| The Glade 4.0 https://gladerebooted.net/ |
|
| The physics of home heating https://gladerebooted.net/viewtopic.php?f=2&t=1675 |
Page 1 of 1 |
| Author: | Hopwin [ Wed Feb 03, 2010 8:43 am ] |
| Post subject: | The physics of home heating |
I have a question for people smarter than I am. Everyone says you can reduce your energy costs by lowering your thermostat and I am wondering if that is true. Constants: outside temperature is <50 degrees insulation performs at the same level regardless of internal temperature(?) the equilibrium temperature for my home is less than whatever I set the thermostat to Variables: If I set my thermostat to 62 then the furnace will kick on at 61 and raise the temperature 1 degree to 62 If I set it to 67 then it kicks on at 66 and again raises the temperature 1 degree Is this logic flawed somehow? Is the rate of heat loss faster at higher temperatures somehow? Is the relatively closer outside temperature versus thermostat setting a difference-maker? |
|
| Author: | TheRiov [ Wed Feb 03, 2010 8:49 am ] |
| Post subject: | |
Insulation slows heat transfer, not stops it heat transfer is a function of (among other things) the difference between two temperatures. Your house loses more energy the higher energy it has. Think of it in terms of a pure blackbody radiation scenario (no convection, no conduction) The radiant energy from something 500K is far less than the radiant energy from something 1000k. Since the higher temperature radiates more, you must pump more into it to maintain. Insulation doesn't change this fact. It mearly slows heat transfer, not stops it. |
|
| Author: | Gorse [ Wed Feb 03, 2010 8:53 am ] |
| Post subject: | Re: The physics of home heating |
Thermodynamics tells us that the greater the temperature gradient the faster the change will occur as two unlike temperatures interact (in this case, your house verses outside temperature). Lowering the temperature lowers the gradient thus extending the amount of tie it takes the inside temperature to drop to the threshold temperature. What is probably even more important is how well insulated your how is, but that's not the question you asked. (Guess I need to post faster). |
|
| Author: | Rafael [ Wed Feb 03, 2010 9:17 am ] |
| Post subject: | |
The funance must cycle on more often, because as pointed out, the rate of heat transfer due to conduction, is directly proportional to the difference of temperature (the temperature across the boundary of the walls between the inside ambient and outside). Convective heat tansfer due to winds is much more complicated to quantify because it hinges on fluid mechanics which means the geomtry of the house, direction of wind etc. come largely into play. Radiative heat transfer is largely negligible and is proportional to the fourth power of temperature difference. The only part that is really relevant is the roof - the roof exchanges radiation with the sky since its view factor is significantly greater than 0. The walls of your house do not because they are largely perpendicular to the sky and thus have a near 0 view factor. |
|
| Author: | Ladas [ Wed Feb 03, 2010 9:17 am ] |
| Post subject: | |
Other things that can have a significant effect on your energy costs, if that was the intent of the post and not just an "out of curiosity" question... 1) Location of your thermostat compared to your supply and return, or in relation to your most "used" space. 2) The design load of your units. Short cycling a system will cost you a lot more energy than a system that runs constantly to maintain the same temperature (thermostat location can also cause short cycling). |
|
| Author: | Ladas [ Wed Feb 03, 2010 9:19 am ] |
| Post subject: | |
BTW, for those interested, it is far better to insulate the roof at the rafters than at the ceiling. |
|
| Author: | Rafael [ Wed Feb 03, 2010 9:29 am ] |
| Post subject: | |
Ladas: Our senior design project was to redesign reach in refridgeration units like you see in supermarkets. Our first suggestion was the design was stupid because it's large, non-closed refrigerated space which basically cools the supermarket. They didn't like that feedback. Our next idea was to remove the digital thermostat and replace it with an analog feedback system and throttle-able refridgeration components (compressor and motorized expansion valve) so instead of cycling, the system would just throttle itself appropriately. However, we quickly discovered that the thermodynamic effeciency of the refridgeration cycle in all aspects was worse. So we had to find a margin we could "overrun" the refridgeration without wasting capcity and losing effeciency to scaling but not lose effeciency due to shortcycling. When we failed that, we did the default undergraduate strategy: don't refine and improve the concept. scrap it and make something more complicated. Our next iteration, which wasn't really an iteration but a new idea, was using a combined cycle, ammonia-alcohol absorption refridgeration type system. On paper it looks good, but these things are only used in large scale applications (like cooling a brewhouse for example) for a reason... Ah, those were the days. Now I get paid to be boring. |
|
| Author: | Screeling [ Wed Feb 03, 2010 9:38 am ] |
| Post subject: | |
By chance anybody in here know much about light wave physics (from the B&M course of intro physics) that might be able to help me out with a problem? I'm thoroughly stuck. |
|
| Author: | Ladas [ Wed Feb 03, 2010 9:41 am ] |
| Post subject: | Re: |
Rafael wrote: When we failed that, we did the default undergraduate strategy: don't refine and improve the concept. scrap it and make something more complicated. Heh. Is that strategy learned before or after the equation that Tonnage = Sq. Ft./X? |
|
| Author: | Rafael [ Wed Feb 03, 2010 9:43 am ] |
| Post subject: | |
Is it simple optics or electromagnetic mechanics? |
|
| Author: | Rafael [ Wed Feb 03, 2010 9:46 am ] |
| Post subject: | Re: Re: |
Ladas wrote: Rafael wrote: When we failed that, we did the default undergraduate strategy: don't refine and improve the concept. scrap it and make something more complicated. Heh. Is that strategy learned before or after the equation that Tonnage = Sq. Ft./X? Typically, it's instilled sometime during ME 161: Introduction to Design, where you write lengthy and abstract reports about your design process during each iteration using absurd design concepts that can't be taught, only learned. Strangely enough, most of our classes taught very few "industry standard metrics". We typically just looked at the processes in fundamental forms that were easily translatable back to theory and math, and just stuck with those units. The Internal Combustion Engines course I took taught it as a matter of necessity: engine design is a very established and regimented discipline. |
|
| Author: | Screeling [ Wed Feb 03, 2010 10:46 am ] |
| Post subject: | |
The question is: Quote: You need to use your cell phone which broadcasts an 830 MHz signal but you're in an alley between two massive radio wave absorbing building that have only a 15m space between them. What is the angular width in degrees of the electromagnetic wave after it emerges from between the buildings. I calculate a wavelength of .361 and divide it by the opening width. I take the arcsine of that to get the angle width. I'm getting 1.37 degrees and the book is getting 2.9. One guy stated I need to double my angle because the angular width of the central maximum is 2(theta). I've looked at the book and I don't understand where he gets that. |
|
| Page 1 of 1 | All times are UTC - 6 hours [ DST ] |
| Powered by phpBB® Forum Software © phpBB Group https://www.phpbb.com/ |
|