Ask The Energy Rater - "Response to a Reader"
I really enjoy hearing from those reading this column. I generally get several emails each week with questions or comments. I personally respond to each one and attempt to answer the questions. I recently had a reader write in with a question that I thought could apply to many who read this column. We have many, many “retirement” type communities with manufactured or modular type homes where the occupants are only here part of the year. Here is a question from someone in that category. I share this question with their permission.
“We are "snowbirds" but we own our 1987 28 x 48 manufactured home in Central FL. This home sits empty from June through October. We have a 4 ton air source heat pump, 4 yrs. old. We have found that it has good and bad features. In cold weather, it produces nice warm air--for a while-- then goes into the defrost mode and blows cold air for a while. That must be the nature of the beast. At least that's what the "experts" tell us. Now--my question to you: we leave the thermostat in the a/c mode, set at 83 degrees while the house is sitting empty. When we return Nov. 1 there is dampness present. Should we invest in a humidistat?”
The first thing I did was determine the square footage of the home. I multiplied the length times the width and discovered the home has 1344 square feet of conditioned space. I have no other information than what you have in this email. So, I had to make some assumptions. By the year, 1987, I know this modular home is not built to the current energy code. In most cases, however, manufactured homes are insulated and are fairly tight construction. A full manual J calculation would have to be done to determine the proper size of air conditioner. My home is 2050 square feet of living space built in 2002. I have a 3 ton air conditioning unit. I despise rules of thumb for air conditioner sizing, but the old rule of thumb was 1 ton of AC for every 500 square feet. That doesn’t work with newer construction, but, may be fairly close on older construction. If a full manual J calculation was done to determine the size of the AC unit, I’m thinking it would be 2 – 3 ton maximum.
What is happening while the family is away is this. The AC unit is too big for the space. It will run to cool the home to the 83* and turn off. Because it is so big, it can achieve that temperature too quickly. The air conditioner needs to run 20 minutes per hour to remove the humidity from the air. If it is “short cycling,” that is only running 5-8 minutes and satisfying the temperature, the moisture (relative humidity) is never removed and continues to build up the entire time they are away. Upon arrival back, the house will be at the desired temperature but will feel clammy because the relative humidity will be too high. The temporary solution may be a dehumidifier. The long term is replace the unit with a properly sized AC. For all energy related questions or comments email sranck@fpuc.com.
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