Sizing a Heat-Recovery Ventilator (HRV)
GBA Editor
| Posted in General Questions on
> My new house with in-floor radiant and basement is approximately 28,000 cubic feet. I calculated a 200 cfm HRV unit:
cubic ft of house = 28,000
28,000 x .85 (to account for interior walls and furniture) = 23,800
23,800 x .5 (air change rate) = 11,900
11,900/60 (to convert air changes per hour to cfm) = 198.33
> My HVAC guy recommends a 300 cfm unit and is not concerned with oversizing the unit as much as undersizing it (he’s used to installing 2,000 CFM heat pump systems in a house my size).
Is there a problem with OVERSIZING a HRV?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
80 to 100 cfm continuous will be more than enough for this house. 200 cfm peak for source point and winter humidity control if needed will be plenty.
Hi Dan Morrison,
I realize that it is not a simple matter to make climate and or location a required field when asking questions.
How about at least suggesting that folks provide their climate/location?
In humid climates during shoulder seasons and cooling season Overventilation can indeed be a problem.
John
I'm building my house in Seattle, WA. (http://en.wikipedia.org/wiki/Seattle#Climate)
thanks
Frank,
I honestly do not know enough about HRVs to answer your question.
I assume that the ventilation rate can still be adjusted lower?
From what I have read I would not reccomend setting the flow any higher than ASHRAE 62.2
described in this article
https://www.greenbuildingadvisor.com/blogs/dept/musings/designing-good-ventilation-system
Frank,
We need to know two items of information you didn't provide in order to answer your question:
1. Square feet of occupiable floor area.
2. The number of occupants.
According to the formula provided in ASHRAE 62.2, your house needs 50 cfm of continuous ventilation (assuming an area of 3,500 s.f. and two occupants). If you have high ceilings, your area is probably less.
Assuming an area of 3,500 s.f. and three occupants, your house needs 58 cfm.
You don't need 200 cfm of ventilation. Overventilation incurs a serious energy penalty.
Martin
1. The living area is 1975 sq ft w/ high ceilings. The unfinished basement is 1000 sq ft.
2. 2 adults, 2 active kids and a greying dog
Along with the energy penalty, are there other considerations with an oversized HRV?
thanks
Frank,
Start with ventilation at the ASHRAE 62.2 level — 50 cfm in your case. If you notice any IAQ problems, you can increase the ventilation rate. But I'd be cautious about overventilating.
Frank,
I meant to add: there's no real problem with an HRV rated at 200 cfm, as long as the ventilation rate is adjustable. For example, the RecoupAerator 200DX is an ERV that is rated at 50 to 200 cfm; it performs well at 50 cfm.
You need to know the minimum as well as the maximum cfm rating of the equipment you are considering.
I think that besides the "energy penalty" there may be a durability issue associated with overventilating in a humid climate......during cooling season
overventilation makes home more humid,
homeowner feels uncomfortable , cranks thermostat down more
colder surfaces,more humidity ...not good
I realize that this may not be related to the size of the HRV...but rather the operation.
Yeah I know, the air condtioner is reducing the humidity... but still .. not good
I agree with Martin that an ERV would work better with your humid climate and I've specified the RecoupAerator 200DX w/ great results in a moderately humid climate. If you have separate AC/dehumidifier, for more humid climate, then an HRV would work and 62.2 calculations are correct.
I'd like to resurrect this old thread because I am also struggling to size a ventilator for my house. I'm in Western NY zone 6.
I have 2400 sf house with 1600 sf finished basement (4000 sf total). 3 bedrooms, 4 occupants. And 46,800 cubic feet of airspace (which includes basement)
There seems to be 3 ways to calculate the size. ASHRAE 62.2 , the International Building Code, and by cubic feet of house.
Problem is I get wildly different numbers.... Which is right?
ASHRAE = 70 cfm
IBC = 150 cfm
Volume = 232 cfm.
And lastly a contractor who told me 130cfm. Not sure how he calculated that.
