Determining make-up air requirements
I have built a very ‘tight’ house with minimal air leakage. Since the original design avoided any externally-vented appliances or unsealed combustion appliances, make-up air was not an issue. I have installed an HRV for air exchange. However, I am now needing to install a vented kitchen rangehood for compliance reasons (I originally planned to install an unvented hood), and am questioning what effect this will have on the house if I do it.
The building is about 10,800 cubic feet of interior volume, the blower door testing yielded an ACH50=0.4, and the proposed rangehood would draw about 270 CFM on high setting. Based on this scenario, the rangehood would draw out many times more air than could leak back in without significantly depressurizing the space, correct?
I’m wondering what effect this will have on the HRV, as well as what measures are normally taken in a situation like this.
My goal from an energy standpoint is to avoid ventilation other than what passes through the HRV…
Replies
Wire it to a relay that will start the HRV when the hood is turned on.
The HRV only provides air exchange, not make up air. I don't see how this will work.Shawn
If there is a pressure differential the HRV will correct that differential. The question is whether it will do it rapidly enough.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
What I've done in our house is to set up a dampered makup air system with a Honeywell EARD5 24V damper, which is activated via a voltage detect device (no direct attachment to the 1100 cfm hood fan wiring). When you turn on the variable speed blower a device detects the voltage and opens the motorized damper. Total cost about $250 and works great.
View Image
I understand what you are saying about intake thru the ERV (we have one too) but the intake grill in our case is very close to the hood, making tempering unnecessary - at least in our climate.
Jeff
Edited 10/16/2009 8:18 am ET by Jeff_Clarke
Jeff - does that simply install in (through) the wall as a stand alone device, or is it part of another dust system?
Standalone:View Image on the outside,
standard 4" to 5" transition ...
EARD5
(pictured above)
5" to 4" transition
then a 4" drop down to a grille right behind a microwave niche next to the range.
Here's the key - current sensing relay - http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR4395 -
View Image
It's adjustable in sensitivity - you take the hot feed to the fan motor through the loop above - maybe an extra turn IIRC.
Jeff
Edited 10/16/2009 3:37 pm ET by Jeff_Clarke
Seems like you'd get serious air movement around that damper when it's closed. Does it have any type weather stripping the damper closes against to mitigate that movement?
Enters through unheated attic so it doesn't matter - but I do think it closes off fairly well.
Jeff
You can get them fully weatherstripped for fairly tight closure. I recently bought a backdraft damper w/ a high quality tight fitting seal and a damper that moves freely. Very nice!
So you are implying that when the range hood is on, dampers in the exhaust air part of the HX close? ... 'Course doesn't that screw up your HX process where you have lots of ventilation air coming in, but no exhaust air going across the HX?
Hmmm - by HX (Heat Exchanger??) do you mean our ERV (Energy Recovery Ventilator)??
No dampers close or open there - they seem to be far enough (50' +) away from each other not to cross-interfere.
Jeff
Edited 10/19/2009 8:35 pm ET by Jeff_Clarke
Yes, I do. The HRV has a HX in it to recover heat from the exhaust and put it in the ventilation/outside air intake (or vice versa).
I misread your post and thought you were implying dampers somewhere in the HRV system. You were saying provide a DEDICATED outside/ventilation air makeup system to serve the range hood using the motorized damper controlled/triggered by the range hood fan use. Sorry.
Yup.
I think that's a great idea, Jeff. Now you got me thinking about dedicated make up air for the bathroom fans.
You have a problem - and your stated goals will only make it worse.
Any sort of baffle that you pass cooking fumes through will collect all the moisture, oil, whatever that the hood is trying to vent. Don't count on the filters to get it all. The collected grime will soon become a rancid breeding ground for all manner of vermin - and a fire hazard as well.
That's but one of the problems with the idea of an 'airtight' house. Energy conservation is fine, but the key is controlled ventilation, not no ventilation.
Some parts of the house create things you want to get out of the house, as quickly as possible. The bathroom and kitchen are the most obvious such places.
If you're making the house 'tight,' then you need to allow fresh air to come in. It almost becomes necessary to have a dedicated air inlet to supply fresh air to every area served by an exhaust fan. For a kitchen exhaust fan, this almost has to mean an inlet either under or behind the stove.
Some commercial range hoods also supply fresh air to the outer face of the hood; when you turn the exhaust on, you also turn on the fan supplying fresh air to the hood.
The same issue arisies with bathroom exhaust fans; that moisture can't be removed, no matter how big the fan, if there isn't some way for fresh air to enter. Your challenge there is to heat the make-up air before it contacts the bather.
Every air intake presents a series of on-going challenges. For example, my furnace is supplied by a duct in the floor, which penetrates the side of the slab near the front door. Many folks living in this type of place will try to conceal, fill, or close off that intake, as they do not know what purpose it serves. Conversely, this duct is a magnet for blown leaves, and provides a direct route for vermin to enter the house. Every try to light your pilot light, only to have a black widow come out?
Still, that's the key: In a 'tight' house, everything that needs to have air exhausted also needs a supply of fresh air. I'd forget about trying to extract every bit of energy from this airflow; just make sure to direct it where it is needed. Chances are that such a directed supply will hardly interact with the conditioned air in the house: it will come in, do it's job, and be exhausted.
I don't think you can fulfill your goal of having non-HRV supplied ventilation.
I would use the same thing Jeff suggested. We have used that Honeywell system and it works as you would expect.
One suggestion I read a long time ago was to bring the make-up air in behind the refrigerator. At least a portion of the time, the cold exterior air will be somewhat tempered by the coils on the fridge. This will not eliminate the problem of bringing untempered air in, but will mitigate the concern somewhat with no cost to you. It would likely even help the fridge to run more efficiently when the range hood is on.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA