Hello, brand new user here, but I've been lurking for a while... I’ve been pulling what little hair I have left out, trying to get useful energy cost numbers for my HVAC system. I need to replace the existing system and want to see if it makes economic sense to install a higher efficiency unit. However, my energy usage calculations are way out of line with my historical usage and I can’t figure out what the problem is. Any assistance would be greatly appreciated! My home is a single story, slab-on-grade double brick house located in southern Arizona. It was built in 1950, with 1400 square feet of conditioned space. It has a hip roof with a fully insulated attic (house ceiling and attic rafters both insulated with R-19 bats). The windows are the original single-pane casement windows. The existing HVAC unit is a 12 SEER, 4 ton, 80% fuel efficient Rheem natural gas furnace/AC on the roof. I plan to install a split unit in the attic (the original furnace was attic mounted). I used HVAC Calc 4.0 and two other methods to estimate my heating and cooling loads. The HVAC Calc results are: [INDENT]Heating: 56,656 [BTU/hr] = 1,490 [BTU/deg*hr] (38° temperature delta) Cooling: 26,170 [BTU/hr] = 1,090 [BTU/deg*hr] (24° temperature delta) [/INDENT] To get [URL="http://www.erh.noaa.gov/cle/climate/info/degreedays.html"]Heating Degree Day (HDD) and Cooling Degree Day (CDD)[/URL] values, I downloaded hourly temperature data from the [URL="https://www.ncdc.noaa.gov/cdo-web/"]NOAA National Climate Data Center[/URL] and summed the differences from my 24-hour thermostat settings. I checked those values against the NOAA HDD and CDD values for my city; my calculated values are within 5% of the HDD/CDD values when corrected for my thermostat settings. For two peak months, my values are: [INDENT]January (avg 51.8°): 12,100 [deg*hr] = 504 [deg*day] (NOAA 65° HDD: 383 [deg*day]) July (avg 87.0°): 7,740 [deg*hr] = 323 [deg*day] (NOAA 65° CDD: 683 [deg*day]) [/INDENT] To check my value against the July NOAA CDD value, I subtracted 65° from my average indoor temp (76.6°) and multiplied by 31 days { (76.6°-65°)*31 [days] = 360 [deg*days] }, and subtracted that value from 683 CDD to get 323 CDD. I calculated my estimated energy use, in BTUs, as follows: [INDENT]January heating: 1,490 [BTU/deg*hr] * 12,100 [deg*hr] = 18,000,000 [BTU] July cooling: 1090 [BTU/deg*hr] * 7,740 [deg*hr] = 8,400,000 [BTU] [/INDENT] According to this [URL="http://www.eia.gov/tools/faqs/faq.cfm?id=45&t=8"]link[/URL], one Therm = 100,000 BTU. I converted my heating energy to Therms, as follows: [INDENT]January heating: 18,000,000 [BTU] * 1 [Therm] / 100,000 [BTU] = 180 [Therm][/INDENT] According to this [URL="http://www.eia.gov/tools/glossary/index.cfm?id=S#sea_ener_ef_ratio"]link[/URL], an air conditioner SEER number is the ratio of BTU output to Watt*hour input, that is: SEER = BTU / W*hr; or, W*hr = BTU / SEER. I converted my cooling energy to W*hr, as follows: [INDENT]July cooling: 8,400,000 [BTU] / 12 [BTU/W*hr] = 700,000 [W*hr] = 700 [kW*hr] = 700 KWH (the 12 [BTU/W*hr] value is my existing unit’s SEER rating)[/INDENT] The problem is, my average January heating usage, in Therms, is 20.5 Therms; the highest monthly bill that I’ve had over the last two years is 50 Therms. Over the course of a heating season, I average 84 Therms, but my calculations predict 570 Therms, a 680% error. On the AC side, my average July bill is 1280 KWH. Over a season I average 5630 KWH, but the calculations predict only 3750 KWH, a 67% error. If both calculated numbers were high (or low) I would assume that there was something wrong with my heat load analysis, but the heating is way over and the cooling is significantly under actual usage. [B]What am I doing wrong?[/B]:wall: Thanks in advance for any help!

I am about to install trench heaters in my house. I already know something about them but my question is whether they are just heating type or can work as an air conditioners and emitate also cool air? Regards, Bruno