Goal
The following is ‘most’ applicable to mixed humid climates. My climate is north central <!—-><!—-><!—->Texas <!—-><!—-> and I have 4,000 sq. ft. of conditioned space.
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I set out to build a house that was relatively cheap to air condition, without spending a lot of money on the AC equipment, and not requiring a lot of non-conventional (expensive) building techniques.
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Results
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I have my thermostat set to 74 degrees and 42% humidity. I do not hear the AC going on or off. The house stays at a constant 74 degrees and the humidity between 39% and 43%. The average high for my last electric billing period was 96 degrees. Keeping my house at that temperature and humidity cost me about $60 for the month – at 14.5 cents / kwh.
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We can set the thermostat where it is the most comfortable and not worry about the electric bill.
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The AC equipment consists of a 14 SEER two ton heat pump for the whole house, and a 19 SEER mini-split for the master bedroom only. With the mini-split turned off, the two ton heat pump will cool the whole house down to 70 degrees on a design day – 98 degrees outside with the dew point at 68 degrees. The day I ran that test, the official high for the day was 101, the mini-split was off, it was 70.3 degrees inside, and my wife had to put a sweater on.
The mini-split is used to keep the master bedroom at 70 degrees for more comfortable sleeping, but is not needed otherwise.
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The house is close to standard frame construction with 2×4 walls, cellulose insulation, and polyiso sheathing on the walls.
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Due to the slope of the land, and some very desirable trees, we used pier and beam construction. The resulting ‘crawl space’ is conditioned with a stepped concrete floor and a ceiling height that varies between 6 and 10 feet.
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All the windows are shaded during the summer. The only exceptions are three very small windows that start to get some sun at about <!—->6pm<!—->. They are very low E glass. The other windows get zero sun during the summer months. This was done by situating the house properly, 3 foot overhangs on the roof, and large porches.
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All of the HVAC equipment and duct work is in conditioned space. Mine is in the ‘crawl space’. Because it is in conditioned space, the duct work does not have to be insulated.
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Air tight construction. I put a lot of personal effort into the air-tight drywall approach. The house is very tight.
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Some extra insulation. R49 blown in cellulose in the attic. Cellulose in the walls of the main floor. 2†of polyiso sheathing on all of the exterior walls.
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Replies
bump
Can you tell me what the specs are on the mini split, BTU, ac only or heat pump, brand, etc? What kind of siding does the house have? Do you have any photos of the house construction showing the polyiso and siding detail? Sounds like a successful project.
Kenny
I have the Fujitsu 18RLQ mini-split in the master bedroom. It is a 1.5 ton 19 SEER heat pump.
I tried to get the 12RLQ 21 SEER, but was not able to find a HVAC contractor that would put that in. Very few companies were willing to bid putting a 2 ton heat pump in a 4,000 sq. ft. dwelling and the company I did go with hedged their bet by specifiying the 18RLQ instead of the 12RLQ.
The house is almost entirely brick. This requires the two piece commercial brick ties and long screws through the 2" thick polyiso insulation into the OSB sheathing.
The OSB sheathing is covered with a self healing peel and stick roofing membrane that covers everything from the foundation to the roof line. The polyiso insulation is attacched using 3" cap nails.
I have hardi-plank on the front porch wall. furring strips are installed with 4" screws through the polyiso and then the hardi-plank is attached to the furring strips.
The window and door openings were oversized and then framed into size with 2x6 to bring the framing even with the polyiso.
What size screws were used #8,#10,#12 etc, to attach the furring strips? And what was the spacing of each screw in the furring strip? Thanks,
Kenny
http://www.mcfeelys.com/product/1040-FMY/Flat-Head-Steel-Yellow-Zinc-Screws
The framer put the screws in and I don't know what spacing he used.
pics?
jt8
"A little 'enthusiasm' and all problems seems small!"
sorry - no pictures
Did you have a blower door test or a thermal scan done?
Did you get the house on the energy star program?
What is your electric bill a month in the summer?
I have not had a thermal scan or blower door test run - at least yet.
I am not convinced that a blower door test will have enough payback to make it worthwhile, but I haven't ruled it out yet either.
I have a total electric house. My electric bill for last month was $178. Approximately $25 of that is for the separate window shaker AC unit on the dog house. I have spoiled dogs. Approximately $100 is my normal bill for lights, hot water, laundry, etc... So my AC cost for the house for the month is approximately $50.
Several changes have been made since that billing period, so next month will be different. I swapped out the contractor installed thermostat with a Honeywell IAQ about the time the new billing period started.
The contractor installed thermostat was flaky. It claimed that the indoor temperature swing between turning on the central AC and turning it off was 2 degrees. My calibrated thermometer said the actual swing was about 5 degrees.
The Honeywell IAQ thermostat keeps the temperature right at my set point 74 degrees and the humidity very close to my set point of 42% - using the variable speed blower on the air handler to control humidity. I have the CPH set so that the AC only comes on once per hour. I have the fan set on circulate so that the blower runs 35% of the time to even out the temperature through the house and bring in some fresh air.
