I Need New Furnace--Carrier, Lennox, or Trane?

[Mark1]

I love a good materials question;
Duplex stainless steels[1][2][3][4][5] are a family of stainless steels. These are called duplex (or austenitic-ferritic) grades because their metallurgical structure consists of two phases, austenite (face-centered cubic lattice) and ferrite (body centered cubic lattice) in roughly equal proportions. They are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as Type 304 or 316.

A 2205 SS is one type, or even a lower grade 2102 duplex is best for combustion gas corrosion. These type of alloy stainless is found in quality HX for furnace application. If you ask the installing tech, he will likely not know the difference. This is primarily needed in the secondary HX that sees the condensing gases. The hotter primary HX can even be aluminized steel, since it does not have the same type of corrosive exposure due to temperatures that maintain H2O in vapor, but still an issue.

Nice summary Happyras, although, I'm a little surprised with the use of duplex within a furnace(?). Typically, duplex is never used at higher temperatures. In fact many engineering codes don't allow for it use above ~600 Deg F. due to embrittlement concerns. I guess the service temperatures most be below 600 Deg F, and the far superior resistance to Cl-SCC must be driving the material selection.

An example on SS materials;
Weber makes Gas Grills and uses 316 SS, mine lasted 15 years until it was damaged by something falling on it. It did not rust out. Chinese grills (you know the cheaper ones) are made from low cost low alloy stainless, at best 304 ss, mostly worse. They rust out in about 3 to 5 years with use. Gas has corrosive biproducts. YGWYPF.....

In terms of stainless steels, my rule of thumb is always default to the use 316 for services where pitting and crevice corrosion are a concern at lower temperatures, due to the presence of Mo within the steel. However, at higher temperatures, I would default to the use of 304 SS over 316 SS due to its superior resistance to high temperature oxidation from the increased alloying of chromium. For my grills and fire-pits I actual prefer 304 SS, although I will admit that mine would be more susceptible to attack from bird droppings, etc., so there are always trade-offs in life.

 

[Happyras]

Nice summary Happyras, although, I'm a little surprised with the use of duplex within a furnace(?). Typically, duplex is never used at higher temperatures. In fact many engineering codes don't allow for it use above ~600 Deg F. due to embrittlement concerns. I guess the service temperatures most be below 600 Deg F, and the far superior resistance to Cl-SCC must be driving the material selection.

Of course you are correct on use at higher temps, that is why the primary HX is typically a coated mild steel seeing the hottest exhaust point, and the secondary is some SS alloy due to condensate and lower temps of the exiting exhaust. If you look at higher grade exhaust duct for on-demand tankless gas Hot Water systems, the material in the liner is typically LD 2101 or equiv duplex SS.

I used 316 in many large industrial evaporator systems and crystallizers for suction ducting to compressors, depending on chemistry and temps. The only application I saw with similar chemistry and use of 304 ss, was in Italy and I was a little surprised it had lasted as long as it did. Good catch on the high Mo properties. We primarily ran low pressures on the steam side which correlate to below 600f. where i found a sweet spot for duplex.

Now back to the Furnace issues, Trane is great, lots of hype and ads and typically higher end trained techs resulting in good installs. When it comes to coils in higher pressure systems, it boils down to quality control at the factory. Jury is out on coil design efficiency, fins versus spiralized tubes ect.

 

[Dean56]

I went with Lennox last time, about 4 years ago.

I also had a Lennox HVAC (with natural gas furnace) installed 4 years ago. It has the variable speed blower and I love it. No issues at all.

When I was looking to buy a new unit, I was told to stay away from Carrier. Very poor quality, not very reliable. YMMV

 

[Safe Harbour]

Heat Pump

Get a heat pump not a furnace. You’ll save goobs of money!

 

[Dawg52]

I have an 18 y/o Goodman furnace. In the 8 years I have owned the house no problems whatsoever. Two years ago I replaced my defunct Rheem a/c with a Goodman. So far so good with it.

 

[DenverDoug]

I need a new furnace

I suggest you consider replacing your NG furnace with a heat pump. Not only will it be the most efficient heating and cooling system you can buy but the tax credits make it a best option in most cases. Heat pumps give you 2-3 units of energy for every unit you use. A 95% efficient condensing NG furnace produces 0.95 for every unit of energy you use. And heat pump models are available that perform to -10 F and have backup for lower temps. An 220 V circuit will be required however.

 

[I am He]

I went with a single stage 96% Goodman. The one thing I'd insist on is a commission report being handed over at the end of the installation stating that they've measured gas, temps, and all that stuff.

I'd also look for an installation manual which might mention that the unit should be slightly slanted toward the front, maybe about a 1/4". I believe this will also keep the secondary heat exchanger draining out which might prevent its degradation over time.

Auxilliary drainage from the PVC pipe might also benefit the condensate issues that arise too often.

Mine had a bit of an issue with one of the pressure sensors which I remedied by repositioning it a bit higher than was stock and since then has been working like a champ.

 

[landis207]

If you like Nest Controllers, Stay away from Trane. They have proprietary wiring that only works with Trane wall controllers. We have an AC/Heater in our FL Townhouse and it works well, but I strongly prefer the Nest over the Trane controller.

 

[braumeister]

If you like Nest Controllers, Stay away from Trane. They have proprietary wiring that only works with Trane wall controllers. We have an AC/Heater in our FL Townhouse and it works well, but I strongly prefer the Nest over the Trane controller.

My Nest thermostat works just fine with my Trane heat pump system.

