HVAC T-stat Location Experiment

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The long axis of my single story house lies roughly NS. The S end is a few points W of S and is two bedrooms-wide, one of which is used for a small office where I type. This being Summer & central Tx, it's rather hot. Adding to this heat inside the office is the desktop confuser power supply. The machine is an old Dell Dimension, and it's currently on its 3rd PS. I don't remember the wattage and don't wish to open the case, but it's around 450W. With the confuser on 24/7, the heat generated by both the PS and the confuser itself begins to add up. Particularly in a small 14' x 11' room with a flat 8' ceiling (1,232 cu. ft). I spend most of my time during the weekdays here...in the hottest room of the house. The t-stat is only 18' away in the living room, but it might as well be in another time zone...further North. The LR has a high ceiling, large ceiling fan and is open to the kitchen, dining area each with a separate A/C duct feeding it. There's about a 3°-delta-T between the LR and this one. A few years back, I performed a load calculation on my house, then designed and installed an all-new duct system with dampers. It made a huge difference as previously I was only getting about 1.8tons of cooling out of a 3ton unit, all-the-while paying a 3ton e-bill. The old duct system was not only too restrictive, but compounded by the fact that the air was distributed to several branches by three triangular duct-board boxes, known to cause high turbulence making an already bad problem worse. They were all replaced with metal Y's w/dampers. Much lower loss and smoother airflow. I recall that the load/duct system calculations indicated the air registers in this room and the other bedroom on the S side are undersized. At the time, I decided to use a reducer instead of upgrading to a larger register box and messing with sheetrock work. I wanted to see how much difference the ductwork itself made. Well after having lived with it for a couple of years now I know: This Winter it'll be time to do the sheetrock work and upgrade both register boxes from 6" to 8". In addition, the current boxes are side-buckets. I'll go with them again if I can find them, but last time I checked all I saw were tops. While writing the other day, it occured to me to time the ON and OFF cycles. During the heat of the day, both ON and OFF lasted about 9 minutes. I seem to recall that a residential A/C system just begins to reach its peak efficiency after it's been running a minimum of 10 minutes. While recently cleaning out some stuff, I found a remote t-stat sensor I'd forgotten about. Still in the box and with all paperwork. My t-stat is an Enerstat SHP-1 that is heat-pump compatible, uses an exterior temp sensor to accurately set the balance point and is capable of using up to 6 remote thermostats to average temps across several locations. Plus it's made in Canada! Canada I thought it quite unique when I bought it back in the mid-90's. Even plain HP t-stats were hard enough to find. This one was digital + programmable. After reading the instructions, I realized that the remote sensor could be used in place of the t-stats temp sensor OR they could be wired to average the two. This morning I wired up the remote temp sensor and installed it in the very short hallway leading to three rooms, all with air registers. With the t-stat set for 78°F, the A/C kicked on and ran for 27 minutes straight before shutting off for about 5 min. then ON again for another 30 min. It has mostly followed this pattern since. The office-room-temp has dropped from 82°F to 78°. Outdoors it's currently 98°F, Rh is 47%, Heat index is 111°, solar heat index is 130° and attic temp is 99°. In the kitchen/eating/living room area it's 76°. A few key points here: 1. A t-stat only measures temp where it is (obvious). However, this doesn't necessarily mean that the temp is uniform throughout the house. Mine's only ~ 1,500 sqr. ft and 18' away it was 4° hotter. Sun exposure, orientation, duct work, insulation, desktop confuser = 24/7 heat source, floor plan, etc. all matter. 2. Just lowering the t-stat from 78° to 76° isn't the same as moving the temp sensor. 3. The dampers to the two South rooms are wide open and the dampers on the North side are reduced to divert more air to where it's needed this time of year. (When the heat is on, I have to reverse this). But due to the t-stat location, and house airflows, more air is still needed to the South side (which the load calculation showed). So the 6" buckets will need to be replaced with 8" (78% larger). 4. Given this day and age, I think it's a good idea to use multiple temp sensors throughout a house instead of just one. 5. Another idea is to install a small, intake ceiling duct in each of the two Southern bedrooms and an ouput duct in the Northern bedroom. An in-line quiet fan is then installed in this separate duct to continuously circulate warm air from the South side into the cooler North side bedroom. A small quiet fan is much cheaper to run than the a/c blower.
 
