240 line voltage thermostat for baseboards seems to be self heating

I am constantly finding 240v line voltage programmable thermostats that are "under heating". Room temperature is quite below the temperature indicated on the thermostat. They appear to be maintaining their setting temperatures, as judged by their own readout, but I finally realized with an infraRed gun that the thermostat box itself was often 10 deg C hotter than the wall adjacent. I further identified that this "heated box" syndrome was hotter when it was controlling more power -- ganged baseboards. I am getting nothing from the manufacturers. This is happening with several models of Aube but I have received complaints of other brands also maintaining room temperature considerably below the set points. Is it possible that the wiring itself inside the box behind the thermostat is heating the thermostat and kicking out before the ambient temperature is reached? When I look behind the problem thermostats there appear to be a pretty tightly packed set of wires.
 
One way to answer this question would be to remove the thermostat from the box, pull it as far away as the wires will permit, give it time to achieve thermal equilibrium, and check its temperature again.

I am not suspicious of the wiring itself being the majority contributor of heat. In order for the thermostat (which has five of its six sides out in the open air) to be 10°C above ambient, the wiring (which transfers heat only to the back side of the thermostat) would need to be generating temperatures many times greater than that. Were that the case, you'd have obvious problems, such as arcing, discoloration or smoke.

I am suspicious that a semiconductor switch in the thermostat is the culprit. A triac is likely to have a one or two volt voltage drop, which turns into a lot of heat when several amps are flowing through it. (relatively speaking, relative to the size of the thermostat and the surface area available for heat dissipation)

It's disappointing that the simplistic software in the box doesn't compensate for the self-heating effect. It wouldn't be difficult to implement.
 

kwired

Electron manager
Location
NE Nebraska
One way to answer this question would be to remove the thermostat from the box, pull it as far away as the wires will permit, give it time to achieve thermal equilibrium, and check its temperature again.

I am not suspicious of the wiring itself being the majority contributor of heat. In order for the thermostat (which has five of its six sides out in the open air) to be 10°C above ambient, the wiring (which transfers heat only to the back side of the thermostat) would need to be generating temperatures many times greater than that. Were that the case, you'd have obvious problems, such as arcing, discoloration or smoke.

I am suspicious that a semiconductor switch in the thermostat is the culprit. A triac is likely to have a one or two volt voltage drop, which turns into a lot of heat when several amps are flowing through it. (relatively speaking, relative to the size of the thermostat and the surface area available for heat dissipation)

It's disappointing that the simplistic software in the box doesn't compensate for the self-heating effect. It wouldn't be difficult to implement.
I have to agree, if power line switching is done via semiconductor there will be significantly more heat given up compared to a dry contact in good working condition. Compensation for this probably isn't necessarily always consistent either. As you mentioned current will impact the effect, so accuracy will be different with a 500 watt heater than it is with a 1500 watt heater controlled by same thermostat in same conditions.
 
... Compensation for this probably isn't necessarily always consistent ... accuracy will be different with a 500 watt heater than it is with a 1500 watt heater controlled by same thermostat in same conditions.
If the software is so dumb that it uses only one fixed offset for self-heating, (or apparently, even less than that) it probably won't be very accurate or consistent. But if it uses a dynamic model that measures the room temperature when the heater is switched off, measures how fast the thermostat's temperature rises when it's switched on and how fast it cools off, it could be quite accurate.
 

StarCat

Senior Member
Location
Moab, UT USA
The Age of Degerated Technology

The Age of Degerated Technology

I am constantly finding 240v line voltage programmable thermostats that are "under heating". Room temperature is quite below the temperature indicated on the thermostat. They appear to be maintaining their setting temperatures, as judged by their own readout, but I finally realized with an infraRed gun that the thermostat box itself was often 10 deg C hotter than the wall adjacent. I further identified that this "heated box" syndrome was hotter when it was controlling more power -- ganged baseboards. I am getting nothing from the manufacturers. This is happening with several models of Aube but I have received complaints of other brands also maintaining room temperature considerably below the set points. Is it possible that the wiring itself inside the box behind the thermostat is heating the thermostat and kicking out before the ambient temperature is reached? When I look behind the problem thermostats there appear to be a pretty tightly packed set of wires.
Most Digital " Tech " = insert your favorite 4 letter word.....
I have watched it all keep getting worse and more ridiculous for the last 30 years.
I installed Honeywell Analog models in my home to keep away from the digital stupidity on 240V Line Voltage.
Never heard of Aube. Honeywell still makes one of the best, although not the quality they once were.
I would think they would be your best bet. Typically with wall mounted thermostats the opposite problem is seen due to cold air incursion from behind the mounting point. I like the cheap electronics answer. Excellent. We live in an age where many things no longer work, others barely work, and most are a colossal waste of a Technicians time. The level of time wasting that never was seen in the past is now for some reason taken for " normal." This is a tell on the insanity of the current times.
 

Besoeker

Senior Member
Location
UK
I am constantly finding 240v line voltage programmable thermostats that are "under heating". Room temperature is quite below the temperature indicated on the thermostat. They appear to be maintaining their setting temperatures, as judged by their own readout, but I finally realized with an infraRed gun that the thermostat box itself was often 10 deg C hotter than the wall adjacent. I further identified that this "heated box" syndrome was hotter when it was controlling more power -- ganged baseboards. I am getting nothing from the manufacturers. This is happening with several models of Aube but I have received complaints of other brands also maintaining room temperature considerably below the set points. Is it possible that the wiring itself inside the box behind the thermostat is heating the thermostat and kicking out before the ambient temperature is reached? When I look behind the problem thermostats there appear to be a pretty tightly packed set of wires.
How close is the thermostat to the source of heating it is controlling?
 

