Greetings,

Thanks in advance for any help that can be provided!

We have a hydronic system, with 2 Weil Mclain HEii boilers, a WM DHW tank and Tekmar controller. The system alternates boiilers, has DHW priority and uses outside temp sensor. Have 2 heat zones of radiators, and one zone of in-floor hydronic with mixing system.

At the end of last season, we began to have a symptom that in the morning only (after overnight setback down to 62 degrees) system would not come on. Noticed because there was no hot water for showers. Simply cycling AC on/off restored everything. This would happen intermittently, and it was close to spring and I didn't follow up.

This winter it's a nightly occurrence.
Every morning I go downstairs, both HEii's have red flashing error code. Power off and on, everything works. Odd that it NEVER happened throughout the summer when the boilers were only being used for DHW. Something about being in heating season makes the difference. Also, never happens during the day, once I flip the power in the early morning, system works fine all day, both heat and DHW, until overnight, and then fails again overnight.

Although not a conclusive test (I only tried it once) last night I didn't setback and kept the temp at 70 all night, the system ran normally, and I did not need to cycle the power.

Boilers are 15 years old, Tekmar is about 3-4 years old.

From reading on the internet, was ready to replace the controllers on the HEii's but now I'm not so sure?

Thanks!!