Figure 3-3 shows that the heating element of the EWH is turned ON and OFF in order to keep TH (t) within a tolerance band (+/- Δ) of Td. When the heating element is ON, TH(t) rises until it reaches (Td+Δ), which is equal to 122.5°F(50.28°C). When the heating element is turned OFF, TH(t) decreases until it reaches Td-Δ, which is equal to 117.5°F (47.5°C). The heating element then is turned ON again.
Figure