Tado Cooling – Intelligent AC control

There's the other element here which is just plain being lazy and wanting to be comfortable 100% of the time. No, I am not calling you this. Just making a general statement. I've walked into my home when it is 80 to 85 degrees F. I don't need the air conditioner to turn on an hour before I get home to get it down to 72 degrees (or whatever). I am perfectly fine getting home and turning it on (if the programmable thermostat didn't) and waiting 15 to 20 minutes while having a cold glass of water as it the temperature goes down.

Yet another reality is that the cooling process is far more efficient the longer you can wait. What I mean by this is that a good air conditioning unit is cycle limited to somewhere around a 50 degree F delta between outside and inside temperatures. That's governed by the physics of the heat exchange circuit. And so, in simple terms, if it is cooler outside your A/C unit will be able to cool your house much faster because it is able to pump cooler air into your home. There's a huge difference between trying to cool a house with 60 degree air vs. 50 degree air.

Finally, one thing you can probably do as a renter is to install a misting system around the A/C condensing unit (usually installed outside the house). Wet air has greater heat capacity than dry air and so the system will operate more efficiently. It is easy to rig a 12 or 24 volt solenoid valve to open when the compressor runs so the mister can provide moist air to the heat exchanger. This alone is bound to produce results significantly better --again-- than a Nest. And, of course, it wouldn't cost much more than $30 to $50 to implement.

In places where people can't be bothered to at least attempt to understand the situation and at the very least spend some time programming their thermostat, setting reasonable temperatures and taking five minutes to open windows at night, well, yeah, Nest might serve in a range from placebo to small savings.

My key argument is that a single thermostat isn't a solution to a systemic problem. Most homes are not designed to be efficient in this regard, which is sad.

You're probably right, had I figured out how to use it (that's what I replaced with Nest). Like most things it's probably most efficient when I put the effort in, but that's the magic of Nest -- a huge work:savings ratio. You're right about laziness. Banking on laziness is a great business model -- find something people are lazy about, automate it to be maybe 60% of optimal and charge $$$$ for it.

> Wet air has greater heat capacity than dry air and so the system will operate more efficiently.

I grew up in a house with a swamp cooler now that I think about it. It worked pretty well.

We just had an offer accepted on a home, so I'm bookmarking your comments. Maybe I can implement some of them soon!

Two small units

For the sake of argument, let's make everything otherwise equivalent. We'll say the small units are 1.5kw each

When upstairs and downstairs call for heat, you use ... 3kw When upstairs calls for heat alone, you use 1.5kw When downstairs calls for heat alone, you use 1.5kw

Ducting/blower loss from this setup (assumes two completely separate duct runs): 0%

One large unit:

3.0kw to make life easy (in reality, if you use small units, you often need less)

When upstairs and downstairs call for heat, you use ... 3kw When upstairs calls for heat alone, you use 3kw When downstairs calls for heat alone, you use 3kw

Ducting loss from this setup: 25-30% (at least, it's just the way it goes due to longer duct runs, more tees, etc. If you have, say, flex runs, it'd be even more)

It is essentially guaranteed to run for more than half the time of the smaller units (due to ducting loss, etc), making overall cost more.

Note that if you had mini-splits as the small units, the comparison would be crazily in favor of that. you'd have no loss due to ducting, they would almost certainly be DC based, and would be completely variable to actual load (even a "variable speed blower" on a large unit isn't), not 1.5kw on/off. Smaller unit efficiency is also almost always greater than larger unit efficiency.

Also note this is all a comparison of efficiency. It may be more cost effective to do one or the other (though it depends how long you own the house, insulation, blah blah blah)