Need some input on kitchen exhaust in tight house.  The house will be 2300 sq ft and constructed with 2 x 6 walls and energy heel trusses.  The entire envelope(walls and roof)  of the house will be sprayed with foam and the ceilings will have an additional 2' of loose cellulose sprayed in.  Planning to use a 36" range hood that uses an 8" duct straight up thru the tile roof.  My concern is that Im not sure if this will dramatically reduce my energyy effieciency or allow alot of air to enter thru the exhaust.  The manufacturer. Pro-Line, said the unit has a back draft built in.  I'm not sure if this is enough and if not what are my alternatives or am I worrying too much.  Tom

If it has backdraft control, you need to find something better to worry about.

Such as...

In super tight houses your hot water/heater burners need to be direct-vent/sealed combustion to not backdraft whenever the kitchen exhaust or dryer is running.

If the dryer doesn't have backdraft control it can become the infiltration point when other exhaust fans are running.

If your house is THAT tight you'll need an active ventilation system/HRV, which becomes the low-impedance entry point for exhaust vent makeup air.

Hope that's all been designed-in and accounted for.

Sounds like you'll be north of R100 on the roof/attic, which is likely overkill if you're not also going high-R on the walls. Got any exterior foam (rigid-board or sprayed) for thermal breaks on the 2x6 framing? If not, skip the roof foam (keep the cellulose) and give yourself a couple inches of iso or XPS between the studs & siding with that money.

Dana1, suppose the question posed by sgtnarc1 was dealing with items that had their own separate vents. (Dryer vents as normal on the side of the house, hot water heater and furnace vented, though possibly combined due to close quarters and that the range hood has it's own exhaust. Would the HRV system then be needed?

A tight house requires a HRV or ERV or another sourse of fresh air.   People breath, fart, perspire, dogs, perfume,  cooking and bathroom odors etc are all chemicals in the air you are breathing.  In a tight house they don't go away without being removed.  Have you every walk into a motorhome  that had been vacent for a while?  That stale smell when you open the door.  The HRV eliminates that and insures you always breath fresh healthy air.  Yes they require electricity, but not much.  More importantly they exchange air, but transfer ~60-70% of the heat to the incoming air.

A tight house also needs make up air for things that vent to the outside.  Things like gas furnaces should get air directly from the outside and should have completely sealed combustion so there is no chance of backdrafting into the house. 

I think any well built house should have a HRV or ERV.  Cheapest health insurance out there.

Eric

Posted By Cgallaway on 19 Mar 2010 06:47 PM
Dana1, suppose the question posed by sgtnarc1 was dealing with items that had their own separate vents. (Dryer vents as normal on the side of the house, hot water heater and furnace vented, though possibly combined due to close quarters and that the range hood has it's own exhaust. Would the HRV system then be needed?

What eric anderson said.

Whether separately vented or commoned into a single flue, if the combustion air comes from conditioned space, dryers and hood-ranges, etc. are a backdrafting hazard in a very tight house.  The tighter you make your house, the more important direct-vented/sealed-combustion appliances are.  And when you're that tight active ventilation is a must.  But that's not a down-side- the end result is higher indoor air quality, lower utility bills.  Relying on a uncontrolled ventilation via flue stack pressure effects and random air infiltration is a crummy way to get ventilation air from the get-go, even if it's been the "standard" for millenia.

How does the HRV makeup the air lost by the kitchen exhaust hood in this example? It's my understanding that an HRV works much like a plate:plate heat exchanger, exhausting stale air and replacing it with fresh outside air with the plates transferring heat.

In the case of an exhaust hood, especially one of the "consumer professional" range hoods, nearly 700CFM is being exhausted. Is the HRV capable of responding to this level of depressurization or is a dedicated powered make-up air system necessary?

If you have 700 cfm exhaust, you need make up air, lots of it.  In a tight house that would be a huge number.  In my house that would be 3 air changes per hour just from the hood. 

