Usually, subject to your local code requirements. All of our furnaces are compatible for horizontal installations and are approved by the manufacturers for use in attics. 

When installing a high efficiency furnace be certain that the ambient air in the attic does not go below freezing as this may cause the condensate drain to freeze and create a blockage. 

There must at least enough heat in the attic to maintain the temperature above freezing for condensing-type, high efficiency furnaces. (90+ efficiency). This limitation does not exist for standard efficiency (80%) furnaces.