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Old 07-18-2019, 01:49 PM
 
4,149 posts, read 6,838,078 times
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Quote:
Originally Posted by ddm2k View Post
This is exactly what I'm talking about. I don't know where they got that number, but it works something like this:

20 degree difference between return air and vent air, maybe. But you're not sucking in that air from the outside, are you?

Home HVAC is like a car's a/c stuck on recirculate. Once the cabin (home) air begins cooling down, you'd expect the unit to cool it further. It may take some time, but it should get you there, every time.

Want to prove them wrong? Get a window unit and see how cold those babies get!
Not true

Again, this goes back to sizing of the unit. Yes, the air itself is kind of closed loop/recirculate (in reality, most residences are very leaky actually, so you are infiltrating a quantifiable amount of outside air at whatever hot/humid/cold/dry condition it is outside), but regardless of the fact that that is only somewhat true, the entire time this process is going on, your home is losing or gaining heat to the outside (depending on the time of year).

The way you present it, it seems like you should just be able to get the house to 65 if you want, even if it's 100 outside and that it would get there, if you give it enough time. This ignores the basic principles of thermodynamics. If you could figure this out, you'd be a billionaire If someone's home unit could actually hit 65F on a 100F day (who doesnt live somewhere like phoenix or dallas etc.), your unit is way, way over-sized, is probably killing the compressor via short cycling, is doing a poor job humidifying the space, and is wasting a lot of money by operating the compressor well out of its peak efficiency point.

It's an equation. In simple terms, in the summer, your house is gaining heat at a rate of X BTU/hr due to ambient:indoor temperature difference and direct solar heat gain + all internal loads (people, electronics, cooking, lights, etc.). Your air conditioner is capable of removing Y BTU/hr of heat from the space. Your house will continue to cool until the rate of Y BTU/hr removal is equal with the rate of X BTU/hr of heat gain into the space. After that, the house can no longer cool the space *even if the whole was completely air tight and truly recirculate only (which it is not)*. On an ASHRAE 0.4% design summer day for your location, that balance point is typically around 75F (per Manual J). Somewhere between 72 and 77 F is usually acceptable. If that 0.4% ASHRAE day is 95F ambient and your house was sized based on 75F on a design day, then on that 95F day, the house should be able to achieve 75F, assuming normal activity inside the house. (On a design day, X = Y).

If the outside temperature is only 90F, then yes, you may be able to get it to 72F with the same unit. This is because that unit can still remove Y BTU/hr of heat and meanwhile the house is only losing 0.9X BTU/hr. At the same time, if it is 105 outside, you may only be able to get to 77F. This is because the house is gaining heat at a rate of something like 1.1X BTU/hr while the AC can still only remove the same ol Y BTU/hr it has always been rated for, at best (in reality, it eventually will actually be reduce to some fraction of Y as the outdoor temperature rises above the rated ambient temperature of the condensing system). Because of this, the balance point for the system is higher. Instead of balancing at 75, the house now balances at 77.

As another example, if it's only 90F outside but you have a party with 25 people over, are running the oven nonstop, and have windows open, then you still will not achieve 75F on a 95F day. There are a LOT more variables than you are accounting for. Hence the reason college provides an entire 4 year degree in this topic and entire careers dedicated to HVAC design (with specialties such as focusing on designing the equipment, designing the systems, designing the control sequences, designing the control devices, etc. etc.).

Last edited by Sunbather; 07-18-2019 at 02:27 PM..
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Old 07-18-2019, 01:57 PM
 
4,149 posts, read 6,838,078 times
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Quote:
Originally Posted by Quietude View Post
I regret being able to rep you only once for this.

Yep, uber-stats do an absolutely amazing range of things. 98% of which are essentially useless. But this kind of 'fifty-seven functions in one' nonsense, headed by There's An App For It!, is just marketing-driven feature creep leaching bux from the marks.

Note that I write as an electronics designer who appreciates the convenience of good tech, has designed hardware user interfaces and remote control systems, doesn't think most remotes are indulgent and built two 'smart houses' when it was a nuts-and-bolts effort, not a shop-aisle-6-at-Lowe's job.

