The cathedralized attic as air-preparation area?
I’d appreciate your thoughts on this strategy for a dedicated outdoor air system in my mixed-humid climate. The idea would be to have radiant heating and cooling with a separate dedicated outdoor air system for ventilation and dehumidification (“the perfect HVAC” per Building Science Corp). I offer specific numbers for my situation, but the question is more general as an approach.
1) the 2-story house is tight and wrapped in a class 1 vapor barrier (perm<0.1) 2) the attic is cathedralized, with external insulation above the deck (R-50) 3) There is spot ventilation directly to the outside in bathrooms, laundry room (continuous) and kitchen (e.g. net continuous total 50 cfm) 4) an air handler in the attic takes attic air and vents through ducts down into the first and second floors (200 cfm). This creates a persistent positive pressure in the living quarters (+150 cfm). The air returns through grilles in the upstairs rooms back into the attic. The net result is to localize the negative pressure to the attic year-round. The attic has a trickle gable window or other pressure-sensitive vent that allows in make-up air (50 cfm). 5) the attic has a stand-alone dehumidifier (dew point 55). The entering humid make-up air (50 cfm) is dehumidified and mixed with the rest of the attic air (attic is ~4500 cubic feet). 6) a portion of the heating/cooling radiant tubing also courses through the attic to keep the attic air at house temperature. 7) an ERV in the attic exchanges the conditioned attic air at 40 cfm (4 occupants per code, total 40+50 make-up=90 cfm) to total 200 cfm (large party). The fresh air is mixed in dehumidified attic air before being vented by the air handler into the house. In the summer, entry of humid air is thus controlled into the attic space where it is dehumidified. In the winter, the warm air rising into the attic is recycled to warm the house and cold make-up air entry is warmed in the attic prior to distribution. My main concern is whether 90 cfm fresh air mixed into 4500 cubic feet prior to distribution will be sufficient ventilation. At base 4 occupants, the distributed attic air (200cfm) will be 98% recirculated and 2% fresh air (90/4500), which is only 4 cfm of fresh air coming through the ducts into the living space. Nevertheless, it is still true that the house as a whole is getting the right cfm and is well mixed and well distributed throughout the house. Of course, there may be some other glaring problem I have not thought about.
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Replies
Richard,
1. What's your climate?
2. You said you are "wrapping your house in a class 1 vapor barrier (perm<0.1)." Why? Where is this low-perm layer? Inside or outside the framing? How are you building your wall assemblies and roof assemblies?
3. Why have you chosen a balanced ventilation system (exhaust plus supply) without any heat-recovery? What's wrong with an HRV?
4. Your conditioned attic is within your conditioned envelope, so blowing attic air into your house doesn't necessarily pressurize your house.
5. Your complicated ventilation scheme attempts to re-invent the wheel. Not sure what the advantages are, if any.
Martin,
Thanks for the response, and sorry to cause confusion.
1. I stated my climate is mixed-humid.
2. I would be wrapping outside the framing (sorry, didn't realize one could wrap a house on the inside). The structure is all masonry (part brick retrofit, part addition with AAC), then vapor barrier then outsulation. My basic point was that it would be tight, would have radiant heating and cooling (1.7 tons each per Manual J), and just needs ventilation, dehumidification and distribution.
3. I listed ERV. Does that not have heat recovery? I thought it had both heat and humidity recovery. Nothing wrong with an ERV, but I might save on dehumidifying incoming air with an ERV.
4. The intended benefit of blowing air into the lower floors would be to position (control) the make-up air point of entry to be in the attic, where I can condition it as it enters. This would follow the basic recommendation of Joe Lstiburek's ""Humidity control in the humid south," where he has an active fan bringing in make-up air into a "pre-conditioning" area. (Figure 12) (http://www.buildingscience.com/documents/reports/rr-9302-humidity-control-in-the-humid-south/?topic=/doctypes/researchreport). The intended benefit of my approach is that by turning the fan into a pressure-sensitive air inlet, I can control all make up air to come in in the attic for pre-conditioning.
