How to predict single values in R

You’re using the incorrect X variable name in your call to predict (it should be called the same as the X variable you used when you created the initial model)

Although not necessary, its typical to have meanValuesHeatingPower and meanValuesOutsideTemperature as columns in the same dataframe, and then pass that dataframe in to data when creating the model

The following works for me:

df <- data.frame(meanValuesHeatingPower = c(1,4,3,7), 
                 meanValuesOutsideTemperature = c(4,3,4,7))

linearModel<-lm(meanValuesHeatingPower ~ meanValuesOutsideTemperature, data = df)

pred<-predict(linearModel, data.frame(meanValuesOutsideTemperature = c(1)))
print(pred)