Designing a MUA system for a 600 CFM hood fan

[MagnusEisengrim]

Hello. I am attempting to get a building permit for a a home renovation (my own home). The HVAC system is not being touched but I do intend to install a new outside-venting 600 CFM max fan over a gas rangetop. I rec'd a deficiency letter from the permit office because of concern over house depressurization associated with the fan. My house is old (1960s) so I know there will be a fair amount of leakage.

Depressurization as I understand it has two main concerns associated with it. 1) Backdrafting of chimney exhausted gas appliances and/or solid fuel appliances and 2) comfort of the occupants. (Soil gas can also be a problem but not where I am)

I have no appliances of the type described in 1).

So my plan is to install a make-up heated air system that is locked to the fan operation, but I am limited in both physical space and capacity on my electrical panel.

So knowing just enough to be dangerous, I found an HRAI form that gives an air infiltration flow based on floor area, a flow factor, and an allowable pressure drop. Floor area is 2600 sq.ft (both floors + basement + half of crawl space is permitted) x 2.5 ACH (old house) x 0.037 (flow factor associated with -10 pa). Or 2600x2.5x.037 = 240 CFM.

So if that is a valid approach I have 360 CFM to make up. However the 600 CFM hood fan will not generate 600 CFM with a deltaP of 10 pa. How do I determine what the flow rate might be reduced to?
As well their is downstream ducting from the fan: 1 foot of 6" pipe, one 90 degree elbow and then an exhaust cap - assume some small pressure loss will occur there too?

In order to make up the ~360 CFM I plan to install a FANTECH MUAS750 along with a 6kw heater - the MUAS750 is rated for 750 CFM max and can be set to provide air so that a slight (-5, -10 pa) pressure is maintained. That device has an 8" collar, but I need to run the ducting from the ceiling of the first floor to the basement as 6" round (or maybe rectangular, but assume round for now). That run is intake + two eight foot sections of 6" pipe and two 90 degree elbows and then would be transitioned to an 8" duct before feeding the MUAS. The downstream ducting will be large in size so can be ignored for now. How can I determine if 6" line is ok from a deltaP/code point of view? A 6KW heater should provide a deltaT of 6/360*3193 = 53F/30F for 360 CFM, so I should be ok there. I need to design for a -18C day, that still gets me to +12C.

Any thoughs/help/etc? Am I way off base? I should note that the fan is capable of 600CFM bust has four speeds. The high speed/flow would only be used rarely.

Thanks.
Jeff

 

[lukaiENG]

in my jurisdiction, the requirement for needing to provision make-up air was if the KEF is over 400 CFM.
If I were you I would select a 400 CFM KEF like I did. We never run KEF past 2/3 speed so I've never had any flow issues.

 

[MagnusEisengrim]

In Ontario the limit is 160 cfm. More than that and you need to take some other kind of action.

 

[willard3]

Rather than making dangerous and expensive mistakes and misunderstandings with free engineering from a forum, hire an HVAC Engineer.

 

[dbill74]

Where did you come up with needing 600 CFM in your hood? That is a lot of air for a residential hood/range.

I'm with Willard, you need to find someone locally that can help you with your design.

Too much infiltration can damage your house by forcing air through cracks in the structure or compromising weather sealing around doors and windows.

 

[MagnusEisengrim]

Well I appreciate the comment on hiring an HVAC engineer (and I did consult one of how they deal with "large" hood fans - hence my intention to install a MUA system) I was hoping to get some feedback on basic calcs. In the meantime I believe I've managed to find the relevant ones and complete a blower test on my house. My house leaks ~4.5 ACH (as opposed to the 2.5 ACH I could find to use w/o a test) which gives (at 600 CFM) about 14 pa depressurization. With the downstream ducting losses on the fan (6") that drops the 600 to a maximum of 560 CFM (I could not find loss info on the actual wall vent cap, so it will really be less than this). So if I supply 300 CFM of make-up air the steady-state depressurization will be ~ 7 pa. Given that I have no chimney-vented gas appliance and no solid fuel burning appliances this should be just fine. As well I am in a low-risk location for Radon (although I did get a test kit to measure it - takes 3 months to do the test). And finally the amount of time we will select the max flow is quite limited; only when things are going wrong (burnt stuff) or very short times of high heat searing.

And in terms of 600CFM being *a lot* of air for a residential hood, I do not agree. In new kitchens homeowners and contractors are installing 600-1200 CFM hoods over commercial style gas ranges/cooktops (wolf, bluestar, Viking, etc). The true concern is that many are doing it without understanding the depressurization risks.

If anyone has more constructive feedback, I would appreciate it.

 

[GT-EGR]

Your question covers quite the range of HVAC topics which is why getting a professional to competently address all of them is best - it will be difficult to successfully get all these inter related items to come together with whatever answers you’ll get here. These are some of the questions and issues you are facing

First off - what is the basis of your 600 CFM exhaust, is that from the exhaust hood manufacturer based on your range hood size, installation type, and location? You need to confidently finalize this number first or everything else after that will be off too.

I don’t do residential work so maybe I’m overkill, but your makeup CFM should meet or exceed your exhaust. Right now it looks like you are allowing your house to go negative to keep with a lower makeup air unit size - while you’ll save some fan install money, you are just going to be bringing in that outside air to unknown locations in your house rather than directly supply treat and filter it. Also the air change calc is theoretical and you may find your house tighter and end up needing a higher household negative pressure to suck the makeup air into your house cracks.

As for pressure drops, you have to account for every fitting and straight duct length and component that the air has to travel through from start to finish. Than apply your design CFM through it and SMACNA can give you straight duct losses and fitting losses and intake louvers for your situation. Just make sure your makeup air unit clearly covers what you consider external and also what it has internally (filter, coil, etc) as well.

Once you install the makeup air unit you’ll select the blower speed and test out the pressure you get and then adjust the blower speed until you get the internal pressure you want

 

[lilliput1]

We use propane gas grill out on the deck outdoors for grilling.