New 2 stage furnace making house very uncomfortable

Garvdog

I have a new 2 stage furnace that replaced an older single stage unit. Both were high efficiency models, although the old one was a very early hi-eff model and was maybe 92%, new one is 96%. Both are the same size, 100k BTU. The new unit always leaves the house feeling cold and drafty, it always feels much colder than the t-stat says due to cold floors, surfaces, bedsheets, it’s like the whole house now radiates cold. I believe the problem stems from the fact that the new unit blows lukewarm air for a relatively short time. It only stays on for a few minutes and by the time the burner shuts off I’m lucky if the temp gets to 95 degrees at registers farther away from the furnace. I suspect the new unit was oversized as the air blows harder than I would expect for low stage.

As a note, I know my attic (2-story house) is not very well insulated, and the second floor is, and has always been, a few degrees colder than the first. But this feeling of constant drafty cold everywhere is new since the furnace was replaced. I don’t want to spring for a big insulation upgrade until I know that it would solve this new problem. I just wish the air coming from the registers was reasonably hot like my old furnace.

Furnace: 100k Carrier 2-stage 96% Performace series
House: 2-story 2450 sqft in PA, Furnace is in basement so ducts are all in conditioned spaces

GW

Hmmm seems a little big for a 2450 sq ft,

I would went 80000 or maybe 60000

two stage stat? 

Is your physiology changing? 

Is the stat accurate?

The two furnaces would be somewhat similar. You’re kinda in the minority saying a two stage is less comfy than a single stage. New furnace has a much more powerful blower motor 

 but the big BTU is a head scratcher 

bburd

The blower speed may be set too high. 

Jamie Hall

bburd said:

The blower speed may be set too high. 

I'm wondering that, or if the new furnace on first stage simply doesn't need to get the temperature up as high. That will give a feeling of a cold -- or cool -- draught (think a nice summer breeze vs. still air!) even if it isn't.

HVACNUT

Agree with all of the above. 
You now have a 2 stage thermostat and its definitely wired at the furnace for 2 stages?
Does it reach the set temperature?
Is the fan set to "auto" at the thermostat?
I've seen some really restrictive supply registers in older homes. Can your registers handle the velocity?
How is the installation itself? Duct connections are tight? Not pulling air from the basement?
A good steam humidifier will help.

EBEBRATT-Ed

@Garvdog

I will guess the contractor never did a heat loss load calculation and just replaced the furnace with the same size.

Have the contractor check the temperature rise of the furnace, check the blower speed and the gas input and did they do a combustion check?

mattmia2

I'm curious what the correct answer to this is. When my new furnace was installed 20 years ago i turned the blower speed for low heat down one speed because it felt cold and drafty but i now wonder if that made it not get the claimed efficiency or caused higher vent temps than the pvc is rated for.

bburd

The temperature rise specs for the furnace should be on the nameplate or in the manufacturer’s literature.

JUGHNE

Do you have a 2 stage tstat?

If so turn it up 10 degrees above room temp, furnace should high fire and fan speed be accordingly.
Then turn it down to just 1 degree above room temp, furnace should go down to low fire and the fan speed drop also.

There are usually adjustments for fan speeds for high and low fire. Those can be selected as needed.

Garvdog

Thanks all. Unfortunately this furnace only has 2 fan settings on heat mode, “comfort” and “efficiency” speed. It’s already in “comfort” mode ( the lower setting). I guess the manufacturer won’t let it be adjusted with the same flexibility as in cooling mode to avoid overheating the burner. It does seem to blow too much air for my preference but I did verify it is in the lower stage. Looking at the manual, if it ever does come on in high stage it will blow like a hurricane. The CFM setting for high is something like 1900 CFM, much more than it is set up to blow in cooling mode. I am now pretty convinced this is oversized for the house and ducts. Would a smaller unit stay on longer and hence have more time to come up to temperature, making h house more comfortable?

Garvdog

By the way, I wish it did have more adjustability of fan speeds because I would do exactly what @mattmia2 did above. I’m pretty sure that is what a previous owner did to my old furnace, since ironically it ran at a lower fan speed at its single stage than this one does in low stage, despite being the same btu rating- and was much more comfortable. Really wishing they had put in an 80k btu unit now. And no, a load calc was not done (to be fair, none of the contractors I got quotes from did one, just walked around the house, counted supply and return ducts, etc).

GW

Sounds like you have the 59TP6B100 

I’m a Carrier guy and only install their furnaces that have adjustable fans. Ironically it’s the “comfort” series these days.

thats pretty sad though. You’re on 1st stage  and it’s blowing heavy CFM- so sad

kinda curious- what size air filter? Curious what kind of Feet per Minute the filter is dealing with 

you can ask for a smaller furnace, if they are true professionals they will come back. 

pecmsg

That blower should be a ECM. Speeds are set up on the DIP switches according to estimated static pressure.

What's the full Model #?

