We have studied the phase behavior at normal pressure of NH4HSO4 (AHS) over the 100-420 K temperature region using ac calorimetry on single crystal samples. From the changes in the ac-calorimetric curves recorded on successive thermal cycles of the samples, it is inferred that metastable phases can be induced in the previously reported region of its intermediate ferroelectric phase (154 K < T < 220 K) at 1 atm. An additional thermal anomaly was found to occur at 220 K that seems to indicate the coexistence of the low and intermediate phase in the 154 K < T < 220 K temperature range. No thermal anomalies were observed in the paraelectric region 270 K < T < 420 K, where it has been claimed that some proton transport occurs in AHS.