Kitchen Hood Venting & Pressure

[vailconst]

I was hoping someone may be able to educate me on the static pressure differences in venting a kitchen hood up towards the roof or down towards the basement. Why would a blower be warrantied in the first arrangement, but not the second? Much appreciated....

 

[ChasBean1]

I think you might have it backwards - heated air can rise up the flue but you're in for problems if you expect it to go down toward the basement without a fan. In either condition, I would use a blower to exhaust the volume specified by the hood maker to optimize its capture performance.

Static pressure differences? For each elevation increase of forty feet, there is about a 0.5 in. w.c. pressure drop (depending on where you start - this is approx. sea-level data). It's thermal differences that create a flue, though. With all temps equal, pressure drops through a duct rise should be equal to pressure drops at the same gradients outside the building, creating a static condition, or no air movement within the duct.

I have no clue if this answers what you ask - I don't want to spam you with extra information...

 

[vailconst]

I think I had it the right way, they won't warranty an arrangement where the hood is ducted down into the basement. They (Best by Broan tech support)told me thier hoods are only warrantied when the ducting runs up or horizontal due to static pressure problems. Which I didn't understand. We've got a 1200 CFM blower in this hood (I should have made that clear before)which, neglecting effects of the duct going down inside a wall for 8 feet, is more than adequetly sized for our venting requirements.
We're working at 8000' so I'll assume 48' height change is equal to .5" w.c. change. This means a .08" w.c. increase from the hood down to the basement in my desired ducting arrangement. Do blowers not work effectively when attempting to move air from a lower static pressure area to a higher pressure area? Even in such a small pressure differential? Thanks again....

 

[ChasBean1]

You'd also be fighting buoyant forces of heated air from the stove. They want to go up naturally. Then there may be an issue of grease drainage. My main question is why would you want to duct this to your basement??

 

[vailconst]

This is a remodel project. The current hood venting runs into the basement and then out above the foundation wall. Venting the hood up, while not impossible, would be a much more involved task and that's what we are trying to avoid. The blower filter is designed to trap all the grease before it gets into the ducting. Basically, is there some reason why exhausting the blower 8' down a wall and 25' across a basement won't work? The blower can certainly handle exhausting 8' up a wall and 25' of horizontal run, why not the other way??

 

[HighPotential]

Can you run it up to the ceiling joists then horizontally to an outside wall?

 

[vailconst]

Anything is possible! Reusing some of the existing sheetmetal and avoiding drywall damage for cost savings is the idea here. Unless someone convinces me of why my desired arrangement won't work so well I'm going to forego the blower warranty and have it installed that way. That's the way the original builder ran it and it seems to work now?

 

[lilliput1]

You better look at BOCA or International Mechanical Code with regards grease hood exhaust. It is required that the discharge be minimum 40" above the roof. Also the duct must be enclosed in a fire rated enclosure once it leaves the space. The duct could be welded steel with grease traps & fire rated enclosure. 3M makes flexible duct wrap that is 2 hour rated. You can also use presinsulated double wall grease ducts. You have to follow required clearances to combustibles. You have to provide access doors to allow cleanibg of the duct.

 

[ChasBean1]

lilliput, I agree with your observation but this is a house. Personally, I have to beat my 70's vintage stove exhaust and wait about 5 minutes for it to come up to speed - when it does run it sounds like there's a lawn mower in the kitchen, and it vents to the attic! (codes weren't so important back then). My last house I bought the cheesy contractor put the stove vent unit in (to look good) but there was no exhaust duct! There was also a phone jack on the wall with no wiring to it... okay, I got taken. If there's no deep fat fryer and it's a typical stove, and it's mechanically exhausted as you say and it's not pitched to trap grease at a low point, and will save you a ton of headache, go for it.

NOTE: be prepared for a nice cold draft blowing back onto the stove during winter if there's not a good backdraft damper. Hopefully it won't be strong enough to blow out pilots and cause an explosive hazard.

In the end, I personally would bash up a little sheetrock, duct it right and preserve the warranty. But that's just me. If it's your own residential, I think we should be allowed some leniency and to assume some risk - it's our own necks at stake. If you're performing a service for a customer, you should (in fact, you're obliged to) do it the right way without question.

 

[vailconst]

Thanks for your input...This is a customer situation, but our local inspector said he would pass either venting arrangement. I'll just have to make our owner aware of the situation and let him decide. I'm just trying provide him with a correct assesment of how well re-using his current ducting will work. Anyone care to venture a guess as to how many CFM's will be exiting the end of the duct? 1200 CFM blower round exhaust > 90 degree bend > 3.25 x 10 rectangular duct > 8' down wall > 90 degree bend > 25' of 8" round duct > ??CFM's exiting basement?

 

[ChasBean1]

mass flow in = mass flow out, so we can say scfm in = scfm out. We lose pressure along the run, not flow. Pressure restriction will decrease system flow (equally at both ends, not at just the outlet) depending on the manufacturer's fan curve.

 

[Vicsidhu]

mass flow in will be equal to mass out. Very true.

But with increase in static the blower will reduce its intake cfm, thus lowering its exhaust.

How much you will get at the end of the duct?
1. Calculate total static
2. Read fan curves and read the cfm against the calculated static loss on the correct curve.
3. This is the cfm you will get at the fan.

This is how I would do it.... Always looking for better ways....

Vic