GW

The HOs description of the blower is accurate 

motoguy128

Low stage should be around 1200 CFM for about 50F temp rise with a 70% firing rate. However, if its a X13 constant torque blower not a fully variable ECM, they might only allow med lo as the lowest setting which could be as high as 1400CFM in some cases. While low is a very low 1000CFM 50% speed, which would be too low in your application.

80k furnace likely has a 3 ton blower with a 70 or 75F temp rise and allows med hi at 1100CFM would be allowed and low maybe at 800 CFM on a med lo speed.

Sometimes its the details that gets you. For example many mfg offer a 60 & 80 or similar with the same blower. They simply certify different max temp rises and use the same airflow settings for both. So if designing ductwork for a either furnace, assuming you have a 2.5T AC, you have the same max airflow requirements for both.


The Carrier 80k have a 4 ton blower and becomes a nearly universal furnace with the wide range of blower speeds. The 60k is identical but with only a lower max temp rise.

GW

Moto he got a 100k

Speaking of Moto- me this spring, New bike and the backgrounds had not yet come in.

captainco

Regardless of the stage the furnace should still have a Delta T of 60 degrees in both if it is set up correctly. That means the input and blower speed are correct.
Just for the recorded, no furnace made today operates at its rating if the gas pressure is at factory settings. If you use the formula CFM X Delta T X ADCF to calculate output you will come up short. Airflow based on static pressure is a guess.
Having had several two story homes with questionable ductwork, running a constant blower will definitely help balance the temperatures. The nice thing about a 2-stage furnace is that is operates in low speed between cycles.

Oversized equipment is not a reason for being uncomfortable. Poorly set up equipment and incorrect airflow are the reasons. When you are old like me oversize equipment is better because my house heats up twice as fast. Okay, I do not recommend oversizing, but I am definitely against underfiring. Even an ASHRAE study from thee 80's stated that equipment can be oversized up to 200% with little or no increase in operating costs.

Garvdog

I believe it’s a combo of high CFM (@GW is correct, even in comfort mode the CFM is around 1400) and the unit not running long enough to get the temperature rise up ( at most registers it doesn’t even get up to a 30 deg. Temp rise before shutting off. Even with outside temps in the low 40’s it only runs for about 5-6 minutes with the house set at 73. The air temp in the house is correct but it gets there too fast and leaves the house feeling drafty. This just wasn’t an issue with the old unit- the air from the registers was noticeably warm right away and the air came out at a lower velocity, which I think is making a big difference. My old furnace had the old-school blower where you could adjust the taps to get different speeds. I suspect a previous owner bumped it down to low speed. The burner may have been running too hot for all I know but the house was comfortable. The new unit has no such flexibility when in heating mode. I am honestly pretty surprised it takes this thing so long to produce noticeably warm air- but I don’t think it would be as much of an issue if it ran for longer periods at a lower speed- which is what I thought a 2 stage unit was supposed to do.

Garvdog

@GW is correct about the model- 59TP6B100

Garvdog

...and to be clear, the blower doesn’t have the same adjustability in heating mode as it does in cooling mode. In low stage it runs close to 1400CFM as @GW guessed. It also takes quite long to come up to temp, and is already shutting off after about 5-6 minutes. So, I never get a temp rise more than about 20 degrees at most of the registers. Very unlike my old furnace which blew at a lower velocity and the air at the registers was noticeably warm right away.

GW

are you 1 million % sure you're running 1st stage?

If you're feeling ambitious, open the bottom cover and look for dip switch section 1 (there are a 2 or 3 dip switch banks I recall), and see what switch 2 is on or off. Then look at the low voltage wires, do you have wiring on W1 and W2? Those answers will settle this once and for all.

Just seems odd that 1st stage is blowing you away.

captainco

90 degree air feels pretty cold on a 98.6 body. If you are not getting at least 110 to 120 degrees from the registers you really have a poorly set up system; You -would have an efficiency of less than 60% with a 20 to 30 degree rise.

Garvdog

@captainco exactly! It seems like it should be heating up much faster to get to that 110-120 degrees. It never gets the chance. And yes @GW I am sure it’s on 1st stage. It’s definitely running about 1400 CFM as advertised- slightly less than when it’s in AC mode. I wouldn’t say it’s “blowing me away”, but it’s not the gentle percolating of warm air I was used to, and expected out of low stage. More like a summer breeze that starts at an uncomfortable 75 degrees and slowly gets up to 90-95 after a few minutes then quits. The only time I measured 120 was when I cranked the heat up a couple degrees to make it stay on for 7-8 minutes, then it passed 120 at the register closest to the furnace.

Garvdog

And to answer @GWs earlier question- the filter is 20x24x4- but the main return plenum it connects to thru a smooth elbow is only 12x24. So it steps up from 12 x 24 to 20x24. To be fair, the house does have lots of returns in many rooms, and lots of supplies, but they all tee off of one main duct near the furnace. I’m thinking this thing is just too big for the ductwork. Even if one could get it to run hotter (by, say, adjusting the gas valve) the fan still can’t be slowed down, so it will just shut off even sooner.

Hot_water_fan

https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

See what the real heat loss and furnace size should have been using this! 

