HTP Boiler Not Firing With Outdoor Sensor Attached

mattbee24

I just installed an UFT-100W boiler. For some reason, when I hook up the O/S it won't fire. If I unhook the O/S it fires just fine. I checked and double checked all the settings and everything seems to be right. It is currently 50 degrees outside, 66 inside. The thermostat is set to 68. I have the max/min outdoor temps at 68/15. The max/min supply temps are at 120/85. Warm weather shutdown is 68. It turns on the CH pump, but never attempts to fire. The boiler shows the outdoor temp as 50 and shows the water temp should be 93. Is there something I'm missing?

NY_Rob

I would try setting the max CH water temp to 150 and the min CH water temp to 110. See if that works.

Also, the factory default for "Setpoint Differential" (item 17:bo) is 30F deg, so if your still at the factory default for 17:bo and your current setpoint is 93F the boiler won't fire till the CH water goes below 63F (your CH water is probably at 66F, same as your indoor temp).
I have my "Setpoint Differential" (item 17:bo) set to 20deg and that seems to work fairly well.

Also FWIW.... you literally get no heat from 85deg CH water unless you have radiators designed for low temp systems. And, unless you have lots of radiation on each loop 85 deg CH water would probably cause short-cycling.

DZoro

Double, check your wire location for the outdoor sensor. Sometimes can be mistaken wiring location with another sensor location. Can look like the right spot but....

mattbee24

Ahhh...setpoint differential...never thought of that. I'll give that a shot and let you know.

This is in a concrete slab, so I'd like to keep temps as low as possible.

mattbee24

NY_Rob....that was the problem...thank you!!

NY_Rob

^ glad it worked for you.

That should be an excellent boiler for slab... good luck with it!