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Old 02-03-16, 07:05 PM   #11
ToddT
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I'm no engineer to help with enthalpy and please forgive if my comments are too simplistic, but as an efficiency nut, I have a few points to suggest.

First, go for as true a white roof as possible. Years ago, a friend with some mobile home rental units painted the roof of one silver and one white. He then turned off the AC and put a thermometer in each unit. Want to guess the temperature differential? The white one was 32 degrees F cooler than the "reflective" silver roof. Can you imagine if it was a black roof?

When I lived in Louisiana, I experimented with water on the roof and on the AC outside unit. An alternating brief water spray followed by a 15 minute evaporation sequence will keep the roof cool with low daily consumption (.1 gallon per square feet per day). Even with high humidity, I recorded a 12 degree drop in interior temperature at the ceiling. However, this benefit is less the more R-value you have in the roof.

As for the AC units, I put a fine fog mist water spray on the outside unit. I saw a 7 degree drop in the air coming out of the central AC register. However, many newer window units are designed to splash the condensate up on the condenser coils.

To assure no condensation, what would be the humidity goal inside? I find 50% is the magic number for comfort but equipment is different. I have a feeling anything much over 60% R.H. would have a tendency to sweat.

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Old 02-04-16, 01:35 AM   #12
jeff5may
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Sorry I didn't see this before. I live in Elizabethtown, maybe 75 miles North of bowling green. For your interior conditions, the toughest time of year for you will be spring and summer, due to the high humidity at night. Even though the outdoor temperature drops at night, the air is no good for supply due to the moisture. During extreme cold snaps, the temperature might plummet to around zero overnight. We have very few subzero cold snaps in the area, ever. The worst one I can remember, it got down to -12 one night during a 3 day cold spell. 2 days later, temps went up, and we had a 50 degree high.

I'm with mejunkhound on this one. Fill your block walls with perlite, vermiculite, cellulose, or something that insulates. Paint on a vapor barrier (inside or outside but not both) and throw some foam and light-colored metal on the roof. This will serve you well to take the cooling needs down during the summer.

As far as the corner seasons go, a dehumidifier may keep the dewpoint down if you are going to try to use outdoor air to provide cooling. Just run your supply air through the dehumidifier on its way in. It will knock at least a few degrees off of the dewpoint. Due to the small size of the building, you need to keep that intake air under control, or the space will quickly be tainted with bad air.

I'm not sure using outdoor supply air would be an advantage during typical spring and summer nights over just extracting the btus out of the building. Let's say you have some cool May air overnight that bottoms out at 58 degrees with a dewpoint of 57. Bringig the dewpoint of that nice cool outdoor air closer to 50 might cost you more than the thousand watts the transmitter is producing. A 5000 btu window air conditioner only uses around 400 watts, running constantly. Of course, there will be plenty of cool, dry days and nights where outdoor air will make for awesome supply air.
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Old 02-04-16, 07:58 PM   #13
JRMichler
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If you still want enthalpy control, check out the Honeywell C7400 enthalpy control.
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Old 03-16-16, 12:25 PM   #14
gtojohn
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Short answer for your question, find a psychometric chart. The harder question would be applying it and making it work for you situation.
I was a broadcast engineer in my previous life. We had a 98kw vhf transmitter. Similarly we used 100% outdoor air. Honestly there was no good reason for that other than it had always been that way and the higher ups had little concern about inefficiency. Humidity and lack of it is your biggest concern. Too dry and static electricity becomes a threat. The more outside air you bring in the more energy required to reduce its temperature and humidity. You'll have greater control of humidity with less outdoor air. I would suggest a humidity operated damper to bring in more fresh humid air once humidity is below 20%. Probably more relevant would be dehumidification control with your variable speed blower. Lower fan speed more dehumidification. Most have a dehum terminal on the control board. Where I live its so humid its nearly impossible to dry air below 20%. Unless your building is unusually tight you're already bring in enough outside air to keep the humidity up. Good luck!

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