Do I really need an ERV?

pschusterSeptember 17, 2013

Hi everyone,
Am installing a geothermal system. Have two quotes from contractors I have confidence in (IGSHPA certified, checked references). Both are quoting 5 ton ClimateMaster Tranquility 30.

The building is a sport court with 30' ceiling at peak, approx. 2,000 sq. feet. We live near Baltimore, MD, climate zone 4--mild. Building is super-tight. Spray foam + blown cellulose in walls (r30) & ceiling (r50).

One quote came in $6,000 more than the other, but included an ERV. Obviously, I liked the cost savings of the lower bid, but am worried it shows lack of design care, as, after reading on this site, it seemed an ERV would be a good idea, particularly since it is a sport court. I asked the contractor who underbid about this, and he suggested we could always add an ERV later, but that it didn't seem necessary to start with one. Thoughts?
Many thanks to all for everything I've learned here!

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Is this a new building? Are you replacing a system that wasn't supported with ERV? Was there indication the building needed freash air while using old system? I believe the ERV would give minimual benifit with the 20' ceiling but if it was desired later,there should be little or no difference in cost because unlike a home or 2k sq ft office building with several rooms,air distrubution is simple.

    Bookmark   September 17, 2013 at 2:56PM
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Yes, it is a new building. Was worried w/my kids playing racquetball (think 2 sweaty teenagers) & a tight building, air could get stale. W/out ERV, will there be enough fresh air coming in from outside?

    Bookmark   September 17, 2013 at 3:45PM
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@ Klem1 - Actually the high ceiling makes this a more difficult design. Being a sport court, the ductwork and associated grilles are going to be located higher in the space. This leads me to believe the unit being proposed is UNDER sized based on the cubic ft of the space.

@ps - save yourself some headache and find a commercial air conditioning design-build contractor and not a residential contractor. If you are putting in wood floors, this brings in another design aspect of humidity control. A residential unit is NOT really what you need here. Adding additional equipment seems silly when you can buy commercial grade units that encompass all these features. This will also require upsizing your HVAC unit to compensate for the re-heat that will be utilized for humidity control. The Commercial grade unit also incorporates outside air in the unit and can modulate via a CO sensor to get more outside air in the space when more people are there. This will eliminate the need for an ERV. A simple exhaust fan will get your air changes. The formula is (x)60 / cubic ft of the space to get the air changes. (x) is the CFM of the exhaust fan. You should be able to work it backwards to come up with (x). You could always have this interlocked with the light switch or keep it running 24/7.

I just sized a unit for a QC lab smaller than your sport court and it sized out bigger than 5 tons with a normal 9 ft ceiling.

A failed HVAC in this application will cause you a warped floor.

On your insulation, Give me some details how the contractor is getting R30 wall ratings please

    Bookmark   September 17, 2013 at 4:03PM
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Cecash1, did try to get a commercial Hvac k'or that I know & respect but he said job was too small.

R30 is due to 2x8 walls. 1&1/2 in cc spray foam & the other 6 &1/2in spray cellulose.

    Bookmark   September 17, 2013 at 5:51PM
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To answer your question.........No, you don't specifically need an ERV, but you do need some way to get the desired air changes. To accomplish this, you need to bring in outside air and exhaust air. The ERV will help to "pre treat" the air using the latent cooling from the exhaust air. You could bring in raw outside air into your R/A plenum, but you would have to account for the higher entering air temperatures and add more cooling. Then you would have to exhaust air. You will want the building neutral to slightly positive.

Its sad to hear that a contractor thinks that job is too small. Business must be booming in your neck of the woods for contractors to be turning away work.

Did any of these guys do a load calculation for you, or are they shooting from the hip. The size they quoted you sounds like the basic 400 sq ft to a ton of air sizing. Those design temps you are talking about are not in that range.

    Bookmark   September 18, 2013 at 3:16PM
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    Bookmark   September 18, 2013 at 4:09PM
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Same opinion,just repacksged out of necessity. ERV is increasingly reccomended as energy cost increases and tempatures become more extreme. Your situation is neither. The very nature of the system is cheap energy and longivity in exchange for high initial investment. Coupled with relitive mild climate in Baltimore,the $6k doesn't return investment equal to the geo-thermal. The 20' ceiling makes passive ventilation very effective useing inexpensive and cheap to operate equipment and air drawn from outdoors can be conditioned inexpensivly. If you were in Mn using fossil fuel or electric,ERV would be high priorty. Stratification of tempature layers are prominatly figured into desigh of the system. That's why a 2k sq ft room with 20' ceiling doesnt require twice the capacity of 2k sq ft with 10' ceiling because of double the cubic feet inclosed in the room. I'm confident if you ask the contractor who included the ERV system,they did so in order to come closer to state of the art comfort more so than because it was as energy wise as the geo-thermal. I would do the same unless told at the onset to balance cost to budget on the system. Based on your statment both are repital companies,I think they didn't realize you would be receptive to techinal details. Most people aren't simply because they don't understand anyhow. Just let them know you will feel better knowing more details.

    Bookmark   September 18, 2013 at 5:52PM
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I would be inclined to agree with your stratification statement if I knew how the contractor proposed to condition the large open space. Being a sport court, I assume they are proposing the ductwork HIGH in the space. If this is the case the cubic ft come into play and the stratification is minimized.


Did they give you some kind of design drawing of how they proposed to condition the space?

    Bookmark   September 19, 2013 at 9:52AM
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