I am designing a green home and have the envelope specified. However, I am not sure what to do about specific exhaust requirements, particularly the stove exhaust hood which needs a significant amount of air. My current hood (loose house) uses 1200cfm when turned up full (rarely).

Even with a tight home you don't have to concern yourself with individual appliance exhaust. If you are building a fully tight building envelope you do want to install an Air to Air Heat Exchanger that will pull out the moisture that gets trapped in these home and supplies fresh air.

But one wants an exhaust vent over the stove for other reasons, like grease and particulate matter from combustion. It's not good enough to just filter this, rather you want to vent it out. At least that's what building science dot com says. But I've read that some stove exhausts are so strong they are actually creating a negative pressure in the home. Any ideas?

Panel Guy,

Thanks. I am assuming from your answer that after the hood's filters the outlet is vented into the exchanger so the only vents through the envelope are associated with normal air exchange. It also suggests that using the hood is a significant energy hog, even with the exchanger.

Jelly,

I know from experience that even in my current loose house you have to watch out for backdrafting. The hood moves a lot of air. I certainly don't want to recirculate that air back into the house. One of the services the hood performs is to signifantly lower the prevalence of cooking smells.

I may be missing something here or reading too much into the previous comment, but I would not want to run kitchen hood vented air through a heat exchanger.  I would have two concerns:  Grease lowering the efficiency of the exchanger and a chance of fires from the condensed grease in the exchanger. 

Alton,

I was not suggesting that you vent the kitchen hood through the heat exchanger. Vent the hood directly to the outside as with any single appliance that requires direct venting. That hood will not move enough air that you have to concern yourself with back drafting.

An Air to Air heat exchanger is a necessity for tight home construction, whether it be advanced framing, SIPs, ICFs or a very concious stick framer doing the best with what he's working with and actually achieves a tight home. You are going to see major changes in the HVAC industry as it begins to understand how much better we are building structures in the 21st century. Mandates through the national building codes are coming.

Hmmm. In a very tight(SIP or ICF) house? I don't agree.

PC (panelcrafters),

If the hood vent is attached to an airplane engine then there is a problem. If the hood is a commercial type that moves a large volume of air, it may be a problem. It has not been my experience as a general contractor that builds full SIP and ICF building envelopes that there is any standard kitchen stove hood vent that creates a problem for lack of air to vent itself. If you know differently, share your experience.

Let's put numbers into perspective.

A 2,000 sf house with 8' ceilings has a volume of 16,000cf.  You will find recommendations of residential air change rates varying from .3 air change/hr up to 1 air change/hr.

For our 16,000cf example house, .5 air change/hr=8,000cf/hr=133cfm.  Most ERV/HRV are between 100 and 300cfm. 

Running 1200cfm kitchen exhuast on high can be a big deal, especially if you have a tight house and gas appliances or a fireplace.  Add up the amount of air required by exhaust fans, fireplaces, gas appliances, etc.  While these do not run continuously, they need to be considered along with return air paths, etc. in the design of a home.  In a home (2x6 stick) I used to own in Minneapolis, I needed to open a window to make the fireplace draw.

Since air infiltration is a bigger factor than insulation in having an energy efficient home (IMHO, at least), having a tight building is really important. 

The question for me is how much adjustment the current ERV/HRVs permit in varing the amounts of supply air vs. relief air in oreder to keep a positive or negative environment.  Anyone have first hand experience?

PC is correct in that I doubt most people would detect a difference due to an exhaust fan running or even an ERV/HRV running at full supply or full exhaust, but I think a lot more attentiontion will be paid to residential HVAC in the coming years.

In commercial applications, kitchen hoods have their own make up air and the kitchens are usually designed to be a bit negative compared to the surrounding areas to prevent food odors from spreading.  Most commercial buildings are designed to operate slightly positive to minimize air infiltration.  The energy needed to condition outside air and the fan horsepower spent to keep a building positive is usually well spent AND it improves the indoor environment.

Bruce

I forgot to add that most residential exhaust fans operate at low static pressure, meaning that unless there is ventilation air available to replace the air they are exhausting, they will not work at their design capacity.  An exhaust fan in a tightly enlosed bathroom will not work very well.  Open the door a bit to provide some ventilation air and its efficiency greatly improves.

Bruce

ERVs will not compensate for exhaust fan negative pressure

Not sure I understand completely, but what is the bottom line?  What system is recommended for kitchen exhaust?
I have an ICF home with a standard hood, Vent a Hood, and detect no problems.  But always looking for new info.

Thanks,  John

Thanks for all the input. I found it all useful. Here's my take on the problem in the context of a tight home. This is not an expert opinion just my solution to the problem.

1. Vents hoods need their own makeup air. They tend to run way below max, but as a safety feature, a stove fire automatically kicks them in to their maximum setting.
2. To lower the volume requirements for the hood, I will design a stove setting that has at least partial enclosure at the sides. I've noticed this design before. It looks good, but I haven't thought of it as a practical issue.
3. The makeup air needs to be delivered as close to the stove as possible. That way I can minimize the amount of conditioned air the vent removes. My current kitchen has wings that allow you to slide a saucepan sideways onto them. It might make sense to route some makeup air through them and put an enclosure outside the wings.
4. In the home I am designing backdraft should not be a problem. There will be no fireplace and any gas burning devices will have their own air supply, but I think I will pay attention to the pressure issue any way. I see a lot of good advice here that suggests there is substantial benefit to maintaining a slight positive environment.

Thanks again for all the advice. This site has been incredible in giving me both context and practical solutions.

Throwing in my 2 cents.

In our last home we before we had an HRV if we had the kitchen exhaust fan on, even on low and the convection natural gas water heater kicked on, it would not start a draft, you could feel air falling out of the chimney.

After we added the HRV we never had an issue. My theory was that even if we were dumping 150CFM out and pulling 150 CFM back in through the HRV when the exhaust hood kicked on a lot more air came in through the HRV, and probably less out as well. You could easily feel the difference in the amount of air, if I had to guess it was about double or 300 CFM coming in. Granted it wasn't making up for all the air getting sucked out the exhaust hood, but I would rather a bunch come in through the HRV then backwards down a chimney or leak in from where ever. I am sure some still leaked in, but we were much better off then before the HRV.

Basically and HRV or ERV is on open 6 inch line in and out with a small fan to help move it. If the house is positive pressure more will leak out through the HRV / ERV, if it is on the negative side more will come in through the HRV / ERV.

300cfm is reasonable for a standard 30 inch wide electric or gas range and oven.  Many people want the roaring quasi-pro gas ranges, and those vent hoods can definitely affect a tight house.  Yeah, OK, those ranges are not really in the spirit of green, but if I built houses for a living, I think I'd have to give 'em what they wanted.

Here is an appliance FAQ on Gardenweb forums that discusses the required vent flows for ranges, and mentions make-up air is for larger vent hoods.
http://ths.gardenweb.com/faq/lists/appl/2003055538004617.html

"The rule-of-thumb suggests 10CFM per 1000BTU of burner capacity"
"Ona alternative solution is to install a special intake duct near the range:"

of course, in a tight house i'd want a separate outside air supply to my furnace, too, jut to avoid issues with negative pressure.

Regards,
Doug