EcoRenovator - View Single Post - Need help with Nest-HRV integration

bbro62 · 01-20-16, 06:31 PM

Thanks for your reply. The HRV unit is long out of warranty so I'm not too worried about that. But you are right, the manufacturer has not responded to any of my queries as expected. Other than the control logic, the HRV is a pretty simple device: a two-speed fan and a damper controlled by a stepper motor, that I've already had to replace once, are the only moving parts. There is a thermistor to trigger damper closure for defrosting and the control board also manages a 20 minute timer cycle activated by wall switches in the bathrooms. The fan and damper are controlled by 3 relays via the control board. The unit is a Carrier HRVCCLHU1150-A and the installation manual is here: http://dms.hvacpartners.com/docs/100...0E/HRV-1SI.pdf.

The raspberry pi work is already done. I have a working prototype of an Outdoor Reset override connected to my boiler to allow my system to recover from nightly temperature setbacks. Surprisingly, the engineers at the boiler manufacturer have been very helpful and interested in this project. You can find details of that project and why I did it here: https://github.com/blbrock/Nest-ODR. Basically, the raspberry pi polls data from my thermostats every 10 minutes via an API. If a call for heat > 1.25F above room temp is detected, the pi activates a circuit that introduces additional resistance to the ODR thermistor wire. It does this in three stages at hourly intervals. Once the call for heat is satisfied, the unit returns to normal ODR mode. That's a tangent but I added a little routine to check the outdoor temperature and adjust the target humidity setting for the thermostat I want to control the HRV. This has been working for a few months now but without anything connected to the HRV to actual do anything.

I posted a more detailed outline of my thinking on the Arduino Forums here: Nest thermostat HRV integration which I have also copied below. I think I overwhelmed them with too much information as I have not received any replies. My current thinking is that I will need to use a logic analyzer to capture the signals sent from the wall controller and then replicate those with a microprocessor. There are only three control states I need to replicate: fan high, damper open; fan low, damper open; and fan off, damper closed. I'm pretty comfortable designing circuits with switching logic for relays and transistors but more advanced signal logic is all new to me. An Arduino can be used as a logic analyzer which is appealing because I could then use the same device as the future controller. But that's where I start getting in over my head. But it seems like it would be similar to the process of reverse engineering IR signals that I've read a couple of tutorials on.

To be honest, I've also thought about ripping out the control panel from the HRV and replacing it with one that isn't such a black box to me. I like the HRV but have never been happy with the control features and know I could do better. But I'd rather figure out the signal logic - in part because it is fun.

-----------------
posts on arduino forum

I'm trying to assess the feasibility of a project I thought would be simple, but was wrong. First, I am completely new to micro controllers but have some hacking experience and am near the end of a project to integrate my Nest thermostats with a modulating boiler via a raspberry pi. That project (nest_odr) is on github but still a work in progress.

Now I want to setup one of my thermostats to control my Carrier HRV. I thought it would be a simple matter of attaching the thermostat to a couple of 24VAC relays to run the fan at low speed using the thermostat's fan timer function, and at high speed using the thermostat's dehumidifier function. But it isn't that simple. The HRV uses a wall controller and 4 wires to communicate with the control board on the HRV. The wall unit ultimate controls three relays in combination to control fan high and low speed, and damper to control fresh air vs. recirculating exchange. I have two options for controls:

Option 1: Brute Force. This option is to control the 120VAC fan and damper control wires via relays directly. It requires converting the 24VAC control voltage from the Nest to 12VDC to allow proper isolation of relays, but would basically duplicate the relays on the existing control board. There are two disadvantages of this option. One is that is involves household current which isn't a huge deal, but it's nice to do everything from the low voltage side if possible. The second is that it would defeat the unit's defrost cycle. The defrost cycle senses the incoming units air temperature and closes the damper to recirculate warm air to defrost the system as needed. The high voltage relays would be behind the unit's controller and therefore would keep the damper open regardless of temperature. So I will need to figure something out here if I go with this option.

