This is a tricky situation, but you're on the right track thinking about return line temperature sensing. Here's a breakdown of potential solutions and considerations

Understanding the Problem
Conventional Thermostats

Not suitable because you're trying to indirectly control heat based on return line temperature rather than air temperature.

Goal

Maintain a minimum return line temperature to ensure the towel rack and tub pedestal provide sufficient heat.

Challenge

Finding a thermostat or controller designed for liquid temperature monitoring, adjustable setpoint, and on/off control of a hydronic zone valve.

Possible Solutions

Here's a prioritized list of options, from most likely to work well to more unconventional:1. Adjustable Liquid Immersion Thermostat with Relay Output



How it Works

These thermostats are designed to be submerged in a liquid (with a thermowell, like you have) and have an adjustable setpoint. When the liquid temperature drops below the setpoint, the thermostat triggers a relay. This relay can then open the hydronic zone valve.

Advantages

Specifically designed for liquid temperature control. Adjustable setpoint allows you to fine-tune the temperature. Relay output is perfect for controlling zone valves. Relatively simple to install.

Where to Find



HVAC Supply Houses

This is your best bet. Look for suppliers who specialize in hydronic heating equipment.

Industrial Control Suppliers

Companies like McMaster-Carr, Grainger, or online suppliers specializing in industrial automation. Search for "liquid immersion thermostat," "adjustable immersion thermostat," or "pipe thermostat."

Plumbing Supply Stores (Limited)

Some plumbing supply stores carry basic immersion thermostats, but they might not have the adjustable setpoint or relay output you need.

Example

Look for a thermostat with these features: Adjustable temperature range: (e.g., 80-180°F or similar) Capillary tube and bulb sensor for immersion. SPDT (Single Pole Double Throw) or SPST (Single Pole Single Throw) relay output. You'll likely use the "normally open" contact to energize your zone valve. Suitable for the voltage of your zone valve (usually 24VAC).

Wiring

You'll need to wire the relay contacts to switch the voltage to your zone valve actuator. Make sure you understand basic electrical wiring or hire a qualified electrician.2.

Digital Temperature Controller with Sensor and Relay Output



How it Works

This is a more sophisticated option. You use a digital temperature controller that accepts input from your 1-Wire sensor (or another compatible sensor like a PT100 RTD). You program the controller with a setpoint and a hysteresis (the difference between when it turns on and off). The controller then activates a relay when the temperature drops below the setpoint.

Advantages

More precise temperature control. Can display the temperature on a digital readout. Often has advanced features like alarms or data logging.

Disadvantages

More complex to program and set up. May require a separate power supply. More expensive than a simple immersion thermostat.

Where to Find



Amazon/Ebay

Auber Instruments or Inkbird are two brands to consider.

Industrial Control Suppliers

Companies like AutomationDirect, Omega Engineering, or similar.

Brewing Supply Stores

Temperature controllers are commonly used in brewing applications.

Example

Search for "digital temperature controller with relay output" or "PID temperature controller." Make sure the controller is compatible with your 1-Wire sensor (or be prepared to buy a different sensor).3.

Programmable Logic Controller (PLC) or Microcontroller (Arduino/Raspberry Pi)



How it Works

This is the most advanced (and likely overkill) option. You would use a PLC or microcontroller to read your 1-Wire sensor, compare it to a setpoint, and control a relay that operates the zone valve.

Advantages

Extremely flexible and customizable. Can integrate with other smart home systems.

Disadvantages

Requires significant programming knowledge. Most expensive and time-consuming option. Overkill for a simple on/off control application.

When to Consider

Only if you're already familiar with PLCs or microcontrollers and want to integrate this into a larger home automation system. It's generally not worth the effort for this single purpose.

Important Considerations and Recommendations



Zone Valve Voltage

Make sure the thermostat or controller's relay output is compatible with the voltage of your zone valve (usually 24VAC). You may need an interposing relay if the voltages don't match.

Electrical Safety

If you're not comfortable working with electricity, hire a qualified electrician to install the thermostat or controller. Incorrect wiring can be dangerous.

Setpoint Adjustment

Choose a thermostat or controller with an adjustable setpoint so you can fine-tune the return line temperature to achieve the desired heat output from the towel rack and tub pedestal.

Placement of Thermowell

The thermowell's location in the return line is important. It should be in a location where it accurately reflects the average temperature of the return water. Consider the flow rate and the potential for stratification (temperature layering) in the pipe.

Hysteresis (Differential)

If using a digital controller, set a small amount of hysteresis (e.g., 2-3°F). This prevents the zone valve from cycling on and off rapidly, which can damage the valve and wear out the actuator. An Immersion thermostat has hysteresis designed into the mechanical bimetal.

Monitoring

After installation, monitor the system carefully to ensure it's operating correctly and that the towel rack and tub pedestal are providing the desired heat. Adjust the setpoint as needed.

Documentation

Keep clear records of your wiring and settings for future reference.

Consider a Time Delay

Adding a time delay relay can also help prevent short cycling. This would delay the zone valve from opening for a short period (e.g., 5-10 minutes) after the temperature drops below the setpoint, giving the return line temperature a chance to stabilize.

Recommendation

Start with option #1: a good quality

adjustable liquid immersion thermostat with relay output. It's the most straightforward and cost-effective solution for your application. Go to a reputable HVAC supply house or industrial control supplier, explain your situation, and they should be able to recommend a suitable thermostat. Make sure it has an adjustable setpoint, a relay output compatible with your zone valve, and a probe length that works with your thermowell.Good luck! Flag for review