HRV supply and exhaust air temperatures
Sorry if this has been done before, but I can’t find it. There are numerous discussions on efficiency, and rentability, and such topics, but I have not seen anything with the exact, real life temperatures.
What confuses me, because I am probably thinking completely wrong about this, is how the efficiency is calculated?
I will be using Celsius in this example..
Outdoor temperature is 0 °C
Indoor temperature is 20 °C
HRV efficiency is a claimed 90%.
What is the expected temperature of the supply air after the HRV? So the air actually coming through the ducts into the living space?
Zehnder and other sellers all claim it’s gonna be 18 °C.
Which makes zero sense to me. How is this possible? Shouldn’t the air streams”equalize” at 10°C and that would be it?
I know I am missing something obvious, but I don’t know what 🙂
I am sorry once again if this ha sbeen discussed many times before, but I simply can’t find it now 🙂
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Replies
Davor,
This is a common misconception about how different types of heat exchangers work. Good H & ERVs use something called a counter-flow heat exchanger, which can achieve very high recovery efficiencies. You're thinking of a parallel flow setup, where the two steams would achieve equal temps (if given enough distance), since they flow together in the same direction. By using counter-flow, the air leaving the HX (fresh supply air about to enter the house) is right next to the room temp air that just entered the HX, so it's "seeing" the warmest part of the HX, and given sufficient area can get very close to that temperature, thus very high efficiency.
This may help too: https://www.cpesystems.com/blogs/news/parallel-vs-counterflow-heat-exchangers-for-non-engineers
And if you're looking for the math, HRV delivery temperature can be found by using the formula:
DT = ODT + [(RAT - ODT) x %HRVeff./100]
Where: DT = HRV delivery temperature, ODT = outdoor design temperature, RAT = exhaust air temperature, %HRVeff. = HRV sensible effectiveness
Thank you to all.
Counterflow vs parallel has solved all my dilemmas. I can have peace of mind now.
Cramer, that link you provided might be the best explanation ever for something that I thought was complex, but it turned out it's really not :)
Thanks again!