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Is a fully dedicated duct system for HRV truly the best option?

KSJeffery | Posted in Mechanicals on

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 ( 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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  1. KSJeffery | | #1

    Oh, I should share that in MN ventilation formula is 0.02 cfm per 100 sq. ft., plus 15 cfm * (# bdrms + 1). That is how I came by 169 cfm total ventilation required. I, like Joe Lstiburek, feel this is too high (no one smokes, and the dog only farts in my kids' rooms!), so another factor in my decision is that I want a system that I can "micro-manage" and tailor to suit me. Unfortunately, that still means venting the bathrooms (but each one has a window!)

  2. KeithH | | #2

    I'm also trying to settle on an HRV system (if I succeed in tightening the house enough to bother). Unlike you, since I also have an ECM variable furnace, I felt it would be more efficient to use that piece of equipment instead of installing a separate ducted system (though I'm dealing with a retrofit not new construction). So my question is: Why do you feel it would be inefficient to use your furnace blower?

  3. UBuildIt_Indy | | #3

    You need to ask somebody to do static pressure tests on using duct work that big to move 169 CFM. Also, Wouldn't it be ineffective if the HRV used the full ducting system for its use? It should pull out of the wet rooms and put back into the bedrooms and living, to work as intended.

  4. KSJeffery | | #4

    Keith - there are several reasons why I feel using the furnace air handler (AH) is inefficient. The biggest, however, is energy consumption. Insofar as the two fan motors are already running in the HRV, running another fan - even if connected to a highly efficient variable speed (ECM) motor - is more energy. Also, since such a system mixes and recirculates more than it ventilates, it cannot be used to exhaust bathroom moisture. Thus, more ventilation is the form of bath fans - exhaust ventilation at that - is needed. This will require supply air, likely met via infiltration of unconditioned air.

    Allen - yes, I am in agreement. As I just mentioned in responding to Keith, using HVAC ducting will not really help exhaust moisture laden air from bathrooms. Also, low flow in large area ducts without using the AH would result in very low flow velocities.

  5. GBA Editor
    Martin Holladay | | #5

    First of all, if you had installed a simpler heating system, then the extra cost of installing dedicated ductwork for your HRV would probably not be an issue. But I imagine that it's too late to rethink your choice of a ground-source heat pump, plus a gas furnace, plus a hydronic heat distribution system, plus ductwork for the furnace. Talk about redundant and expensive!

    If I were you, and I wanted 20 cfm continuous exhaust ventilation from my most important bathrooms, for an HRV system exhausting 84 cfm, I would choose 4 bathrooms (or laundry rooms) for ducted HRV exhaust. If the system is properly balanced, each of those 4 rooms will be exhausted at a rate of about 20 cfm. Most HRVs have a user-controlled booster switch which can be installed in each of these bathrooms to bump up the exhaust rate when desired.

    If you have more than 4 rooms that need occasional exhaust -- let's say there are 6 such rooms -- the 2 rooms that aren't served by the HRV can get a simple bath exhaust fan controlled by a wall switch.

  6. KSJeffery | | #6

    Martin - No, it's not too late to rethink my system, and I'm doing precisely that. But that's a discussion for another thread, which I will start shortly and add the link (

    I know you are a strong advocate for using the HRV for bath exhaust, but others like Robert and AJ Builder endorse separate exhaust (and windows!). I wonder if anyone has good, real-world experience at different exhaust rates. Bath fan manufacturers espouse 8 ACH for sizing their equipment, which would be about 150 cfm in the case of the Mstr Bath, Clearly, one should not expect to replicate the fog-clearing performance of a stand-alone bath fan unit with an HRV, but on "boost" if one can achieve, say 60 cfm, is that good?

    And, is it worth the effort to set up zones via motorized dampers to get such a target?

    I guess I worry that without dampers and zoning, and just picking the 4 most import rooms to vent, 20 cfm from the HRV won't cut it. (Not to mention that ventilating the bathrooms now becomes the rate-limiting step when it comes to choosing whole house ventilation settings, rates, minutes of operation)

  7. wjrobinson | | #7

    Kent, you are mixing simple solutions with complex, low cost with costly.

    I install low cost systems. I have a HVAC contractor friend who installs complex systems, infact last year I suggested them for use on a project similar sounding to yours but five times larger with a cost I am guessing close to $200,000 just for the HVAC. Commercial boilers, back up boilers, radiant heat, HRVs, vents, fans, multiple AC set ups, 3 200 amp panels....

    I suggest a large home like yours should be handled by the best HVAC pro you can find. Then go with his plan not yours.

    Or.... start over and go simple and smaller.

  8. KSJeffery | | #8

    AJ - I agree with your take on this. I have found that when you hire a hammer, everything looks like a nail. So, indeed, I've put a halt to this project and am re-evaluating before I get locked into an expensive and unsatisfactory solution. I understand simple, but what do you mean by "smaller"? The size of the house? Or the size of the HVAC?

  9. mackstann | | #9


    One Panasonic spot ERV in the ceiling for each "cluster" that runs 24/7 at 10, 20, or 40 cfm, as deemed appropriate. No ducts other than the two per ERV that go outside.

    One normal exhaust fan in each bathroom with timer or humidistat control, for use during/after showers and smelly activities.

  10. KSJeffery | | #10

    Hmmm... interesting! I had not encountered these before. However, I'm not sure an ERV is appropriate for bath ventilation in zone 6, as I would think I'm trying to remove moisture, not reclaim it.

  11. fitchplate | | #11

    Gents ... As Dana pointed out elsewhere, the Panasonic ERV WhisperComfort will not work on the coldest days of Zone 6 which is when its most needed.

    32 F - 20 F the unit goes into a ERV/exhaust alternatiing cycle.
    Below 20 F is works on an exhaut only cycle, no heat exchange.

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