Newbie question
Beemer
| Posted in Expert Exchange Q&A on
Hi, I’m building a new house in Minnesota. We’re installing a 48” gas range and a 48” Best hood. The hood has a multiple speed motor, the highest is 1100 cfm….which is about double what local building codes permit.
The hood has an 8” vent. If I use a duct reducer and take it down to 6” how much will the cfm be reduced?
TIA.
Bryan
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Replies
Look up the static pressure (exhaust duct and makeup air) on the spec sheet. That'll give you flowrate.
Easy peasy! (Hah!)
You might then ask us what your static pressure is...
Back to your question-- you're wondering what the increase in static pressure to the whole system when you transition an unspecified length, geometry, and path of sheet metal duct to an unspecified length, geometry, and path of 6" sheet metal duct.
Can you help us fill in some of those blanks there?
And given this forum you should be ready to account for your choice of exhaust air system and quantity, and perhaps cooktop choice, and also have an idea of how you're going to handle makeup air. (just fair warning LOL!)
You're going to make things noiser by restricting the flow like that. What happens when trying to stuff more airflow through a smaller duct is that the air velocity in the duct is increased, which increases noise. It's similar to what happens when you hold your thumb over the end of a garden hose: the water shoots out at higher velocity, due to the smaller open aperature because of the blockage from your thumb, which reduces overall flow but also increases noise (and back pressure in the hose).
Another issue here is that you're probably not going to satisfy your building department officials by putting smaller duct onto a unit like this, because it's likely to violate the installation instructions for the hood. If the hood says "install 8" duct", and you install 6", then the inspectors can fail the installation since you didn't install it in the manner described by the manufacturer.
I'd recommend you select a hood with a lower airflow, or possibly configure the motor to operate with a lower maximum flowrate in the hood you have. 1100 CFM is a lot if you're building a tight (as in well air sealed) home, which is likely if you're reading the forums on GBA. Makeup air becomes more critical in a tight house, since a tight house will leak less air, and "accidental" makeup air -- which is what you're using if you don't provide a specific way for makeup to enter the home -- is going to come in through all the little leaks throughout the house.
This being GBA, I'd also recommend you look into an induction cooktop :-)
Bill
Sadly both the gas range and the hood are already purchased. I’d installed the same hood over the same range back in Chicago…. But the was a 60 year old house which leaked like a sieve.
Yikes.
And to answer the above question, the duct opening is 8” and the length is perhaps 12” with a 90 degree turn to the outside exhaust.
Bryan
Nothing too exotic with the length and turns and such.
Background question: what are you actually wondering/Worried about?
It sounds like 'it is was it is.'. You'll get some reduction in flow rate with the duct transition. As a rule, most of the hoods in my projects never achieve rated flow most of the time on maximum. That doesn't mean they don't achieve their intended outcome (from the occupancy perspective) which is some nonzero amount of exhaust ventilation over the cooktop.
So you get lower flowrate... So what?
Luke,
Isn't that a pretty cynical way to look at a range hood install?
No. I don't think so.
I wasn't being rhetorical with the question. I want to know why Bryan cares about this so we can speak to that concern or underlying question. It's not cynical, it's empathetic
Hi guys, I guess I have two concerns. The first is the unlikely possibility that a future inspection could flag the non conforming hood and the second is what actual effect this hood would have on the house if run on high.
1. It’s a moderately large house by my standards, about 4,000 sq ft with 9’ ceilings, so a lot of cubic feet.
2. I’d never run the hood on high for more than a few minutes.
And worst case I could have a make up air system installed, but I’m trying to avoid this expense.
Thanks for all your thoughts and wisdom.
Bryan
The volume of the house (cubic feet) doesn't have much to do with the range hood. The area of the exterior walls, basically the leakiness of the building envelope, is what determines how much "accidental" makeup air you have available. That's really difficult to guess at. A blower door test will give you some real numbers. You have a pretty high flow hood though, so you'll probably need a makeup air intake to deal with that. There have been discussions on here before about various ways to do that.
You will want to avoid any "natural draft" gas appliances (this usually means water heaters) too, since you risk backdrafting with those. "Sealed combustion" units have their own supply of outdoor air, and are much safer when you're worried about negative indoor air pressure.
Bill
Bryan,
There are two types of issues. Practical and code.
Installing the range hood with too small ducting increases the static pressure on the fan, creates more noise, makes the risk of the ducts leaking higher, and reduces the flow-rate.
You are well over the 400CFM where make-up air is required by the IRC, and installing the smaller duct is also a code violation. For me you need a pretty good rationale for violating building codes on a new build.
https://codes.iccsafe.org/s/IRC2015NY/part-v-mechanical/IRC2015-Pt05-Ch15-SecM1503.4#:~:text=M1503.-,4%20Makeup%20air%20required.,not%20less%20than%20one%20damper.
Malcolm:
Do you have the code reference on the duct sizing issue?
Only if you have it ready to hand ...
I'll bookmark it for my work as I will find it helpful
Luke,
The IRC requires all materials and equipment to be installed according to the manufacturer's instructions. Not doing so is a c0de violation.
Ahh yes Excellent. Of course
I didn't get a chance to read through the installation instructions to note that. My bad