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More Q&A Spotlight
Our Question of the Week focuses on a query from “DC,” a Texas reader who wants to know which residential ventilation system will provide the “most bang for the buck.”
DC knows that a tight home requires a mechanical ventilation system to provide fresh air. But how does one choose among the bewildering array of options? And are there any performance advantages to expensive ventilation systems?
Armando Cobo recommends a balanced ventilation system without heat recovery, explaining, “I would spend NO money on an HRV or ERV — you do not need it in your climate.”
Martin Holladay recommends a central-fan-integrated supply ventilation system. Such a system requires an AirCycler control wired to a motorized damper controlling the flow of fresh outdoor air to the return plenum of the home’s furnace.
Robert Riversong recommends a balanced system with an ERV or a supply-only system.
FURTHER RESOURCES
Designing a Good Ventilation System
Can Houses be “Too Insulated” or “Too Tight”?
Exhaust-Only Ventilation Systems
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10 Comments
DOE research on the subject
It just so happens that the DOE Building Technologies Program has researched this issue. You can read their report here: http://www.nrel.gov/docs/fy05osti/36643.pdf. The main issue tackled here is the tendency for humidity control to be a major factor in the comfort of energy efficient homes with ventilation systems, in Texas. Basically, these homes tend to have efficient envelopes, which reduces the amount of sensible cooling required. At the same time, the introduction of ventilation air increases the indoor humidity. This increases the ratio of latent to sensible cooling. The result, is that the traditional air conditioner cannot remove sufficient moisture (they are equiped to deal with lower ratios of latent to sensible cooling). So, a separate dehumidifier is required in efficient homes with mechanical ventilation. Read the report, but I believe the lowest cost upfront and the lowest operating cost system was the exhaust only ventilation with a stand-alone dehumidifier in the closet of the first floor.
Also worth reading is a report by the guys at LBNL: http://epb.lbl.gov/publications/lbnl-62446.pdf. They don't investigate the humidity issue in much detail, but they found the exhaust only systems to be the lowest cost in Houston. Though, these systems tend to produce lower overall air change when compared with balanced systems. So, that is certainly part of the cost-control feature.
Humidity Is Not A Big Problem In My Part Of Texas
Brennan - Texas is a big state with three different climate zones one of which has both warm-humid and dry areas. I live on the border where Zone 3 meets Zone 4 and are not near the warm-humid line. We are well over 500 miles from Houston. Your advice may be sound for Houston but would likely not apply to my location. Here is some climate data I posted with my original question:
Climate is fairly dry. Not as dry as Arizona, New Mexico or the El Paso region of Texas but much drier than the eastern half of the state. We're practically a desert compared to Houston.
Lubbock Climate Data:
Annual HDD = 3431
Annual CDD = 1689
Annual Days w/max temp over 90 = 80
Annual Days w/min temp below 32 = 93
Average annual rain = 18.6"
Average annual snow = 9.9"
Average wind speed = 12.4 mph
I would be interested in any reports you know of that better pertain to my climate.
Thanks for the input.
Martin - I wonder if folks would change their votes if they knew my exact location rather than just "Texas"?
DC
Looking at the general trend...
DC, thanks for the update on your climate info. I am not aware of any statewide studies that have been done in Texas, which would compare the internal climate zones. What I can say is a generalization, which I believe applies. If you look at the LBNL research, it suggests the general trend of exhaust only systems having lower costs--this includes fan power usage, increases in heating/cooling costs, and distribution of air. So, for the lowest cost system, I would still recommend exhaust only. But, this doesn't take into account the proper mixing of the air, or its distribution. This distribution tends to have a large energy penalty, because the central blower fan is usually used. So, an ERV that doesn't use the central blower may be of use (http://www2.panasonic.com/consumer-electronics/shop/Building-Products/Ventilation-Systems/model.FV-04VE1_11002_7000000000000005702). But savings will be minimal, because these units tend to recover heat for heating better than they exchange heat for cooling (66% vs 33% approx).
Another consideration is that with exhaust only, you will be pulling make up air through leakage points in the walls, floor and ceiling, rather than through a dedicated/safe duct. In Houston, this poses a problem, because condensation can occur as this warm humid air travels through the air conditioned walls. This should not be a concern in your region. But by having a balanced system, you can rest more assured that your ventilation air isn't full of dust or mold. These systems increase the total air exchange, which increases space conditioning and fan costs, but also results in better ventilation.
So, long story short. If keeping operation costs to the minimum is your priority, then use an exhaust only. If maximizing the ventilation air is important, and you are willing to suffer an energy penalty (though not a huge one), then a balanced exhaust/supply system can work. Just don't use the central blower to distribute the air, because it will increase costs dramatically. Wish I could be more specific.
On ventilation I would use a
The reason for my answer to DC, in Lubbock, TX, is that the HVAC system can run at LOW SPEED by the IAQ TStat using very little energy. A separate HRV system for a whole house normally costs around ±$1,500 installed and the “new” spot ERVs are in the ±$600 installed but you may require 2 or 3 spot ERV depending on the size and design of your house. Houston is a whole different story.
motorized damper
In a Michigan home would a motorized damper used to provide make up air to the clothes dryer be a good method to reduce the conditioned air expelled by the dryer? We do alot of laundry in my house with children.
Response to Dan
Dan,
Unless you can find a way to build a sealed dryer, introducing exterior air to the laundry room isn't going to change anything. The dryer will still be exhausting conditioned air.
Ventilation and wood heat
I live in a hundred year old house in northern PA. (HDD 7200) The house is about 1800 sq. ft. and I heat it with three full cord of wood. My CFM50 is 1500 and I have a dryer, two panasonic 80cfm bath fans and a range hood. With the house in winter mode and all the exhaust stuff running, The house depressurizes impressively! 10-12 pascals. All of the fans are never running at the same time but it is common for the dryer and range hood to be running. The woodstove/chimney have very good draft and the only conditions that require me to open a window is when the outside temp is 50F or higher and the fans are running and I am starting a fire. Once the fire is started and the chimney is warmed there are no signs of backdraft even when the bath fan is on with the dryer and range hood.
If a "certified" building performance specialist reviewed this situation, they would start sucking air and declare an evacuation emergency.
I could install an outside air supply on the quadrafire but I suspect that I would still need to open a window when starting a fire on slightly cool days. I do not look forward to cutting a hole through the tile under the stove.
I would be interested to hear what folks suggest. ( my water is heated with the sun-no combustion-the sun also makes 90% of my electricity)
Response to Steve
Steve,
It's your house, and it seems that you know how to operate it. As long as you are operating your house, I don't think you have an emergency.
But -- if your wood flue ever gets blocked by a bird's nest during the summer, you could run into problems with a smoky fire that emits CO. Under this scenario, you definitely don't want backdrafting.
The standard answer to your question is, "You should install a duct to convey outdoor air to the woodstove (for combustion makeup air)." Just because you don't look forward to a task doesn't mean that the task isn't necessary or wise.
Martin's suggestion
Thanks for thinking about this situation. Your statement "as long as you are operating your house" is important. I may not always be the one living here.
If the chimney gets plugged, a combustion air supply duct will not prevent backdrafting.
How does one pressure relieve 350 CFM of exhaust ? i know opening a window works but is there another automatic choice ? I don't think a 6" auto damper will do much.
Response to Steve
Steve,
Shelter Supply sells make-up air units:
http://www.sheltersupply.com/corporate/default.asp?cwpID=16
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