Electric Furnace (only) blower on continuously.

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Old 11-25-13, 12:54 PM
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Electric Furnace (only) blower on continuously.

Fan is on constantly (warm out was coming out of the floor registers also). Digital T-stat control module was removed from its base to eliminate t-stat as the problem.

Relay part number: 9400-13Q152 See pic below.
With long side of the rectangular relay positioned vertically in front of me (facing the quick disconnect terminals),
#1 & #3 terminals are at bottom (#1 at bottom left).
#2 & #4 terminals are at top (#2 top-left).
#5 & #6 terminals are just below #2 & #4). #5 terminal at left side.

#1 & #3 are the coil terminals, and are good (continuity). With no power to coil, the only other terminals that have continuity are 5 & 6. With 24vac applied to coil, the only two terminals with continuity are 2 & 4.

I just checked the sequencer with ohm-meter. With no 24v power applied to base, both sets of terminals (base & the only pair on top of that) have continuity across them, but someone just told me this "sequencer" is actually a limit/sequencer. After a second look, I realize now that the bottom of this "seq." looks to be a typical limit switch, with the quarter (25 cents)-sized sensor button on the bottom. Part Number is 64TX11-208487 (limit/sequencer)

Since the only other components in this furnace control box are a contactor relay (not sticking) and the XFMR, one or both of these units (Fan Relay and limit/sequencer) would have to be bad, correct? Changing them both would take care of the problem (FAN on constantly), unless there were a short circuit in the wires to fan motor, Correct? Warm air is also coming out of the registers when fan on.
 
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Last edited by plumducy; 11-25-13 at 02:13 PM.
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Old 11-25-13, 07:10 PM
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Disconnect G (or equivalent) from the low voltage terminal strip, if applicable.

Do you have the model number of the furnace? (see i could find the schematic)

#1 & #3 are the coil terminals, and are good (continuity). With no power to coil, the only other terminals that have continuity are 5 & 6. With 24vac applied to coil, the only two terminals with continuity are 2 & 4.
Sounds like the relay is doing what it should be.

Check for NC/NC labels/symbols to determine what the relay should be doing.

Trace the hot lead from the fan motor to determine what interrupts power to it - might be a sequencer and a relay, or just a relay depending on how your system is designed.

The old electric furnaces may not bring on the fan without a call from the t-stat.

The new ones as far as I know have a sequencer or fan control board bring on the fan.

Since the only other components in this furnace control box are a contactor relay (not sticking) and the XFMR, one or both of these units (Fan Relay and limit/sequencer) would have to be bad, correct? Changing them both would take care of the problem (FAN on constantly),
I don't recommend troubleshooting by replacing parts until it works.

Both might be good - other things can cause the fan to run when it shouldn't.

Does the air stay warm long after a call for heat or drop to room temperature?
 
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Old 11-25-13, 07:37 PM
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Thank you muggle - I'm going back again tomorrow for a more thorough check on what's happening with this unit. Will have the model and serial then.

I'm normally not a fan of parts changing until it works, either, but in this case I'm wanting to get it fixed before Turkey day (the limit/seq. has to be ordered in case that's the problem) and just figured since the only other parts in this control box are the contactor (not sticking) and transformer, those two parts (fan relay and limit/sequencer) would solve my problem (limit/sequencer has to be ordered, which I did today - pick it up tomorrow before going back to check out system again). If that' not part of the problem - glad to have the extra part for future use.

One thing I didn't do before however (will tomorrow) was to check for a grounded heat strip (there's only two). Digital t-stat control module (Honeywell) was removed from its base. There's no air conditioning connected to this electric furnace & t-stat. It's a smaller mobile home and there's a package unit A/C with separate t-stat, so only two-wires from electric furnace thermostat to control panel, RH & W, but as I said the digital control module was removed from its base and fan still runs with heat at the registers.

Given all the above, other than short circuit in the wires feeding the blower motor, what else could keep the blower and heat on constantly?

I've checked the limit/sequencer for continuity across the two only sets of terminals it has, and both have continuity (zero ohms resistance). Isn't the sequencer part of this limit/sequencer supposed to be OPEN until its disc warms up and closes the path across its terminals?. In other words. I'm assuming that the contacts at the base of this limit/seq. are NC for the limit function, which then heats up the sequencer function for the two contacts above the base.

In other words, if the seq. part of this unit functions like other sequencers (closes only after delayed timing), seems then that this limit/seq. is bad. Or maybe my gray matter is just working overtime and muddying the waters? I'll be looking at the wire paths like you suggested.

Does the air stay warm long after a call for heat or drop to room temperature?
Afraid I can't answer that yet, sorry.

No NC/NO labels on the relay. No diagram on relay.
________________________________________________
I just found this online:
621867R Nordyne 24V relay Part # 621867R

Double pole single throw blower and fan relay. 24V coil. With the power off, terminals 5 & 6 will be closed and terminals 2 and 4 will be open. With 24V to terminals 1 & 3, terminals 5 & 6 will be open and terminals 2 and 4 will be closed. Be sure to remove wires from terminals 2,4,5 & 6 before testing.

Also known as # 9400-13Q152. Replaces 621867, 6218670, 628337, 6283370, 660190, 104786000 & 105521000.
This is exactly what I found with my continuity checks on this relay, so apparently my relay is good (which I guess is what you said also).

Maybe I can google some more for the proper operation of the limit/sequencer, or maybe you know that...
 

Last edited by plumducy; 11-25-13 at 09:04 PM.
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Old 11-30-13, 12:50 PM
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I know I shouldn't admit it, but the problem was a short circuit in the t-stat wires, due to some recent paneling work by the owner's contractor. Another lesson well-learned.
 
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