Can formaldehyde be stored at a higher temperature than room temperature?
Trimerization of formaldehyde can happen at high temperatures, but appear to require catalysts. There is a patent for synthesizing 1,3,5-trioxide at high temperature. This patent uses phosphoric acid catalysts.
According to the patent:


  The process of the invention is advantageously carried out at reaction temperatures of from 80 to 160° C., preferably from 90 to 145° C., at an inlet partial pressure of the formaldehyde of from 0.5 to 5 bar absolute, preferably from 0.5 to 2 bar, and at an inlet concentration of formaldehyde of from 1 to 100% by volume, preferably from 20 to 100% by volume


There is also an early study which explored the kinetics of the reaction but for a different catalyst. In "Vapour-phase trimerization of formaldehyde to trioxane catalysed by 1-vanado-11-molybdophosphoric acid" by Kern and Emig, they use a molybdophosphoric acid catalyst. They used a tube reactor and varied the temperature from 
102.5 to 110° C. According to their study:


  The 
  conversion 
  observed 
  increases 
  extremely 
  with 
  decreasing 
  temperature. 


However, they comment that 


  the 
  temperature 
  dependence 
  cannot 
  be 
  attributed 
  to 
  thermodynamic 
  effects 
  only. 
  It 
  has 
  to 
  be 
  related 
  to 
  the 
  properties 
  of 
  the 
  heteropoly 
  compound. 


Another study is "Synthesis of Trioxane Using Heteropolyacids as
Catalysts" by Mazamoto et al. Their process uses heteropolyacids as catalysts and they were able to obtain a conversion % of 22.9 at 110° C. Where 
$$\text{Conversion %}=\frac{3[\text{trioxane}]+2[\text{methyl formate}]+[\text{methylal}]}{[\text{feed formaldehyde}]}\cdot 100$$