Need help with Nest-HRV integration

bbro62 · 01-18-16, 12:05 PM

Can anyone point me to information about how to replicate the signal logic of an HRV wall controller with a microcontroller?

I have a Carrier HRV which works well but I've never liked the controls. The unit is designed to run constantly at low speed and kick into high speed when 20 minute wall timers in the bathrooms and laundry are activated, or the humidity rises above the humidistat setting on the wall controller. Obviously, running the unit constantly wastes a lot of heat and since optimal indoor humidity depends on the outside temperature, the crude and inaccurate dial humidisat is next to worthless. For several years I've compensated for part of the problem by building a homemade timer circuit to activate the 20 minute timer function at set intervals to keep the indoor air fresh without running the HRV constantly.

I've recently installed Nest thermostats which offer better solutions to both problems. The Nests include a fan function that allows a fan to run at user defined duration at hourly intervals. The nest also has a dehumidifier function that can control a fan using the built-in humidistat. I have a raspberry pi configured to monitor outside temperature and adjust the Nest humidity setting accordingly. The pi also will let me control the fan based on other scenarios I might conceive in the future.

The Nest can provide the added features I desire but I'm having trouble figuring out the actual control. I've looked at using simple relays but that messes with defrost and other functions provided by the wall controller. So I'm trying to figure out the signal logic used by the wall controller to control fan speed and damper opening. I can muddle through standard switch logic pretty well but microcontrollers and serial communication signal logic is all new to me. Does anyone know where I might find information about communication protocols for these devices or how I would go about analyzing the outputs from the wall controller to figure out the signal logic?

01-18-16, 12:05 PM	#1
bbro62 Lurking Renovator Join Date: Jan 2016 Location: N/A Posts: 10 Thanks: 2 Thanked 0 Times in 0 Posts	Need help with Nest-HRV integration Can anyone point me to information about how to replicate the signal logic of an HRV wall controller with a microcontroller? I have a Carrier HRV which works well but I've never liked the controls. The unit is designed to run constantly at low speed and kick into high speed when 20 minute wall timers in the bathrooms and laundry are activated, or the humidity rises above the humidistat setting on the wall controller. Obviously, running the unit constantly wastes a lot of heat and since optimal indoor humidity depends on the outside temperature, the crude and inaccurate dial humidisat is next to worthless. For several years I've compensated for part of the problem by building a homemade timer circuit to activate the 20 minute timer function at set intervals to keep the indoor air fresh without running the HRV constantly. I've recently installed Nest thermostats which offer better solutions to both problems. The Nests include a fan function that allows a fan to run at user defined duration at hourly intervals. The nest also has a dehumidifier function that can control a fan using the built-in humidistat. I have a raspberry pi configured to monitor outside temperature and adjust the Nest humidity setting accordingly. The pi also will let me control the fan based on other scenarios I might conceive in the future. The Nest can provide the added features I desire but I'm having trouble figuring out the actual control. I've looked at using simple relays but that messes with defrost and other functions provided by the wall controller. So I'm trying to figure out the signal logic used by the wall controller to control fan speed and damper opening. I can muddle through standard switch logic pretty well but microcontrollers and serial communication signal logic is all new to me. Does anyone know where I might find information about communication protocols for these devices or how I would go about analyzing the outputs from the wall controller to figure out the signal logic?