Air handler blowing cold air when heat pump is in defrost mode

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Old 12-06-14, 12:57 PM
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Air handler blowing cold air when heat pump is in defrost mode

My air handler (Trane Tam 7 with electric heat as backup) blows cold air when the heat pump (Trane XL15i) is in defrost mode. I checked the connections at the air handler and they were connected as per the installation manual. I then verified the connections at the heat pump and I noticed that they did not follow the installation manual exactly. Specifically, the
1) Black (X2) from heat pump was left unconnected and capped: see photo
2) Orange (HP) connected Orange (AH) – OK
3) Red (HP) connected to Red (AH) – Ok
4) Blue (HP) (common) connected to Black (AH) (common) goes to Blue on AH terminal – OK
5) Yellow (AH) connected to common terminal (Red) of Outdoor thermostat (SPDT) (see picture)
6) White (W1,W2, W3 from AH) connected to NO terminal (blue) of ODT
7) Yellow (HP) connected to NC (yellow) terminal of ODT

When ODT is above set point (12 degrees F), the Yellow (AH) and Yellow (AP) are connected. When ODT is below set point, the Yellow (AH) and White (W1, W2, W3) from AH are connected

I guess the reason for this is that when the ODT is below the set point, the energized Yellow from AH (during a heat demand) will be connected directly to the White of the AH causing the electric heat atomically come on.
But because the Black (X2) from the heat pump is not connected to the White (W1, W2, W3) of the AH, then when the HP is in defrost mode the energized Black wire (X2) does not make connection and the air handler does not turn on the electric heat, but the fan starts blowing cold air instead.

I’m thinking that the right way to wire this is the following: (can someone confirm?)
1) Remove the white wire (AH) from the ODT and connect it to the black wire from HP.
In this way, when the unit is in defrost mode, the Black wire from the HP will energize the electric heat. The “downside” is that when the ODT is below the set point, and the thermostat calls for heat it may take a little longer for the electric heat to be turned on by the thermostat (i.e. as the thermostat will automatically energize its White terminal) when it detects that the temperature is not going up.

An alternative is to leave the white wire connected to the blue ODT terminal and jumper a wire from there and connect it to the Black (X2) wire from the HP. Any danger in doing this?
1) When the HP is in defrost mode and the ODT is above the set point, then the connections are as per the installation manual. The electric heat will get turned on through the connection of the black wire (x2) to the white wire (W1, W2, W3).
2) If the ODT is below the set point and there is a heat demand then the electric heat will automatically come on through the connection between the Yellow (AH) and White (AH) connection. The Black X2 (HP) will also get energized. Is this a problem?
3) Can you have a case where the ODT is below the set point, HP is in defrost mode, and there is a heat demand? In this case the yellow (AH), Black X2 (HP), and White (AH) are all energized and the electric heat is on which is what is wanted in both cases.

Appreciate if someone can confirm my thinking here and if the 2 alternative connections are valid. Also is there any danger in leaving it as is to the health of the heat pump? Is the worst case here that cold air is being blown during defrost mode and it does not affect the defrost operation?

Also with the existing connections, if the HP is in defrost mode, and ODT is below the set point and there is a heat demand, then the Yellow (AH), and white (AH) are energized and in this case the electric heat will turn on. The problem occurs if the ODT is above the set point, as no electric heat will be turned on since the W (AH) is not energized.
 
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  #2  
Old 12-06-14, 03:50 PM
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That's lot of information! When in defrost mode the heat pump should bring on the back up heat. The ODT should lock the heat pump out at or below setpoint.
 
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Old 12-06-14, 06:02 PM
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Ok so by jumpering a wire to both the
Black X2 (HP) and white wire from the air handler would work and not cause any side effects? I.e. I would connect a wire to the middle ODT terminal and connect it to the loose black wire from the HP
 
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Old 12-07-14, 01:03 PM
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This site is pretty silent? Anyway I connected as in my last post and problem solved.
 
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