5. I've seen many drawings but none for one that does not include a forced air HVAC system. But I'm thinking the Aprilaire ERV-into-the-air-handler idea is probably best. The difference is that my air handler is not a central forced air heating/cooling system but a simpler ducted fan for ventilation alone. My design would allow me to mix the fresh air from the ERV with the attic air to temper it before distribution, and would bring all make-up air into the attic for preconditioning. I hope that is clearer and you can guide me if I'm missing something.
Richard,
1. In most cases I'd be very wary about installing a low-perm vapor barrier on the inside of the insulation, but if all of your insulation is rigid foam, you should be okay.
2. Sorry I missed the reference to the ERV. If you've got an ERV, that's your ventilation system. You don't need separate exhaust fans from your bathroom and laundry room, because of course you want to reclaim the heat from your exhaust air. That's the whole purpose of an ERV or HRV. So, install dedicated ventilation ductwork; pull your exhaust air from your bathrooms and laundry room through your ERV in order to perform the needed heat exchange. Deliver fresh air from your ERV through dedicated ventilation ductwork to your bedrooms and living room.
3. Hydronic in-floor cooling is experimental; obviously you must be aware of the danger of condensation on your floors. You'll have to do a good job dehumidifying. Your dehumidifier can be almost anywhere in the house -- a stand-alone system or hooked up to your forced-air system; it doesn't really matter. If your dehumidifier ever conks out, your floors will get very slippery, very fast.
4. Your complicated pressure balancing scheme -- so much exhaust ventilation directly outdoors, so much "pressurization" from the conditioned attic (which is actually within your pressure envelope), and a "trickle gable window" (whatever that means) to allow leaks into your attic -- is mostly hogwash. The ERV does the pressure balancing for you if you set it up right.
Richard,
Your system will not work, but not for the reasons Martin suggests.
And I beg to differ with him in integrating bathroom and laundry exhaust into a central ERV. If the ERV's purpose, in addition to energy recovery, is to dehumidify incoming air than you certainly don't want to recycle humidity from source points such as shower and laundry - those require independent exhaust fans (as Bruce Davis - an expert in the field http://sivadhome.com/documents/Davis-Resume.pdf - explains in "Use and Misuse of ERVs" http://www.advancedenergy.org/buildings/knowledge_library/ventilation/erv.pdf)
The problem with your creative system is that it dehumidifies and provides fresh air in the attic only, which is then diluted 3:1 for distribution throughout the house, and then you're using a cooling system which almost guarantees condensation on the radiant surfaces. You may not have sufficient dehumidification. You certainly will not meet ASHRAE 62.2 ventilation standards. And, on top of that, you're using a cooling system designed to make a bad situation worse.
Eliminate the radiant cooling, which then requires central AC ducting. Supply fresh, dehumidified air to the whole house with an ERV, which then requires additional centralized ducting for ventilation. Use separate exhaust fans for laundry and bath and kitchen hood, which then requires additional dedicated ducting.
Or, use some variation of the BSC system, with interlocked supply and exhaust fans overbalanced on the supply side to maintain positive pressure.
Martin's comment from Sept 7
Martin,
I have read the same advice(as yours) from other sources.
However, I have recently been reading this book :
http://www.buildingsciencepress.com/%2FThe-ASHRAE-Guide-for-Buildings-in-Hot-Humid-Climates-P35.aspx
According to Lewis Harriman... It is better to Dry the ventilation air BEFORE it mixes into the living space air.
The book has a commercial focus but I would think that most of the concepts apply to residential as well.
On another note.
I live in a tight house in a Hot/Mixed Humid Climate.
I have a standalone dehumidifier but I only need to use it for a few hours during the year in the spring and fall.
So standalone is simple and cheap.
John,
The Lewis Harriman quote is only half the battle.
