Most energy efficient setback point for house in Dallas?

[paul42]

You are lucky to have a/c that will do the quick pull down your scheduling. On part of this group thinks long runs are more eff. and make the last longer. It's interesting to here them talk against the longer run you have with the setup and setdown. Their fear that more customers will want a/c systems that are able pull down the temp 6-8^ in a few hours. Most here are recommended small systems that are undersized because of inability to control humidity during cool wet weather.

Temperature setup saves energy provide the time is long enough. If the outside temperature is 20^F warmer than inside, a given heat transfer is established. When the inside temperature is raised 10^F, the heat transfers at 50% slower rate. The heat gained going rising up 10^F will have to be removed at nigh and is not a savings. But the slowed rate of transfer once the higher temperature is reach is all savings. So, a couple hours are probably not worth it. But 10 hours is different story. Learn to read your electric meter. Do it both ways and compare the results. A 4 ton a/c uses 4-5 KW per hour. Saving several hours 15-20 kw/day. I feel you will have better humidity control with an over-sized a/c like yours by seting the temp up during the day and getting a long run instead of short cycling through mid day. The equipment will last just as long if installed properly. The major limitation is your equipment ability to pull the temp down when you arrive and to have several hours of high temperture stability. I am doing some data logging but I only get to about 95^F in WI. I can cut my a/c by 50% by using temp setup during a hot day. It be less for your. Read your el meter and post the results. Regards TB

You will save a lot more money by building the house right to start off with. My house will not warm up enough during a one day setback to make any significant changes to the heat gain. Get the house right and the HVAC is a lot easier.

[adamk]

Paul42;

How did you build your house to get this ability ?
I heard you have a basement - where in DFW did you get a basement ?

Adam

[Carnak]

You will save a lot more money by building the house right to start off with. My house will not warm up enough during a one day setback to make any significant changes to the heat gain. Get the house right and the HVAC is a lot easier.

house has more of an impact than any HVAC system

About the only time I think setting up the temperature will actually cost you more trying to recover is when the home has a some thermal mass and you are in a cold place, or of course the heat pump and the auxiliary kicks in. Best to try and recover when the sun is shining.

In cooling it would take thermal mass, lousy attic insulation and high infiltration to make it not worth setting it back every day.

Thermal mass would imply, concrete, blocks or log construction.

[beenthere]

Oh, I don't know if this affects the recommendations but we have 12 ft ceilings in most of the house, so even more air mass to cool.

12' ceilings gives more to to pull down, but its also more to maintain. So 12' ceilings don't cause you to save less back using temp set back.

[Shophound]

house has more of an impact than any HVAC system

About the only time I think setting up the temperature will actually cost you more trying to recover is when the home has a some thermal mass and you are in a cold place, or of course the heat pump and the auxiliary kicks in. Best to try and recover when the sun is shining.

In cooling it would take thermal mass, lousy attic insulation and high infiltration to make it not worth setting it back every day.

Thermal mass would imply, concrete, blocks or log construction.

I wish your words, "the house has more of an impact than any HVAC system", could be cemented into the brains of every architect, architectural student, and residential builder.

I would like more firm data to back up my opinion, but I'm inclined to think that two of the three you mentioned; lousy attic insulation and high infiltration, would work against any setback program for energy savings. Stick frame homes, even with brick veneer, do not have a great deal of thermal mass, to my understanding, vs. an all concrete block or concrete home. The way to resist thermal extremes in stick built homes is to reduce infiltration and heat gain or loss. That's about it. Long pulldown times from a setback high for stick built homes that are marginally insulated, with leaky, attic mounted ductwork (and often air handlers), and unchecked infiltration does not strike me as a good recipe for energy savings.

[Carnak]

I wish your words, "the house has more of an impact than any HVAC system", could be cemented into the brains of every architect, architectural student, and residential builder.

