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I have a Healthy Climate HRV3-150-TPD installed and the defrost damper is constantly in recirculating position. During start-up test, the damper opens and closes normally, so I know the actuator and the damper function well. However, after the test cycle, the damper closes fresh air intake and stays in that position constantly. I have tried changing DIP switches with no effect.

The installing contractor is no longer around. Lennox is useless as they won't talk to me unless I have a gas fitter license (the last time I installed their product until they learn to service customers). Talking to other contractors was no better as they were telling me (without bothering to come) that my HRV was obsolete and no longer serviceable, which is a BS. They are happy to sell me a new HRV rather than troubleshooting mine.

Any suggestions how defrost damper can be forced out of recirculating mode without breaking it off or buying a new HRV?

Does the terminal block give you a flashing code?

I would first rule out a faulty wall control that may be forcing recirculation mode by disconnecting the wires at the HRV.  Install a jumper between ON and RED and another jumper between HI and ON.  Then power up the HRV and see if the damper stays open.

You could also check the P4 Blue connector on the HRV board and make sure the thermistor is secured.  It may be that the thermistor is bad and telling the HRV to enter defrost mode.

Lennox doesn't list that part # in their manual but I believe it's a Honeywell 208365 Thermistor Assembly.

Thank you for the prompt response. I'll give it a try.

Hi Admin,

I followed all your suggestions. After I disconnected three control wires (red, orange, and green), the HRV went through regular self-test sequence, after which the damper opened fresh air intake and stayed open (progress!), but the fans stopped. Connecting jumpers between ON and RED and HI and ON returned the HRV to the previous state, although this time the damper didn't quite close fresh air, but stayed slightly ajar, something I haven't seen before. I then restored control wires, checked P4 blue connector and, whether I had it pulled out or inserted back in, the HRV went back to what it did before: after test cycle closed fresh air and started running fans.

The red light on the terminal block does give a code sequence during regular operation: 3s on, 10s off.

Does it still sound like a thermistor fault?

No, it sounds like a control board problem.  Are you certain you had a wire making contact with all 3 terminals, ON, RED, HI?

I attached a troubleshooting chart below.

You would need Lennox Part #Y2164 ($415.79 CAD),

https://www.hvacpartsshop.com/y2164-hvr3-200-transformer-circuit-board/

I realized that last time I disconnected only three control wires (red, orange, and green), so this time I disconnected all six (including two blacks and a white) and then interconnected Red, Hi, and On with jumpers. After self-test, fans run on high and damper went into recirculating mode (closed fresh air). Similar to what has been the normal state but with fans on high. The red light is still giving same code: 3s on, 10s off.

I'd like to show a couple of photos, but I haven't found a way to insert them on this page.

If you click the User Actions button, it will let you reply instead of quick reply, and then you are able to add attachments.

Try disconnecting the sensor and see what happens.  I'm wondering if it will give us an error code.

Thank you for suggestion. The first photo shows terminal block in its original condition. The second photo is after I disconnected control wires and installed jumpers between Red, On, and Hi. Fans are running on high and the damper is in recirc position. 

I just checked the thermistor connection again. Whether I have P4 blue connector in or out, the behavior doesn't change. Red light gives the same code - 3s on, 10s off. Fans are running and damper is in recirc.

I don't have the resistance values for the outdoor temperature in order to test the sensor.

When you unplug the sensor the terminal block LED should flash rapidly for 3 seconds.  If it doesn't that's another indicator the controller board is bad.

The terminal block LED doesn't change the pattern when I unplug the sensor. Also, I measured 3.5 kohm impedance on sensor terminals. Doesn't guarantee that the sensor works properly, but at least it's not a short or a break.

If I look at replacement boards, the link you sent earlier for a $415 replacement kit seems to be for a set of 6 boards and none of them look like the one in the HRV. I found this one:
https://gasexperts.ca/product/lifebreath-replacement-circuit-board-kit-pn-99-rnc3-3spd/
It's listed for  different make of HRV, but the board looks identical, and I mean completely identical. Would it be reasonably safe to try this one?

Photo of the control board cover showing the layout.

I think the site I posted uses generic pictures but the part number is right.  I do remember hearing that Lennox HRV's are made by Lifebreathe so you may have luck with that part.

I'll give it a try and let you know if it worked. Thank you for your help and patience.

The replacement control board arrived, I installed it, and everything seems to work fine. The fans are running and the defrost damper is in the non-defrost position. I'm keeping fingers crossed that the defrost cycle works normally next winter. The red light is still giving the same code: 3s on, 10s off. I suspect it's not a code but rather a status signal.

As I was suspecting, it's the same control board for Lifebreath, Lennox, Waterfurnace, Aldes, Honeywell, Airflow, and Titan HRVs, just slightly different DIP switch setting for some of them. Funny that you can buy it labeled Lifebreath for half the cost of when it's labeled Lennox. What a scam!

Anyway, thank you for helping me troubleshoot this problem. Looks like I finally have my HRV working properly and am much less hesitant to take it apart and rework electronics. Another lesson learned - don't get stuck on branding when looking for replacement parts - most of them are made at the same factory.