We're adding on to our house in Atlanta and have decided to replace our 10-year-old package HVAC with a new system. Putting in a new system in the attic should get us a better fit, capacity-wise, and it'll be sealed up much better than the leaky ductwork in the crawlspace.
We had an independent consultant do load calc.s and a blower air door test. Based on reducing some of the measured infiltration by sealing the leaks, he came up with 31.5k Btu's sensible heat gain, 8.3k Btu's latent heat gain, and 54.4k Btu's heat loss.
Now it's time to coordinate with the HVAC sub, and that's where I can use some help. He typically installs Carrier, so although I've lurked and read the Carrier complaints, I've also read that the installer is the key to any mfr.s equipment. So we'll go with Carrier.
Our builder included a 4-ton system in his proposal (heat pump, I believe, though I don't have it handy) based strictly on rule of thumb. We'd like to use a standard AC and gas furnace, with both at higher efficiency (13 SEER minimum, 93% AFUE).
Here's what I haven't figured out from Carrier's site: to put this system together, we basically need a compressor, matched coil, and gas furnace, right? I'm assuming the furnace fan circulates for the AC also. Might be obvious, but this isn't my biz.
Next, it seems that upflow/downflow is prevalent in Carrier's better furnaces. Beyond that, I just get confused in the model numbers. But it appears that the Performance series is the way to go. I'm not clear on whether you can have an Infinity control w/o Infinity equipment. And we might zone sometime in the future, so that flexibility would be useful.
So, to sum up, I could use a check on some of my assumptions, model number choices to go with the load, and correction on anything I'm mistaken about. Also, talking to the HVAC sub this evening briefly, he made some comment about not wanting a higher-efficiency furnace in the attic because of condensation freezing up in the system. Can anyone shed some light on that comment?
thull in ATL
Heating with a heat pump is significantly cheaper in the Atlanta area than even the highest efficiency gas furnace- and that gap is widening lately with gas prices rising. Good heat pumps with good control systems don't really have the comfort drawbacks that old/cheap ones do. Dual fuel is popular around here, too (use a heat pump on the mild days and a gas furnace on the coldest days)- but it still costs more to install and operate dual fuel than an all-electric heat pump system. Atlanta winter electric rates are cheap, and are likely to stay that way. Gas rates have gone up by a factor of three since 1999, and there's no expectation that they aren't going to continue to skyrocket.
Your assumptions about how a gas-electric split system is set up are correct. In a heat pump system, you have an air handler that takes the place of the furnace; it's got the blower motor and controls, but no gas burners. It also has a big electric heating element that kicks in some extra heat to help the heat pump along on the coldest days, when the heat pump alone wouldn't be enough heat.
High efficiency (90% or higher) furnaces have to deal with water that condenses from the combustion process. Put one in an unheated space like an attic, and there is the risk that the condensation water in the drain pipe will freeze up and either cause an overflow or burst, leaving you with wet insulation and drywall and a furnace that's out of commission... all during subfreezing weather. If you are hell bent on gas, consider using dual fuel and an 80% efficient furnace. The furnace efficiency doesn't matter much with dual fuel, because the heat pump does the vast majority of the heating, and 80% furnaces don't have the problems with water. Because they are less efficienty, their flue gases are hotter, and the water vapor produced by the combustion process goes out the flue as steam. That eliminates the freeze danger in the attic, but keeps heating costs way down compared to using an 80% furnace as the only heat source.
You can have the Infinity control with 1) the Infinity 80% gas furnace, 2) the Infinity 90something% gas furnace, or 3) the Infinity air handler. With any of the three you can choose a wide variety of air conditioners or heat pumps to go outside; they don't have to be the Infinity two-speed outdoor unit. The Infinity system works really well, and Infinity zoning is a major improvement over traditional zoning systems.
If you want to zone it down the road, make sure the contractor knows about it. If you spend a little extra now, they can make the zoning an easy addition later. If not, a lot of ductwork may have to be tossed out and redone to do a proper retrofit of zoning.
[Edited by wyounger on 09-23-2005 at 10:22 AM]
My main reason for trying to keep the gas furnace is that we have a 150A electrical service (fuse box replaced and installed w/o permit by previous owner) that we'll be pretty much maxing out in terms of breakers and load with the new space and meeting code. I'm unclear on whether a heat pump will add load (existing package unit is on a 30A breaker).
