ERV vs HRV decision using historical data on psychrometric chart
I’m trying to finalize the decision between an ERV and HRV for my home. Would appreciate feedback on the approach below which I haven’t seen used before.
I found software called Climate Consultant that will plot climate data from energy plus on a psychrometric plot.
https://www.sbse.org/resources/climate-consultant
This is the median temp and humidity for every hour of the year taken from 1991-2005 data. I have the plot attached, this is from Portland Oregon so Climate Zone 4C. Each green dot is an hourly value.
My target indoor conditions are 70F and 45% RH. I set RH limits to 30% and 60%, which are the points beyond which I would definitely run a humidifier or dehumidifier. I set these wide here for some wiggle room. Ideally I stay in the 40-50% range.
On the plot below the yellow dot is my target condition, and the blue lines just highlight where 70F is. I put red horizontal lines that intersect 70F at 30% and 60% RH, to show what regions of the graph can be brought into my comfort zone at 70F with only sensible conditioning. The areas outlined in orange are the times of the year where sensible conditioning alone will not work. That is, an HRV would be insufficient.
This seems like a clear indication that an ERV is preferrable in my case. Especially because I’m pretty committed to using the Broan AI series at this point, and they advertise nearly equivalent sensible efficiency between their ERV and HRV models.
I know that I’m not accounting for internal moisture load and latent cooling from the AC – those will bring up some of the low RH conditions in the winter and high RH conditions in the summer, but given the loose limits I used I don’t think that changes the picture much.
EDIT: changed image to PDF
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It's hard to read the chart -- the site software limits the size of images, if you post them as PDF they're easier to read -- but it looks like you're on the right track. I'm assuming the right axis is absolute moisture content or dew point or some equivalent measure of moisture in the air.
The one thing I would caution about is that each person puts about five pints a day of water into the air through breathing, bathing, cooking, etc. Often you will need to be using ventilation to remove that moisture which means that you need to be replacing indoor air with air that has significantly lower moisture content. It's hard to quantify, but the band of acceptable air exchange will probably be pushed down from where you have it.
ERV’s are the correct choice for nearly every climate zone except the most coldest, most driest regions. If your area has even just seasonal humidity, you would be better served by an ERV, otherwise you are taking a latent energy penalty by bringing in the outdoor humidity with the air. Total system efficiency is better with an ERV than HRV, the sensible transfer is a little less but it also has latent transfer which needs to be accounted for.