GW

Ok that duct work is way bigger that what your house requires- wow 

that’s prolly why they gave you a 100k. Gotta fill the bag. But yes- 100k is a lot of energy for a 2450 square foot house 

All your vents are open? Still seems like something is amiss 

still curious how you’re so sure you’re on 1stage. Have you heard it bump to 2nd stage? You have a two stage stat? They clicked the correct dips? Some installers can’t do the technical thing 

If you’re old furnace was high Efficiency, I suspect it was horribly out of spec (running hot). If it was chimney vented then all of this makes much more sense 

Garvdog

@GW a 12x24 single return duct is way bigger than what my house requires at 2450 sqft? I felt sure you were going to say it was way undersized for the furnace. And yeah, I do suspect my old unit was running hot. The top surface of the unit near where the pvc air intake came into the burner box used to get too hot to touch after running for awhile. Probably the fan was set too low for the size of the unit to make up for the fact it was oversized. That unit was in the house 10 years before I bought it so I can’t vouch for what previous occupants and their hvac guys did.

captainco

Anyone writing an article about sizing equipment based on its AFUE rating or the factory input and output, has never checked a furnace in the field correctly.
Typically a 2-stage furnace operates at 60% in low fire. The average measured efficiency in the field is 50% in low fire at factory settings. So a 100,000 btu furnace in low fire would be 60,000 btus input and 30,000 btus output.
If we depended on the fabricated rating of furnaces and sized them exactly, people would freeze.

Just an after thought. If the furnace is cycling that quick it may have a high limit in it that is tripping or defective. Some furnaces are designed so we can't make them run properly.

Garvdog

It’s cycling because the stat is telling it to shut off. It keeps a pretty narrow band and I usually just get my house to maintain temp at the same number. I suspect it rises fast because there is a large return vent above and pretty close to the stat. It was never an issue with the old unit but again, the fan didn’t run as hard. For @GW, no way it is running in high stage. That would be almost 1900CFM- I would notice that hurricane of air. It is indeed a single stage stat- the contractor claimed “Carrier likes us to have the furnace control the staging”. So apparently the staging is based on timing...it will go into high stage after about 16 minutes of runtime- which I’m convinced based on what I’ve observed so far will never happen given the current runtimes.

Hot_water_fan

Anyone writing an article about sizing equipment based on its AFUE rating or the factory input and output, has never checked a furnace in the field correctly.
Typically a 2-stage furnace operates at 60% in low fire. The average measured efficiency in the field is 50% in low fire at factory settings. So a 100,000 btu furnace in low fire would be 60,000 btus input and 30,000 btus output.
If we depended on the fabricated rating of furnaces and sized them exactly, people would freeze.

Maybe the part when Dana discusses oversized factors covers that? Low fire efficiency shouldn't matter for freezing anyway, that's the whole point of multi-stage equipment. If efficiency is so low at low fire, that would result in more gas used and therefore larger sizing based on the article.

Judy_Smith

Uh, in case its of interest (consider it rhetorical) my small early-50's ranch style house used to be very drafty and cold. Subsequently, I discovered that when I kept the living room door closed (thus creating a barrier between L.R. and the long-hallway plus rear bedrooms), that helped alot toward preventing the L.R. drafts from circulating to the bedrooms and kitchen. Note the L.R. has a ridiculous picture window which can get drafty despite its facing south. Prior to that epiphany, the entire house was drafty. Now its simply cold, but not as drafty. I had tried an insulation guy, but he didn't want to insulate, due to mold issues, which i'd rather not discuss, as this is the least of the nitty gritty's draining my life.

HVACNUT

Mold in your home shouldn't be the least of your nitty gritty's. It should be near the top of the heap, king of the hill, A number one. It's your health your talking about. Now skedaddle, and no more tomfoolery. 

pecmsg

Your return can be extremely large, it will only draw in what the supplies can deliver and no more!

Dan's Rules, you can only take in what you can deliver!

GW

True that, I dumb it down a little, what goes up must come in

Where that supply air goes and where the return air comes from, that’s a different topic.

Garvdog

Good point about returns. I do mostly close down two supplies in the two small rooms right above the furnace as they get a ton of hot air otherwise. It helps balance things out. Also of note is that my small finished basement (not part of the aforementioned 2450 sqft) has 3 supplies that I mostly close- but the basement has no returns in it, so if anything opening those up would create more of an imbalance I would think as they would take air from upstairs and push more downstairs.

jfk

I'm really worried. Had carrier two stage 96 installed today - (previously had one stage). I DONT want a colder/cooler breezy drafty house ! Is there a consensus on what exact tweaks and can have the installers do to avoid the problem you all are talking about? (Eg- tell them to open valves fully? Lower the power speed ? Other ? Help!

GW

I’ve done a lot of these carrier two-stage furnaces, and I’ve never had a complaint. There’s something unique going on here, little bit difficult to point it out without being there. Apparently, the old furnace was running a very large temperature rise, extra hot air. It will certainly have an effect on general comfort. Plus, the installer should’ve downsized the furnace one or two steps.