Why do cyclic ethers have higher boiling points than their acyclic isomers?
The data I could find suggests that cyclic ethers have higher dipole moments than their acyclic counterparts. 

$$\mathbf{Four~carbons}$$
\begin{array}{c @{} c} \hline
\text{THF} &amp; \mathrm{1.63~ D ^1, 1.75~ D ^2} \\
\text{1-butene oxide} &amp; \mathrm{1.89~ D^2} \\
\text{ethyl vinyl ether} &amp; \mathrm{1.26~ D ^2} \\
\text{diethyl ether} &amp; \mathrm{1.15 ~D ^3, 1.15 D ^2} \\
\text{methyl propyl ether} &amp; \mathrm{1.11 ~D ^2} \\\hline
\end{array}

$$\mathbf{Five~carbons}$$
\begin{array}{c @{} c} \hline
\text{Tetrahydropyran} &amp; \mathrm{1.58~D ^2, 1.87~D ^4} \\
\text{2-Methyltetrahydrofuran} &amp; \mathrm{1.38~ D ^5} \\
\text{dipropyl ether} &amp; \mathrm{1.21~D^2, 1.00~D^6} \\\hline
\end{array}

This seems consistent with intuitive expectations based on conformational models where, in the cyclic ethers where the molecular geometry is constrained by the ring, the lone pairs are pointing in the opposite direction from the carbon skeleton. Whereas the lone pairs and carbon chains in the conformationally more mobile acyclic isomers are less geometrically fixed and therefore less "directed" in space. 

It seems likely that the higher dipole moments in the cyclic compounds would lead to greater alignment / ordering in the liquid phase, which in turn would lead to higher boiling points.

References:


  1: http://en.wikipedia.org/wiki/Tetrahydrofuran
  2: https://physicalchemistryrosamonte.wordpress.com/material-balances/material-balances-on-a-crystallizer/physical-properties-of-pure-methanol/dipole-moment/ 
  3: http://en.wikipedia.org/wiki/Diethyl_ether
  4: http://www.drugfuture.com/chemdata/tetrahydropyran.html
  5: http://www.stenutz.eu/chem/solv28.php?s=1&amp;p=20
  6: https://books.google.com/books?id=G6jaBwAAQBAJ&amp;pg=PA50&amp;lpg=PA50&amp;dq=propyl%20ethyl%20ether%20dipole%20moment&amp;source=bl&amp;ots=VA52gqJ0kn&amp;sig=GM8tua_QpXccFW3VXpy9qiNf7rk&amp;hl=en&amp;sa=X&amp;ei=rzRvVZ__Cc32yQT9gIPACQ&amp;ved=0CDsQ6AEwBQ#v=onepage&amp;q=propyl%20ethyl%20ether%20dipole%20moment&amp;f=false