Option 2: Low Voltage Control. I think a better option would be to create an interface that replicates the signals being sent from the wall controller. So if the Nest was calling for the Fan, the interface would call for low speed fresh air exchange. If it was calling for Dehumid, it would send a signal for high speed fresh air exchange. That way the unit's wall and on board controllers can still do their functions, including defrost. The biggest disadvantage of this option is my ignorance of micro controllers. I have no idea what is actually being sent across the 4 communication wires to signal the unit. Actually 3 wires because 1 is just a 12VDC supply. I've also been able to determine that the other wires operate at 5VDC with varying current depending on control function. But I have not been able to find any documentation that would help decipher what is happening there. It must be fairly standardized though because all of the HRV manufacturers seem to use the same controllers.

Option 2 appeals to me because it is more interesting for personal growth, it involves only low voltage signals, and it would probably allow me to replace the ugly wall controller all together. It would also allow adding more advanced controls like using humidity or occupancy sensors. So I'm looking for advice on whether this project is feasible for a noob like me. If so, any tips on at least figuring out the signal logic of the controller and how to replicate it with an Arduino.

Thanks in advance for any advice, even if that advice is to stick to my day job.

-------------------------
After more research, I think I'm starting to figure out the microcontroller/logic bit of this project and have more specific questions than my original rambling project description.

It appears my HRV is using serial (probably TTL) logic, possibly current modulated since my poking around with a multimeter indicated changes in current at different controller states. It seems my first challenge is to decipher the HRV wall controller signal logic to know what I need to replicate with the microcontroller. What is the best/ least expensive way to do that? Can this be done with an inexpensive logic probe? There are only 5 possible states for the controller:

Fan High, Damper Open
Fan Low, Damper Open
Fan High, Damper Closed
Fan Low, Damper Closed
Fan Off, Damper Closed

Can I methodically record what is happening on each of the two signal wires in each state with a logic probe to map out the logic? Or am I missing something? I'm pretty comfortable with switching logic but the more sophisticated digital logic is all new to me.

Also, initially I intend to add the thermostat based controller while leaving the original wall controller in place. This would add a second transmitter to the serial communication which I've read enough to know is very bad. Is there a good way to isolate two serial transmitters so they don't try to communicate with the receiver at the same time?

