Tempstar Blower randomly not coming on.

[pvstansel]

1st time post, and new to diagnosing HVAC. Tho I'm pretty handy.

I have 2x Tempstar Heil NTxx series units (both about 15 yrs old) (dual zone home).
Both units are installed horizontally in the crawl space.
Natural Gas. System also has 2x AC compressors.
Up until now, both units have performed flawlessly.

On one of the units, I'm noticing the following behavior (note that this behavior does not happen consistently, often the unit works just fine).
(the fan setting on the thermostat is set to "AUTO")

Description of the problem (again not happening consistently).

1. The furnace kicks on, there is the expected delay, and then the blower comes on and everything works fine.
2. After about 6 or more cycles, the furnace will kick on normally, but the blower will never come on. And the furnace will keep running.
3. From the thermostat, if I turn off the unit for a period of time, and then back on, it seems to correct itself for awhile.
4. I am able to manually turn on and off the blower (fan) from the thermostat.

What could be causing the blower not to not consistently come on after the expected delay during a heating cycle

Thanks in advance for any ideas. I can provide more information if you let me know what would be needed.
Paul

[Freon]

You have the worst kind of problem... an intermittent problem. I strongly suspect the fan relay for the heating cycle (there's another relay for cooling) is causing you the problem. It could be a logic error causing the relay not to energize or it could be a connection that's loose causing the 120 (or 240) volts AC not to get to the motor when the relay closes.

Can you post the make and model of the control board... the board your wires motor attaches to. If you have the patience and a multimeter, see how easy it is to measure the voltage at the terminals where the motor wires connect to the control board. These connection points (spade lugs) should be labeled Heat, Cool and Neutral. You want to CAREFULLY measure AC voltage between the Heat and Neutral when the blower is not running but you know it should be. I suspect you will measure zero volts. You should also listen for the Heat relay to close approximately 50 seconds after the burners first ignite.

Another test is, at the thermostat, set the Fan to ON, not AUTO. Then see if it runs continuously, as it should. That will help eliminate (although not completely) the motor as the source of the problem

[pvstansel]

Freon,
Thanks for the reply.
The problem and behavior continues to be intermittent.

I plan on getting up in the crawl space today, and will get the info on the control board. Fortunately I live in San Diego area where the climate is mild.

Couple of things

If you strongly suspect the fan relay for the heating cycle, I'd be willing to just purchase this part. How would I go about purchasing it.

Reminder that I have dual identical units. I could potentially swap the fan relay from one unit to the other. Would you advise this.

While the unit was mis-behaving, (The furnace being on, 30 second delay, and no blower kicking on, and the furnace stays on) I did turn the fan from AUTO to ON at the thermostat. The fan did come on, but the air coming out of the fan was not heated air. So perhaps this is another clue.

[Freon]

The relay is usually part of the control board. And the problem could be with the coil, involving bad logic or a loose solder joint, or it could be with the contacts of the relay. Sadly it may be easier to replace the control board but we'll see about that later.

You say the fan does not come on but the burners stay lit. If that's the case, then you should see the burners turn off due to the high temperature safety shutting the burners off. When the temperature in the heat exchanger cools, the safety will close and the burners my re light, depending on if you're locked out. It's all in the control board programming.

At the thermostat if you leave the fan set to ON and not Auto, you should be fine. The relay for the ON setting is usually the same as the AC and you thereby use a different relay. And you really won't use much electricity with the blower running constantly.

Once we know the control board, it will help in finding the best solution.

[pvstansel]

Freon, thanks again for your time, help and patience.

The controller board is a Honeywell.
I do have the instruction paperwork, and the controller board is on page 20 of the NTC5 Series NTN5 GNJ (NOx Models) Installation Instructions.

There is a number stamped in black ink - near the Honeywell logo 9528

Another number stamp in black 03 95
Above the 03-95 is a stamp t 9 0

There is a white sticker on the Honeywell board with the following on it.
ST9120C4016
HQ1009836HW

Overall the unit is in very nice shape. And no obvious solder problems or corrosion on the board.

[Freon]

Here is a link to your Honeywell ST9120C4016 Fan Control board - dot forwardthinking dot honeywell dot com slash related_links slash combustion slash st9120u slash install slash 69_0644.pdf
The small black boxes are the relays. And the motor connections for the main blower are clearly marked. Usually when you set the Fan to ON at the thermostat you are powering the Cool connection on the control board. A test you could do is set the Fan to ON for a while then to Auto. The purpose is to verify the blower runs consistently. Assuming the blower is using the Cool motor speed, you will have established the Cool speed of the motor is good.

Now turn off the power and TEMPORARILY switch the Heat and Cool motor connections so the cool motor lead is on the Heat terminal of the control board. Now if you still have intermittent blower behavior during the heating cycle, you know it's the board, not the motor.

If it is the board it may be worth removing and checking all the solder joints. Label ALL wires so you know how to reconnect.