Buderus R2107 outside temperature reads very high

Stelcom66

I have a Buderus R2107. Last night for the first time since the spring I set the thermostat to Auto mode. I noticed the 'Summer' indicator was on which at a certain outside temperature sets the furnace to run at a reduced rate/temperature. The display on the furnace showed the outside temp as 90 degrees F, it was probably in the upper 40s. Putting the furnace in manual mode still wouldn't activate the burner. Today I read the resistance of the outside sensor - about 20k ohms = 77 degrees. The outside temp was actually about 50. For now I disconnected the sensor and put 2 resistors in series in place of it which are about 32k ohms which = 30 degrees. This took the system out of summer mode and it did run.

Can these thermistors (think that's what it is) go bad? Seems like there's no standard for temperature & resistance - saw various charts with various values. Online I saw a replacement unit for about $60! That's of course the entire housing. Looking online they average about $2, but it seems there's no standard, one spec I looked at said 'Available in 7 different R-T curves'. I wonder if I could somehow determine what the R-T curve should be for mine. I can't use the existing one as a reference since it's wrong - or could it be the circuitry in the furnace itself? I wouldn't normally turn the furnace on this early, I'd have the wood stove going but I need to replace the door gaskets first.

Paul Pollets

Replace the outdoor sensor. If the R2107 is over 10 years old, the computer itself can start to glitch.

GW

buy a few of those $2 resistors and see which one ohms the closest to what you have for actual temperature, if you want to go the cheap way

In manual and no fire? That's a glitch

The 2107 is a beast, extremely reliable, I've installed scores

Stelcom66

Good idea buying several thermistors. Someone on another forum attached an Installation/Service manual which shows the temperature/resistance ratio. Lines up with what I found - approx. 32k Ohms = 30 degrees F. Maybe I could find the proper thermistor based on that.

But get this - I installed a toggle switch on the sensor housing to switch between the resistors I put in and the thermistor. The circulator was always running since the system was referencing 30 degrees, it got into the low 60s today so I preferred not to have it running. The switch I assume would switch between 30 degrees and about 90, but now the display shows the correct temperature (?). Nothing felt loose, when I first pulled a lead off and put it back this morning it still read 90. Will just have to keep an eye on it, I don't consider it fixed. I should order a thermistor (or several) anyway.

Stelcom66

As for not firing up in manual - maybe because it thought the temp was 90 it didn't? I turned the temp on the control up, but not all the way up.

mercedes

Hello

If you want the original Supplyhouse.com has them in stock

https://www.supplyhouse.com/sh/control/search/~SEARCH_STRING=buderus outdoor sensor

or page 38 of the Logamatic R2107 Service Manual. The graph chart shows about 80kohm thermistor
https://www.bosch-climate.us/files/HS2105_Service_US.pdf

I know it show for R2105 but it is the same for R2107 the chart.

Stelcom66

Hi,

Thanks, good to know where to get the original sensors. It appears the R2107 uses different sensors, one with a certain curve/range for outside, a different one for the inside sensor. Yes it appears the outside sensor is 80K or higher. I'm looking at a local electronics store's site right now, (fortunate to have them with Radio Shack being gone) there are many different thermistor values - from a few K to 100K. The R2107's thermistor is too small to have any indication of what value it may be. If 100K is a standard, maybe it's that.

It's still accurate - just checked the display which was 46 degrees.

Stelcom66

Looks like it could be a 10K thermistor based on a chart I found. The resistance I made, about 32K ohms resulted in 30 degrees which is in line with the 10Ks.

Stelcom66

The 'OA Temp' reading (outside temperature) on the display has been correct. The only other concern I had was once when setting the control to Night Mode and to the minimum temperature I got 'Remote 1 Err'. Turned it back to Day, then Night again - so far Ok. Good to see a couple of posts that these Buderus R2107 models are generally reliable, I can only hope mine will be.

GW

do you have a remote, or, a Buderus BFU room sensor?

Stelcom66

Good question, it's a temperature control with a circuit board with and multi-step control for setting the night temperature difference. Also buttons and LEDs for night, auto and day mode. So I think it's considered a remote.