Saturday, July 14, 2018

Thermostats, Location, Location, Location

When I was researching supercooling as a way of limiting my power bill I kept getting readings and such that just didn't make sense. One side of my house was comfortable, the other ... not so much. Sure it was the sunny side this time of year, but it has really good windows and good curtains. The only two major problem areas were a couple of windows that faced directly into the after noon sun.

I fixed those years ago with reflective insulation that I put in for the summer and (sometimes) take out for the winter. It's easy to work with and will last forever if I just give them a little attention every once in a while. I wandered around the room and noticed that the interior walls were about 2 degrees warmer than the rest of the house and there was heat getting in through the foundation.

The sun bearing down on the exposed slab foundation would heat up the concrete slab and transmit heat inside the house. That's something that might go unnoticed if I had carpeting, but the tile felt warmer as you approached the wall.

So, basically, it was like any other house on the sunny side. Why was my AC running all the time and having little impact on that room?

A little background: I have a house shaped like an "L" and two AC units. The one on the north is a 3 ton and the one on the south is a 5 ton. Both of them going could cool the house, but when they cycle, it doesn't seem to handle it. I had two thermostats that were located by the AC 'experts' that did my house and were in the top middle of the price range. I replaced them with highly programmable thermostats that were in the high range of prices. These wouldn't do the job of handling peak periods well, so I got a high priced "learning' version. It was crap.

The learning thermostats only learn what the programmer wants them to learn, and it was great at anticipating my needs. It decided I liked the AC on at 07:00 and off at night ... exactly the opposite of what I wanted. They tried to predict what temperature I wanted the house at and must have been born in Nova Scotia because they wanted to keep it at around 65F.

I sent those things back before the 30 day refund period was up. What a waste.

Then, I built my own <link>; I could control them any way I wanted to. Over the years I tried recirculating the air around the house with them, experimented with differing hysteresis curves, used various smoothing algorithms on the temperature readings, etc. Basically, I tried everything I could think of to get the environment the way I wanted it.

Nothing worked very well. I would still wake up with the bedroom running around 85F at night.

But, during my experimenting with supercooling the house (look at the posts on supercooling by using the contents search on the top right of the page) I noticed a whole bunch of things that really shouldn't be happening. First the south AC unit was short cycling. Short cycling is where the AC comes on and then shuts off in seconds or minutes. This is NOT a good thing.

Cycling like this can radically shorten the lifetime of any motor driven device. It causes undue wear on the bearings to be jammed up by the quick start of the motors. It can cause fan blades to slip and that scores the shaft. The air start up can loosen vents and cause rattles. Basically all the things that destroy devices like this. This behavior can also cause a room to be too cold and then too hot within minutes.

The very most expensive AC units start slow and ramp up to speed, they will run slowly to keep the air moving, they even open and close vents based on the need for cooling in a particular area. But, I'm not Bill Gates; these things are way out of my price range.

So, I started looking for the cause of this situation on my south AC. TLDR (too long didn't read) the thermostat was in the wrong place. The thermostat was in a hallway like most of the thermostats in the world close to an overhead air return for the AC. Over half the air for the house went through that return. The temperature would drop rapidly in that location when the AC turned on and cause shut off in a matter of a couple of minutes. Then it would rise rapidly back up because it was only the breeze that cooled it off and repeat the cycle.

Another factor was that I run in and out of the garage a lot. Each time warm air would hit the thermostat and cause the same problem. Add to that the garage is on the other side of the wall and it doesn't have AC. When it heated up, the wall was warmer than the air in the house; cycle started all over again. Obviously, the thermostat had to be moved to get better performance.

Based on this research, I modified the house code to save the activity of the two AC units and looked at them for a day to see how bad it was. It was pretty bad. Here's a chart of my findings on the first day for the south AC unit:


The tall vertical stripes are when the compressor was running, and the jagged line down the middle was the temperature the thermostat was reading. Each shaded long vertical strip is the compressor chewing up power and wearing itself out. Here's a closeup of a couple of them from the middle of the chart:


The stair steps in the temperature is due to the granularity of my recording the data. I only mess with one degree changes, so it comes out like this. So, you can easily tell that the hysteresis is three degrees and it takes very little time to overcome that with the AC unit essentially blowing down the hallway from all the rooms. Similarly, the temperature will shoot back up in a similar fashion because that short a cooling interval doesn't really cool anything except the air the it is blowing around. The gap in the middle is because the cumulative impact of a series of cooling cycles finally does cool things down a little.

When I looked at the North thermostat, it was a different set of environmental factors that added up to a different problem. This thermostat was in a short hallway outside the master bedroom (my room darn it) while the AC unit was venting into that room. There were also a couple of vents that dumped air into the rest of the house to aid there as well. When I looked at it, it looked like this:


Same things on the graph, compressor run period and temperature recorded by the thermostat. Notice the compressor runs a lot even though the temperature isn't changing? Part of that problem is that the thermostat isn't really looking at the AC activity, it's looking at a completely different area. The rest of the problem is that the two vents into the rest of the house are letting almost a great a volume of air get out there as it allows inside MY ROOM.