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I noticed that desktops produce a lot of heat. I only have it on when needed. I don't believe in the 24/7 thing unless one is using it as a server. Laptops are hugely more efficient. Tablets even more. Speaking about AC cycling. This always bothered me. When my AC kicks in, it blows hot air first due to ducts on hot attic. I always thought longer cycles would be more efficient. But, I'm too lazy to replace the old thermostat. Now, since your AC runs 3x longer with the same OFF cycle, your bill will be higher, right? I just measured the ON cycle and it was 16 min. It was actually longer than I thought. It's a hot day today. Also, how do you keep 99F in attic? Mine is more like 120F (full sun).
 
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sleddriver

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Originally Posted By: simple_gifts
Do you have awnings over the windows? For some reason a reasonable, low cost option of preventing sun from hitting windows has largely been forgotten.
I use a solar screen on two West facing windows that are hit only late in the day. On the South windows the only time Sun hits them is during the equinox. The Fall Equinox is worse due to the heat in Sept. No doubt higher quality windows would help. They'll come later.
 

sleddriver

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Originally Posted By: friendly_jacek
I noticed that desktops produce a lot of heat. I only have it on when needed. I don't believe in the 24/7 thing unless one is using it as a server. Laptops are hugely more efficient. Tablets even more.
Agreed. WXP may have a power-on/off feature I've forgotten about. For the longest, I never left them on 24/7, then began to. Time to go back to the old ways! I'm going to tape a small thermometer probe so the PS exhaust fan hits it to measure the temp. I can also plug the confuser into a WattsUp and measure the draw over time.
Quote:
Speaking about AC cycling. This always bothered me. When my AC kicks in, it blows hot air first due to ducts on hot attic. I always thought longer cycles would be more efficient. But, I'm too lazy to replace the old thermostat.
That's what I've heard. I normally leave it on 78 or 77° while here as it's fine, very dry indoors and with fans running it works. However, not everyone does this. If I set it on say 72°, it would run much longer before kicking off, the air would mix better, and eventually the office would cool off. No doubt the bill would go up as well. So the key is to provide enough CFM of cold air to the rooms with the highest cooling load while it's running. Changing out those buckets for larger ones will help that. More air will flow to those two rooms and less to the others just by removing the smaller buckets. No doubt I'll have to rebalance everything though.
Quote:
Also, how do you keep 99F in attic? Mine is more like 120F (full sun).
Yep, that full Sun will get you. I have some large trees to the East and one to the West that blocks most Sun except for mid-day. I also have 2-yr old light colored shingles that meet the energy star rating, a ridge vent, enough soffit vents to properly flow the RV, and radiant barrier paint sprayed on the underside of the roof decking. Shingle color (and radiant barrier) makes a HUGE difference...I wince whenever I see these new houses going up around here with black or dark brown shingles. They must be from up North. We don't have ice dams here, nor much of a winter. That's just nuts in this climate......
 
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Originally Posted By: sleddriver
Yep, that full Sun will get you. I have some large trees to the East and one to the West that blocks most Sun except for mid-day. I also have 2-yr old light colored shingles that meet the energy star rating, a ridge vent, enough soffit vents to properly flow the RV, and radiant barrier paint sprayed on the underside of the roof decking. Shingle color (and radiant barrier) makes a HUGE difference...I wince whenever I see these new houses going up around here with black or dark brown shingles. They must be from up North. We don't have ice dams here, nor much of a winter. That's just nuts in this climate......
I live in south and black shingles are universally used here and even required by my subdivision code. I know it's nuts, but that's how it's here. My friend just did all metal roof in bright red and it looks great, I guess he has no restrictions in his neighborhood.
 

JHZR2

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I like that concept of the added duct with the small pusher fan. Primarily because I like the idea of more air mixing through the home, without more stagnant spaces.
 
