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Cold air during defrost cycle. What's wrong?

Posted by stant98 (My Page) on
Wed, Jan 12, 11 at 17:55


So I recently had IAQ stat(9421) + EIM installed. I have an all electric heat pump + air handler (1 cool 2 heat):

Air handler:
Product # FC4BNF036000AEAA
Model # FC4BNF036

Heat pump:
Product # 697CNX030000ABAA
Model # 697CN030-B

Heating and the stat work ok, but I have noticed that, during defrost, cool air is blown from the registers.

Both "emergency heat" and "aux." heat work ok.

Could someone help figure out what could be wrong (may be 1 wire was missed to be wired)?

Manual for my air handler:

Decription of my the air handler's "smart heat" low voltage terminal board starts on page 46

The schematic for the board is in Fig. 39�Smart Heat Printed-Circuit Board (PCB).

Diagram on page 51.

Manual for my heater package:

Decription of my heater package and its functions starts on page 4

Diagram on page 12.

My condensing unit is not equipped with outdoor thermostat.

What I don't understand is this:

How does the air handler board "know" to activate backup heat during defrost?

On the air handler board, there is W2D terminal (defrost input from outdoor unit).

There is also w2T terminal on the control board - it's used to activate emergency heat or auxiliary heat.

What I don't understand is how does the signal for defrost is processed by the air handler's control board and how should the wiring be set up for it to activate emergency heat during defrost.

From outdoor unit, W1 wire goes up to W2D terminal on the board.

From the EIM, Aux wire goes to w2T on the board.

Should there be another wire from EIM "Aux" terminal to the board w2D?

Thank you!

Follow-Up Postings:

RE: Cold air during defrost cycle. What's wrong?

Here's a discussion right here that is extremely informative that has just been going on concerning a similar problem.

RE: Cold air during defrost cycle. What's wrong?

There should be a wire that comes off of the outside unit that is the signal that it is in defrost mode and on my IAQ it goes to one of the stages of strips. Defrost starts and activates that one set of strips with power (24v) coming from the outdoor unit to the strip lug on the IAQ. The thermostat is actually 'out of the loop' as far as defrost works on my system.

The thermostat makes a call for heat and the heat pump starts.. the heat pump makes a call for defrost and sends the signal back to the wire that activates the heat strips via the lug on the IAQ control. With this in mind the first thing to check is if everything is okay on emergency heat to find the control wire from the outside unit and make sure it is connected and connected correctly.

***NOTE**** I did upon first install with the new unit and staged strips have an issue with 'cool' air at defrost but not COLD. The strips are staged at 70/30 and the defrost wire was connected to the 30% stage of the strips and wasn't enough to temper the air. I switched it to the other stage of strips and now have 90 degree air instead of 70.

RE: Cold air during defrost cycle. What's wrong?

I have a burnt spot on my ckt board in the air handling unit for my carrier fc4bnf036 heat pump system. I purchased a new board. But I cannot see how this new one wires in i need schematics or maybe there are 2 boards.
Please help

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