ERV and humidity
JC72
| Posted in Mechanicals on
This is more for my own curiosity.
I’ve read that ERV’s are generally suggested for warm humid climates and I’ve also read they can cause humidity levels to rise within a home. My understanding is that with an ERV some moisture is transferred from the stream that contains higher levels of moisture to the stream that contains less. Does the system medium look to balance the amount of humidity between both streams?
Example, Stream A is at 40% RH , Stream B is at 20% RH, ~10% is transferred from A to B where both end up at ~30% RH)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Chris,
As you wrote, humidity is transferred between the two air streams that pass through the ERV core. The transfer of humidity is not based on mathematical percentages -- that is almost a technical impossibility. The amount of humidity transfer depends on a variety of factors, including air temperature, the moisture levels in the air steams, and (most importantly) the characteristics of the vapor-permeable membrane through which the moisture transfer occurs.
For more information, see HRV or ERV?
Whether you say an ERV will cause the humidity to rise depends on what you are comparing to. Assuming that the outdoor humidity is higher than the indoor humidity, then running an ERV will increase the indoor humidity relative to not ventilating. However, it will increase the indoor humidity by less than ventilating at the same level with other ventilation methods, such as HRV, exhaust only, or supply ventilation.
An ERV has a moisture transfer efficiency. As Martin mentions, that is not a constant but depends on conditions and the design of the machine. If the efficiency is 100% and the two airstreams have equal flow rates, then the air delivered to the interior would have the same humidity as the air extracted from the interior. At 50% efficiency, the air delivered to the interior would be halfway between the indoor humidity and the outdoor humidity.
Okay so can an ERV increase the amount of humidity of the incoming stream over and above the humidity of the outgoing stream?
Ex. Outgoing is 50% RH, Incoming is 60%RH and once mixed the incoming RH is increased to something over 60 percent.
Chris, what do you mean by "once mixed?" ERVs don't intentionally mix the two airstreams (although some types allow some parasitic mixing). Also, it is not clear how you are defining the humidity of the incoming and outgoing airstreams since the humidity changes within each airstream as the air flows through the ERV.
The humidity of the air delivered to the house from the ERV will be somewhere between the outdoor humidity and the indoor humidity. Depending on the efficiency, it might be higher or lower than the humidity of the air leaving the ERV towards the outdoors. Ideally, the two airstreams would be flowing in opposite directions such that the humidity difference between the streams is as high as possible over the full pathway. That is called counterflow. If the airstreams flowed in parallel in the same direction, it would be impossible for either airstream to get past the halfway point because the humidity difference would go away as the airstreams got close to equal. In practice, it is more common to see crossflow exchangers where the airstreams are perpendicular to one another.
Remember that hotter air can hold a lot more moisture than cooler air -- something that is made clear by consulting the psychrometric chart. (Here a link to a relevant article: How to Use the Psychrometric Chart.)
If the outdoor air is at 85 degrees F and 60% RH, that outdoor air will be at 100% RH if you cool the air to 70 degrees F.
It's easiest to understand if you consider a scenario in which the temperatures inside and out are similar. In that case you can think about it just in terms of relative humidity. It works on the same counter-flow principle that the heat transfer uses, so it's possible in your initial example, with a good system and a low air flow rate, to bring stream A down to below 30% RH, and to bring stream B up to above 30%. That would correspond to a moisture transfer efficiency greater than 50%. I think moisture transfer efficiency is typically on the order of 60%, which means the result is not very different from the 30% you estimated, but the moisture transfer efficiency can vary a lot, so don't put too much stock in my 60% guess.
In your 50/60% example, the incoming air could only get above 60% if it was being cooled on the way in, because of indoor temperatures being cooler than outdoor temperatures.