 

[Retired Expat]

I suggest you consider replacing your NG furnace with a heat pump. Not only will it be the most efficient heating and cooling system you can buy but the tax credits make it a best option in most cases. Heat pumps give you 2-3 units of energy for every unit you use. A 95% efficient condensing NG furnace produces 0.95 for every unit of energy you use. And heat pump models are available that perform to -10 F and have backup for lower temps. An 220 V circuit will be required however.

I was surprised this wasn’t suggested more. When my oil furnace goes I will install inverter heat pump and if needed expand my solar system. In the long run NG and oil is only going to go up and likely electric rates so I should make out OK. Then there is doing your part for the planet which I feel very strongly about so when I read about not buying the most efficient system you can, buy the cheaper but a polluting 80% Afue I just cringe! Some things can’t or shouldn’t be just about $ and cents!

Oh and when my hot water heater died I installed an inverter heat pump unit! Yeah pricey up front but a great investment and I live the cooling drying aspect in my basement

 

[ERD50]

... Then there is doing your part for the planet which I feel very strongly about so when I read about not buying the most efficient system you can, buy the cheaper but a polluting 80% Afue I just cringe! Some things can’t or shouldn’t be just about $ and cents! ...

While we all would like to do 'the right thing', I don't think it's that simple.

An 80% AFUE Natural Gas furnace uses less raw materials to make in the first place, so that needs to be considered (maybe a minor point though - depending on how long/cold your winters are). The high eff furnaces have more things to go wrong, will, on average, probably have a shorter life than the basic 80% models (I've owned both, plan to stick to 80% in the future - cringe away!).

And while a heat pump can move heat far more efficiently than an NG furnace can produce it, you have to consider that much of that electricity is being produced by an NG plant, and the best (co-gen technology) are 60% efficient, and then you have distribution losses.

Some things can’t or shouldn’t be just about $ and cents!

Correct, but everything needs to be considered. A couple reservations I have with heat pumps is their cold weather performance - I know this has gotten better, but if you need to supplement with resistance heat, that gets expensive, and is less efficient use of NG than an 80% NG furnace. Second, I've had power outages, but never a long NG outage. I can run my furnace fan and electronics from an inverter if needed.

I'm also wary of the complaints I've heard about the lower duct temperature from a heat pump. I like a nice warm blast of air from the registers. I routinely turn our heat way down at night and when we go out. The fact that it feels warmer almost right away with that warm air blast means I don't hesitate to do this, and save energy.

But I know people who have complained about the lukewarm air at the ducts of a heat pump. It doesn't feel warm if it isn't much above body temperature and blowing on you (as anyone who has fanned themselves on a hot day knows!) - they ended up kicking up the heat which engaged the resistance heat, and then wondered where their savings went!

Heat pumps have their place, but it's not all unicorns and rainbows.

-ERD50

 

[ncbill]

For me it's not just cold weather performance that is an issue, but the much lower outlet temperature as well.

E.g., 105-110ºF versus 125ºF for a gas furnace.

Heat pumps have to move a larger quantity of lukewarm air versus a smaller quantity of hot air with the furnace for the same increase in room temperature, so they often feel "drafty" versus a furnace.

Unless outlet temps have been increased significantly with newer model heat pumps I'll continue to lock out my heat pump and use the 80% gas furnace instead all winter long.

Not to mention the super-annoying loud noises a heat pump makes on defrost...that's another thing that needs to be fixed.

 

[Happyras]

My opinion, but a properly set up heat pump will have a 30 degree rise at the register from the cold air intake. Typically 20 to 30 degrees pending efficiency at outdoor temp. I look for 90 to 100 at the register to be in range. mini splits are a little different, they run quiet and lower flow. A gas furnace should run 110 to 120 at the register if properly sized for flow, or about a 40 to 50 degree rise. I do an annual calculation on cost per therm of gas, versus cost per KWHr average and do a lock out at the break point of efficiency. I sized my HP knowing I would switch over to gas fire when it was cold out. Typically here in WA-Seattle area I set this at 35 to 40 degrees, pending SEER of the HP. Typically they have a COP of 3.5 while outside is above 40, but this drops below 3 quickly into the 30's for a 16 SEER central system.

The mini-splits I have installed all are over 22 SEER, and produce well down to 20F no problems, so they have not needed any dual fuel or back up heat plan.

 

[Safe Harbour]

Current Top Heat Pumps

https://neep.org/sites/default/files/ColdClimateAir-SourceHeatPumpSpecificationProductListing-Updated11.30.18.xlsx

I looked up my Mitsubishi MXZ-4C36NAHZ heat pump unit:

Temperature Min Capacity Max Capacity Min Cap COP Max Cap COP
(degrees F) (BTU/hr) (BTU/hr) unitless unitless
47 22,500 45,000 6.0 4.0
17 22,500 45,000 4.3 2.8
5 22,500 45,000 3.7 2.4
-13 24,000 36,000 2.8 2.2

Good down to -14 F. The maximum capacity is still 80% of nominal. And if you're worried you put a simple electric strip heat auxiliary on the unit and you're covered no matter what. Hard to beat that. Remember the HP never quits working it's just less efficient.

Also most people actually prefer the comfort of the lower temperature air from the heat pump because, especially with a variable speed blower, they run continuously at much lower speeds. This means consistent temperatures. No getting blasted by hot air every time the thermostat turns on. The HP will save you lots of cash in fuel and keep you much more comfortable.

 

[Happyras]

One caveat is that these stats do not account for defrost cycles, that unit is not a DYI, but not hard to install if you can silver solder and have a vacuum pump and gauge set. However, that is a 4.0 COP at 47f, which is a very high SEER rating. I would guess its close to 19 or higher?

Had to check your table, yup comes in at 19.1 SEER and 11.3 HSPF for region IV, nice unit......