HVAC School - For Techs, By Techs
Superheat in Heat Mode: Why Heat Pumps Get Weird - Short #274
· Summarized
Key takeaways
- Superheat is essential for the effective operation of heat pumps, particularly in heat mode.
- Cold temperatures contain heat, allowing heat pumps to extract energy even in frigid conditions.
- Measuring superheat in heating mode is challenging due to the variability of outdoor temperatures and environmental factors.
- Different metering devices affect superheat stability, with TXVs and accumulators playing critical roles in preventing compressor damage.
- Operational factors, such as defrost cycles, must be understood to accurately assess superheat and system performance.
Summary
Understanding Superheat in Heat Pumps
Superheat is crucial in determining the performance of heat pumps, especially in heat mode. The podcast tackles a common query regarding superheat values in cold environments. Bryan explains that cold temperatures still contain heat, dispelling the myth that cold equates to the absence of heat. He emphasizes that even at low temperatures, heat pumps can extract heat from the outside air, which can be measured through superheat between the outdoor coil and reversing valve inlet. However, measuring superheat in heating mode poses unique challenges due to varying environmental conditions.
Instability and Challenges of Measuring Superheat
In heating mode, superheat values are influenced by fluctuating outdoor temperatures, creating instability. Bryan discusses that superheat is easier to measure in cooling mode because systems are optimized for stable conditions. In heat mode, the outdoor coil operates as the evaporator, leading to variable pressures and temperatures. Furthermore, frost buildup and weather changes complicate measurements, often causing considerable swings in superheat readings.
Metering Devices and Their Impact on Superheat
Bryan describes the differences between metering devices found in heat pumps, such as fixed-orifice pistons and TXVs (Thermal Expansion Valves). He discusses how these devices control refrigerant flow and affect superheat stability. While TXVs aim to maintain superheat, they encounter challenges due to environmental changes, which can lead to insufficient or excessive superheat that can harm the compressor. Accumulators are also highlighted as crucial components to prevent floodback in systems.
Operational Considerations for Heat Pumps in Cold Weather
The podcast dives into operational scenarios like defrost cycles that introduce additional complications to measuring and interpreting superheat. Bryan explains that during defrost, the system reverses and the expected pressures and temperatures change, rendering simple superheat calculations ineffective. He encourages listeners to rely on their experience and the overall system's behavior rather than just numbers to assess performance.
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Show notes
In this short podcast, Bryan answers a listener-submitted question and explains why heat pumps get a bit weird: when it comes to superheat in heat mode. He also explains how we can move heat from outdoors to indoors, even in temperatures below freezing (cold temperatures just have less heat, not zero heat).
In order to move heat in very cold conditions, we need very low suction pressures and cold coils, which gives us a high compression ratio. Superheat will be affected by these conditions. Remember: superheat that is too low can cause floodback, and superheat that is too high can cause the compressor to overheat.
Superheat is easy to check in cooling mode, but it's harder in heating mode, especially since the suction line is at the outdoor unit. In heating mode, we can only measure superheat between the coil outlet and the reversing valve inlet, which is a very short run of tubing.
The superheat will also be less stable in heat mode in cold weather, and it will have a wider range of "normal" values than cooling mode, depending on the conditions. Superheat could even drop to zero with some fixed-orifice metering devices (which would cause floodback). That's why many of these heat pumps have accumulators, which collect liquid refrigerant to protect the compressor. Even though TXVs attempt to maintain superheat, you may still see some variation in heat mode during cold weather. EEVs are common in ductless systems and are highly controllable but maintain relatively low superheats by design; fast metering control, intelligent logic, accumulators, and low refrigerant charges allow them to avoid floodback in low temperatures.
In any case, spikes and drops in temperature can cause the superheat to jump or collapse because the load changes (as the outdoor coil is the evaporator). Frost buildup on the coil also inhibits airflow and heat transfer, causing the superheat to change as the evaporator pressure and temperature drop. Defrost also introduces chaos to the equation. All of these should influence your judgment when checking superheat to diagnose or commission a system in heat mode.
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