Using air exchanger to cool upstairs
I live in New Brunswick (Canada), climate zone 6. We have cold winters and hot humid summers.
The upstairs of the of my 2-storey houses gets quite warm in the summers. I have an HRV but I turn it off during the summer months to prevent heat and humidity from coming into the house. I don’t run an AC or heat pump during the summer and open the windows at night to bring in some fresh air.
I was wondering if I can/should disconnect my air exchanger from the outside supply line to circulate the cold air from the basement upstairs and return the hot air to the basement (disconnecting the return to avoid depressurizing the house). This would bring hot humid air to the cold basement but I have a dehumidifier that runs in the basement that maintains a constant humidity level and drains to a floor drain.
Would doing this cause and any mold or other issues I am not thinking about?
Any other better suggestions?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Hi Milco.
I'm basically giving your post a bump in hopes that one of our more mechanically-oriented members sees it. I have seen houses designed to use de-stratification as a cooling technique, but have also heard experts criticize it's efficacy. If the second floor is really uncomfortable, my guess is that you need to cool it mechanically. Let's see what others have to say.
Your HRV doesn't move enough air to make much difference (say 840 BTU/hr sensible, the latent component is likely to be unused unless you add a humidifier). Move lots of air and it becomes a complicated geo-exchange modeling question that is unlikely to work out.
Hi Milco,
So the problem with this one, is one of physics. Let's go through the maths shall we?
We can calculate the BTU of cooling you can provide by looking at this equation:
BTU/H = CFM x 4.5 x (Eh – Lh)
Eh is Enthalpy of the air entering, and Lh is the enthalpy of the air leaving.
Normally we would do this across an AC coil, but as you're basically using a fan, we'll be looking at the air you're blowing upstairs vs what is coming downstairs.
Let's say your basement is a nice chill 68 degrees F at 45% humidity because you've been using that dehumidifier. That nice cool air is what we're going to be blowing upstairs... Look up those values in a Psychometric calculator, and you'll get an enthalpy value of 15.9 BTU / lbs of dry air.
And your upstairs is quite warm at 80 degrees, at a relative humidity of 50%.. and we get a value of 23.7 BTU / lbs We can now punch this into our equation, and get
BTU/H = CFM * 4.5 * (15.9 - 23.7)
Simplifying we get:
BTU/H = CFM * -35.1
This means that for every CFM of air flow we provide here, we will generate about 35 BTU of cooling. So far, so good right? Now my HRV provides about 100 CFM of air flow, but that's at really low static pressures, so lets say we actually get around 70 CFM.
That means that we can achieve about 2500 BTU / hr of cooling... Sounds great right! And this is where everything goes wrong.
You see, you're not just pumping cold upstairs. You're also pumping hot downstairs. That's going to raise the temperature and humidity of your basement in short order, just as much as it's going to cool the upstairs. You need to run your dehumidifier, but that will only deal with the humidity by making the basement even warmer.
In practice, when I tried this, I was able to destratify the house a fair bit, by spending a lot of energy to make the entire house uncomfortable. It took about 500 watts running the dehumidifier and HRV fan on recirculate to provide about 1000 BTU of cooling to the upstairs as the temperatures converged, which gives you a COP of about .5, or an EER rating of about 2... Which is really really dismal.
Conversely, my A/C unit provides 30,000 BTU of cooling at around a COP of ~4, and an EER rating of around ~19. This is why most people will push you towards mechanical cooling solutions.
Well put... looks like even a cheap a window shaker for the really hot days would be better that a complicated mess of ineffective fans.
Hi Stephen - thank you for this. Super helpful.
I'm curious if you think it'd make any difference in my situation to do the same rough idea... I'm in Climate Zone 5 and have two mini-splits sized for heating located on the first floor. In the summer, they don't keep up with dehumidification downstairs (or upstairs) even window units don't help enough.
My thought was to duct from the basement/first floor, a dehumidifier supply to the upstairs, with a return either in the basement or first floor to circulate heat/cool/humidity seasonally.
Since my AC is way overpowered/oversized - would this possibly work?
I can provide more details if it seems worthwhile, but it might just be a simple "no the math never works". I used your calc above to determine I could in theory move ~8000/BTU/hr of cold air with the dehumidifier I'm looking at using ~225CFM to estimate.
Thanks everyone for the replies!
Steve thanks for the detail and numbers, it makes it easy to quantify the excepted results.
I like these kinds of problems. :)