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southeast US
Originally Posted By: friendly_jacek
I just measured the ON cycle and it was 16 min. It was actually longer than I thought. It's a hot day today.
As sun went down the AC is cycling 11 min on and 20 min off. Is it about right? I never measured it before. The indoor temp is 77F and outside is 88F.
 

sleddriver

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Originally Posted By: JHZR2
I like that concept of the added duct with the small pusher fan. Primarily because I like the idea of more air mixing through the home, without more stagnant spaces.
My neighbor has a Broan installed in the ceiling of their garage to improve airflow. Problem is it's undersized. The ceiling grill is rather large as well. However other options are avail. Panasonic also makes these. In my situation, it would constantly move air from South to North and more evenly mix the air and remove hot air at the ceiling. I've seen these used effectively in two story houses where hot air is removed from the highest point and directed to the bedrooms on the first floor. This was back in the 80's in a passive solar house. I remember thinking "What a great idea."
 

sleddriver

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Originally Posted By: friendly_jacek
Originally Posted By: friendly_jacek
I just measured the ON cycle and it was 16 min. It was actually longer than I thought. It's a hot day today.
As sun went down the AC is cycling 11 min on and 20 min off. Is it about right? I never measured it before. The indoor temp is 77F and outside is 88F.
I don't know. Your set-up depends on many things. I only know my own. I'm hoping a HVAC contractor will chime in here, but they tend to be pretty tight lipped. So I'll call a guy tomorrow who'll be able to answer this. Good luck!
 

sleddriver

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Update I: After a few days of living with the new remote temperture sensor, and turning off the desktop in the evenings, I'm encouraged by the results. Previously, my office room temperture would slowly creep up during the day as the direct Sunlight bore down on the roof mid-day. With the t-stat being in it's living room condition, it was unable to detect this. Delta-t between the two locations usually ranges about 3°F, however it can rise to 4 or 5° during these mid-day/early afternoon times. By remoting the t-stat temp. sensor closer to this room, the t-stat is better able to keep the room temp more even. Makes sense. Also the unit doesn't run as long nor as often as I originally reported. It has been hot: 104°F a few days ago and 102° yesterday. Previously, I'd set up a large floor Vornado fan to blow air from the living room into this one. I'd also tried blowing air the opposite direction to blow the warmer air down the short hallway into the living room. However, there is just no substitute for CFM's of cold air blowing into a warm space over sufficient time to remove the heat and mix the air in the space. To make it permanent, I'll have to get in the attic and pull some wire. The main impediment is a Texas attic is no fun during August...even if it IS well ventilated!
 
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The t- stat needs to be near the return air filter grille location at 5feet off the floor. If your home is a 3 ton unit, you should be hanging beef in all the rooms. If you designed your duct work using a duct calculator and have adequate attic insulation, you should be freezing to death. Of course I do not know your window insulating rating or wall ratings R value. In Florida we designed at 600 sq ft of living space to one ton of airconditioning. You unit is should be sufficient ..placement of supply grilles is very important and you can use curve blade supply grilles to direct the air as oppose to fixed supply grilles,,this makes a huge difference on air flow. One more point, allow 200 squares inches per ton of air for your return air filter grille size. You need a return air grille of at least 20 x 30 filter size and the return to trunkline size also. I have been out of the business now for 10 years, but thats what we did years ago, never a complaint. Our best complaint was the unit make the homes to cold,,easy fix,,,lol.
 

sleddriver

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^^^ The relocated sensor is 6' away from the main (20x24) air return. The unit itself is an upflow located in a small closet in the small hallway (8'long by ~ 4'wide). When I installed the new duct system, I added a 12"x12" additional return as the main one above was bowing the filter. I did a Manual J load calc and used Manual D to design the ductwork. The supply grilles do use adjustable curved blades and are of high quality. Re: Air return filter grille size. I currently have 624 sqr. in, so I meet your requirements. I could have gone with a 12x24 though on the new one. Re: Size. My 3ton is probably too large now, given all the updates I've done over many years to make the house more efficient. Personally, I think oversizing is a big mistake, particularly in a humid climate, where there are both sensible and latent heat loads to consider. Indoor Rh needs to remain low compared to hot, muggy outdoor conditions. In order for that to happen, the air handler has to run long enough to condense the humidity out. I'm amazed when I go to my brother's place in Houston. He has a new two-story house, very high ceiling living room. His house is humid and hotter to me at 74° than mine is currently at 79°. I don't think it was designed correctly as they had a gathering during the Summer and the system was incapable of providing enough cooling to keep it comfortable. Further his bills are >> $$$$. It wasn't until I began educating myself deeply about HVAC that I understood just how poorly my 2nd system was installed back in 1990. Idiots...yes it's that old, but still works great. I hired a kid to install a TXV I purchased. Made a huge difference in performance, humidity removal, system life, etc. I also added a sight glass so I could easily check the charge without gauges. Thanks for the reply!
 

