As far as I was aware, the fixed plate/capillary core style ERVs were definitely not a de/humidifier, they can only equalise the humidity level between the outgoing air (from the house) to the incoming air (to the house):
Hence why I was thinking I was going to need a dehumidifier on the intakes from the house since they are meant to be located in bathrooms where humidity is high, causing it to exchange this higher level of humidity in comparison to the intake from the outside (if indeed it’s not that humid outside).
That’s a good point to make. Somewhat of a semantics argument, but ERV’s take moisture out of the incoming air when in cooling mode, so in that sense it’s operating as a dehumidifier, but it doesn’t remove any moisture from the air already in the building, so in that sense it’s not acting as a dehumidifier. And yes, as I suggested previously, a dehumidifier in the indoor intake (or potentially both intakes) is a good idea.
A desiccant enthalpy wheel with a heating coil for regeneration on the other hand will actually reduce the humidity of both air streams, which would be considered a dehumidifier by any definition. The amount of dehumidification in either case is often not enough to forgo a dedicated dehumidifier elsewhere in the system though.
I stumbled on this thread because I’m struggling with the same thing bern1al was. However, I’ll give my problem statement and you can guide me to a solution.
I have a crawlspace under my cabin. Mud floor with vents and a turbo fan powered by a ST Zigbee switch. When it rains, I do get moisture or sometimes even water inside this area. I would like to automate the exhaust fan with a certain set of rules to eliminate mold, etc. Also, this will keep the humidity from entering the house.
Rules:
1- Outside temp above 40 (so I have a bit of freeze buffer)
2- humidity in crawlspace > humidity outside.
I have local weather in ST so I can get the outside RH and temp from that. I also have Temp, humidity sensors in the crawlspace. So you see, I have the sensing worked out.
My problem is, the CS normally runs 10 to 20F cooler then outside. This I think is where bern1al was getting stuck. I can’t simply say if (RH outside) < (RH in CS) then fan = on. Because, since the CS is cooler, the RH of each in and out are not comparable. I need to convert to absolute humidity. For those about to say, it makes no difference, I would argue. Go here to see a great interactive psychrometric chart. That’s the chart of RH to AH.
Here is a real example of what I’m saying. Measurements at same time same day. Outside
T= 80F
RH= 80%
AH= .0178 lb/lb (pounds H2O per pound air)
Crawlspace
T=75F
RH=95
AH= .0178 lb/lb
So, in the example above, it’s the exact same AH but at only 5 degrees cooler, the RH jumps 15%. So therefore, I can’t compare RH to RH.
So, I need to compare like units, therefore, I need to convert to AH. Then use that for my control comparison. Any of you wizards know how I can get to AH? I do have WebCore for the automation, I’m just not a mathematician to be able to write the formula or maybe there’s something I’m missing, like a function in WebCore.
To get the actual absolute humidity you also need the barometric pressure. A formula that will give you a good estimate of AH using only RH and temperature would be AH={6.112 x e[1] x RH x 2.1674}/(273.15+T) where RH is percent relative humidity (ie 60 for 60% not 0.6) and T is temperature in degrees Celsius. The output is grams of water per cubic meter of air. This should be close enough for your purposes for most ambient weather conditions.
That said, I think your logic may be a bit flawed. It seems you’re assuming that the absolute humidity is the controlling factor for mold risk, but it’s not. In your example the air has the same AH outside and in the crawlspace, but the crawlspace air has a drastically higher risk for mold than the outside air, and in that situation I’d be doing whatever I could to swap that air with outside air. The biggest risk for mold is the differential between the dew point and current temperature. The greater the difference, the lower the risk of mold, the smaller the difference, the higher the risk. In general, RH is a more useful tool to estimate mold risk than AH, so you could likely get by just fine without converting anything.
If your goal is to reduce mold risk, I’d be making my rules regarding when to run the exhaust fan based on RH or dew point rather than AH.
Thanks Jason for your reply. Also, I know folks don’t live on forums so no sweat with not getting right back.
I’ll see what I can do to incorporate the formula into my app or DH. I like to tinker so it will be a good challenge and learning task.
I totally understand what you mean about getting the humid air out, and actually, I agree however I may have missed some info. We are both on the same page I think.
I know I want to decrease the RH. The problem I have is the area I’m working with is much cooler than outside. The air I would be pulling in would be from the outside. Without taking into account what the outside AH is, if I turn on the blower only based on outside and inside RH, I would potentially be pulling in air with more water. Thus I could cause even worse, condensation! The crawl space is always cooler and I can run the blower all day, it won’t budge the temp. My goal is certainly to pull in air with less water, but only when it will actually have less water. If I simply base my comparison on RH outside vs inside, I could end up pulling in air with a greater AH and I would effectively INCREASE the crawlspace RH once the air cools to that space’s temp.
Does that make sense?
As you mentioned, in my example, both did have the same amount of water. I was more using that to illustrate how temp can effect RH. My app would only turn on the blower when outside had less AH.
BTW: I do have 2 sources of baro. I have the local WX in my smartthings and also, the sensors do baro. These little things are great: Xiaomi Aqara Temperature Humidity Sensor.