Heat Pump Cooling Mode Conflict

[kecarbaugh]

I have a 10 zone radiant floor hydronic heating/cooling system using a heat pump.  Most of the time will be spent in heating mode, but occasionally will operate in cooling mode.  While connecting the Nest Thermostat is not a problem when heating, the concern I have is when I switch the heat pump to cooling mode.  There is not a means of insuring all thermostats have switched over to cooling mode, therefore a risk exist of a zone still in heating mode will continuously call for 'heat' and will be provided cool water continuously.

I've come up with a setup using two relays per zone (first relay's COM is connected to Y1 and the NO connection feeds the COM of the second relay and the NC feeds the W connection of the zone controller.  This relay is activated during cooling mode. The second relay's NO connection will also feed the W connection of the zone controller and is activated by the OB signal.) I would post a diagram, but this post doesn't appear to provide an option to submit a diagram.

While the above setup will work, it takes 20 relays to function.  This is based upon the recommended connections to the Nest thermostat for heat pumps, and because it uses the OB to signal the heat pump to cool it complicates the setup compared to using separate signals for heat vs cooling modes.  I could eliminate half of the relays if I could just use the connections that a standard furnace would use with separate signals (Y & W) to call for heat and cooling. What I don't know is if I use the standard furnace connections is there logic with the Nest that would not operate the heat pump properly?

Any help would be appreciated!

[alejandom · Accepted solution]

Hi @kecarbaugh,

While the Nest Thermostat is designed for heat pumps with OB (Orange) signal for cooling, it can also be wired for systems with separate Y (Cool) and W (Heat) signals like a traditional furnace. This would eliminate the need for half the relays in your initial plan.

Here's what you need to consider:

• Nest Thermostat Compatibility: Check the compatibility checker on the Nest website to ensure your specific Nest thermostat model works with separate Y and W systems. https://store.google.com/intl/en/widget/compatibility/thermostat/
• Wiring Configuration: You'll need to follow the Nest wiring guide for separate Y and W systems. This will involve connecting the wires from your zone controller's Y terminal to the Y terminal on the Nest and the wires from your zone controller's W terminal to the W terminal on the Nest. Make sure to follow the instructions carefully and ensure your system is off before making any wiring changes.
• Nest Settings: Within the Nest app, there should be an option to select the type of heating and cooling system you have. Make sure to choose the option for "Y" for cooling and "W" for heating.

Safety Considerations:

• Zone Interlock System: Even with separate Y and W calls, it's highly recommended to install a zone interlock system. This prevents a zone from calling for heat while another zone is calling for cool, protecting your heat pump from malfunctioning. An HVAC professional can install this for you.
• Professional Consultation: While the wiring itself might be straightforward for someone comfortable with electrical work, consulting with a qualified HVAC technician is recommended, especially for complex hydronic systems and to ensure proper installation of the zone interlock system.

Conclusion

Using separate Y and W connections with the Nest thermostat can simplify your setup and potentially avoid needing extra relays. However, prioritize safety by installing a zone interlock system and consider consulting a professional for a comprehensive evaluation.

Regards,

Mario.

[kecarbaugh]

Mario,

Thank you for the info!  I have another question. Configuring the Nest Thermostat (Model GA02083-US) for a standard furnace using Y & W, is there a configuration that would also set the OB terminal based upon the thermostat current mode (Cooling or Heat) as well as the Y & W calls?  The reason why I ask is that I would like one of the ten thermostats in my system to determine if the heat pump is in heat mode or cooling mode, and I would use this signal to activate the valve in the heat pump as well as activate relays for each zone so that the Y or W calls would be routed properly to the zone controller.  Since Y & W are only active when calling for heat/cool I can't utilize those signals for this purpose.