Is a fully dedicated duct system for HRV truly the best option?
So, I’ve been researching the proverbial pee-pee out of this question: dedicated ductwork for my HRV or simplified installation? Here are the pertinents:
• Climate Zone 6
• 2700 sq. ft. story-and-a-half house, along with a 2000 sq. ft. unfinished basement
• 4 Bdrm, 3 ½ bath house
• Double stud walls, spray foamed exterior sheathing and cathedral ceiling, very “tight” and efficient house planned.
• Geothermal ground source heat pump, with gas furnace back-up (Xcel Energy “dual fuel” program allowing electricity to be purchased at 40% rate for geo.)
• Primary heating is hydronic, but there is ductwork throughout for cooling and forced air back-up heat.
The layout of the house is typical in that moisture producing rooms are in two clusters, on opposite ends of the house:
• Cluster one = Basement bath, laundry, powder room and kitchen are all located immediately adjacent to mechanical room.
• Cluster two = Mstr Bath and second bathroom vertically stacked at other end of house, 40 ft from mechanical room.
Now I’ve read every article I can in JLC, BSC and GBA to help me decide whether to go with dedicated ductwork for the HRV, which was my initial choice. HVAC contractor will support whatever I choose, but candidly states dedicated ducting is, in his opinion, inefficient.
Ventilation req’d = 169 cfm total, or 84.5 cfm continuous, with Venmar Solo 1.5 being recommended (capable of 150 cfm on high, 66 cfm low). First question: running 45 minutes “low” and 15 minutes “high” should meet ventilation requirements, yes? (0.75*150 cfm) + (0.25*66 cfm) = 87 cfm per hour.
Second question, then, is how do I make this system work for exhausting the bathrooms? My understanding is bathrooms should have 50 cfm “demand” exhaust minimum, or 20 cfm continuous. If I send dedicated exhaust to each moisture cluster, then best I could hope for would be about 11 cfm continuous in bathrooms (66 cfm ÷ 6 locations), or 14.5 cfm “averaged continuous” (87 cfm ÷ 6 locations).
If I treat each “cluster” as its own zone, I figure the problem could be solved by connecting the bath exhaust timer switches to a zone control damper, shutting down one the other zone and redirecting the full exhaust flow. Ideally, then, the Mstr and 2nd baths would see 75 cfm on high (with both baths exhausting if either timer is activated). Going the other way, the basement bath could see a max of 37.5 cfm, unless another motorized damper is used to focus flow more preceisely. Is this how the inadequate ventilation problem is typically solved?
Well, the HVAC contractor feels the solution is to connect the HRV exhaust and supply air stream to the HVAC, and use the furnace blower to move req’d ventilation air. “Demand” ventilation of the bathrooms would be via traditional bath exhaust fans.
Here are the talking points to aid in making the decision:
• Interlocking the furnace blower to HRV seems wasteful, even though the blower has a variable speed (ECM) motor.
• I would like to keep envelope penetrations to a minimum, so the idea of foregoing bath exhaust fans appeals to me.
• Redundant ductwork seems wasteful.
• BCS outlines all sorts of potential pitfalls to using anything but a fully ducted system (http://www.buildingscience.com/documents/information-sheets/info-611-balanced-ventilation-systems). I won’t elaborate upon these at this point as this submission has already consumed to much screen space as it is! Suffice it to say the “getting it right” with a simplified approach can be challenging.
• Cost differential estimated at $2000-$2400 more for dedicated system.
• Finally, I am a believer in the KISS principal.
So, my third question is this: Fully dedicated HRV ductwork, or not? Why? Or why not? What is the most important factor driving your advice?
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