And has a new thermostat that has an on-off switch and up-down buttons, and thinks that's a little fancy for the simple, day-after-day job it does. I can't conceive of a need to pull out my phone and have a conversation with it.
I respect your background as an electronics designer and I understand your position. But as an HVAC design and controls engineer, I respectively disagree.

The thing is, you seem to think that more complex in this situation is a waste. In reality, Nest or Ecobee is still like a child's toy compared to the complexity of the HVAC control sequences we write, implement and study in every other application outside of residential (commercial, healthcare, industrial, etc.). And the real-time, proven cost savings by utilizing these sequences is massive.

For the price, the Nest and Ecobee and similar bring a lot of functionality above a typical 5/2 and absolutely have the opportunity to save money with paybacks of just a few years given the right situations.

There is however nothing wrong with sticking with something simple if you don't feel like messing with something more complex. It's not about the need to pull out your phone and have a conversation with your thermostat (if you have successfully used the smarts of a smart thermostat, you shouldn't need to be doing that, except for a couple different scenarios). But there is also no need to lampoon something just because you don't see personal value in it.

When implemented properly in the right situations these thermostats absolutely save money. The funny thing is, once you get them setup and get the control loops tuned in to your house, they require a lot less interaction than a traditional thermostat, in my experience. But they do still give you the opportunity to communicate with them remotely when your schedule changes or something happens out of the norm.
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Old 07-18-2019, 02:03 PM
 
Location: Wonderland
67,666 posts, read 60,197,371 times
Reputation: 100999
I love our two Nest thermostats.

My husband and I have very unpredictable schedules and so we really would rather not program our thermostats to go up or down at a particular time. So what I do is while we're out and about and probably an hour or so from coming home, I just open the app and switch the thermostat back to the temperature I want the house to be when we get home, and it's there or close to there when we get home. The thermostat senses when we're away and switches to "eco" till I change it or we come back home, which ever is first.

I find them very, very easy to use and they are so small - I really like them. We replaced some ridiculous system that was so complex I was constantly confused by it. The Nest is easy to read and easy to operate and I just love it.

We also have a Nest doorbell and some outdoor Nest cameras. I didn't even realize it when we bought them, but you can talk on them (and also hear what's going on in their range). I love those too! Recently we had some sprinkler system guys come out to fix something but I had to be on an appointment so I couldn't hang around waiting for them. So when they showed up, I got a notification on my phone, and I was able to talk to them that way. I was also able to let them into the garage without being there. It was great!

I can't help you install it but I hope you love yours as much as we love ours.
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Old 07-18-2019, 02:24 PM
 
17,472 posts, read 17,292,541 times
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My wife is home all day and I work rotating shifts. For us an automatic digital thermostat is best. Set the heat temperature, set the cool temperature, and put stat to Auto. Key is to have no less than 3 degrees difference between heat and cool. In our area for a few weeks twice a year we can wake up with heater running and need AC in the early afternoon. With this thermostat it means when the switch happens it only runs enough to raise or lower air temperature by one degree. Some families forget and come home to a house over 80 degrees because they forgot to switch the AC before leaving for work.
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Old 07-18-2019, 02:51 PM
 
Location: Aurora Denveralis
8,712 posts, read 6,664,144 times
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Quote:
Originally Posted by KathrynAragon View Post
My husband and I have very unpredictable schedules and so we really would rather not program our thermostats to go up or down at a particular time.
Why bother? Exceptionally fussy management of temperature setback saves... very little, in any modern, well-insulated house. Keeping the interior at X degrees uses more or less the same amount of energy as heating/cooling it from a standby temperature on demand.

Set back the temp manually when you are going to be absent for more than a day. I suppose if your gone a day or more on a regular basis, a programmable thermostat helps, but really - how much hassle is it to adjust the temp for comfort somewhere between once a day and once a week?

