Furnace Options- 1stage or 2 stage?


Old 02-25-18, 12:43 PM
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Furnace Options- 1stage or 2 stage?

Hi all; , I’m currently getting quotes and doing research on new HVAC to replace my current 20 year old set up and was hoping some of you who have been through this process could give me some feedback, especially given the seemingly dizzying amount of options to choose from!

For those of you who have replaced their furnace and AC, I was wondering if you have a 2-stage furnace and if you have had any issues with it? Some contractors say the 2-stage does a great job of evening out temp between 1st and 2nd floors whereas others recommend getting a single stage (95% efficiency) due to problems of not getting enough heat to supply ducts far from the furnace during the low stage.

I currently have an 135,000 BTU, 80% efficient single stage furnace and a 5-ton AC for an approximately 3000 sqft house (no zones). All the HVAC contractors I have talked to agreed the furnace is likely oversized (although it heats the house just fine). I am, however, getting conflicting advice on if the AC should remain 5-ton or 4-ton. Some recommend 4-ton and argue the smaller unit runs a bit longer but does a better job dehumidifying thus allowing the thermostat temp to go up a bit.

Accordingly, I and am debating between the following options:

Furnace (either Carrier, Heil, or Goodman)
100,000 BTU 2-stage (96%)
100,000 BTU 1-stage (96%)
120,000 BTU (same options above)

ACsame brands as above)
4 ton AC 1-stage
5-ton AC 1-stage

Furnace Questions:
Did you go with 1-stage or 2-stage? Do you like the 2-stage? Does it heat upstairs during low stage? Have you noticed energy savings? How is the sound? If you have similar sq footage, what BTU size furnace did you go with? Some tell me 2-stage furnaces are more prone to breakdown and other say this is untrue—what has your experience been?

AC Questions:
If you have comparable sqft, do you know the size of your AC and are you satisfied with it?

Brand Questions.
Most of what I have read say brand is not as important as a quality install by a reputable contractor. However, still curious if you have had any problems with a specific brand.

Thanks in advance!
Old 02-25-18, 01:09 PM
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Increased tonnage/BTU should not be considered an upgrade of any kind.
Equipment is sized via manual J, manual S and manual D. You should not purposely oversize because you have insufficient ductwork. It is unreasonable to expect a single unzoned unit to properly condition 2 floors within the same temperature.
There is also no best brand. The best unit is the one that’s properly sized and selected for its environment.
Further explanation-






A free manual J calculator-

HVAC Load Calculation - Maunualj - Whole House Loadcalc

The sizing, to be more specific, is done via-

Manual J-structure heat loss and gain
Manual S-equipment selection
Manual D-duct design

It should not be assumed the original equipment was sized correctly. It rarely is.
2 stage equipment is nothing new. And it’s not more prone to breakdown. We have progressed into fully modulating equipment these days.
Old 02-25-18, 01:31 PM
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Thanks; the current furnace actually keeps both first and second story pretty even through manipulating the dampeners.

I am only shopping for HVAC due to the increasing age of the equipment and desire for more efficiency.

The options I am looking at, at least for furnace, are all less BTUs then one I currently have. I am just wondering if 2 stage furnace would be able to supply enough heat upstairs and would like others experiences with this. Or does the 2 stage, due to its longer run time, keep temps pretty even in house?

Old 02-25-18, 01:38 PM
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2 stage is supposed to be more for occupant comfort. Now if it keeps temps even in the house is dependent on the duct it’s connected to.
Don’t close dampers. Especially if you go with a new system with a variable speed fan. They need to move the proper amount of air and are very expensive to fix when they overheat and fail due to inadequate airflow.
What size is the ductwork connected to the furnace?
Are there returns in every room?
100,000 BTU could still be too big for your home.
Old 02-25-18, 01:53 PM
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We have 6 inch round ducts, returns in every room (4 upstairs) as well as basement. Aside from those returns, I have 7 on main level of house. I don’t close dampers all the way, except 2 on main level where there are already supplies fully open. This pushes more air upstairs and I can keep both levels at 70 with little temp variation. Is this OK? Not doing so creates temp difference between floors, especially in summer.

Thanks for the replies!
Old 02-25-18, 02:01 PM
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Depends on how it affects the CFM and static pressure of the system.
How was the suggested size of 4 tons/100k btu determined? Was a 5 ton/120k btu system being offered as an upgrade?
Old 02-25-18, 02:07 PM
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I had 4 contractors come our for a quote on new equipment. Only an upgrade in the sense of efficiency and maybe comfort.

