I just installed a new digital thermostat (HoneywellRTH230B) and I'm trying to figure out if it is malfunctioning, needs to be calibrated (if that's possible), or if I wired it backwards (if that's possible). This is a simple 24-volt 2-wire installation; thermostat is connected to the transformer in a relay box on a gas furnace (hot water radiator system).
I removed the old analog thermostat some time ago while I was doing other rewiring work; so I didn't label the thermostat wires. Apparently it wouldn't have mattered because the wiring between the thermostat and the transformer didn't make any sense (an artifact of previous heating systems), and there were no labels on the terminals of the old thermostat anyhow. I also replaced the wiring between the thermostat and transformer. There are two leads (well-labeled with "24V thermostat") coming from the transformer terminals, to which I connected my wiring. The terminals are labeled "T" and "Tv" (I don't know what these mean exactly). On the thermostat end, one wire goes to Rh (with jumper from Rh to Rc removed), the other to W. So one question is, which end of the transformer is which? Does T connect to Rh and Tv to W, or vice versa? Or does it not matter? (My understanding was that coming off a low-voltage transformer, it shouldn't matter.) Currently, Tv is connected to Rh and T to W, and the furnace responds to the thermostat commands.

However, there are two problems I don't quite understand with the thermostat. Let me preface this by saying that the thermostat is installed in an ideal location- it's on an inside wall, not near a radiator or drafts, in the middle of a open-layout first floor, no dimmer switches nearby. I have another digital thermometer nearby (which I've always believed to read about 2 degrees(F) above actual temp) and an analog thermometer in a container of water which has been sitting long enough to match room temperature. Firstly, the thermostat shows the room temperature to be anywhere from 2 to 7 degrees higher than it actually is... anywhere from 2 to 5 degrees higher than the digital thermometer and 2 to 7 degrees higher than the analog thermometer. This first made me think that the thermostat was simply not calibrated correctly; but the degree to which it reads higher than actual temp is variable. The second oddity is that when I set the thermostat to start heating the house to a higher temp, the thermostat's reading of the actual temp goes up RIGHT AWAY, like within 15-30 seconds- clearly impossible.

So, is it possible that I wired the thermostat backwards, and if I did, could that be causing the problem with the thermostat reading? (I'm not even sure that could even make sense, unless there was another thermostat in the relay system itself and the reading I was getting was actually by the furnace. I have not tried switching the wiring.) Or, is my thermostat just calibrated incorrectly or and/or malfunctioning, i.e. a piece of junk? There is nothing in the instructions for my thermostat to indicate how to calibrate it (though it claims to be accurate within 1 degree), and there are no function buttons to do this.

Thanks in advance for the assistance. This seems like it should be simple to figure out, but I'm confused.

Inside walls are best BUT did you do this from your manual. And yes it can make a difference since inside walls connect to the attic or basement usually.


If the hole in the wall is too big, insulate it
using a non-flammable material in order to
avoid air draughts behind the thermostat

Your manual

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1800.pdf


Did you do this setting?


3 For optimal heating control, use the setting that matches your system as
follows: 2=30 min (steam, gravity), 3=20 min (hot water, 90%+ high-efficiency
furnace), 4=15 min (gas or oil), 5=12 min (alternate setting for gas
or oil), 6=10 min (electric).

I for one like accuracy but this thermostat makes no allowance to adjust the temperature display. You will have to spend bigger $$$ to get on that does.

Hvac- Thank you for your response. Yes, I have the manual and I did set the optimal heat setting to 4. I thought about setting it to 3, but it is not a 90+ furnace. Maybe I should try 5? I did also consider insulating behind the thermostat, but the wall cavity leads directly into the garage, which is cold; so if anything, the thermostat should be reading a colder temp than actual. And even if the surrounds were causing a misread, it still doesn't explain why the temp reading starts going up so quickly after I turn up the heat.

Take it off the wall, connect via a couple of wires and place on the floor in a box with your independent thermometer. Compare reading after an hour or so,

Keep it Simple- Thanks for the response. I will try that and post back.