Radiant heating and cooling systems offer superior comfort and energy efficiency, but their true benefits can be significantly enhanced by intelligent control strategies. Modern hydronic controls, when utilized properly, can manage heating and cooling efficiently, which leads us into a discussion about exergy.
Incorporating both ambient air and floor temperature sensors can improve system exergy, leading to more efficient energy utilization and reduced operational costs. As Robert Bean explains, “By minimizing exergy destruction through precise temperature management, these controls elevate radiant systems to a higher standard of sustainable climate control.”
The first law of thermodynamics focuses on energy-in versus energy-out. This can vary. For instance, a pure electric heating system has a 100 per cent efficiency compared to a gas boiler at 95 per cent.
Energy quantity is crucial, hydronic champions are increasingly focusing on the quality of energy or its exergy. Think of exergy like a rechargeable battery; energy is the total charge in the battery, while exergy is the battery’s useful work potential.
A fully charged battery has high exergy, capable of powering demanding devices, while a nearly depleted one has low exergy, making it only suitable for weak tasks. To make our battery last a lot longer, the tool needs to be as efficient as possible during each use. For example, if I am putting in a small screw, I don’t put the drill on its highest impact settings and burn through the battery. Using the drill efficiently with the right setting means the tool’s internal design efficiency minimizes exergy destruction as it converts the battery’s high-quality electrical energy into useful mechanical work, preventing unnecessary waste and maximizing battery life.
Using lower temperatures
In a hydronic heating system, we have more control over efficiency. Our smart radiant controls excel by operating in a low exergy manner. Meaning, radiant systems use lower temperatures than a forced air heating system. This minimizes the destruction of exergy, directly translating to more efficient heating and cooling, maximizing the useful output from every unit of energy consumed, and thus, often using less overall energy.
A dual-sensing thermostat, as opposed to an air-sensing only thermostat, monitors both the ambient room air temperature and the mean radiant surface temperature. A thermostat relying solely on air temperature is prone to calling for heat until the air setpoint is met.
Without floor temperature feedback, it risks overshoot, where the radiant slab overheats. This leads to uncomfortable floor temperatures and residual heat that can cause the room to overshoot the setpoint. To quickly overcome air temperature drops (e.g., door opening), the boiler or hydronic heat pump might be asked to provide unnecessarily high-water temperatures, increasing the temperature difference between source and space.
The floor sensor in a dual-sensing system allows an upper limit to be set, preventing damage and discomfort. This preserves exergy by avoiding the input of energy into a space that already has sufficient useful energy (e.g., stored heat in the slab). If the floor is already warm or sufficient heat has been stored, the thermostat can reduce supply temperatures or flow, maintaining a smaller temperature difference between the slab and the room, minimizing exergy destruction and improving overall energy efficiency.
Radiant cooling
Let me just state for the record that radiant cooling is a good idea, but don’t do it lightly! Radiant cooling presents the challenge of condensation if surface temperatures drop below the dew point. Air-only thermostats cannot detect this risk until it’s too late. Designers often compensate by setting higher cooling surface temperatures or using independent dehumidification.
Dual-sensing thermostats, with real-time floor temperature feedback and integration with humidity sensors, can calculate the local dew point. The control then adjusts the cooling water supply or flow to keep the floor just above the dew point. This enables higher cooling surface temperatures while still achieving comfort and avoiding condensation. The result is reduced temperature gradients and preservation of useful cooling potential, enhancing exergy efficiency.
The exergy benefits of dual-sensing thermostats translate into reduced energy consumption and improved second law efficiency, lower operating costs through efficient heat generation/distribution, extended equipment life due to moderate operation conditions, enhanced occupant comfort through precise control, and improved sustainability via reduced carbon footprint.
Why don’t people use dual-sensing thermostats?
The first reason is often a lack of awareness. At no time when I was on job sites as a lad growing up in the industry did floor sensors even enter the vocabulary, and no, that is not the fault of the installers.
Education is a top-down flow of information; it is our job to empower our installers with information, not for them to come to us with questions. Even when awareness exists, it’s sometimes a cost discussion. “But this thermostat is $100 more than the one I see at the hardware store!”
Hydronics is not a DIY project, and saving $100 on a thermostat can add thousands of dollars in cost to the operation of the system. More importantly, you will have the unintended consequence of making your customer very uncomfortable. The mean radiant temperature of a space is one of the leading contributors to occupant comfort. While mean radiant temperature is the technical term, the practical takeaway is clear — effectively controlling the radiant surface with a floor sensor significantly enhances comfort while also saving money for the building owners.
The last most common reason was a technology gap. Air-sensing thermostats have had Wi-Fi and remote access for a while, and most homeowners investing in hydronics want that. Good news: that excuse is no longer relevant. You can get a Wi-Fi enabled hydronic thermostat anywhere now.
Dual-sensing thermostats are crucial for optimizing the thermodynamic performance of hydronic radiant systems. By precisely controlling both ambient and surface temperatures, they minimize exergy destruction and elevate the system’s overall efficiency, comfort, and sustainability. As demand for efficient climate control grows, the intelligent control offered by dual-sensing thermostats will become an essential standard.