Outdoor temperature sensor not reading correct temp

eurban

Hello,
My 14 year old Burnham Freedom FCM120 boiler is still humming along but I am having an issue with the outdoor temperature sensor. It is reading a good 15 degrees too warm. I have replaced the sensor with a replacement part (updated Burnham part number) AND replaced the wiring from the sensor to the boiler but the issue still remains. The new sensor is a 10k resistance variety which is presumably the same as original. The sensor is located on the north facing of my home and is not in the direct sunlight. It has worked correctly for many years although I have had to replace it once previously due to non functioning / incorrect readings.
For the short term I have just disconnected the sensor and set the supply water temp to a reasonable amount for the 40degree weather we are having at the moment. When I disconnect the wires from it, the boiler does recognize that no sensor is connected and doesn't try to alter the supply water temp set point from what I set it to.
Any thoughts to the cause of the high reading? Problem in the MCBA? The new Burnham sensor from SupplyHouse is listed as a direct replacement for the old part number and is specced at 10kohm resistance. Any chance that is the wrong resistance? This seems doubtful because the previous sensor (that I thought must be bad) is also reading too high by essentially the same amount.
Thanks for any wisdom on this matter!
Eric

PC7060

Where is the sensor located? I found my ODR read high when it was attached to the foundation of the house. I mounted mine to the A/C condenser which solve the major heat deltas however it still tends to read a few degrees high.

I just adjusted the curve a bit to compensate.

eurban

The sensor is 3 feet off the ground on the brick exterior wall of my 1920s home. This location has not been an issue in the past. I don't believe I have the ability to adjust the outdoor reset curve on my boiler but I could be wrong . . .

Jamie Hall

Bad connection somewhere or some other problem with the wiring.

eurban

Wiring from boiler to sensor was all just replaced. The terminals on the MCBA look fine. Wires are firmly pushed into the spring connectors on the new sensor. Wiring wise what else is there except the internals of the MCBA? Temperature readings are the same as before I changed the sensor and replaced the sensor to boiler wire.

JUGHNE

You could actually measure the ohm value at the boiler and again at the sensor to see if any major difference, this would tell if sensor or board is at fault.
Chart showing ohms @ temp usually come with sensor.

EBEBRATT-Ed

Yes @eurban what @JUGHNE said is correct.

You need an accurate meter to measure the ohms at the sensor and at the boiler (they should read the same)

Then find a temperature chart for the sensor. It should have come with the sensor or in the boiler manual. Measure the temp at the sensor and then compare to the chart.

HomerJSmith

Brick masonry stores heat. Maybe some insulating foam between the brick and the sensor may help. I could be a faulty board. Checking the resistance, ohms, at the sensor and at the board and compare it to the resistant chart vs degrees would be informative as suggested.

The minimum size wire for the cable is 18 ga and not running it close and parallel to romex cable.

ratio

It used to work and now it doesn't. New sensor, new wire.

IIWM I'd get a precision 10K resistor, install it in place of the outdoor sensor, & see if the control says 25°C/77°F. If that's good, look for a new source of heat near the sensor; if not replace the board (or try to calibrate it out).

eurban

Ok a few updates.
I checked the resistance of the new sensor and it's reading was about 6 degrees too high for what I would expect for the outside temperature. I found an online 10k temp sensor chart for Rheem sensors which I would guess should be the same for all 10k sensors. This may help to explain the high reading I'm getting.

Following PC7060's suggestion I moved the sensor from the brick wall and attached it to a 4ft piece of 3/4" pvc pipe that I hammered into the soil a foot or so away from the house. After doing this the temperature reading did seem to drop around 5 degrees so having the airflow around the sensor does seem to help . . .THANKS!

Now my reading on the boiler is about 10 degrees too high some of which can be attributed to the sensor reading high. Thinking about it, doesn't really make sense that it is a wiring / connection issue since the temp is reading high which means if anything the resistance is LOWER than it should be. Bad wire / connections would be the opposite right? Also it doesn't seem possible to check the resistance with the wiring all connected up. When the wires are connected to the MCBA the resistance is changed.

I suppose I could return the new sensor but I don't think that it is likely that the replacement would be better. Maybe instead I'll just get a 4 or 5 or 6kohm resistor to add to the wiring to correct the temperature problem. I'll need to check it a bit more to get the actual temperature error and then figure out how much resistance to add. I guess it doesn't have to be super accurate.

Anyways unless someone else has any more brilliant ideas I think adding a resistor is my best bet.

BTW, I read over the manual and I can't find any way of adjusting the outdoor reset curve.

EBEBRATT-Ed

@eurban
adding a resistor should work I guess you could put it in series or parallel if you do the math right.

I would still measure the resistance of the sensor outdoors, then hook up the wires, take the wires off the board and check the resistance of the sensor and wire . That will tell you if you have a wire issue

eurban

EBEBRATT-Ed said:

@eurban
I would still measure the resistance of the sensor outdoors, then hook up the wires, take the wires off the board and check the resistance of the sensor and wire . That will tell you if you have a wire issue

Thanks. I'll do that.

eurban

ratio said:

It used to work and now it doesn't. New sensor, new wire.

