Buderus R2107 reading high outside temperature

Stelcom66

I posted about this happening about 3 years ago. A couple days ago for the first time since then the system again was reading a constant high outside temperature, this time 104º when it was about 75º outside. I installed a resistor on a switch 3 years ago to create a reading of 30 degrees. Moved the switch and the temp did read 30º and the circulator started. I then moved the thermistor back and forth a couple times, put the switch back to normal and then the temp was correct. After that I did loosen/retighten the thermistor connection.

My original plan was to buy a couple different types of thermistors to have on hand. Never needed to, and now unfortunately my company made us part time last year due to the pandemic but kept us that way. Every bit of expense now especially counts. I may still order a few online or check a local store that (luckily) is an electronics components store.

I do wonder if anyone has seen this with a Buderus R2107. I'm hoping it is just the thermistor itself and not the control unit. It seems to be given when I connect the 32k ohms it reads 30 degrees.

ratio

See https://forum.heatinghelp.com/discussion/168102/buderus-outdoor-sensor.

Note that there are several different kinds of 10k thermistor (type II and type III AKA CP and AN) and with the range you need for an outdoor sensor it's going to make a difference. You should be able to figure out the exact type by sticking a few other resistors in place & seeing what temperature pops up. Use the table at https://www.workaci.com/sites/default/files/category-files/Thermistor Temp Curve_7.pdf to figure out exactly which type you've got, then order on of these: https://www.workaci.com/catalog-new/outside-air-2

Stelcom66

Thanks for the informative references. My system may use a ACI/10K-CP type thermistor - that column shows about 32k ohms at 32 degrees. Like you said, I have many resistors on hand and can see what they end up showing for a temperature. I even have a step type resistance wheel with many values on it. They're probably only 1/4 or 1/8 watt but that should be fine to test with. Back in the day Radio Shack probably sold an assortment 10 pack. Boy do I miss them!

Big Ed_4

Normally it's the outdoor sensor it could also be the wire .....

Stelcom66

A lot has changed in 2 years, the local electronics store closed in late 2022 and the mail order store I've used for over 40 years is going out of business. Working full time at a new job though, with retirement not far away.

I got tired of the inaccurate high temperature situation. Especially when it was in the 30s and the temperature read in the upper 50s, putting the furnace into 'Summer' mode, where it would not run. The switch I made replacing the thermistor with 32k ohms put the temperature right at 30º, so the furnace would run. but during warmer days unnecessarily. And, with the switch in the 30º position, the circulator constantly ran.

@ratio - Thanks for the information again. I ordered 3 10k ohm negative coefficient thermistors from my go-to electronics store that was going out of business. Of course the shipping was more than the components which were $1.10 each, but I still saved almost $70 over buying the replacement Buderus outside temperature sensor. With the new system the temperature read 77º, which seemed kind of high. After a few minutes it dropped to 73º which seemed about right.

So if anyone has a defective thermistor with a Buderus B2107, you'll want a 10k ohm negative coefficient.

109A_5

Hello @Stelcom66,
Seem like you have it all figured out, good.

The Buderus R2107 installation and service manual shows the sensor curves. Which could be matched up with manufactures data sheets for an off the shelf sensor solution.
http://s3.supplyhouse.com/product_files/R2107-Install.pdf

www.digikey.com,
www.mouser.com,
www.newark.com
sell electronic components, as far as I know they are not going out of business.

Stelcom66

Now I see it, tried editing the 9:08pm but got 'Failed to find discussion for commenting'

109A_5

Hello @Stelcom66,
If you like having the fixed resistor as a troubleshooting aid. I'm wondering if a fixed resistor of 20k to 25k value for a test of about 50 to 43ish degrees would not keep the circulator running continuously. I would experiment with a resistor substitution box and / or a ten turn potentiometer and an Ohmmeter, if you have either or can get them used inexpensively.

"All it takes is a small inexpensive component like that to throw off the whole operation. "
Even worse when it is just a bad solder joint and a HVAC tech wants to replace the whole controller.

Stelcom66

109A_5 said:

Hello @Stelcom66,
Seem like you have it all figured out, good....

Hello @109A_5

Thanks - and thanks for the Buderus manual! I have the 'Operating Instructions' pdf but I see the installation manual has many more details. Good reference of the sensor curves - coincides when I rigged up the bypass switch replacing the thermistor with 32k ohms, equaled 30º. All it takes is a small inexpensive component like that to throw off the whole operation.

Also thanks for the electronics stores, the one I don't think I've heard of is newark.com. Do to see Digikey and Mouser are still around, and hopefully will be here to stay. Even if there was a local Radio Shack store back in the day, the thermistor being somewhat of a specialty component may have needed to be ordered anyway.

Stelcom66

I don't need to troubleshoot now since I've replaced the thermistor. A while back I bypassed the toggle switch that was putting 32k ohms in place of the thermistor, it was intermittently open which caused an OA Err condition which also made the circulator run.

Yes a resistor to simulate around 43-50 º may have not activated the circulator. I have potentiometers and one of those rotary step resistors that provides a pretty good range. And yes just a cold solder joint can do a whole circuit in. A couple years ago had a co-worker remove the instrument panel from an older Ford Windstar, He researched and found a common solder failure, He gave it to me, all I did was touch it up at the spot. He re-installed it, he did the hard work! I've only done surface mount once though, that's pretty tough for me to do.