I also have a 19 SEER, inverter driven, variable speed, Fujitsu, mini-split in the master bedroom. Before I installed the Honeywell IAQ, I had to turn the mini-split off during the day to control humidity. The mini-split does not remove as much humidity as I would have thought, and while it was on, there was not enough load on the whole house system for it to control humidity either. Now that I have the humidity under control, I can run the mini-split all day and take advantage of the efficiency of the 19 SEER system. It switches between off and low speed all day to keep the master bedroom at 70 degrees.
What is the roof color and composition?
medium gray asphalt shingles, radiant barrier sheathing.
R49 cellulose in the attic, all of the duct work is in the basement.
Manual J claims the heat gain through the ceiling on my house is 1,618 BTUH - and it does not consider the radiant barrier sheathing.
Please post some pics :-)
What area of Fort Worth are you in? Is it a true basement, or is it a split level type setup where you can walk out ground level on one side of the house? I'd like to see some pictures of the crawl space/basement if you get a chance to take some.
I live in Farmers Branch.
David
It is not a true basement. It is a VERY tall crawl space. About 6' tall on one end and 10 foot at the other end. It has a stepped concrete floor, and it has 2x6 cripple walls.From the outside, the west end of the house looks like a two story structure. From the east end, it appears to be one story.
Congratulations Paul. You could have skipped the cellulose in the walls. On the house I showed you here in Austin we did a cost analysis on adding r19 in the walls and the savings were only $7.30 a year. This is with seer 18 equipment so with the 14 seer it would have been about 10$ on a 7000 sq ft home. That is a 411 year payback without figuring interest on the capital investment.Enjoy your new home. Ya done good.RayThere is only one planet earth. How many chances do you think we get with her?
How did you do your analysis? Did you use software?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Thanks again for all of the help!At the time, adding the cellulose to the walls seemed like cheap insurance. The numbers all said it would work, but putting a two ton heat pump on 4,000 sq. ft. in Texas seemed fairly extreme. The cellulose does add quite a bit of noise damping as well.I've added a CO2 monitor to my experiment. I'm still trying to come up with the best method to add the right amount of fresh air to the house.
Because it is in conditioned space, the duct work does not have to be insulated.
Paul, I am very fascinated with what you and Ray Moore are doing (with your wall construction). I am confused about the need to insulate the ductwork.
I have visited Ray Moore's project(thank you Ray) and I believe that Ray knows his building science as well or better than anyone else in Texas. Ray's ductwork( like yours) is in the conditioned space yet Ray chose to insulate his ductwork.
Some say that there is condensation potential on the ducts even when they are in the conditioned space.
Paul & Ray please elaborate about the ducts.
Are either one of you using a dehumidifier other than the ac?
Paul, how do you plan to handle humidty in the shoulder season?
Paul..do you have any gas appliances?
John
I do not have any gas appliances.I do not have any problems with condensation on my duct work. It is not required by code.Some dehumidification is provided by my heat pump water heater which is located in the lower level.The majority of the dehumidification is supplied by the AC. I have a variable speed blower in the air handler and a Honeywell IAQ thermostat. The thermostat is set to also control the humidity. When the humidity is above the set point, the thermostat lowers the blower speed on the air handler. I have found this combination to be very effective at limiting the humidity in my house. I have it set at 42% and it stays between 38 and 45%. Normally, it is between 40 and 42%.A nearly air tight house makes dehumidification simpler. Most of the humidity comes from the outside air. it is easier - and cheaper - to dehumidify the outside air before it is mixed with the whole volume of the air in the house. If the fresh air intake goes to the return on the AC, then the house is slightly pressurized which prevents the outside air from coming into the house except at the fresh air intake.The internal heat loads are a much greater percentage of the AC load on an air tight house. With my 'relatively' small heat pump, it is able to dehumidify without over cooling the house, even in the shoulder season
Edited 7/27/2008 8:29 pm by paul42
Yesterday, it was 103 when I came home from work. The house was 74 degrees, 40% humidity, and the basement was at 70 degrees. My little two ton heat pump was still cycling - i.e. not running continuosly. It was a FANTASTIC feeling!
Thank you for the kind words.You need to use insulated duct for two reasons. There will be times when large amounts of humid outdoor air are introduced into the home. People will leave doors and windows open at the wrong times. There are also spot humidity issues such as bathrooms and kitchens. You don't want the duct system to become a drippy dehumidifier. Perhaps more important than that is that you want the air to still be cold when it gets to the end of a long run instead of picking up heat along the way. Insulate all your supply duct. Return duct in the conditioned space is another issue and it does not need to be insulated, though your building official may disagree with this. We want to be in complete control of our indoor environment. Yes, I have supplementary dehumidification included in all my projects, as well as filtered fresh air intake and exhaust hood makeup air.There is only one planet earth. How many chances do you think we get with her?
Ray,
Have you noticed when the greatest demand for the dehumidifier occurs?
I would assume the shoulder seasons or perhaps summer mornings....
SpringThere is only one planet earth. How many chances do you think we get with her?