Being able to dink with this from your phone is, as noted above, almost wholly a solution still searching for a problem. You're using tech upon tech to do something that's not really necessary in the first place.
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Old 07-18-2019, 03:24 PM
 
Location: Knoxville, TN
2,516 posts, read 1,865,958 times
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Quote:
Originally Posted by Quietude View Post
Smart thermostats have to be the most overrated piece of tech in years.

Nest is the Bose of the field - oversold and overpriced.

Stay with a good digital 'stat with easy-to-read numbers and simple controls. There is next to no advantage to anything more complex.
You obviously have never been out of town, had your HVAC fail and the temperature in your house plummet to 45 degrees. Fortunately, I had no freezing pipes, but I now have a Nest and can monitor the temperature in my home and promptly address it if a similar calamity should occur.
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Old 07-18-2019, 03:27 PM
 
4,149 posts, read 6,838,078 times
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Quote:
Originally Posted by Quietude View Post
Why bother? Exceptionally fussy management of temperature setback saves... very little, in any modern, well-insulated house. Keeping the interior at X degrees uses more or less the same amount of energy as heating/cooling it from a standby temperature on demand.
.
That's actually a pretty broad-brushed myth with a lot of 'but' and 'if' statements attached to it. Thermostat setbacks are a proven way to save money in most cases if the homeowner actually understand how to use it and understands the limitations. (such as not setting it back to 82 and then resetting it to 70 at 530pm every day). Now, the thermostat companies and others largely claim way too big of a savings. Most people are not going to save these '30-40%' numbers like the marketing people will tell you. In reality, the average homeowner is most likely going to save somewhere more around 10% annually.

There are a lot of good studies out there if one cares to read and understand the limitations and variables in each of the studies. I've done a lot of studies myself and have proven savings (and found the right 'balance point' for my house regarding setback to maximize energy savings, before the pull-down started eroding any savings). The savings are noticeable in my house vs leaving the thermostat set at the same point all day. 15-20% lower kWh draw on days with roughly the same hourly ambient temperatures. Noticeable.

Of course, the most energy savings come from having a properly sized system (something most residential AC contractors do a poor job of) and from not keeping one's house at 70F all summer long, not from setbacks. But proper scheduling and setback still saves money.
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Old 07-18-2019, 03:28 PM
 
Location: Aurora Denveralis
8,712 posts, read 6,664,144 times
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Quote:
Originally Posted by JOinGA View Post
You obviously have never been out of town, had your HVAC fail and the temperature in your house plummet to 45 degrees. Fortunately, I had no freezing pipes, but I now have a Nest and can monitor the temperature in my home and promptly address it if a similar calamity should occur.
Google the White Knight's mousetrap.

I'll concede that remote monitoring and control has its place, especially for things like vacation and weekend homes. But putting this forward as a general reason to put in an insanely over-featured tech toy isn't really the best argument.
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Old 07-18-2019, 03:30 PM
 
4,149 posts, read 6,838,078 times
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You are correct - the best argument is the energy savings.

Again, you have the apply them in the right way. Understand setback and pulldown limitations, and understand how a thermostat works differently with different equipment (variable speed compressor vs 2 stage vs single stage and heat pump vs gas furnace etc.). You're not always going to see wintertime savings with a basic heat pump or electric resistance system, so setback on these kinds of systems won't always net you any savings for example. Gas heat on the other hand you can save quite a bit.

Last edited by Sunbather; 07-18-2019 at 03:44 PM..
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Old 07-18-2019, 04:12 PM
 
Location: Knoxville, TN
2,516 posts, read 1,865,958 times
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Quote:
Originally Posted by Quietude View Post
Google the White Knight's mousetrap.

I'll concede that remote monitoring and control has its place, especially for things like vacation and weekend homes. But putting this forward as a general reason to put in an insanely over-featured tech toy isn't really the best argument.
Maybe it isn't the best argument FOR YOU, but maybe it will be some food for thought for those who travel frequently or have second homes and are wondering about the overall value of installing a unit. It's pretty cheap insurance against a disaster. I have no idea how long my house was without heat, but I do know it was 45 degrees when I arrived home at noon on a sunny, but cold, day. I felt very lucky the HVAC was the only thing I was replacing.
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