Working through efficiency metrics

We had this interesting spell of hot weather in July, including a two-day heat wave. To improve comfort in the garden unit, we ran a dehumidifier. Except that at one point, it became counterproductive.

With daytime temperatures consistently over 90 degrees and nighttime temperatures barely dropping down to 70 degrees, the dehumidifier produced too much heat for our comfort levels.

That led me to look into efficient air conditioning options for emergencies like this.

The Energy Star program is a good starting point. I opted to research portable air conditioners for their ease of use and flexibility.

The first lesson was to understand the efficiency metric, which is EER (Energy Efficiency Ratio). It is basically the cooling output of the unit in BTU (British Thermal Units) per hour divided by the electrical input in watts.

Say your air conditioner has a cooling capacity of 10,000 BTU/hr and consumes 875 watt. The EER rating for that unit would be 11.43, which is pretty good – meaning it’s pretty efficient.

While searching for efficient portable air conditioners online, I came across some interesting EER ratings, that, once I did the math, where obviously exaggerated. Buyer beware! It’s always a good idea to check the claims from the sales literature.

But there is more. The common window units have their compressor on the outside of the window – the building exterior. The compressor produces a lot of heat when compressing (cooling down) the refrigerant.

With a portable air conditioner, the compressor is now in the building interior. Some portable air conditioners come with an exhaust hose, to remove the heat from the compressor to the building exterior. Other units come with two hoses — a supply hose and an exhaust hose. Outside air is sucked into the unit to cool the compressor while the resulting hot air is exhausted to the outside.

The units that have the highest EER rating seem always to be those with the exhaust hose only. One may conclude that these efficient units may be a good purchase.

Not quiet, as I learned. If the exhaust hose is removing air from the conditioned space to the outside (and in the process cools down the refrigerant), then make-up air will have to infiltrate into the building because of the negative pressure that is created.

That make-up air coming from the outside is typically hot and humid. As a result, the air conditioner has to work extra hard. And yes, it does that extra hard work very efficiently, based on the high EER rating.

The units that come with a supply and exhaust hose have a slightly lower EER rating, say 11.2 compared to 12.6. Because of the supply hose, these units don’t suck hot and humid air into the building and thus won’t have to work as hard as the units with the exhaust hose only (which may well negate the slightly lower EER rating).

That said, we did not really have a choice. Our building envelope is as good as airtight, which means that we have to have a unit with the supply and exhaust hose, and that is what we bought to give it a try.

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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