Tekmar 360 Erroneously at Maximum Mix Target

Hello All:

I'm new to the forum, with a question about the Tekmar 360 and Maximum Mix Target. Quick summary: I've twice now seen the 360 mix target at an irrationally high value, given the outdoor and room temperatures. I'm wondering what could be causing it to do this. Now, a little background:

We built this house 30 years ago. It has high mass concrete radiant floors, and the original heating system was a Buderus G205 Boiler, Indirect DHW, and Buderus 3220? Logamatic control. Floor heat water temperature is controlled via an ESBE MG 4-way mix valve, controlled by the 360. Nine months ago I had the old, failing Buderus controls replaced with a Tekmar 360 / 306V set of controls, keeping the Buderus Boiler, which is still in excellent condition.

The Tekmar system has been mostly working well, properly using outdoor reset for determining mix temperature, which all seems correct and the house is comfortable. However, as stated above, I have twice now see the 360 trying to set the mix temperature at the MIX MAX value, when the outdoor temperature did not justify anything near that high.

My 360 settings:

ROOM OCC = 68F, ROOM UNOCC= 65F, OUTDR DSGN= -10F, TERMINAL UNIT = 1, MIX INDR (indoor design temp) = 68F, MIX DSGN (design supply water temp) =130F, MIX MAX = 140F, MIX MIN = 70F, BOIL MIN = 130, BOIL DIFF = Ad, WWSD OCC = 63, WWSD UNOCC = 60

First observed instance:

11/29/25, Outdoor temp reading from the 360 (and outdoor thermometer) 15F. the Mix Temp actual was 130, the Mix Target said 140, which is 10 degrees higher than the design value of 130F at -10F. My subsequent data-gathering over the last two months has revealed a heating curve that at Outdoor 15F should put the Mix Target = 118F. The boiler was not able to keep up with the heat demand, and the 360 was cycling the mix valve open and close trying to maintain the Boiler MIN value of 130F. This was the first time I had seen this situation, I didn't think it was right, didn't like the idea of 140F water going out into the floors. But I left it alone, and by the next morning it was behaving "normally", which it seemed to do for the next two months

Two months later I found it doing it again: 1/22/26 This time Outdoor temp reading on the 360 was 11F, Mix Target again was at 140F! The heating curve should have been setting the target at about 120F. Again the boiler was not able to keep up with the demand, with a Mix Actual averaging about 114F, given the 360 again playing boiler protection. This did not seem right! So, I changed the 360 settings over to a manual Mix target, no outdoor reset.

For the next four days I would set the mix target manually, as outdoor temperatures changed from day to night, (This was the coldest four days of the winter so far, getting down to -4F, of course)

I haven't been able to get the installer to come look at the old 360, so ordered a replacement and swapped it in a couple of days ago. It's been only a few days, but I haven't seen the problem again, (yet).

My simple question: What might make the 360 decide to ignore its setting curve and jump right to the MAX MIX setting? It seems to report the correct outdoor temperature. It reports the correct indoor room temperature, The design value settings have always been unchanged from what they were programmed as given above. What might it be? I can only suspect an intermittent electronics hardware problem. Which would mean a warranty claim to my installer.

I would appreciate any insight from the forum. Is there some quirk of the 360 that might cause this? or is it hardware failure?

Thanks in advance.

Best regards,

RC Harris