You have some good questions and I thought the first reply did a very good job of answering them, so I wont repeat here but rather ad to a couple of points. I live in NJ so I dont know how cold it gets in atlanta but if there is a potential for freezing temps then I suggest you re-think putting that 90+ in the attic. If you insist on having it up there, just take some extra precautions to ensure you do not have a problem with freezing. For example, insulate the flue pipe and the drains very well, and ad heat tape on to the drains. Try not to run your flue pipe horizontally. Do not terminate your drain outside, try to find an indirect drain inside your house (ask the plumber to provide you one in the attic). An Ideal situation would be to put the furnace in a small room in the house and just run the ductwork in the attic.
You cannot have a Infinity control without infinity equipment just like the first reply states. If you are thinking about zoning then I strongly suggest using the infinity line. Or else you will be stuck using an aftermarket zoning system (not that its such a bad thing but carrier makes a very nice zoning system that works with the infinity line)
Models numbers are pretty easy for example:
1. The 100 is the input of BTU's
2. The 20 reflects the blower size, 20 is for 2000 CFM which is a 5 ton drive.
3. The 58 means its a furnace
4. The MVP reflects the model for example the MVP is 90+ Infinity, the MTA is 90+ Performance, and the MXA is 90+ Comfort series.
5. The F means it comes with a media filter cabinet
Proper selection and matching of equipment can get pretty complicated especially with carrier. I dont mean to sound disrespectful but It sounds like your second guessing your HVAC Contractor. I think you should have more faith in that person and let that person do their job, because like you said its not your "biz" If you do not think they are competent the perhaps you should hire someone else? If you dont understand something, I am sure your HVAC Contractor would be happy to explain it to you.
I would recommend a 80% Variable speed 2 stage gas furnace(if you insist on staying with gas) and run a Heat Pump as a dual fuel system... Best of both worlds..
I live in ohio, so we don't like 90% in attics due to the fact that the condensate traps can freeze.. But in your climate it may not be an issue..
Also make sure there is plenty of access to unit in the attic to service the unit safely.. I personally hate attic installs,, if you can make room in a closet or utility room in your home it would be best...IMHO..
OK, now there's a good reason to stay with gas backup.
The sweet spot, then, will be with the nice 80% gas furnace (58CVA "Infinity" series)- that gets you variable speed- and a heat pump. I'd encourage you to use the Infinity control. You don't have to use the Infinity outdoor unit unless you want to aim that high.
I live in the Chamblee area and have the 58CVA furnace, Infinity control, and the 38YXA heat pump (Performance 13). It's a great setup, and the operating cost has been outstanding in comparison to gas heat alone. It was installed by Estes last August.
Who is the HVAC sub, by the way?
Oops. I never answered your question about how going all-electric would affect the load on your 150 amp panel.
Yeah. Given your heat loss of 51k I would specify a 15 kilowatt electric heater for your backup. That would run on a 90 amp breaker, leaving you very little room to run an electric stove, clothes dryer, etc., before the main breaker popped. And that would be in addition to the 30A circuit for the heat pump itself.
So yes... you'll either have to stick with gas backup or upgrade your electrical service, which pretty much ruins the economic argument for using electric backup.
Thanks. That's the info I needed. Looks like the choices are either 93 AFUE furnace plus heat trace on the drain line (which sounds like it still makes some folks itchy) or heat pump (say Performance 13) plus gas furnace backup-Infinity 80 would eliminate condensation concerns and make Infinity control an option.
wyounger- the sub's company is Tru(e?) Comfort Heating and Air. My contractor has used him in the past.
Here's the next followup question. I gave the layout drawing to the sub last spring before we finalized pricing and signed the contract with the GC. It's basically a ductwork schematic showing sizes, register locations and airflow per register (supply and return) overlayed on a plan drawing of the house. This was all done based on the Manual J calc.s, but I hadn't given the sub the total loads.
I'd always figured that we'd put the quote for the equipment the GC would've supplied against one for the system I want, and we'd pay the price difference for the equipment. I realize that there might be a little more setup effort for the system I want. No argument there.
Well, the sub is now saying that there's something special about the system because of Manual J requirements (i.e. he wants more money). Yes, this makes me nervous that he apparently doesn't typically go by Manual J.
And my BS meter got pegged by the vague statement that the air pressure or something would be different.
Finally the question: given a duct layout showing AHU and compressor location, as well as airflow, are there other complications about the equipment that I'm missing? Like I said, I understand a difference in equipment cost and maybe some time for setup, but otherwise, am I right in thinking that this guy asking for more money (a lot, according to GC) is mostly voodoo?