The HRV does not have a damper on it so if the house is under negative preasure, air enters the house through the HRV.  The HRV core has some resistance so it is not the equivenlent of a 6" opening, but it  does let in alot of air.  I don't know how to quantify the amount.   The fans increase the flow.  When you have negative preasure in the house, but both fans are runing at equal power, levels more air will still flow into the house then out, decreasing the preasure differential.

I am not suggestion that the HRV be the method of preasure equalization.  It does help though. 

Eric

An HRV becomes the de-facto low-impedance entry point for makeup air when exhaust-only fans are running. There isn't any heat-recovery happing in that process. But when the negative pressure goes away (exhaust fans off, flues closed), the HRV resumes it's balanced heat-recovery mode.

I think sgtNarc1's question concerned the possible loss of energy thru the hood vent pipe when the hood fan is turned off. The backdraft damper is probably a flimsy hinged aluminum flapper held closed by gravity or maybe a weak spring. When its windy, negative pressure can be produced in the vent pipe which is open to the roof and cause the damper to open. Heat flows out in significant amounts over time as compared to other leaks in a tightly built house. The question is: how to prevent this heat loss without eliminating the hood altogether? Good question. Are there products on the market to address this problem?

http://www.energyfederation.org/consumer/default.php/cPath/30_742_1215
is a good quality roof cap with a tight sealing damper

for dryer vents and bath exaust vents, I use:
http://www.energyfederation.org/consumer/default.php/cPath/30_742_110
these seal nicely and are paintable so they can be matched to the siding.

I hoe this answers the original question.
Eric

I installed the FanTech roof cap from EFI that you referenced, Eric. Maybe it's due to the orientation of the house and the location of the cap high on the roof, but when the wind picks up (frequently, here) I can hear the damper opening and closing. What is needed is a damper that is opened by an electric motor or solenoid when the hood fan is turned on, and closes tightly when turned off. Any vent or flue traveling vertically through the roof will be a major air leak in a tightly built home due to the large pressure differential that develops. That's why I chose an electric fireplace over the gas log kind. No flue, no lost heat. I don't use bathroom exhaust fans either. The bathrooms are ventilated by exhaust ducts connected to the HRV. For my dryer vent I used the one you referenced above. It works very well.
Clark

Clark,
I have put 2 of them in houses and have not heard any complaints.  I can see it being a problem  on a pitched roof on the backside relative to the wind direction because of negative preasure differnential.  Unless it is a very tall house I can't see stack effect generating enough preasure to cause it to open.  You could also jury rig something so the exaust is  higher off the roof, the requirements for chimney exaust  heigh are designed to prevent them oporating under positive or negative preasure

Eric

Posted By SgtNarc1 on 19 Mar 2010 11:11 AM
Need some input on kitchen exhaust in tight house.  The house will be 2300 sq ft and constructed with 2 x 6 walls and energy heel trusses.  The entire envelope(walls and roof)  of the house will be sprayed with foam and the ceilings will have an additional 2' of loose cellulose sprayed in.  Planning to use a 36" range hood that uses an 8" duct straight up thru the tile roof.  My concern is that Im not sure if this will dramatically reduce my energyy effieciency or allow alot of air to enter thru the exhaust.  The manufacturer. Pro-Line, said the unit has a back draft built in.  I'm not sure if this is enough and if not what are my alternatives or am I worrying too much.  Tom

Sgtnarc1;

you will get more air coming in/out when you open and close an exterior door

Posted By cmkavala on 26 Mar 2010 05:35 PM

Posted By SgtNarc1 on 19 Mar 2010 11:11 AM
Need some input on kitchen exhaust in tight house.  The house will be 2300 sq ft and constructed with 2 x 6 walls and energy heel trusses.  The entire envelope(walls and roof)  of the house will be sprayed with foam and the ceilings will have an additional 2' of loose cellulose sprayed in.  Planning to use a 36" range hood that uses an 8" duct straight up thru the tile roof.  My concern is that Im not sure if this will dramatically reduce my energyy effieciency or allow alot of air to enter thru the exhaust.  The manufacturer. Pro-Line, said the unit has a back draft built in.  I'm not sure if this is enough and if not what are my alternatives or am I worrying too much.  Tom

Sgtnarc1;

you will get more air coming in/out when you open and close an exterior door

But, the heat lost through a leaky hood vent adds to the losses due to opening and closing an exterior door, losses which are unavoidable.  Why not minimize this waste with a good back draft damper?

