HVAC School - For Techs, By Techs
Non-bleed TXVs and Hard Starts - Short #257
· Summarized
Key takeaways
- Non-bleed TXVs prevent refrigerant migration during off cycles, which is critical for system efficiency.
- Hard start kits may be necessary for certain compressors, but they should be chosen based on OEM recommendations.
- Quality matters in components; prefer OEM or high-quality aftermarket parts for reliability.
- Avoid using hard start kits to compensate for voltage drops or existing issues in the system.
Summary
Understanding Non-Bleed TXVs
In this episode, Bryan dives into the mechanics and advantages of non-bleed TXVs, also known as hard shut off TXVs. He explains how these devices work to prevent refrigerant migration during off cycles by shutting off to maintain zero superheat. This functionality is especially crucial in air conditioning systems with longer line sets, as it helps keep liquid refrigerant out of the evaporator, suction line, and compressor crankcase.
The Role of Hard Starts
The necessity of hard start kits in relation to hard shut off TXVs is discussed, particularly for different types of compressors. Bryan advises on the scenarios in which a hard start kit may be beneficial or needed, emphasizing that the OEM's recommendations should generally be followed. It’s highlighted that while some compressors may struggle to start under the conditions created by the non-bleed TXV, proper preparation and understanding the system requirements can alleviate potential issues.
Choosing the Right Hard Start Kit
This section focuses on the practicalities of selecting hard start kits, especially when dealing with older or struggling compressors. Bryan shares his preference for high-quality components and the importance of avoiding aftermarket kits unless absolutely necessary. He provides insights on how to assess the need for hard start kits based on compressor type and system conditions, cautioning against their use in inverter-driven systems.
Avoiding Common Pitfalls
Bryan wraps up by discussing common pitfalls when dealing with hard shut off TXVs and hard start kits. Key cautionary points include ensuring that there is no existing issue with voltage drop that could affect compressor performance, and stressing that hard start kits should not be used as a workaround for inadequate system specifications. The importance of thorough diagnostics and adherence to manufacturer recommendations is reiterated.
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Show notes
In this short podcast episode, Bryan talks about non-bleed TXVs and whether they need hard starts.
A non-bleed (hard shutoff) TXV is a metering device that shuts off during the off cycle; it closes shut when there is no superheat. This strategy prevents refrigerant migration, especially in cases where we have long line sets. Liquid refrigerant stays in the condenser and is kept out of the evaporator, suction line, and compressor crankcase.
When we have a non-bleed TXV, the compressor may have a hard time starting under the conditions created by the hard shutoff TXV (high head pressure, low suction pressure). Reciprocating compressors are more likely to have this problem than a scroll or rotary compressor. If you have a short line set, 230+ voltage, and a scroll compressor, you probably won't need a hard start kit. You may need a hard start kit if you have a reciprocating compressor, a longer line set, or 208V power.
The OEM will recommend a hard start kit that is suitable for the equipment. Aftermarket hard start kits are okay if a compressor is old and struggling, but they're not the preferred option and should be as close to the OEM's recommendation as possible. Hard start kits typically consist of a start capacitor and a potential relay. Positive-temperature coefficient resistors (PTCRs) also exist, and these have a resistor that heats up and eventually takes them out of the circuit. Hard start kits should not be added to inverter-driven equipment and are not meant to compensate for voltage drop issues; they can cause more harm than good if they're used when they're not necessary or not used correctly.
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