lot more powerful and noisy fan outside and have some peace and quiet inside.

 

czizzi,

The OTR microwave is a Samsung. The louver does open and it blows when running, but it is not a strong wind. When I had the pipe work partially installed it seemed great, but once installed, less so.

 

joecaptian and Bud9051,

It is a 1946 duplex, and there is no other option other than having pipe work run exposed through the kitchen.

If I ran exposed pipe in the kitchen there would only be 1 90 degree elbow. But i am hoping that is last resort.

Esand1

I have a 2 1/2 foot x 5 foot service shaft that runs basement to roof, anything installed there is out of sight and won't be heard. All the wiring and plumbing comes up through this area.

 

Bud,

Should I be concerned?
I do have a naturally drafted oil boiler for a hydronic heating system.

 

Not really. Not in a 1946 duplex.

 

In my work as an energy auditor (retired) we would measure the worst case depressurization by activating all exhaust appliances with all doors open or closed (which ever tested the worst) and then fire up the smallest naturally drafted appliance. The most sensitive is a gas water heater. An oil boiler is more powerful, but that 300 could tip to backdraft with too many appliances on.

Since you are not getting the flow you expect, test it again with a nearby window open. If the house is too tight then you might be having trouble getting enough return air already. And that would complicate the boiler drafting concerns. Do the window test and let us know.

Now, you said: "I have a 2 1/2 foot x 5 foot service shaft that runs basement to roof, anything installed there is out of sight and won't be heard. All the wiring and plumbing comes up through this area." That sounds like a major bypass for basement air to flow directly into the attic. If that is the case, before you button up this project you will want to close top and bottom of that service shaft. Did you feel the air flowing up through when you were in there?

BTW, if concerns continue about backdrafting, any energy auditor should be able to test it and Canada has plenty of them.

Bud

 

Thanks Esand1, but I am curious, because of the hole having enough drafts? Or because the boiler is strong enough to force an updraft?

 

Bud,

I will do a test this weekend with a pot of boiling water on the outside burner and see if there is a change when the window is opened a crack.

Thanks.

The service shaft has a floor on the ground level and then is wide open to the skylight on the roof.

The draft tends to be down, not up. In the winter you can feel the cool air coming down in to the bathroom.

I wanted to seal it a little better this summer but discovered that for the height of the attic, there is only 1 sheet of drywall on the service shaft side. Zero insulation and nothing closing the back side in the attic. So it got bumped back 1 year due to lack of time.

 

That goes on the to-do list, but at least it isn't warm moist air leaking into a cold attic, that's bad.

best
Bud

 

I did the pot of boiling water test today.

When I opened the windows, wide open, it seemed to work a little better.
With the widow opened just a crack, I saw no difference.

I spoke with the tech support of a local company, Vortex Powerfans (i like the idea of buying locally made as much as possible) and he suggested, as did Esand1, that I install towards the end of the run.
They have a 235 cfm and 450cfm unit that would fit my 6" vent pipe run.

Is 450 overkill?

What would be the best way to allow air to enter the house while using the range vent hood? keeping in mind, it is -20 degrees for most of Jan/Feb and -10 for a good chunk of November/ December and part of March.

Thanks,

 

You will never see 450 CFM from a inline cheap fan.

 

I saw some really cheap units at the local big box.
Can anyone recommend the best fan to look at for the job?

This is the fan I am looking at for the moment:

ATMOSPHERE - VTX600

 

A bit late here but putting anything in the stream of the exhaust for a cooking appliance is asking for problems.
I may help at first but you will just be installing a nice grease trap.

Check the installation instructions for whatever booster fan you want to install and kitchen exhaust booster is likely not on the list.