Posted By Clark on 26 Mar 2010 07:21 PM

But, the heat lost through a leaky hood vent adds to the losses due to opening and closing an exterior door, losses which are unavoidable.  Why not minimize this waste with a good back draft damper?

I made the assumption it would have a back draft damper, you can also use and exhaust fan with integral damper and one on the roof cap or wall cap, providing double the protection

Posted By Clark on 25 Mar 2010 08:48 AM
I think sgtNarc1's question concerned the possible loss of energy thru the hood vent pipe when the hood fan is turned off. The backdraft damper is probably a flimsy hinged aluminum flapper held closed by gravity or maybe a weak spring. When its windy, negative pressure can be produced in the vent pipe which is open to the roof and cause the damper to open. Heat flows out in significant amounts over time as compared to other leaks in a tightly built house. The question is: how to prevent this heat loss without eliminating the hood altogether? Good question. Are there products on the market to address this problem?

In answer to my own question, what's needed is a motorized damper on the range hood vent.  Specifically, a round, in-line, gasketed, motorized, normally closed, 120 volt damper with an end switch.  American Aldes makes them.  One for a 6" duct can be purchased here.  The damper is wired so that it opens fully whenever the range hood fan is turned on.  The integrated end switch is wired to prevent the fan from operating until the damper is opened.  When the range hood fan is turned off, a spring returns the damper blade to its closed position, positively sealing off the duct.

Posted By Clark on 27 Mar 2010 10:48 AM

Posted By Clark on 25 Mar 2010 08:48 AM
I think sgtNarc1's question concerned the possible loss of energy thru the hood vent pipe when the hood fan is turned off. The backdraft damper is probably a flimsy hinged aluminum flapper held closed by gravity or maybe a weak spring. When its windy, negative pressure can be produced in the vent pipe which is open to the roof and cause the damper to open. Heat flows out in significant amounts over time as compared to other leaks in a tightly built house. The question is: how to prevent this heat loss without eliminating the hood altogether? Good question. Are there products on the market to address this problem?

In answer to my own question, what's needed is a motorized damper on the range hood vent.  Specifically, a round, in-line, gasketed, motorized, normally closed, 120 volt damper with an end switch.  American Aldes makes them.  One for a 6" duct can be purchased here.  The damper is wired so that it opens fully whenever the range hood fan is turned on.  The integrated end switch is wired to prevent the fan from operating until the damper is opened.  When the range hood fan is turned off, a spring returns the damper blade to its closed position, positively sealing off the duct.

BAD IDEA........... I am pretty sure those are not intended for range hood use , that are normally 7" dia ducts or 3-1/4" x 10"

I have seen similar fans used for dryer vent boosters or spot ventialtion, but never for range hood applications where grease can accumulate

Take a closer look, Chris. It's not a fan. It's a damper. My fan is a separate in-line unit which is designed for range hood use (sold by FanTech) and has 6" connections on both ends. The motorized damper itself would not be affected by greasy exhaust any more than, and probably less than, a gravity damper.

Posted By Clark on 27 Mar 2010 02:27 PM
Take a closer look, Chris. It's not a fan. It's a damper. My fan is a separate in-line unit which is designed for range hood use (sold by FanTech) and has 6" connections on both ends. The motorized damper itself would not be affected by greasy exhaust any more than, and probably less than, a gravity damper.

Clark;

range hoods do not come with a 6" round duct, I beleive the intent of the damper is for ventilation control for kitchens and other areas, I did not see where it was intended for range hood use, which is different than ventilating a room

use a ductless kitchen hood..............problem solved