Your local building code will ultimately dictate the minimum rate. In my limited personal experience, the ASHRAE rate is inadequate once the number of occupants starts to go up. You can't go wrong in oversizing it, as long as it is adjustable (strongly recommend a unit with ECM blowers so as to maintain efficiency at lower speeds). Occupants are driving the most common contaminant, CO2, so I think that's a much more critical number than floor space. And occupant number that you plan for should include temporary occupants, such as if you have guests staying over or even visiting for more than a couple hours. CO2 levels can snowball out of control in that little amount of time.
My main issue with my indoor air at the moment is VOCs. My house has tested off the charts, literally, for VOCs.
I don't have any building code where I am to provide any type of "follow this rule" guidance.
What does " ECM" stand for, as blowers are concerned?
The only downside to oversizing is cost, as long as it can be adjusted to what you need. If your house is pretty airtight and you don't currently have active ventilation, it actually might not take as much airflow as you might expect to bring the VOCs down. Still, I would lean toward a bigger unit. You can turn it down if you need to.
ECM = electronically commutated motor. These are more efficient and quieter in general, but especially when running at lower than full speed. An ECM is infinitely adjustable, so usually you can program the motor in increments of 1%.
I second Trevor. Go as big as you can afford. The low flow efficiencies improve due to a more HX surface area. But only with ECM fans so you don't pay for it fan energy. My personal top picks are the Vanee Gold series (HRV or ERV), and Renewaire EV Premium L. Both them are electronically balancable with ECM fans so that fact that they can move well over 200CFM doesn't mean you pay more at 60cfm.
Some units like the Vanee AI Series/Panasonic Intellibalance self adjust to the targeted airflow, so you can play with the settings without having to get out a manometer. You could set it high in the shoulder season to flush your home, and turn it down summer and winter for energy efficiency.
I'm getting really frustrated. For the life of me I have no idea how these units are controlled. The only one that seems to have controls is the panasonic intellibalence, with dials right on it to pick your fan speed and adjusting how often it runs.
How do you control the other ones? Other than just min airspeed or max airspeed?? Like if the unit says 40-250 cfm. How do you set it at 90 cfm? Or 130 cfm? Or 180 cfm?
Typically there's a wall controller with some kid of programmable controller. We'd need to know exact models in order to comment any more specifically.
The life breath 170ercd
Anything would be fine as long as I can adjust the cfm to get to a good setting for my house. I liked the panasonic intellibalence because it has simple dials on it to select a cfm and it self balences. But, it may be too small. The life breath 170ervd has a control unit that I *think* makes it adjustable.
I've been reading install manuals for life breath, broan, honeywell, renewaire, vahee. Very few say anything about what their cfm range is, or how adjustable it might be.
The ones John recommended above aren't in production yet from what I can see.
________
This pansonic intellibalence is exactly what I need, features wise.
https://www.supplyhouse.com/Panasonic-FV-10VEC2-Intelli-Balance-100-Energy-Recovery-Ventilator-Cold-Climate
I have no idea if it's big enough tho. I looked back at my notes, and I had two companies give me "quotes", one suggested a 130 cfm unit and the other a 218 cfm unit
So I've had 5 different sizing suggestions...... 70, 130, 150, 218 and 232 cfm.
Both contractors hady.blower door test results at their disposal too.
Can someone please tell me if the 40-100 cfm panasonic is big enough??
The panasonic will work ok at the ASHRAE levels, but won't have a lot of flow headroom or as much effectiveness during boost (the newer Panasonic's support boost now). I would lean towards something that can boost a little higher but keep your baseline flow rate close to ASHRAE's calculated value.
These offer decent efficiency and ECM fans will direct CFM setting.
https://www.vanee.ca/AI-series.html (VanEE, Venmar, Broan all have the same units will slightly different names).
Personally I'm going to be installing a Renewaire EV Premium M(or L), it has good efficiency flow headroom and ECM fans, but it doesn't auto balance.