IIWM I'd get a precision 10K resistor, install it in place of the outdoor sensor, & see if the control says 25°C/77°F. If that's good, look for a new source of heat near the sensor; if not replace the board (or try to calibrate it out).

It would be good to know what the boiler thinks about an exact 10k resistance. Still the reality is that this is a 14 year old boiler and I don't think it would make sense to replace the board (if I find it doesn't read the correct temp with 10k resistor) to correct this issue since it seems to be working OK otherwise. I think the band aid approach of adding the right amount of resistance to correct the temperature is the right price point. I can get 100 resistors on Amazon for $6. Hopefully the error is reasonably consistent. That is something else I can check. Thanks to all for the help!!!!! I'll update.

Zman

Sensors are rated by their resistance a 77 degrees Fahrenheit. Not all sensors have the same range, so some 10k sensors will read differently at higher and lower temps. It is best to use an OEM sensor.

ratio

There are two popular 10K thermostors (type II and type III) and several unpopular ones. They all read 10K at 25°C/77@°F but they diverge the further they are from that. It's going to be important to match the thermistor type on an outdoor sensor, the difference between types II and III, at 32°F, is over 3K. If there's a table for the thermistor in the manual (or google), you should be able to figure out which sensor it's using. Here's a table showing a few of the common thermistors.

eurban

Thanks for that table. I'll see if I can figure out which type of 10k I have and whether or not it is reading correctly . The sensor I am currently using is a Burnham part from Supplyhouse that they listed as being a direct replacement for the original part number. Hopefully it is the correct type. Thanks!

eurban

Small update. I have been getting inconsistent temp reading deviations from actual outside temp but they are always high. Never really less than 8 degrees F but sometimes twice that. I'm waiting for some supplies to move the outdoor sensor much farther away from the the boiler vent to ensure I don't have any heat contamination to the sensors reading. In the meantime I put a 5.1K resistor (measures out at 5110ohms) in place of the sensor and the boiler read 114 degrees F. According to the chart that ratio provided (and also a chart I found on Tasseron's website) regardless of whether I have a type II, type III, type IV 10k sensor that temp is too high for that resistance. Depending on which which type of sensor the system is designed for its around 4-8 degrees off.
Once I move the outdoor sensor I will determine if temp deviation is consistent or somehow varying with conditions inside the boiler. If it is consistent I will either add a resistor to correct or perhaps someone can explain how this might be adjusted inside the honeywell MCBA?. If it's not consistent then I'll either abandon the outdoor sensor or spend for a new MCBA.
BTW the sensor I was sent from SupplyHouse is a labeled Tasseron: TSA00AA which I was not able to decode on Tasseron's website. Anyone know how to determine what type of 10K sensor it is supposed to be? Here's a link to one but no detailed specs: https://www.radwell.com/Shop?source=GoogleShopping&IgnoreRedirect=true&ItemSingleId=127338664&utm_source=google&utm_medium=cpc&adpos=&scid=scplp127338664&sc_intid=127338664&gclid=Cj0KCQiAnaeNBhCUARIsABEee8UhoyC3T6pN7mTa60PTBbUGljKvzxxFBQwSJTGYZ9dCTdSJvA9EAisaAtFNEALw_wcB

And here's a link to description on how to build Tasseron part numbers: https://static1.squarespace.com/static/575837b259827e26b5694200/t/5c5b143d7817f7f9516848da/1549472829910/How+to+build+a+Tasseron+TEMPERATURE+part+number.pdf

ratio

Do you see the resistance varying as well as the displayed temp? Looks to me like it's the right time to pick up a logging multimeter like a Fluke 289.

Test the input at several different resistances, if the board isn't confused you should be able to pin down what curve it's expecting; although that 5k1reading is a little off putting.

eurban

I think I am done messing with it for now. I moved the outdoor sensor farther away from the boiler exhaust and the boiler was displaying about 6 degrees higher than actual (I used my point and shoot thermometer to read sensor case temperature). It did seem to remain consistently high so I played around with a couple of different resistors and the 5.1K ends up being almost spot on to correct it to actual outdoor temperature.
One thing that became apparent was that the house's masonry does radiate out some heat which is particularly noticeable with the wide temperature swings that we have been seeing here. Even though the sensor is now a foot off of the house masonry (on the north side of the the house) on a plastic pipe, it's surface temp is still typically higher by 3-4 degrees than actual air temp so that can explain some of the discrepancy I was seeing earlier.
Thanks to all for the help!

STEVEusaPA

ratio said:

Do you see the resistance varying as well as the displayed temp? Looks to me like it's the right time to pick up a logging multimeter like a Fluke 289.

Test the input at several different resistances, if the board isn't confused you should be able to pin down what curve it's expecting; although that 5k1reading is a little off putting.

You could use a decade box to feed precise resistance to the control and make sure the control is working (sensing) correctly. Can also use it as a temporary measure if you have to wait to order a sensor.