Hi everyone. Last month I upgraded my old Honeywell mercury two-wire thermostats, used for heat only, to Nest Learning Thermostats. So far, I haven’t seen any issues despite not having a C-wire connected. The thermostats power up fine, the display works when I walk by, and I haven’t had any Wi-Fi disconnects or error codes.
I’ve been monitoring the technical details, and the battery voltage usually stays around 3.83V, while the Voc, Vin, and Iin readings are all nominal. The charging current is a steady 20mA, whether the system is running or not.
I’ve read that thermostats without a C-wire might start pulsing the system to get power, which could eventually cause problems or damage the heating system. It’s been over a month, and I haven’t had any issues or heard strange noises from my heating system. The thermostats worked for three weeks before I started using heat, and for the past two weeks, they’ve been actively calling for heat with no issues.
My question is: has anyone else had a good experience like this initially, only to face problems later? Should I go ahead and install Nest Power Connectors in my zone controller to add a C-wire, or just leave everything as it is since it’s working fine now?
I used mine without a C-wire for a couple of years. It worked fine at first, but last winter, the house went cold. The thermostat battery couldn’t keep up anymore, so I’m now using my old Honeywell until I can add a C-wire.
@Astrid
Thanks for sharing your experience. I want to avoid that situation. I just ordered power connectors and plan to install them this weekend. Did you ever consider trying a power connector instead of a full C-wire install?
@TracyJones
I looked into it, but reviews made me think it wouldn’t work well with my system. I’ve considered using an AC to C-wire adapter since there’s a wall outlet nearby, but I’m concerned about fire hazards with low-quality adapters. I’m leaning towards hiring an electrician to add a proper C-wire.
I had similar issues. After a few months, my Nest started losing Wi-Fi, the screen wouldn’t turn on, and I got low battery errors. My AC also stopped cooling properly because the thermostat was pulling power from the system. Adding a C-wire fixed everything, and now it works perfectly.
@Derek
Thanks for the insight. I decided to add C-wires to all my thermostats to avoid potential issues. I’d rather not risk waking up to a freezing house in the middle of winter. Everything’s working flawlessly now, and I hope it stays that way.
I’ve been running three Nest thermostats without C-wires for years. One Learning model for AC and heat since 2018 and two E models for heat only since 2019. No issues at all.
Rodolfo said:
I’ve been running three Nest thermostats without C-wires for years. One Learning model for AC and heat since 2018 and two E models for heat only since 2019. No issues at all.
That’s great to hear! Do you mind sharing your battery voltages? I’m curious how they’ve held up over the years.
I had issues after a couple of years. Symptoms included short cycling, Wi-Fi disconnects, and auto-sense not working. I eventually used an unused thermostat wire to connect a C-wire to my system. It’s been working fine ever since.
@Dwayne
Thanks for sharing. I had a similar issue with my AC and used an extra wire to add a C-wire. For my heating system, I’m planning to use the Nest Power Connectors.
I’ve been using a Gen 3 Nest for heat only in Michigan for four winters now. I don’t have a C-wire but avoid issues by fully charging the thermostat with a USB cable before winter starts. It lasts the whole season without problems.
@AlarmAuthority4
Thanks for the response. I’m concerned about both those points, especially during winter. I’ve decided to install power connectors to avoid potential problems.
TracyJones said: @AlarmAuthority4
Thanks for the response. I’m concerned about both those points, especially during winter. I’ve decided to install power connectors to avoid potential problems.
That sounds like a good plan. The power connectors should provide consistent voltage and save you from issues later.
Barbara said:
How did you set up the thermostat without a C-wire? My heat-only system doesn’t allow me to skip the C-wire during setup.
I have the Learning thermostat, and it let me set it up with just the red and white wires. When I added a C-wire, I reset the thermostat and set it up again. The app detected the C-wire automatically.