You have 4 HVAC units one of which is 5 tons? That is a lot of heating and cooling. How large is your house?

The attic is the worst place to install HVAC equipment. It is a shame that all the equipment was not installed in the basement. Builders like attic installations because it is the easiest and cheapest way to run ducts to the second floor.

I think the equipment is rusting because of a moisture problem in the attic. Is the attic well ventilated? Do you have any black mold on the roof deck? Buy a inexpensive hygrometer and measure the humidity in the attic. If it is high then you may need to add additional ventilation.

Building a conditioned room for the equipment is not a bad idea. You will need to have an fresh air intake for the gas furnaces. This needs to be carefully otherwise you could cause carbon monoxide problems.

I suggest addressing the moisture problem first. Once that is under control then you can decided whether you want to build a insulated room in the attic.

If most of the attic is finished, why not seal the entire remaining attic area to bring all of your ducts into conditioned space?

Any savings realized from extended equipment life will imo be more than offset by increased energy consumption.
Venting the gas burner is a sticky problem because albeit small, fluxulating air pressure in the area. If you decide to do this,for your family's health and safty have an engineer desighn all the supply,returm and vent.

"Any savings realized from extended equipment life will imo be more than offset by increased energy consumption. "

It depends on how leaky the duct system is and how leaky the partition between the attic and living space including the knee walls if you have them. In hot, humid climates, there is often a net every savings from sealing the attic and insulating the roof deck. The OP has to figure out if this approach is recommended in their area. A consult with an energy rater might be a good idea. Check out www.resnet.us. They should know the best things to do for local conditions and the possibility for rebates from government other sources.

"Venting the gas burner is a sticky problem because albeit small, fluxulating air pressure in the area. If you decide to do this,for your family's health and safty have an engineer desighn all the supply,returm and vent."

No engineer is required. Building code covers the use of an interlocked blower for combustion air supply. If the furnace is going to be enclosed and insulated, this will be needed even if the whole attic is not enclosed.

This post was edited by ionized on Fri, Sep 12, 14 at 9:38

Are all of us on the same page? As I understand it the OP is talking about constructing an air tight and insulated mechanical room around 1 heat pump and 1 gas furnace 5 ton cooling coil combo. Said room is to be heated and cooled by one or both units. The goal is to gain energy effeciency, reduce rust on the 5 ton unit and prolong it's life. I believe it's a novel approach with several oppertunities in making wrong turns. When I mentioned seeking the services of a ventilation engineer it was in lieu of taking a harder stance and reccomending Op forget the whole idea. I can't give a link that addresses this situation but my collective knowledge of hvac lead's me to believe it will not give desired results. Finding myself alone in this opinion deserves rethinking if there are rebates to be had. If this qulify's for rebate,point me to it so I might join the informed and lay aside my old school reasoning. A strong case could also be made here if reccomendations for keeping equipment in conditioned space during new construction could be sited. Evidence of gas heat being common pratice for close quarters like computor rooms would cause me to rethink. What concurn's me is the sheer size of a 5 ton blower in such small space with supply and return air shared with living space. I can see where induced combustion helps but it is still akin to balancing a nail on it's piont.
I share the opinion that keeping ducts within conditioned space is a good idea. What I can't see is construction in the attic for the purpose of bringing ducts into conditioned space. Inclosing equipment in an existing attic then conditioning the space sounds unpresidented.

Klem1 - you are spot on with what I'm attempting to do. My basement a 2 furnaces are in an unfinished part of the basement and is surrounded by finished areas. They are be vented outside and the room they are in has that air intake vent described in my earlier post. You are correct that I'm only thinking about that HVAC units in the attic - not the duct work. I appreciate your concern and if I was going to do the project I would most certainly hire an expert to make sure it wasn't dangerous. I think the cost to do this project would be minimal. I don't think it would eat up much energy because my finished attic is only about 800 sq ft, is one big room and a bathroom but has it's own heat pump.

In hot, humid climate, sealing and insulating the attic is an advantage in old homes that have ducts retrofitted into the attic space. This is especially true if the barrier between the living space and the attic is not good to start with. Whether it is a good idea for the OP is not a sure thing. It depends on the local climate and the details of the home.