# How to determine COP for an ERV

| Posted in Energy Efficiency and Durability on

Hello,

I am trying to figure out the COP for ERVs. For example, if an ERV consumes 40W for 70 CFM with a Heat Recovery Efficiency of 96%, how to arrive at the COP for the energy that has been recovered for a space of 20,000 Cubic Feet.

Thanks,

venkat

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### Replies

1. | | #1

It's a net loss, so not sure to what degree COP would be important (it's negative), unless you are talking about one of the ERV/heat pump units. 96% efficiency with a conventional heat exchange core is not possible (at least currently) either.

For example, my setup is running at 50 CFM, currently at 81% sensible efficiency. It's recovering 590 watts from the outgoing air stream, of a possible 728 watts. It's using about 25 watts at 50CFM, so by my calcs the net loss is 153 watts.

2. | | #2

I have always been very skeptical of ERV/HRV efficiency when they claim over 90%.

In the real world copper and aluminum heat exchangers find it impossible to exchange energy when the differential falls below 15°. To my eye no matter how much surface area you have the fibrous cores in ERVs seem very unlikely to do better.

My guess is your numbers will vary from zero% with a 15° differential to 96% with a 130° differential. How often is it -50 outside and 80 inside.

Walta

1. | | #5

I don't know where you're getting those numbers from, but they are not realistic. At 15deg differential you'll get much closer to the full rated efficiency than 0%. I don't even think your premise of higher differential equaling higher efficiency is correct. Higher total energy recovery, sure.

1. | | #6

The way I see it if you look at the attached chart there will be a delta of about 15°F before any real amount of energy will be transferred.

You may find this link interesting.

https://www.alfalaval.com/globalassets/documents/microsites/heating-and-cooling-hub/alfa_laval_heating_and_cooling_hub_the_theory_behind_heat_transfer.pdf

I think the ERV/HRV industry has sold the sizzle and not the steak.

Walta

1. | | #7

I am again confused. That chart is about logarithmic mean temperature difference. The equation therein doesn't even address efficiency. I cannot fathom what you're seeing there that implies anything about a minimum delta for heat transfer to occur.

When it comes to ERVs and HRVs, the only thing needed for a non-zero efficiency is for the delivery air temperature to be higher than the outside air. I can tell you from real world experience that the delta between outdoor and indoor air temperature doesn't noticeably affect the recovery efficiency, as long as the delta is big enough to actually measure accurately (i.e. if the delta is in the same order of magnitude as your measurement uncertainty, you can't infer anything).

Let's look at a scenario based on your 15deg delta. We'll assume a typical indoor temperature of 70. With the prescribed delta, that makes the outdoor temperature 55. In order for a 0% efficiency, the air being delivered to the house by the E/HRV would have to be 55. My ERV has shows the temperature of all four air streams, and I can tell you that's not the case. The delivered air temperature is more like 68, which is right in line with the rated efficiency. Note also that the smaller the delta gets, the less it matters how efficiently you're recovering heat. When the delta is a couple of degrees, there's so little heat there to recover it doesn't matter if you're getting any of it.

1. | | #9

OK then post your 4 numbers when T3 is 48-53°F
T1 = Inlet temperature – hot side
T2 = Outlet temperature – hot side
T3 = Inlet temperature – cold side
T4 = Outlet temperature – cold side

Walta

3. Expert Member
| | #3

Some of the efficiency of HRVs comes from condensing the moisture from the house. Depending on the indoor RH, this can be a significant amount of heat.

This is why HRVs are always more efficient than ERVs in terms of temperatur recovery. Since the ERV transfers this moisture, same size core will always be less efficient on paper.

As for the 90% plus numbers, it is easy, use a silly sized core. You can look at the efficiency graph of something like EV Premium unit.

At min flow rate, it picks up almost 20% extra efficiency, I would not be surprised at say 30CFM it will be 90+. That is a cross flow core, counter flow cores generally do better for the same surface area but are more prone to frost up so need better defrost control.

1. | | #4

Looking at that table, I'd expect at 30cfm the efficiency would be something like 84%. There's only a 3% difference between 100cfm and 67cfm, and the same between 140 and 100. There's no way it's going to jump >9% going from 67cfm to 30cfm. There's likely to be a diminishing rate of returns coming into effect at some point as well.

4. | | #8

Akos is spot on there. Build a massive core, and run air through it slowly. That EV premium L has a very large core, relative to others and is doing it as a simple cross flow. It's about 7" deeper than the core in my "test" HRV setup. I would have already sourced that core if the space issue in my setup was not a constraint.

If you want more efficiency, then this system ( https://www.tempeff.com/wp-content/uploads/2022/09/14094-Tempeff_RGSP-K_Sellsheet.pdf ) for around \$6-7K has solid engineering (regenerative dual core heat recovery) and has been throughly tested by the NRC in arctic conditions: https://nrc.canada.ca/en/certifications-evaluations-standards/codes-canada/construction-innovation/canadian-centre-housing-technology-evaluation-dual-core-heatenergy-recovery-ventilation-systems

Very few have properly evaluated the energy cost of the defrost cycles (NRC did do a heavily instrumented test in the Arctic) but anytime an HRV/ERV is in that mode, it's not ventilating and is incurring a sizable energy penalty (coming from the building's heating system) while warming the core. Then, if running via sensors for CO2, VOC etc, the system needs to run at a higher volume when it is ventilating, taking another efficiency hit. The Tempeff unit is dual core/regenerative so there is no defrost cycle which helps to keep efficiency in the high 80s and low 90s in cold temps at sustained, more efficient (lower) ventilation rates.

Your climate zone will have lot to do with choosing the most cost effective/efficient solution, particulary when enthalpy is factored into the equation.

@this_page, I do notice a small increase in sensible efficiency on the system, 3-5% at a given air flow as temps drop below freezing outside.

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