Outdoor Air Reset Issue

akaDigger

Lochinvar WBN50 W/DHW/radiant floor, installed in 2008. Annual service performed by me (installer/homeowner) a week prior to issue arising. The service was as routine as any in the past. A thorough cleaning and inspection.

I'm having an issue with the outdoor reset. The boiler is not firing due to perceived high outdoor temp. For example, this morning when it was 47°F outside, the boiler display read 62°. The thermistor that came with the boiler, when tested with ohm meter, tests exactly as it should. These readings were tested at the boiler disconnected from the board.

This happened last week too. Then, I tested the thermistor on the bench. All good after soaking it in the refrigerator to get an exact known temperature. After reinstallation, the boiler display showed the correct outdoor air temperature. For several (warm) days after the boiler continued to display the correct OAT.

Then, today, a cool morning I again have an incorrect OA temperature reading.

DHW works as it should.

At this point I'm afraid the Control Module may be acting up.

Thoughts please.

-Dan, aka Digger

Jamie Hall

More likely the sensor or something in the wiring -- you mention damp and cool. Wiring doesn't like that... check all your connections.

hot_rod

Is the sensor seeing sunshine or heat from a dryer vent or some other influence.

pedmec

Is the OA still in its plastic housing and not being affected by the sun? make sure its in the shade.

To do an accurate test give the sensor an ice bath. put it in a bowl with ice an water. that way you know the fluid temperature is exactly 32 degrees. but wrap the sensor to prevent it from getting wet, just to safe.

akaDigger

OA sensor is installed properly in its original case on the north side of the home. It has worked as it should for fourteen years.

Again, the sensor and its wiring test fine.

hot_rod

The sensor is the least expensive part to replace. The control board swap $$ would be my second choice.

You can see all the wiring, connections, any splices?

akaDigger

When I first saw the erroneous reading I was almost elated as surely it was an easy fix. But I did the testing and have ruled out the sensor and wiring. Sensor case shows some fading but the sensor is clean and dry, looks new. No splices in wiring, I installed it myself.

Looking at a cost of between $550 and $650 depending on the part# which we currently are not sure of. Lochinvar support is pretty busy today. It is Monday and peak season 

pedmec

In the knight service manual at 47 degrees you should be getting an ohm reading of 21,760. That's interpolating between 40 and 50 degrees.

HomerJSmith

Where did you measure the ohm reading? The terminal wires at the control board is where to measure. Always make sure the wires are secure in the terminals when done.

Go into the installers programing (password 5309) and check the outdoor warm weather shut-down value.

akaDigger

pedmec said:

In the knight service manual at 47 degrees you should be getting an ohm reading of 21,760. That's interpolating between 40 and 50 degrees.

I just retested it again, DISCONNECTED from the board, 16.3k ohms @58°. That is correct according to the service manual.

akaDigger

HomerJSmith said:

Where did you measure the ohm reading? The terminal wires at the control board is where to measure. Always make sure the wires are secure in the terminals when done.

Go into the installers programing (password 5309) and check the outdoor warm weather shut-down value.

With the wires secure in their terminals on the board, probing the terminals, I get only 150 ohms.

akaDigger

HomerJSmith said:

Go into the installers programing (password 5309) and check the outdoor warm weather shut-down value.

The OA Shutdown is set at the default 80°

pedmec

You need to measure with wires disconnected from board so your not reading through the board.

akaDigger

pedmec said:

You need to measure with wires disconnected from board so your not reading through the board.

This is what I have been doing all along. I only took the reading hooked up because it was recommended by @HomerJSmith

pedmec

I don't think he meant with them connected to the boiler. He meant to measure from the boiler thru the loop. That means you need to measure the resistance of the wires and the sensor. If you had a bad wire, spliced joint or anything to increase the resistance that would be interpreted as a different temperature than what it actually is.

STEVEusaPA

You could use one of these to check the control.
Dial in the ohms you want based on a temp you want to see, and see if the control is reading the right temperature.

https://m.media-amazon.com/images/I/81rH2w2U2iL._SX522_.jpg

akaDigger

Control module, 4-5 weeks. New boiler, 7 weeks.

I began moving set points to compensate for the bad OA interpretation. I got the boiler heating and modulating. Baby steps. I've more tinkering to do. It likely works more efficiently than a default single set temperature. I hate it that I lose my efficiency just as energy prices soar.

On the displayed OA reading, only an eight degree spread last 24 hrs. 62° down to 54° overnight. In reality, it dropped to 38°.

On the upside, cycling at SET POINT MET is redistributing the passive heat from the sun on the kitchen tile.