Greg1707

Senior Member
Location
Alexandria, VA
Occupation
Business owner Electrical contractor
heat at thermostat

heat at thermostat

I have installed many digital wall thermostats connected to one or more baseboard heaters. There is one thing I have notice after returning to the job several years later. In many cases where the homeowner burns candles or is a poor housekeeper there is a dark smudge on the wall above the thermostat. This would indicate air rising through or around the thermostat. I assume the heat is rising because the thermostat is a little warmer than the surrounding air?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
This is why the t-stats on baseboard heaters and some of them on the wall do not have temperature numbers but rather just number settings. You set the T-stats to where it is comfortable.
 

kwired

Electron manager
Location
NE Nebraska
This is why the t-stats on baseboard heaters and some of them on the wall do not have temperature numbers but rather just number settings. You set the T-stats to where it is comfortable.
Most of those are mechanical and don't have internal heating issues of any significance.

Differences like a draft in the wall will make them appear to be inconsistent from room to room.

Mounting any thermostat above the baseboard heater it controls will probably eventually heat up the room, but it may short cycle a few times because of being right over the heater. Mounting any thermostat in "line of sight" of a radiant heater will have similar effects.
 
Next steps

Next steps

Hi Guys, Thanks for weighing in on this. A couple of details -- The thermostat I am zeroing in on is on an inside well sealed wall about 12 feet away from the 1000w baseboard on the opposite wall. Aube is a very popular manufacturer in Quebec. Every Aube I have seen has that dark streak above the thermostat, candles or no candles -- I do a lot of house inspections. I like the idea of stretching the unit forward into the room air to isolate it from the wires -- will do and report back. I actually have two units fairly close together (was two bedrooms made into one) so I can even do some comparative testing with other brands. Then I will take this entire forum thread and take it to the manufacturer for their response.
 
Aube states in their instructions that the thermostat will be warm. I assume that they have differential circuitry to allow for that but have not measured room temperature independently. I have a digital thermometer I can move about so I will take some measurements and post the results. Right now my Aube in the den is not calling for heat and the Aube is cool to the touch. I will get a comparison here and move to the bathroom where I can easily play with temperatures and get some comparisons there.
 

texie

Senior Member
Hi Guys, Thanks for weighing in on this. A couple of details -- The thermostat I am zeroing in on is on an inside well sealed wall about 12 feet away from the 1000w baseboard on the opposite wall. Aube is a very popular manufacturer in Quebec. Every Aube I have seen has that dark streak above the thermostat, candles or no candles -- I do a lot of house inspections. I like the idea of stretching the unit forward into the room air to isolate it from the wires -- will do and report back. I actually have two units fairly close together (was two bedrooms made into one) so I can even do some comparative testing with other brands. Then I will take this entire forum thread and take it to the manufacturer for their response.
In my view Aube/Honeywell stuff is good. I'm not a big fan of line voltage stats, no matter who makes them. When cost is not an issue I think a much better and flexible solution is to use a low volt stat driving a relay. This avoids the inevitable situation down the road when the customer wants to use something like a NEST or such. Aube makes a really good potted heating relay-very quite and durable.
 
Aube states in their instructions that the thermostat will be warm. I assume that they have differential circuitry to allow for that but have not measured room temperature independently. I have a digital thermometer I can move about so I will take some measurements and post the results. Right now my Aube in the den is not calling for heat and the Aube is cool to the touch. I will get a comparison here and move to the bathroom where I can easily play with temperatures and get some comparisons there.
Interestingly my Radio Shack digital thermometer read a couple of degrees higher than either Aube.

Using a Taylor precision bulb thermometer in my den w/o demand the Taylor reads a bit over 70° and the Aube reads 72° but the wall may be a bit warmer as there is a PTAC heat pump running on the other side of the wall.

In the bath the Taylor and the Aube read the same with demand at 74° and w/o demand at 72°.

In the bath I placed the Taylor bulb on the vent of the Aube and with demand the Taylor read 77
° and the Aube 72
°.

So in my case even though the Aube is warm to the touch with demand the circuit seems to disregard it.
 

414Mhz

Member
Location
Canada
I am constantly finding 240v line voltage programmable thermostats that are "under heating". Room temperature is quite below the temperature indicated on the thermostat. They appear to be maintaining their setting temperatures, as judged by their own readout, but I finally realized with an infraRed gun that the thermostat box itself was often 10 deg C hotter than the wall adjacent. I further identified that this "heated box" syndrome was hotter when it was controlling more power -- ganged baseboards. I am getting nothing from the manufacturers. This is happening with several models of Aube but I have received complaints of other brands also maintaining room temperature considerably below the set points. Is it possible that the wiring itself inside the box behind the thermostat is heating the thermostat and kicking out before the ambient temperature is reached? When I look behind the problem thermostats there appear to be a pretty tightly packed set of wires.
I've noticed this too.

The triac based 'programmable' tstats tend to get hot. Many also have wattage limitations lower than traditional line tstats as well.

I don't know how much savings are actually seen from the digital ones, but I much prefer the good old contact on and off !
 

mopowr steve

Senior Member
Location
NW Ohio
I’m not familiar with is Adobe thermostat but one thing comes to mind.........do these thermostats have “anticipators” little thermal elements connected to the thermostat that warm up when the heater is on, with this type line and load connections are important.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
I’m not familiar with is Adobe thermostat but one thing comes to mind.........do these thermostats have “anticipators” little thermal elements connected to the thermostat that warm up when the heater is on, with this type line and load connections are important.
I've only seen the anticipator in standard mercury-bulb and bi-metallic-strip T-stats.
 
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