HRV Duct Sizing?
I am in the planning stage of putting in a Heat Recovery Ventilator (HRV) in a two-story colonial with basement.
I have existing central heat/air located in the basement with a full set of supply and return ducts in every room (except none in the unfinished basement, and no return ducts in the two bathrooms or the kitchen).
I am planning on running an entirely new supply and return system for the the HRV (separate from the furnace/AC supply and returns). I plan on running the HRV dirty-air intakes from the two bathrooms and the kitchen, and then direct the incoming fresh-air into the three bedrooms on the second floor.
This steup was recommended by American Aldes ventilation, a manufacturer of HRVs:
Question #1 - Sizing:
The dirty air room volumes are as follows:
Bathroom1 - Downstairs____119________14%
The fresh air room volumes are as follows:
The HVAC pulls in 238 CFM and outputs 247 CFM at 0.4 (in WC).
I plan to build both the supply and return branches and trunks to be as short as possible, will minimize the number of elbows, and will seal them with mastic/aluminum tape. Both will be run in conditioned/semi-conditioned air spaces (I'm considering the basement to be semi-conditioned).
I plan to run two intake trunks---Trunk #1 (Bathroom1 and the kitchen), and trunk #2 to just the upstairs bathroom2. Trunk #1 will be about 25' long before splitting into 9' sections going to Bath 1 and the kitchen. Trunk #2 will be about 37 feet long in total and will have no branches.
The HRV intakes and outputs are 6" round.
My thought was to run the HRV dirty air intake to a 6" wye, then have an iris damper for both trunk#1 and trunk#2. After Trunk #1 runs to the other side of the house and goes into a wye, I would add an additional iris damper for both branches (bathroom1 and kitchen).
Ignoring losses due to friction, if I adjust the dampers so that Trunk #1 (bathroom1 and kitchen) gets 77% of the 238 CFM, it would receive (0.77 * 238 = 183.26 CFM), and Trunk #2 gets 23% of the 238 CFM (0.23 * 238 = 54.74 CFM).
Then, if I adjust the dampers so that the kitchen gets 86% of the 183.26 CFM, it would receive 157.98 CFM, which would give me an ACH of (CFM*60/Volume) = (157.95 * 60) / 476 = 20 ACH for the kitchen; and bathroom1: 14% of the 183.26 CFM = 25.27 CFM, which would give me an ACH of 13.
The ACH of bathroom2 on Trunk#2 would be 13 as well (25.277 *60)/119.
I've read that the maximum airflow capacity of 6" round duct is 180 CFM.
Is it a sound plan to run 6" duct throughout, and just use the iris's to balance the CFM?
Note: I'll have to switch to 3 1/2" x 10" for the wall stacks to fix in the stud cavities.
Any advice would be appreciated.
I would connect it to your existing HVAC duct system. And have both the HRV and the HVAC units fans run at the same time.
PS: I don't know where you read 180CFM for 6" duct. But there is no HRV made, that could move that much air through a 6" duct. The duct would be too restrictive.
Here is there reasoning on separate system:
Connecting to forced-air ducting? Not recommended!
Contrary to the recommendations of most manufacturers of heat and energy recovery ventilators, American ALDES strongly discourages drawing exhaust air from the return HVAC ducting and supplying the fresh air directly into the HVAC duct system.
Here is why:
In the case of both heating and cooling systems, connecting to the ducting can result in severe imbalance of supply and exhaust airflows as the HRV/ERV operates on low to high speeds, as well as the variable speed operation of modern furnaces and air conditioners. It is virtually impossible to assure the design airflows remaining in balance when the two systems are operated at varying blower speeds, resulting in a large variation of static pressure in the HVAC ducting system.
The exhaust fan in the HRV/HRV is working against the negative pressure in the return plenum. If the return plenum pressure is sufficiently high with the HVAC blower running, it can reduce or even stall the exhaust airflow. At the same time, if the HRV/ERV supplies air to the return plenum, the supply blower has the assistance of the HVAC blower to supply excessive fresh air whenever the HVAC system is operating. This situation makes it diffi cult to assure the HRV/ERV will remain in balance.
If the HRV/ERV is set up to operate only with the HVAC system, then mechanical ventilation is not provided when most needed—in milder weather but when windows remains closed. Ventilation is likely to be excessive in severe weather, when the air handler operates more frequently.
Many brands tell you to install both the exhaust and fresh air into the return. Then the return static and air volume doesn't change.
You also wire it up so that when the HRV needs to run, it brings on the central systems blower. So that you have ventilation anytime you need it. Weather its during mild weather, or severe weather.
This also prevents you from getting a cold draft feeling in the middle of winter. Or a hot air blowing on you, in the middle of summer. Since the fresh air is mixed with room temp air before it is discharged into teh home.
Running a separate duct for the HRV, will give you cool/cold drafts when it runs in the winter. And warm/hot air when it runs in the summer.
HRV's are NOT 100% efficient with heat transfer. So in winter. When its 10°F outside. Do you really want 40 to 50° air blowing out of the vents?
Not as cold as you think....
I agree with beenthere's comment that HRVs are NOT 100% efficient, but I have to take exception to his comment about 40° to 50° air blowing out of the vent.
So, how do you figure out how cold/hot the air would be when brought into the house during the winter?
I found this definition:
Apparent Sensible Effectiveness (ASE) – The term used in the CSA C439M standard for testing HRVs to describe the temperature rise of the outdoor air passing through an HRV. The effectiveness includes the effect of motor heat gain, cross leakage gain and casing gain. It is usually numerically higher than the sensible recovery efficiency of the HRV. When the flows of indoor and outdoor air through the HRV are equal, the sensible recovery efficiency equals the temperature rise of the outdoor air divided by the temperature difference between the outdoor air entering the HRV and is expressed as a percentage.
I interpret that to mean: Temp Rise / Temp Diff = ASE
or Temp Rise / (Inside Temp - Outside Temp) = ASE
and finally, Temp Rise = ASE * (Inside Temp - Outside Temp)
The HRV I purchased, the Fantech 30005R, had a 92% ASE.
I will stipulate an interior temperature of 75 degrees (F).
Inputting the numbers, I get:
Temp Rise = 0.92 * (75 - 10 F)
Temp Rise = 0.92 * (65 F)
Temp Rise = 59.8 F, round to 60 degrees F
So, it's 75 degrees inside, 10 degrees outside, and the air being brought in is at 70 degrees.
I will be installing unit this summer, and I'll update this thread with some actual performance numbers.
And if you had used 70 indoor temp, it would come out to 55.
keep us posted. I think you'll find it comes out colder then you calculated.
Was there any updates to this install? Curious to see what the temps were during winter.
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