A TXV bulb belongs on the suction line near the evaporator outlet in the Danfoss example below. That location lets it respond to the temperature of vapour leaving the coil. The valve itself sits in the liquid line ahead of the evaporator. The bulb and an external equalizer have different jobs; a diagram becomes easier to read when you trace each signal separately.
What does the bulb sense?
The bulb responds to suction-line temperature near the evaporator outlet. In the Danfoss fitting note, it is fastened tightly to a horizontal suction tube as close to the evaporator as practical. The bulb's temperature signal helps the valve regulate evaporator-outlet superheat; a loose bulb or one exposed to another heat source can send a misleading signal.
This is different from measuring the temperature of the liquid line at the valve body. Sketch the liquid path through the valve and evaporator, then mark the bulb on the outlet suction tube. See superheat versus subcooling for the two temperature differences.
Why is the bottom of the pipe a poor location?
For its illustrated horizontal line, Danfoss places the bulb between roughly one and four o'clock, with the precise position dependent on outside pipe diameter. Its note warns against the bottom of the line because oil can collect there and distort the sensed temperature. It also warns against mounting after a heat exchanger, at a heavy component or where external heat or cold affects the bulb.
A paper scenario that says “bulb strapped to the bottom of a horizontal suction line” should prompt you to ask what it is actually sensing. Do not turn one manufacturer's clock range into a universal installation rule. Use the valve and equipment manufacturer's instructions for the exact model.
Is the external equalizer another temperature sensor?
No. In the Danfoss arrangement, an external equalizer connects to the suction line immediately after the bulb. It communicates outlet pressure to the valve; the bulb communicates temperature. Keeping those signals distinct helps you reason about why a valve responds to conditions at the evaporator outlet rather than treating the equalizer as a second bulb.
Draw two arrows back to the TXV: one labelled bulb temperature, the other outlet pressure. Then explain how a wrong bulb location could mislead the valve even with a correctly connected equalizer. The actual response depends on the valve design and operating conditions.
How should a 313A candidate use this example?
Practise identifying the valve body, evaporator outlet, suction line, bulb and equalizer on a manufacturer diagram. Explain each part's job without assuming the Danfoss dimensions apply to every system. For equipment work, follow the actual service documentation and authorized isolation procedures; CCOHS explains lockout. This article is a study explanation, not a field installation procedure.
For exam scope, check the Ontario guide and current-standard note. Red Seal still identifies the previous standard as the current-exam basis on 2026-10-02. The free sample is original, unofficial draft practice pending certified-mechanic review and cannot predict a pass.
Sources
- Danfoss: Fitters notes, thermostatic expansion valvesPages 5-6, bulb/equalizer installation and placement limits; checked 2026-10-02
- CCOHS: Lockout/Tag outAuthorized hazardous-energy isolation; checked 2026-10-02
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent versus newer standard status; checked 2026-10-02
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.