JHZR2

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Your situation is also why I like the idea of a dual or multiple AC setup in the home. It is far less wasteful and far more customizable by space that is occupied. Mini split systems and dual/triple HVAC systems are becoming more common as a result.
 

sleddriver

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My small house is barely 1,500 sqr. ft. A bit small for multiple systems. Having cfm distribution carefully balanced is key in any house and far too often, I think it's ignored. How often do you hear of people remarking about cold/hot rooms? Seems common. At the risk of painting with too-broad a brush, I think HVAC contractors, particularly in the hot, humid South, where we live in A/C, with long cooling seasons have far too long relied on 'rules of thumb' instead of math. Oversizing a system in these conditions means it usually doesn't run long enough to thoroughly mix the air + remove moisture, which is essential here, as the latent loads are sizable. Poor installation and ham-handedness also doesn't help. For example, my present system wasn't installed level, resulting in the upflow condensate pan being tilted away from the drain! Thus it would pool opposite the drain and at times, spill over. Further, the unit could either be installed as an upflow or sideflow. It was installed as the former, however they didn't bother removing the sideflow drip pan, resulting in very uneven airflow on that side of the coil. (Having looked into this, I don't know why any sideflow system would use an A-frame evaporator. Doesn't make sense to me...) Lots of these things just add up to unknowing customers Sierra-Oscar-Lima, paying higher bills, cold/hot rooms, water stains on their ceiling, and buying new systems just because their duct system is inadequate and the evaporator is clogged with dirt, mold and hair. While buying a new system will fix the later, it won't do anything at all regarding the former. Plus the new unit will have to 'breathe' through it. AND on top of all that, HVAC equipment + service is $$$$$$. Their yellow page ads are only equalled by plumbers, lawyers and car sales! They make a nice living!
 
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If you have measured the cfm from each grille and the air flow adds up to the cfm of the unit, you should have been ok. Heat infiltration can still steel your cooling. Usually your utility company will do a free inspection or an HVAC company will,,well at least they did in Florida in Pensacola.
 

sleddriver

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Update: My e-bill runs from about the 19th of the month to the 21st of the next month. As I'm not sure how to format a table here, I'll list it in line. Mid-June to Mid-July I used 1161 kWhrs. Mid-July to Mid-Aug I used 1192 kWhrs. Last Aug bill was about 1210 or so kWhrs, from the course graph on the bill. I'll have to plot usage over the past few years. I don't believe any money has been saved by moving the t-stat temp sensor to its new location. However, it has prevented the delta-T from rising too high, which is good. I still think the solution lies in enlarging both buckets in these two Southern rooms to increase air flow, pressurize the room to force more air out and move it to the return air grille, which isn't that far away. Maybe 9'. I'm still interested in the idea of a separate vent/fan combination running from the S all the way to the N to continuously move air from the warm side to the cooler.
 

sleddriver

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Update II: The t-stat relocation has made quite a difference in comfort for the South side where I work & sleep. I'm also using a large floor model Vornado fan on either low or medium to blow cooler air from the living room into the short hallway where the t-stat is located. This keeps some air moving into there and prevents stagnation. Aug 20 - Sept 20 I used 1059 kWh for a combined cost of 0.121/kWh. I'm locked into Austin's power utility and unable to shop around unfortunately. A few years ago they raised the rates about 18% and adopted one of those multi-tiered plans. Tier 1 500kWh @ $0.0375 per kWh Tier 2 500kWh @ $0.08___per kWh (113% increase over Tier 1). Tier 3 [email protected] $0.09325per kWh (16.56% increase over Tier 2). Then I'm hit with other fees, charges and tax. I try to stay out of Tier 3, but there's no way to stay out of Tier 2. It'll be interesting to see the Winter kWh usage with the solar water heater back on-line, assisting the electric one. Also, since this side will now be warm during Winter, the relocated t-stat will be closer to the warmer part of the house. I might have to relocate the floor fan to push warm air out into the hallway, into the living room.
 
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