t-stat recommendations?

jpf321

I'm sure there is a past thread or something .. I have a crappy ritetemp (maybe should be wrongtemp) .. i have watched it jump 2dF was I walked by.



I'm looking at <a href="http://www.pexsupply.com/Honeywell-TH8110U1003-VisionPro-Thermostat-1-Heat-1-Cool-3909000-p">http://www.pexsupply.com/Honeywell-TH8110U1003-VisionPro-Thermostat-1-Heat-1-Cool-3909000-p</a> .. It is the 1H/1C stage version of the line. I only have 1 heating system .. the Fitzgibbons 402 1-pip Steam... and I don't have a central cooling or humidifcation system .. so the only thing I need to run from the t-stat is the Steam Heating system.



Other links to the product brochures and manuals are here:

<a href="http://www.forwardthinking.honeywell.com/products/thermostats/visionpro_8000/visionpro_8000_feature.html">http://www.forwardthinking.honeywell.com/products/thermostats/visionpro_8000/visionpro_8000_feature.html</a>



Thanks,



jpf

FJL

VisionPro

I use that t-stat in my apt building.  The t-stat is in a common hallway and we put a sensor in one of the apartments.  It has worked well for us for two years.  We're not into the third heating season with it.

SteamHeat

Look For Adjustable Differentials.



I am in the process of testing out a Robertshaw RS6110 that I got from prothermostats.com  So far it seems good. I had a Honeywell RTH6400D from HD, but the Honeywells (according to their tech support staff) do not allow adjustment of the differential and a differential of 0.5 or 1 degree is too small for steam heat. The boiler was going on and off too frequently and for too short a time. I was running up a gas bill just boiling the water. For steam you want at least 1.5 or 2.0 degrees differential. The Robertshaw models with adjustable differential (not all of them are adjustable) allow upto 3.0 degrees differential below and 1 degree above. There are other brands that allow adjustable differential listed on the prothermostats website. The Robertshaw seemed like the best combination of features that were useful to me. One note though, the thermometer portion of the thermostats (Honeywells and the Robertshaw) I have had so far do not seem to change value as rapidly as the digital thermometer I have in the room. There seems to be a noticeable time lag. I have noticed this in four different thermostats. Seems like something designed in. Hope all of this helps you.

jpf321

relativity is an interesting thing ...

thanks SteamHeat & easilyFound .. funny thing is that my stat is currently on a +/-0.25 "swing" (which I believe is the same as differential) .. and I run 41mins to satisfy it (of course about 1/3 of that 41min there is no heat to the rads yet .. so the temp is still falling while the boiler is on).... then off for about 1hr or so (i have been having a hard time getting the timing for the off portion). so i guess right now, I don't need such precise diff controls..but it's a good consideration since I don't plan on having the behemoth in the basement next year :-)

Big-Al_2

Keeps Rising

Yes, I think my cheapo Lux thermostat is set for a differential of 2.  I don't know if that is 2 degrees or .2 degrees . . . but it doesn't seem to matter much.  When the thermostat kicks off at setpoint, the radiators are fully hot and are still pumping out quite a bit of heat.  The radiator closest to the stat is a monster.  It takes a long time to heat up and even longer to cool off again. The temperature on the stat rides up another degree or two past setpoint, and then holds steady for quite a while. 



With the thousands of pounds of hot cast iron in the house acting as a thermal flywheel, I don't think I've ever noticed the boiler coming on more than once every hour or two, even in the bitterest cold days..

FJL

Differential

Just so I'm clear -- what is a differential?  Is it the variance between what the temp actually is vs. what the sensor or t-stat recognizes?  In other words, the temp will be 66 but the sensor may only sense a temp of 65 or 67?

Mike Kusiak_2

Differential

Differential is the difference in temperature between the setpoint when the thermostat calls for heat, and when it is satisfied. For example a thermostat with a 2 degree differential might turn on the heat at 68F and shut it down at 70F. Once the thermostat is satisfied at 70, it will not call for heat again unit the room temp again drops to 68F. The higher the differential setting, the longer the time period between calls for heat. Too small a differential can cause short cycling of the boiler, while to large a setting can cause discomfort due to wide swings in room temperature.

SteamHeat

Another Thing To Keep In Mind...

The relationship between the temperature displayed on the thermometer section of a thermostat analog or digital and the temperature that is being sensed by the thermostat mechanism seems to be different. All four of the thermostats I have tried recently seem to operate this way. I can track my system turning on and turning off with a separate digital thermometer I have, pretty reliably, but the temperature displayed on the thermostat itself, seems to be a time-delayed rough estimate of the room temperature. I do not now how or why it seems to work that way, but that is what I have seen so far.

Mike Kusiak_2

Thermostat operation

Here is a thread that explains the way modern digital thermostats are designed to calculate desired room temperatures and cycles per hour. The last few postings give the best explanation I have seen yet on how a digital thermostat actually works.



http://www.heatinghelp.com/forum-thread/127201/How-does-a-digital-thermostat-really-work-inside



In the case of older analog mechanical and mercury thermostats, the operation of the anticipator will cause the thermostat to end the call for heat at a lower actual room temperature than is indicated by the setting. The anticipator is really a tiny electrical heater inside the thermostat itself which "fools" the sensing element into thinking that the setpoint temperature is reached before the room temp actually rises to that level. This compensates for the time lag due to the thermal mass of the system, and when properly set, prevents overshoot of the desired setpoint temperature. Digital thermostats accomplish the same function as described in the link above.



So in reality, as you describe at any given moment, the actual room temperature is only a calculated approximation of the thermostat setpoint temperature at that time. Averaged over a much longer period, the room temperature should pretty much correspond to the thermostat setting.