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ERV vs. HRV Efficiency – Humidity Loss Considered?

lance_p | Posted in Mechanicals on

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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  1. GBA Editor
    Martin Holladay | | #1

    I haven't had a chance to read the whole document yet, but below is a link to a document that should answer your questions about how ERV efficiencies are calculated:
    Guideline for Calculating the Efficiency of Energy Recovery Ventilation

  2. user-626934 | | #2

    In the HVI listings of ERV's and HRV's, there are separate performance metrics for sensible heat transfer and moisture transfer at the HVI winter test condition. HRV's will shows zero or near-zero figures in the moisture transfer column...ERv's will range from ~30% up to ~80%, depending on model and airflow.

  3. lance_p | | #3

    Martin, thanks for the link. I skimmed through the document and it seems they are most concerned with how the ERV integrates with existing HVAC equiment and the potential impact on equipment sizing.

    John, yes I see the moisture transfer data in ERV specifications. My curiosity lies in how much energy is saved as a result of the latent transfer.

    Let me paint the scene; I live in a house of very average construction in Ottawa, Canada, a subdivision house built presumably to code in 2004. In order to keep my indoor RH above 30% during cold weather I need to operate a humidifier, a whole house unit that mounts on the supply side of my furnace. Water trickles down through a porous grate and evaporates when warm furnace air passes through it, which cools the heated air.

    This house has no ventilation energy recovery. We have a fresh air duct that dumps beside the furnace where (hopefully) most of our make-up air comes into the house.

    If I was to install continuous mechanical ventilation my humidifier would have to operate much more often to maintain a set humidity level. Evaporating that water sucks heat energy out of the furnace supply air. My mock scenario above was an attempt at determining the amount of energy it would take to make up for all the water exhausted if an HRV was put into service in a situation like mine that required winter time humidification.

    IF MY MATH ABOVE IS CORRECT, an ERV with 70% latent efficiency would be saving 46000BTU per day. In an efficient home that amount of energy would represent a significant percentage of the total heating load on the house! Sure, it's an extreme example, but one that's about equal to our coldest weather here (daily average temp of -21C).

  4. ranson | | #4

    Some of the outgoing moisture is going to condense on the core. This would convert the latent heat of the outgoing stream to sensible heat in the incoming stream. I don't know if the HVI numbers account for this. If they don't, you might see an effectively increased SRE in an HRV due to indoor humidity.


  5. Anon3 | | #5

    renewaire got a really nice calculator on their website, check it out

  6. GBA Editor
    Martin Holladay | | #6

    To the best of my knowledge, the energy effects of moisture transfer in the two air streams of an ERV vary from house to house. In some homes (and under some weather conditions), the moisture transfer saves energy; in other homes (and under other weather conditions), the moisture transfer isn't helpful.

    I think you are focusing on the wrong solution to your problem. You are living in a house with a defective thermal envelope, and you are focusing on the choice between an HRV and an ERV. Instead, you should be focusing on fixing the defects in your home's envelope.

    If you are living in a house that is so leaky that it requires a whole-house humidifier, you don't need more ventilation. You need less ventilation. Your house is leaking like a sieve, especially during cold weather (when the stack effect is greatest, and when outdoor air is dry).

    No home should need a humidifier.

    The best solution to your problem is to locate and seal your home's air leaks. This work is best performed with a blower door operating during the air sealing work. This type of weatherization work is referred to a "blower-door-directed air sealing."

    If you don't want to hire a weatherization crew or a home-performance contractor for this work, you can do the work yourself. If you don't have a blower door, the work may miss some important leaks, but it can still be effective. Start with these two articles:

    Air Sealing an Attic

    Air-Sealing a Basement

    Finally, the type of outdoor air duct that you describe -- one that dumps air near your furnace -- is not intended for ventilation. That type of duct is intended to provide combustion air to your furnace. If you can afford to do so, you might consider replacing your current atmospherically vented furnace with a new sealed-combustion furnace.

    -- Martin Holladay

  7. lance_p | | #7

    Martin, I am using my current home just as an example of one that requires humidification in the winter in order to maintain a reasonable humidity level.

    If someone in my current situation were to come to the conclusion that more ventilation was required despite the integrity of the envelope (for any number of Indoor Air Quality reasons, not just %RH), adding ventilation would increase humidifier load and related energy consumption to vaporize all that water.

    My example above may be a little extreme, but consider this. In another conversation here I referred to a study done by NRC here in Ottawa at the Twin House project. During hot humid summer weather an ERV removing moisture from incoming air resulted in a 12% reduction in air conditioning power consumption vs. the same house operating an HRV, likely due to a reduced latent load on the AC equipment and shifting more to sensible cooling.

    Condensing water out of the air takes just as much energy as vaporizing it back into the air, which is the case in my scenario above.

    Has anyone had a chance to go through the math and see if my calculations are correct? If they are indeed correct and I haven't overlooked something, the benefit to using an ERV over an HRV in cold weather could be immense and far greater than is generally understood.

  8. GBA Editor
    Martin Holladay | | #8

    I stand by my statement: there shouldn't ever be a need to operate a humidifier during the winter. Any house that is now operating a humidifer shouldn't try to solve their problem with ventilation equipment.

    Instead, they need to focus on sealing air leaks in the home's envelope.

    -- Martin Holladay

  9. lance_p | | #9

    Martin, I have a feeling that you are correct, but I do have another scenario to add.

    A friend of mine lives in a fairly new house; a custom built ~1500sqft bungalow with his wife and two young children. The house was built in 2010 and, though he didn't build it, the people who did had it built fairly well for their aging mother who unfortunately was in no shape to move in by the time it was completed. She passed away and the family sold the house.

    Though he hasn't had a blower door test done, he's fairly confident that it was built very well. It has a geo heat pump system for year round conditioning and his electricity bills are very reasonable.

    The house also has an HRV. He tells me he runs the HRV very little in the winter because it dries the house out too much. Based on his comments I wonder if he runs the HRV anywhere near what even the 2003 62.2 standard would require, which in this case would be 7.5 x 4 + 15 = 45 CFM average.

    I really should see if I can get some more accurate information from him regarding his HRV usage and his indoor RH (his humidity meter is inaccurate). I do feel, however, that this is a good example of a reasonably well built house that would benefit from an ERV instead of an HRV.

  10. lance_p | | #10

    Forgot to mention, his house is located just North of Toronto, likely an equivalent to the US climate zone 6A.

    So in his case of likely under-ventilating his house because of winter driness, if he was to increase his ventilation rate and use a humidifier to keep humidity reasonable, his energy costs would certainly go up.

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