ERV vs. HRV Efficiency – Humidity Loss Considered?
I’ve been looking into Heat/Energy Recovery Ventilation and comparing specifications of different models. As we know, HRVs transfer heat and ERVs transfer both heat and humidity to incoming air from outgoing air in cold/dry conditions, and do the reverse in hot/humid conditions.
I was looking at the specs and I couldn’t really figure out how the latent recovery of an ERV was being accounted for when compared to the efficiency of an HRV. Perhaps the specs indicate this and I’m just not getting it?
Curious, I did some math to find out what the energy loss would be based on exhausting moist air and bringing in cold winter air (latent load to evaporate water lost in order to maintain RH). PLEASE correct me if I’m wrong with my calculations, metric units:
Indoor Air = 21C @ 45%RH (contains 8.3g/m3 water)
Outdoor Air = -21C @ 85%RH (contains 0.8g/m3 water)
Average Ventilation = 100CFM (converts to 170m3/h)
For every m3 of ventilation we lose 8.3g – 0.8g = 7.5g of water
170m3/h average ventilation X 24hrs/day = 4080m3/day total ventilation
4080m3/day X 7.5g/m3 = 30600g/day, or 30.6kg/day of water lost
To replace that water we need to evaporate 30.6kg of water each day:
2260kJ/kg (heat of vaporization, water) X 30.6kg = 69156kJ/day
69156kJ/day = 19.2kWh/day, or 65,500BTU/day
Am I doing the math correctly? If so, an ERV transferring 70% of the exhausted humidity to the incoming airstream would be saving roughly 46kBTU/day vs. an HRV.
In this scenario I’m considering a Tempeff style ERV that uses alternating charged cores to transfer humidity between the airstreams, so no defrost cycle is necessary. A standard cross/counter flow core style ERV would require some frequency of defrost cycles at that temperature which would either limit the total air exchange per day or require preheating of incoming air.
This is also not considering the moisture generated internally, or I guess in an average leaky home that the moisture generated internally results in a perfect indoor RH with the ventilation switched off completely. In that case, 30.6kg/day of water would need to be added back into the interior air to maintain RH levels.
Thoughts? I know 21C @ 45%RH indoors is rather high for an exterior temp of -21C, but I’d like to think it’s possible with excellent windows and insulated frames.
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