my annual lament on invensys 9700i with a Taco twist re 3 wire themostats

archibald tuttle

So, i'm in that apparent minority of building managers who thought that a remote indoor sensor for a setback thermostat was a great innovation, but very few companies have followed suit and the choices I have found have performed abysmmally.



seems like it would be the worlds simplest thing and an extension of that single format unadjustable 68 deg. thermostat that apartment managers would install to keep the tenants from running the heat through the roof. (literally and figuratively).



I finally focused on the invensys [or robertshaw, whatever the label is this year) 9700 and have about a dozen of them in service.  They have one of the most unergonomic battery compartments ever designed [or not designed] by a product developer. But other than that complaint they had more or less done the job I wanted which was a 2-wire themistor monitoring the temperature in the apartment and the actual thermostat and control mounted near the boiler in the basement.



This provides the convenience when I get a late night heating call of not having to go into the apartment for most service functions. I can monitor the temp. but toggle the thermostat functions while standing right next to the boiler. And the 9700 also has the only digit based security code I've found.  So you can set an actual security code rather than pushing some series of buttons the pattern of which is available either by guessing or looking up the pdf instructions online. (tenants never seem clever until it comes to turning their creature comforts, and these are mostly college students who perhaps revel in beating the system a bit rather than the more pedestrian approach of putting ice trays by the sensor).



All is well in ivy league ville until a year or so in and virtually all of these 9700s start telling me they need batteries, even if I've just replaced the batteries.  So now, after investing 90 bucks a head times a dozen of these things, I'm tearing my hair out over heat calls.  So I call invensys and they sheepishly tell me the answer is to send the things back and they will send me a 9701 because the 9700 has been dicontinued.



So without getting into the four part harmony and the circles and arrows and paragraph on the back of the instructions, the main difference is that a 9701i is 24v 3 wire and the 9700i, may it rest in hell, was a 2 wire battery powered stat. This is a slight hassle, although, for the most part because these are mounted close to the boiler it isn't a big problem to run new wire, however not all controls (see e.g. carlin 60200FR) are set up for 3 wire input these days. usually you have relay interposed that offers 3-wire capability but not always just to make life interesting..



So  now comes the TACO deal.  so I go to hook up one of these 9701i to a SR503exp.  The TT contacts have no polarity indication vis a vis the R and W terminals on the thermostat, which made no difference when it was a battery stat, but makes a hell of a lot of difference in a 3 wire set up.



The 24V out tabs do not indicate their relationship to the TT tabs so I tested for 24V across the R wire (which was on the left hand T tab on the SR503) and found 34V to the 24V tab, but not to the common tab on the 24V tap.  Perhaps this should have made me suspicious because the third wire is labeled Com on the thermostat.  When I hooked up the third wire to tap 24V I got a very stange result.  It showed 24V at the state R to Com but if I reversed my tester leads it went down to 16.  this kind of defies my understanding of AC. Anyway, it doesn't power the thermostat.  On the other hand, if I just tap the 24V leads on the SR503 to R and Com on the 9701, bingo power, but now I'm not tapping the TTs. 



Back to my days of railing about the end switches on TACO zone valves being combined with the operating circuit to save one terminal.  Pennywise and pound foolish when it comes to robust service in multi-zone applications exceding the capacity of a single 24V transformer.  but I digress [on one of my favorite rants, I know they have an independent end switch on the new style zone valve although that reportedly has its own idiosyncracies being a new operating technology for them.  I never liked zone valves anyway because they cost as much as a pump and add another layer of potential failure to get me a phone call in the middle of the night, but since they are the wave of the future I hope they get it ironed out before I actually start using them]



Back in Ivy League Ville, after scratching my head for a while I reversed the TT leads so the red now goes to the right hand T terminal on the SR503 and moved the 3rd wire over to the common terminal on the 24V tap and voila, I'm in business. 



Maybe none of this is news to anyone who has 3 wired one of these before, but thought I'd pass this on with the recommendation that relay manufactuers adopt the same convention as thermostat manufacturers and maybe label their TT terminals  TR and TW.



And of course none of this would have arisen if Invensys were able to make a battery powered thermostat that actually works a few years. I wish I didn't have to think about giving up my job and becoming a product designer every time I bought some new device that was supposed to save energy and instead cost me much more energy in callbacks and cursing.



so here is where you can tell me if you have any other more reliable way of identifying the 3 wire thermostat approach for varying relays other than guessing and/or tell me i've got way too much time on my hands if I'm telling this story, or you've got way too much time on yours if you're still reading.



Of course, I've got an excuse which is I'm at World of Concrete in Vegas and its 5 in the morning here but 8 where I come from , so I'm surfing the net before I go to enter the John Deere excavator competition and get my mind boggled by various concrete solutiosns (including yesterdays find, a really next system for introducing soap foam into concrete to make a lighter mix. Not counting equipment it costs about $12 per yard of expansion and you can make some nice floor material at about 70 lbs per cubic foot a liitle more than 1/3 the weight of conventional concrete and I think about 1/2 the weight of gypcrete.  seems like a real nice compromise of thermal mass and can be used with conventional floor mix or grout mix



the downside is that you got about ten minutes to batch it either into the ready mix truck or your smaller batch mixer. for the smaller batch mixer they've got a $1500 unit that can make the amount of foam necessary in 10 minutes, but for a truck size batch you need a more expensive skid mounted unit to make enough foam in a short enough time.



still, the whole thing is pretty cute.  They can actually push the foam to the point where the concrete is an R2 per inch insulator.  obviously that cuts way down on finish strength, but depending on the application. . .



back to checking out the lights on the strip. i'm only reading the signs, not looking at the scantily clad pictures.

PeterNH

lol

 

"the more pedestrian approach of putting ice trays by the sensor"

A much neater solution would be a very small Peltier.



Peter

archibald tuttle

elegant

. . . and they could probably crib one from the lab.



brian