library(deSolve)
yini <- c(temp = 18, heating_on = 1)

temp <- function(t,y, parms) {
  dy1 <- ifelse(y[2] == 1, 1.0, -0.5)
  dy2 <- 0
  list(c(dy1, dy2))
}

rootfunc <- function(t, y, parms) {
  yroot <- c(y[1] - 18, y[1] - 20)
  return(yroot)
}

eventfunc <- function(t, y, parms) {
  yroot <- c(y[1] - 18, y[1] - 20)
  whichroot <- which(abs(yroot) < 1e-6) # specify tolerance
  y[2] <- if(whichroot == 2) 0 else 1
  return(y)
}

times <- seq(from=0, to=20,by=0.1)
out <- lsode(times=times, y=yini, func = temp, parms = NULL, 
             rootfun = rootfunc, events = list(func=eventfunc, root = TRUE))
head(out, n=200)