Whole House Ventilation
I have a 3 bedroom, 2 story cape cod style house (1st floor 1500 sq ft, 2nd floor 500 sq ft; open floor plan) with a full basement (1500 sq ft). It is insulated with icynene except the 3 sides of the basement below grade, which are concrete-filled block lined with styrofoam wallboard (R 7.5) and 1/2″ drywall. The house is in a semi-rural location in East TN (Mixed-Humid Zone) at 1000′ elevation. Two adults and 4 dogs live in the house. Central heat and air is provided with an electric heat pump. The vast majority of heat in the winter is provided by a Jotul wood stove, which is located on 1st floor and has a separate ducted air supply. I’ve never had a blower door test, but the house seems pretty tight. With temperatures down to the high single digits (°F) the stove will keep the house in the mid 60°s overnight without adding additional wood after 11 PM. During the spring and fall the house, which has many windows, is pretty much wide open. During winter and summer the house is typically closed up.
The HVAC is being replaced with an American Standard Platinum 20 heat pump and air handler, which has a variable speed compressor, and a Lennox Healthy Climate HR 16 Air Filtration Cabinet. My intention is to add an AirCycler g2-k ducted into the return immediately before the plenum with matched exhaust ventilation via a Panasonic Ventilating Fan mounted in the ceiling of 2nd floor and ducted to the side wall in attic space (attic is within icynene envelope). During the hot, humid summer months I will use a stand alone dehumidifier if needed. My plan is to have the American Standard air-handler trigger the AirCycler during the summertime and have the converse during the winter, the AirCycler triggering the air-handler. Latter because the HVAC is not used much in the wintertime. I have three questions.
1. The AirCycler comes in several duct sizes. Currently I have a 4″ passive fresh air duct going into the return plenum of the existing air-handler, but could increase it to 6″ if necessary. Is there an advantage to enlarging the fresh air supply duct from 4″ to 6″? My assumption is in the summertime the heat pump variable speed compressor and air handler will be running at a lower speed for longer durations than an HVAC with a single or 2 speed compressor and exceed the time needed for adequate air exchange; therefore, the smaller duct size will not be a limiting factor.
2. I plan to purchase the Panasonic FV-05 ventilating fan, but am unsure if I should buy the model with the Multi-Speed module (11VKS1) or not? My plan is to match the AirCycler air intake and Panasonic fan exhaust at set-up, but am thinking that on warm spring and fall nights when the house is still open I could use the Panasonic exhaust fan alone as a whole house fan, and it would be advantageous to be able to increase the speed without having to go up into the attic.
3. I read in Ventilation Guide by Armin Rudd that “during humid summertime conditions injecting ventilation air into the return plenum upstream of a relatively restrictive media filter will impede the flow of the ventilation air into the cool central supply ducts.” The Lennox HR 16 Air Filtration Cabinet can use a MERV 16 media filter coated with activated charcoal or a non-coated MERV 11 media filter. Should I restrict the MERV 16 media filter to wintertime use?
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Replies
Several points:
1. If you haven't read it already, you need to read this article: Designing a Good Ventilation System.
2. The way you describe your central-fan-integrated supply ventilation system includes a few eccentricities. In almost all cases, a supply-only ventilation system does not require the simultaneous operation of an exhaust fan (because almost all homes have enough random leaks to allow air to escape from the house).
3. You never described an essential component of a central-fan-integrated supply ventilation system: the motorized damper that must be installed in the fresh air duct. This motorized damper is controlled by the AirCycler control (also called a FanCycler) to prevent overventilation and underventilation.
4. Air flow rates with this type of system must be verified during the commissioning process to prevent overventilation or underventilation. Ventilation rates should not exceed ASHRAE 62.2 rates without a valid reason to do so.
5. To avoid the problem highlighted by Armin Rudd, just introduce the fresh air downstream from the restrictive filter.
6. Follow the control guidelines provided in the article I linked to.
-- Martin Holladay
I did rear the article which you reference and thought I had used the principles in it and other articles to design my system.
My intention is to have a balanced system, not a supply-only ventilation system.
I may be incorrect but from reading the information available from AirCycler the AirCycler g2-k includes the motorized damper. Am I wrong?
I intend to adhere to the recommended ASHRAE 62.2 rates, hence the statement "with matched exhaust ventilation" in the description of set up.
All of the articles I have read recommend introducing the unconditioned fresh air into the return air at or near the plenum. I thought the purpose of introducing the air at this location was to allow it to be both filtered and conditioned (heated, cooled and dehumidifier). Is this an unnecessary requirement?
If you plan to install a motorized damper, that's great. It's essential.
Concerning the duct size: I imagine that the 4-inch-diameter fresh air duct would be adequate, but I don't have enough experience to be certain. (In general, the main problem with poorly designed central-fan-integrated supply ventilation systems is overventilation, not underventilation.) Regardless of which size duct you install, the system needs to be commissioned by an installer who can measure air flow rates.
Concerning the size of your Panasonic fan: I don't think it makes sense for you to be changing the fan speed to adjust the home's ventilation rate. This is a complication you don't need. If you really want to create your own balanced ventilation system from a variety of components manufactured by different companies -- a fool's errand, in my view -- choose a simple exhaust fan with an air flow rating that matches your ASHRAE 62.2 ventilation rate.
That said, here's my advice: If you want a balanced ventilation system, just install an HRV. It's much simpler than your suggested approach.
-- Martin Holladay
Thanks for your thoughts. I researched an HRV and if I was building a new house and built in an independent supply and return duct system I would install one in a NY minute.
Because of TVA (TN Valley Authority) rebate offerings many local HVAC installers have the ability to measure air flow rates. The installer I've hired has assured me he can do this.
My desire to seasonally maximize exhaust ventilation from the Panasonic 2nd story ceiling fan would only occur in the spring and fall when the house is wide open and HVAC is totally off. The HVAC is used continuously during the hot, humid summer, infrequently during the winter and seldom during spring and fall. Case in point-the current 12 year old system has been inoperable since late October and we have been toasty warm all winter because of the wood stove and adequate insulation.
I understand I risk overventilation, but assume if I set up a balanced system my downside will be only a waste of energy. On the upside I'll have some of the freshest indoor air in the community. I have purchased both CO2 and humidity monitors and will adjust air supply to keep CO2 <900 ppm and relative humidity between 35% 50%.
One critique I have of many of the papers I have read on house ventilation is a religious adherance to air exchanges without monitoring the end result, i.e., CO2, VOCs, humidity and cost. Ultimately I want to breath the cleanest air in a comfortable indoor environment at a reasonable price without damaging my house.