01-20-16, 06:31 PM	#3
bbro62 Lurking Renovator Join Date: Jan 2016 Location: N/A Posts: 10 Thanks: 2 Thanked 0 Times in 0 Posts	Thanks for your reply. The HRV unit is long out of warranty so I'm not too worried about that. But you are right, the manufacturer has not responded to any of my queries as expected. Other than the control logic, the HRV is a pretty simple device: a two-speed fan and a damper controlled by a stepper motor, that I've already had to replace once, are the only moving parts. There is a thermistor to trigger damper closure for defrosting and the control board also manages a 20 minute timer cycle activated by wall switches in the bathrooms. The fan and damper are controlled by 3 relays via the control board. The unit is a Carrier HRVCCLHU1150-A and the installation manual is here: http://dms.hvacpartners.com/docs/100...0E/HRV-1SI.pdf. The raspberry pi work is already done. I have a working prototype of an Outdoor Reset override connected to my boiler to allow my system to recover from nightly temperature setbacks. Surprisingly, the engineers at the boiler manufacturer have been very helpful and interested in this project. You can find details of that project and why I did it here: https://github.com/blbrock/Nest-ODR. Basically, the raspberry pi polls data from my thermostats every 10 minutes via an API. If a call for heat > 1.25F above room temp is detected, the pi activates a circuit that introduces additional resistance to the ODR thermistor wire. It does this in three stages at hourly intervals. Once the call for heat is satisfied, the unit returns to normal ODR mode. That's a tangent but I added a little routine to check the outdoor temperature and adjust the target humidity setting for the thermostat I want to control the HRV. This has been working for a few months now but without anything connected to the HRV to actual do anything. I posted a more detailed outline of my thinking on the Arduino Forums here: Nest thermostat HRV integration which I have also copied below. I think I overwhelmed them with too much information as I have not received any replies. My current thinking is that I will need to use a logic analyzer to capture the signals sent from the wall controller and then replicate those with a microprocessor. There are only three control states I need to replicate: fan high, damper open; fan low, damper open; and fan off, damper closed. I'm pretty comfortable designing circuits with switching logic for relays and transistors but more advanced signal logic is all new to me. An Arduino can be used as a logic analyzer which is appealing because I could then use the same device as the future controller. But that's where I start getting in over my head. But it seems like it would be similar to the process of reverse engineering IR signals that I've read a couple of tutorials on. To be honest, I've also thought about ripping out the control panel from the HRV and replacing it with one that isn't such a black box to me. I like the HRV but have never been happy with the control features and know I could do better. But I'd rather figure out the signal logic - in part because it is fun. ----------------- posts on arduino forum I'm trying to assess the feasibility of a project I thought would be simple, but was wrong. First, I am completely new to micro controllers but have some hacking experience and am near the end of a project to integrate my Nest thermostats with a modulating boiler via a raspberry pi. That project (nest_odr) is on github but still a work in progress. Now I want to setup one of my thermostats to control my Carrier HRV. I thought it would be a simple matter of attaching the thermostat to a couple of 24VAC relays to run the fan at low speed using the thermostat's fan timer function, and at high speed using the thermostat's dehumidifier function. But it isn't that simple. The HRV uses a wall controller and 4 wires to communicate with the control board on the HRV. The wall unit ultimate controls three relays in combination to control fan high and low speed, and damper to control fresh air vs. recirculating exchange. I have two options for controls: Option 1: Brute Force. This option is to control the 120VAC fan and damper control wires via relays directly. It requires converting the 24VAC control voltage from the Nest to 12VDC to allow proper isolation of relays, but would basically duplicate the relays on the existing control board. There are two disadvantages of this option. One is that is involves household current which isn't a huge deal, but it's nice to do everything from the low voltage side if possible. The second is that it would defeat the unit's defrost cycle. The defrost cycle senses the incoming units air temperature and closes the damper to recirculate warm air to defrost the system as needed. The high voltage relays would be behind the unit's controller and therefore would keep the damper open regardless of temperature. So I will need to figure something out here if I go with this option. Option 2: Low Voltage Control. I think a better option would be to create an interface that replicates the signals being sent from the wall controller. So if the Nest was calling for the Fan, the interface would call for low speed fresh air exchange. If it was calling for Dehumid, it would send a signal for high speed fresh air exchange. That way the unit's wall and on board controllers can still do their functions, including defrost. The biggest disadvantage of this option is my ignorance of micro controllers. I have no idea what is actually being sent across the 4 communication wires to signal the unit. Actually 3 wires because 1 is just a 12VDC supply. I've also been able to determine that the other wires operate at 5VDC with varying current depending on control function. But I have not been able to find any documentation that would help decipher what is happening there. It must be fairly standardized though because all of the HRV manufacturers seem to use the same controllers. Option 2 appeals to me because it is more interesting for personal growth, it involves only low voltage signals, and it would probably allow me to replace the ugly wall controller all together. It would also allow adding more advanced controls like using humidity or occupancy sensors. So I'm looking for advice on whether this project is feasible for a noob like me. If so, any tips on at least figuring out the signal logic of the controller and how to replicate it with an Arduino. Thanks in advance for any advice, even if that advice is to stick to my day job. ------------------------- After more research, I think I'm starting to figure out the microcontroller/logic bit of this project and have more specific questions than my original rambling project description. It appears my HRV is using serial (probably TTL) logic, possibly current modulated since my poking around with a multimeter indicated changes in current at different controller states. It seems my first challenge is to decipher the HRV wall controller signal logic to know what I need to replicate with the microcontroller. What is the best/ least expensive way to do that? Can this be done with an inexpensive logic probe? There are only 5 possible states for the controller: Fan High, Damper Open Fan Low, Damper Open Fan High, Damper Closed Fan Low, Damper Closed Fan Off, Damper Closed Can I methodically record what is happening on each of the two signal wires in each state with a logic probe to map out the logic? Or am I missing something? I'm pretty comfortable with switching logic but the more sophisticated digital logic is all new to me. Also, initially I intend to add the thermostat based controller while leaving the original wall controller in place. This would add a second transmitter to the serial communication which I've read enough to know is very bad. Is there a good way to isolate two serial transmitters so they don't try to communicate with the receiver at the same time?