Weil Mclain Ultra 105 (series 2) intermittent response to call for heat

JTecosky

I have a 10 year old WM Ultra 105 boiler, with indirect HW. In the past several weeks, boiler (intermittently) simply doesn't respond to calls for household heat from programmable Honeywell Thermostat, but NO error codes, soft or hard. My first thought was that the TT relay was worn out (TT 10 years old) since when I disconnected TT from wall and reconnected, boiler woke up and started heating cycle. I replaced TT with new programmable Honeywell TT, (new TT batteries, too) and checked/resecured the TT wires at the boiler low voltage power strip, but no change in intermittent behavior.

Boiler is operating properly when it does fire up, and I see no problem with DHW call for heat (although perhaps I might not notice that if it, too was intermittent).

Some more details -- it seems to consistently happen first thing in the morning; when I wake the TT says "heat on" but the WM readout is something like 088 -- no error codes, just quiet, as if no call for heat. If I manually lower TT so that stops call for heat, then raise to call for heat, the boiler cycles on and all is fine -- until it happens again.

One more detail -- I have repeatedly reset the boiler and also powered off and on. When I power on, I see U124 code for about a second, but then it disappears. (I read in control manual that this code can mean bad transformer.) Also, don't know if it is significant, but I've observed boiler, when heating the house, overshoot 190 temp and shut off, only to turn right on again with a lower output temp.

Any suggestions for further tests or ideas? I'll replace control module if I have to, but hoping not.

Thank You!

Tim McElwain

Does this unit have the Honeywell MCBA control as the Integrated Boiler Control?

ColdJohn

JTecosky, did you ever get a resolution to your heating issue. Your post is almost identical to what we have going on.

EdTheHeaterMan

What is the possibility of the indirect thermostat failing closed. The WM Ultra has a priority for DHW built in to the system. I have already found this feature to cause No Heat condition. As soon as I disconnected the DHW thermostat the heating circulator kicked in and the home heated properly.

Check that your DHW is not calling for Hot Water when you have the NO HEAT issue. Could be that the DHW thermostat is intermittently failing to satisfy (not opening on temperature rise) once the tank is up to temperature.

Mr. Ed