Being in the hvac business,any furnace builder that does not include a filter is asking for trouble,,or as in this case imo parts sales.Adding a filter is a must.I know of not one rv furnace manufacturer that includes a filter but to say a filter will adversely affect the furnace is crazy.That is one of the first things we added, built a small filter rack and installed it behind the return grill on the fiver,only use the cheap black window a/c filters but that is better than nothing.A plugged filter can cause major adverse affects on the furnace so it must be attented to,also the a/c filter.But no filter will allow airborne particles to enter the motor cooling rings,the blower,and the heat exchanger thereby allowing extra heat to build up, less airflow and thus,new parts are needed.I would rather clean a filter once in awhile and protect the unit than change furnace parts after a few years.This is by no means a knock on keystone as like I said before, no rv furnace builder in any rv we have owned,atwood and surburban;has included a filtering system.Real simple job to do yourself and gives me peace of mind,but just my 2 cents worth.
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