When manufacturers produce products which are UL listed and tested they are tested with the manufacturers components and accessories as a system.  When you are in the field and start mixing brands which are not UL listed when mixed, you are looking for  problems.  A prime example is the mobile home furnace. These products are tested and listed as a package, the floor base, vent pipe and chimney.  A heating appliance is the last thing you would want to compromise the UL listing.
Some contractors in the field purchase a brand X furnace which is made to be installed into a mobile home and then purchase a Miller floor base because it is less expensive than brand X floor base.  The contractor has now installed a furnace which is no longer a UL listed system.  The  same goes with using the correct chimney products made for each brand. 
Don't let the price be the driving force in your business.  Are you really saving anything when you may be compromising your customer's family health and home?  You may want to ask your insurance company if they will stand behind you if you choose to mix brands and compromise the UL listing and something goes wrong.  We stock and can provide you with a complete furnace package: all items from the same vendor. Call us for your next mobile home furnace job.