I would like more firm data to back up my opinion, but I'm inclined to think that two of the three you mentioned; lousy attic insulation and high infiltration, would work against any setback program for energy savings. Stick frame homes, even with brick veneer, do not have a great deal of thermal mass, to my understanding, vs. an all concrete block or concrete home. The way to resist thermal extremes in stick built homes is to reduce infiltration and heat gain or loss. That's about it. Long pulldown times from a setback high for stick built homes that are marginally insulated, with leaky, attic mounted ductwork (and often air handlers), and unchecked infiltration does not strike me as a good recipe for energy savings.

shop I base my comments on experience and some numbers I crunched.

had a log home system sized right on the money for -26F, could not recover if he set it back unless his "return" setting was timed for when the sun was still shining. We hit a cold snap of two weeks in the minus 40s and no problem with temperature as long as he never set it back.

60,000 Btu glow core boiler in a 2000 square foot log place with a 5 foot crawlspace below it.

Setting back overnight when it was the coldest was dumb it would never recover until the sun was up. He went through less propane leaving it be

First place I lived in down here had leaky jalhousie windows, recessed ceiling lights crap attic insulation, concrete block construction. If I left the systems off all day it would be 8 degrees hotter than ambient when I arrived home at 5 and systems ran until midnight just to get it to 80F. If I left them set to 80 was cheaper.

[jax1]

s..... We hit a cold snap of two weeks in the minus 40s and no problem with temperature as long as he never set it back.

....Setting back overnight when it was the coldest was dumb it would never recover until the sun was up. He went through less propane leaving it be

Carnak,
Wouldn't the opposite apply as well?

..... We hit a hot snap of two weeks in the plus 100's and no problem with temperature as long as he never set it back.

....Setting back during the day when it was the hottest was dumb. It would never recover until the moon was up. He went through less KWH leaving it be.

[paul42]

Paul42;

How did you build your house to get this ability ?
I heard you have a basement - where in DFW did you get a basement ?

Adam

Three things in our area.
Put the duct work and air handler in conditioned space.
Shade all the windows in the summer.
Control infiltration.

I have R49 cellulose in the attic, and fairly well insulated walls. I have large porches, a good orientation, and a 3' roof overhang which shades all the windows in the summer. The house is very well sealed, but my largest heat gain is still infiltration and fresh air intake.

Technically, it is not a true basement. It is all above ground. It is a VERY tall conditioned crawl space - between 6 and 10 foot tall in the usable area, with a stepped concrete floor. The house was built on a slope. To save some very desirable trees and give us a LOT of storage space, we went with a pier and beam house with cripple walls. From a property tax point of view, it is an "unfinished basement" and that space is not taxed - YEAH!

Many people looked at the lot before we bought it and could not figure out how to build on it. I looked at the problems and found an opportunity.

[beenthere]

Carnak,
Wouldn't the opposite apply as well?

..... We hit a hot snap of two weeks in the plus 100's and no problem with temperature as long as he never set it back.

....Setting back during the day when it was the hottest was dumb. It would never recover until the moon was up. He went through less KWH leaving it be.

Not really, if the system has to run all day to maintain a set point of 76. Then a setback to 80 won't use more electric.

[Carnak]

Carnak,
Wouldn't the opposite apply as well?

....Setting back during the day when it was the hottest was dumb. It would never recover until the moon was up. He went through less KWH leaving it be.

I would say that one describes the first place I lived in down here, it would run from 5PM until midnight to get to 80 from 95F

The second place I lived in here was a ground floor condo, end unit with long east wall, and a condo above. Shut the AC off all day, come home its 82 inside, and it recovered fairly quick. Place was tight also.

My current place no problem setting it back, it saves money.

[Carnak]

Put the duct work and air handler in conditioned space.

or seal the attic

[paul42]

or seal the attic

That puts the duct work in conditioned space and does a good job of controlling infiltration. And, for many houses, it is the best answer.

However, I could not come up with a cost effective way to seal the attic that worked with my large porches and 3' overhang. The large porches are necessary for my our way of life. Between those and the 3' overhang, our total heat gain through our windows is about 1,200 btu/h.

I have 2,230 sq. ft. of living space, and more than 5,000 sq. ft. of roof. Any sort of foam insulation, spray or 4x8 sheets became cost prohibitive compared to simply blowing cellulose over the living area.

[cem-bsee]

heat transfer takes place whenever there is a temperature difference over a time period.

maybe one has to have a longer recovery time for a given house --
so, maybe one has to bring temp up in 'steps' -- to 'fool' the all- knowing t-stat [designed using the Microsoft theme]