I'd like to start by thanking everyone up front for their time in helping me sort through some conflicting information.
We live in Georgia in a 30 year old two-story house built on a slab (no basement). Each floor has its own HVAC system and both systems are original equipment installed, along with the hot water heater, in the attic. Heat is provided by natural gas furnaces. We've owned the home for around eighteen months now and have come to the conclusion that we need to replace both units. We had some trouble with the AC systems and had both serviced in the spring, which included some refrigerant along with a new compressor fan motor. Now that the weather is turning cold we're also having trouble with the furnaces in that their pilot lights will not stay lit. I have to crawl up in the attic and maneuver around ducts to light the pilots almost daily. It's no fun when the pilot light goes out in the middle of a cold night, especially since we have an 11 month old son. We simply cannot allow the house to get too cold.
One other important piece of information is that I'd really like to have the attic spray foamed and sealed with insulation and a fire retardant. I've read of the many benefits to this and am sold on the notion. The concern I have regarding the question of replacing our systems is whether it's a sound idea to put a gas furnace in air-tight sealed attic? Any possible gas leak could be deadly, be in natural gas or carbon monoxide.
With all this in mind we called out two highly rated (on Angie's List) HVAC contractors to give us competing quotes. This is where the conundrum began. Both stated that it was perfectly safe and in many ways beneficial to install a gas furnace in a sealed attic v.s. an open attic. However, I recieved conflicting information on the proper type of furnace to use and this is where I need some serious help.
The first contractor stated that we need to install 80% efficiency furnaces that, unlike the 90%+ efficiency furnaces, do not chill the flu gas below the dew point and therefore do not create condensation that will need to be drained off via a water drain line. According to this contractor, this is Carrier's own guidlines - 90%+ efficiency furnaces are for basement installation only. The problem is that this condensation drain line can freeze, back up, and trip a sensor shutting the furance off in the middle of a cold night. The quote from this contractor includes Carrier Performance AC systems and 80% efficiency Carrier natural gas furnaces, all controlled by Honeywell thermostats.
The second contractor contradicted the first by stating exactly the opposite. According to the second gentleman, we really should go with 90%+ efficiency furnaces in a sealed attic because they, unlike the 80% furnaces, have a snorkel that will be plumbed through the roof allowing them to breathe in fresh air from the outside. The 80% furnaces do not have the snorkel and instead breathe in air from the attic space, which in a sealed attic will result in negative air pressure and dramatically lower the furnace's efficiency. He did not even mention the issue with having to install a condensation drain line. He also recommended Carrier thermostats as they will properly talk to the Carrier equipment and control everything more efficiently. His quote includes Carrier Comfort AC systems and 90% efficiency Carrier furnaces, all controlled by Carrier Comfort thermostats.
In calling back the first contractor to bring up the issue of the 80% efficiency furnace having trouble breathing efficiently in a sealed attic, his answer is that they will cut a vent between the attic and the upstairs living space so that the furnace in the attic can draw in air from the upstairs living space. This response almost seems to compound my reservation about having the natural gas furnaces in a sealed attic.
So, two questions:
1) Are there definitive guidelines on the type and use of gas furnaces in a sealed space? I'm aware most basements, even unfinished ones, are sealed. But is the condensation drain line issue a true concern for attic installations? I've looked and cannot find much information from Carrier available to the general public.
2) Is it important, efficiencywise, to use Carrier thermostats with Carrier equipment? From what I can gather this might be an issue with their higher end equipment such as the Greenspeed or Infinity multi-speed equipment, but perhaps not so much with the single-speed Comfort or Performance series?
Again, I apologize for the length of this post and thank you for your time!