It's certainly helpful to dehumidify incoming ventilation air with an ERV, but an ERV should not feed into the HVAC coils or the air will pick up condensate when the AC is not running and increase indoor humidity.
During AC cycles, it's valuable to dehumidify return air before cooling, as this will reduce conditioning costs (up to 87% of AC electrical consumption, according on one reputable source, is used for condensing water).
But the only way to dehumidify the interior environment is with a recirculating-air AC or dehumidifier. In other words, dehumidifying incoming outside air helps lower the latent load but still adds additional humidity, so the only way to dehumidify the air in the house is to dehumidify the air already in the house.
Robert,
If I understand correctly what Lew is doing in the commercial buildings.
He is drying the supply air BELOW the humidity control set point.
John,
That might make economic sense in a large commercial building that requires dehumidification and air conditioning year round (because of large occupancy or process energy consumption and windows that don't open, e.g.) and the AC costs are astronomical. But, for residential applications, I don't think it's practical.
It reminds me of the picture I once saw of an old Maine farmhouse which was surrounded on the outside with a Rube Goldberg of cast iron radiators and connecting steam pipes. The house leaked so much that the owner figured it was cheaper to heat the air before it came in.
Richard, a few thoughts and suggestions:
Don't like the radiant cooling for condensation reasons mentioned above.
I believe you are woefully underestimating the ventilation contribution in #3 on your list. Continuous kitchen, continuous laundry, and spot bath fans = only 50 cfm effective? I do like this strategy though.
The ERV will do all your ventilation work for you. Eliminate the damper in the attic.
I would suggest making your attic air handler a fan-coil unit and use it for the cooling load. 1.7 tons is very minuscule, the ducts will be of minimal size (or at least minimal increase in size).
Intentional conditioning of the attic space will not be necessary other than what energy makes it's way there. I have experience with dozens of cathedralized attics. Additional conditioning is not needed. I am in Rochester, NY and my cathedralized attic (R-52 external) is slightly cooler than the second floor in winter and summer.
In regard to all mechanical systems in efficient buildings. I try to keep them as simple and inexpensive as possible, mainly because they are almost not needed. You've handled your point-source pollution well, let the ERV and air handler do the rest.
-Rob
Transfer air (i.e., air from adjacent dwelling units or other spaces such as garages, crawlspaces, or attics) shall not be used to meet ventilation requirements (ASHRAE 62.2 section 6.1).
Thanks to all respondents.
Martin and Rob, you suggest I use the ERV itself to supply make-up air. I know that the RecoupAerator allows for make-up air, but I was hoping to use filters that are washable and don't need replacement. (I know the RecoupAerator does also remove the risk of leakage, since there is no condensation). Is there an ERV that you know of in the 90-400 cfm range that can be made off-balance to provide more intake than exhaust? All the ones I have seen are ERV's with neutral, balanced ventilation.
Robert, is the code also for cathedralized attics within the conditioned space? Can I use the attic as the conditioned room from which I draw air into an A/C central return, just as if I had a central return in a hallway? Attic instead of hallway.
Richard,
You were the one who pointed out that you were planning to use an ERV. Although I provided advice on the best way to install ductwork for your ERV, I actually think an HRV makes for sense for your application.
I'm not sure what you mean by "makeup air." An HRV or an ERV provides balanced ventilation, providing fresh outdoor air in roughly the same quantities as exhaust air is removed. It isn't very hard to adjust an HRV or an ERV (using balancing dampers) to be slightly unbalanced -- so that your are either slightly depressurizing or slightly pressurizing your house, if you want to.
However, if you really don't want balanced ventilation -- if you want a supply ventilation system to pressurize your house -- then maybe you should just go ahead and install a supply ventilation system instead of an ERV, which is designed to be balanced.
The term "makeup air" is usually used to refer to air that replaces the air removed by an exhaust appliance (for example, a clothes dryer, a central vacuum cleaner system, or a range hood), or air to replace combustion air burned by an atmospherically vented appliance (for example, a water heater or furnace).