Fine ! I gotta move the thermostats, but where the heck am I going to put them? Also, it's a real pain to move a thermostat. I don't have a real attic, it's an area that is totally filled with insulation about two feet tall. I really, really don't want to go up there --- ever. I could carve out the wall and move the thermostat and then patch up the wall. That's equally unlikely to happen. Instead I experimented with really long hysteresis values. I was up to 10 degrees at one point. That solution was not satisfactory. I also tried running the fan longer to make sure the air got moved. That didn't work well either; it actually made the problem worse.

Then, I thought about it a bit. I had these temperature sensors <link> all over the house, why couldn't I just use them instead of the temperature sensor inside the thermostat? There wasn't a single reason why not except I would have to change some code in a few places. I took all the code out of the south thermostat related to reading temperature and reorganized it a bit. Now, it would receive the temperature forwarded by the house controller from any of the temperature sensors around the house.  I chose a room where the sensor was on an inside wall away from any breeze from the AC directly. The sensor gets air circulation, it just wasn't directly in the cool air path.

Now, I was ready to test the newly modified south thermostat. The one that was banging the compressor to death with short cycles. Let's see what this accomplished:


Now, that's more like it. Granted, it was a stormy day and not our usual 100+ temperatures, but look, it wasn't short cycling any more. There are long periods of AC activity and the temperature rises slowly instead of a few seconds at a time. This may look like I'm actually using more power, but I'm not. It's actually about the same because the gaps are longer as well. the big thing was that I wasn't tearing the guts out of my AC.

I did the same thing with similar results to the North AC, except I also closed off the vents to the rest of the house. Now my bedroom cools down pretty quickly and stays that way for a while.


Notice that it isn't on all the stinking time blowing air where it isn't needed? The big gap is the 'demand' period where the AC isn't allowed to run 15:00 to 20:00. After the down time, it kicked on and recovered a little after midnight.

The beauty of this is that I can build a couple of temperature sensors specifically for the AC units and put them anywhere I want. If I want priority in the bedroom, put the sensor by the bed. If I want it for the shower, put it just outside; you get the idea. Basically, this is a priority system now that allows me to carry the priority anywhere I want.

<rant>
I hereby copyright this idea !! Anyone is welcome to do anything similar; heck steal my code if you want, but you commercial shark guys out there better get permission to use this. It's real annoying when you find your code, stolen right off the site and only perfunctorily changed in a book that sells for $35 a copy at a book store. I'm looking for new ideas and find my old ones on the shelf...sigh. I have found my code and ideas all over the web and almost every single time it's attributed; that makes me feel good. When someone is selling it, not so much. This stuff is supposed to be free dog gone it.
</rant>

Anyway, the jury is still out on this idea. I need to watch it over time and see if there are problems with the devices talking to each other, getting out of hand unexpectedly, actually working the same way over time. That kind of thing. One thing I did notice though is that a web only version where there is no control station nearby sucks. It's really nice to be able to lower or raise the temperature by pushing a button on the wall instead of starting up a browser and working through pages to get to it. Now, this could be a cheap tablet, and old phone, or something like that if you want to get fancy, but a nearby control is nice. I like buttons.

Also, it's a good thing to have a display that shows everything is working OK. I'm currently using the display on the thermostats, but it could be anything that you can see when wandering aimlessly around the house looking for the car keys. This also helps with the, "It's too hot." comments. You can point to the thermostat and show them that it's only 80 and not hot at all. Folk visiting from the East do this all the time.

Another thing I learned is that AC 'experts' are not to be trusted. These things were professionally installed and came highly recommended. They put one thermostat outside the room it was needed in and the other in a breezy hallway next to the air intake and a garage door. In retrospect, I may be to blame somewhat for not asking enough questions, but these were experts hired for their knowledge and experience. I don't have a suggestion on how to handle this problem except to get two or three experts to present proposals and pick their brains. If you are Bill Gates and can afford to do that kind of thing. I even blogged about the unreliability of 'experts' a couple of times; my experiences are not good with them. Of course, now that I can put the temperature sensor anywhere I want, it doesn't matter as much.

See what happens when you start automating your house? Ideas may come slowly, but actually doing them is entirely possible.

Oh, this is another post that will attract attention from the spammers. If you post advertising in a comment, it will be quietly deleted. Usually the same day. So, don't bother advertising your AC repair company here.

3 comments:

  1. Another great article. I've always been a bit curious as to the variables that go into the decisions determining where thermostats are located. I've come to the same conclusion as you, it seems always to be a WAG (wild ass guess). I hope your efforts provide more comfort, less costly AC maintenance and lower utility bills.

    I've noticed zone thermostats in some high-end homes, but I've never lived in one to appreciate whether they actually work well. Some of these houses use solenoid controlled vents to reduce/cut-off unused areas of a home... great idea, but I'm not sure how the fan/compressors deal with the variable load.

    In a medical office I once renovated, the electric bill was suspiciously high, tracked it down to a torn elbow in a main duct in the basement.

    ReplyDelete
    Replies
    1. I haven't written about it yet, but I used one of those cheap bore cameras that hook to a cell phone and shoved it down a couple of suspicious vents. I didn't find anything to concern me like rats or squirrels and still have the camera for next time.

      When those things got cheap, I was right there in line to get one. I've shoved it down the drains, inside a sparkplug hole, a water drain outside for rainwater, everywhere I thought it might tell me something. Cool Tool.

      So far, the house is just as cool and the AC is NOT on all the time anymore. Time will tell if I did good or not though; I've messed up a lot in these experiments over time.

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