My current 130k btu is definitely oversized, according to them. No one offered a manual J test. These are tract homes and similar models so I am guessing they have serviced these many times before.
Old 02-25-18, 02:23 PM
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Id specifically request a load calculation. Or you could wind up with poor comfort, performance or energy use. Bigger equipment is not better
Old 02-26-18, 07:29 PM
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My experience

I have a trilevel house with a 2 stage gas furnace and a variable speed blower. The furnace is on the lower level of the two story section. The thermostat is on the middle level at the far end of the house. When the furnace is running in the low setting it does not push enough air to reach the theromostat. Result is: I have to keep a ceiling fan running in the bedroom on the second floor all winter to keep it bearable. Repeating what others have told you, duct work and the total system have to be designed properly to get the most from high efficency systems. In my case if I did another install, I would put a smaller unit in the two story section and a separate heat pump in the middle level.
Old 02-27-18, 04:07 PM
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KISS ( Keep it simple stupid)

Hello DVA79 read over your entry, you indicated ".your furnace heats the house just fine". Enough said.

The orginal furnace 135,000 btu @ 80% efficency outputs 108,000 btus. the 100,000 96% efficent unit would output 96,000 btus. A 120,00 btu unit @ 96% would output 115,200 btus.

The choice is yours to go on the low side or the high side.

I'm no expert but everything I've researched indicated a -2- stage furnace is for comfort only no measurable savings.

As indicated by the title line "Keep it simple stupid". The more complicated it is the more to go wrong. In addition the diagnosis involved can be subject to interpretation. Not to mention a -2- stage is going to be more expensive than a single stage unit for parts alone.

I can't address your AC situation, but I had my 20 year old AC unit replaced a -5- ton for 3100 sq ft house. I was told the ac unit was oversized and I need a 3 or 3.5 ton unit ( for better dehumdification, longer run times). A recommended tech said keep the -5- ton unit, I did and have no regrets.

Good luck and let us know how you make out.
Old 03-05-18, 03:57 PM
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Sizing based on what's there is a mistake.

It shouldn't be used to decide what to put in.

If the house is insulated decently, you may only need 80k at 95%+.

Furnaces need to move a certain amount of air to work properly and today's units need to move more air than the older ones for the same BTU output.

So a load calc needs to be done and the duct system needs to be evaluated to see if it can handle the airflow requirements of the new furnace.

Heating "just fine" doesn't mean working optimally. It takes a good 10 to 15 minutes for a furnace to hit steady state output, so grossly oversized units cost more to run. The constant short cycling shortens the life of motors, relays, igniters.

Having a short blast of hot air followed by an off cycle reduces comfort; People can turn up thermostats and do other things to mitigate this problem with poor results

If you've never had a properly sized furnace, you don't really know how these systems are supposed to run; they aren't supposed to have 5 minutes of on time per cycle.

The longer the cycles, the better.

A furnace sized to meet the maximum heating load is oversized at least 95% of the time, so the last thing you want to do it oversize.

2-stage, extends cycle length. The first stage output is 66 to 70% of second stage, so a correctly sized 2-stage runs a lot longer and starts running continuously at 2/3rd load; below that, high fire starts cycling on and off. (a 2-stage thermostat is required to do this properly)

The very long cycles of properly sized 2-stage furnaces reduces the temperature swing and keeps the exterior surfaces consistently warm, so you don't feel cold near a wall or window. (vents need to be in the right places to make this happen)

2-stage in certain cases can cause distribution issues; yes, the contractor has to make sure the specific model furnace being used is a good fit for your house and duct system. Long runs with lots of elbows, having the furnace on one side of the house with one very long trunk line may cause issues.

Some 2-stage furnaces are designed to supply air at a lower temperature on low, (fan speed is almost the same as high) so don't have this problem -> however, can make the supply air drafty on low.

Lennox/ducane/air-flo units come to mind. The spec sheets of the unit have to be checked to see what kind of airflow they move on low vs high fire.

It's also true that higher end furnaces cost more to repair; they aren't necessarily less reliable.

The 2-stage has a different circuit board, venter motor and gas valve. Most better furnaces have ecm blower motors now, which cost a lot to replace.

On the a/c side, the smaller unit would take longer to cool the house down initially but do a better job of dehumidification. The better the dehumidification is, the warmer you can keep it, so smaller units use less energy.

Just as with furnaces, it takes a while for an a/c to reach full capacity. So a unit that cycles on and off a lot doesn't use energy very efficiently.

A/C systems with undersized ducts and low airflow never run at rated capacity and efficiency. A 5 ton system on a duct system that's only good for 3 to 4 tons never supplies 5 tons of cooling.

So get the correct size; if you only need 3.5 to 4 tons, don't get 5.

Proper matching of the indoor coil to the outdoor unit, airflow, refrigerant charge are essential and oversizing is can cover up for deficiencies caused by the installer.

A contractor can say, "oh, I put a 3.5 ton in a house like yours based on a load calc and the customer complained, so you need 5 tons."

No! If the machine is set up right and the duct system doesn't leak like a sieve, it will work fine.

Bottom line, this is a science, no rules of thumb, no blanket statements ("ie 2-stage makes it cold upstairs) get a competent contractor who understands matching the equipment to the house and duct system.

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