Difference between 2-stage and 1-stage A/C and furnace when both have a variable speed fan?

clueless_in_chicago

Hi all,

I'm having a really hard time understanding the effective difference in comfort between having a 2-stage A/C or furnace vs. a 1-stage, when both setups have a variable speed fan/air handler. Maybe I'm butchering the nomenclature, but I can't find anything on it.

My impression is that having a 1-stage system with a variable fan is between a 2-stage and a 1-stage with a fixed fan. Is that right? Does it bridge the gap in humidity removal during the summer? I live in the Chicago area and the summers can get muggy, but if there's not a big difference in the humidity control, I see no reason to go for the 2-stage A/C given the price increase (~$1600).

My HVAC guy who gave me the quotes said that there isn't a big difference between the furnace options, but people really comment on the A/C due to the humidity control.

Thanks!

captainco

Actually 2-stage A/C doesn't dehumidify as well as a single stage. The A-Coil doesn't get cold enough. I get this from a very smart guy in Texas that teaches airflow and btus with my company.
2-Stage furnaces are for people that like to wait to get warm, but they do have an advantage if the right tech is working on them and makes some adjustments.

John Mills_5

Agreed. Sometimes the latent heat ratio isn't too good on low so to me, the argument of better dehumidification isn't always valid. I'd rather get a good single stage and use the dehumidify on demand feature of the variable speed blower or if you are really in a moist area, a whole house dehumidifier is wise. No A/C dehumidifies when there is no call for cooling such as 70 and raining out.

ChrisJ

I'm a bit confused by what has been said here.

For example if you have a 2 stage 3 ton A\C system that runs the blower at 1200 CFM on high and 800 CFM on low, why would the evap not get "cold enough" for good dehumidifcation?

You have 67% of the normal air flow and 67% of the normal compressor output so the coil should run at the same temperature. No?

Most of these also have a dehumification function which forces the fan to run even slower and many thermostats will allow the system to cool up to 3 degrees lower than the setpoint for moisture removal.

All of the charts I've seen for 2 stage systems showed the same SHR in both high and low stages with slightly improved efficiency in low.


My opinion at this point is a 2 stage A\C system should have the same humidity removal as a single stage, but should result in far greater comfort and much quieter operation (inside) most of the season. It should also give slightly improved efficiency.

captainco

Based on field measurements, two stage cooling is not dehumidifying as good as single stage. What you say sounds good it just isn't what is happening.

ChrisJ

captainco said:

Based on field measurements, two stage cooling is not dehumidifying as good as single stage. What you say sounds good it just isn't what is happening.

So you're saying the manufacturers who show the SHR for both high stage and low stage being identical are publishing false information?

That's all I've got to go on right now is manufacturer's specifications.

njtommy

I will take a 2 stage system over a single stage any day of the week. My experience is they provide more comfort then a single stage system and sometimes even a bit of money savings( nothing crazy).

You can and will dehumidify a space running on 1st stage and a lower fan speed. You must dial in your fan speed and make sure you superheat is with in range at the compressor.

ChrisJ

So,

The results are in.
My system most certainly dehumidifies better in LOW stage.

In high stage I typically have an 18 to 20 degree drop across the coil, depending on outdoor conditions.

In low stage however, 21 to 22 degrees is more common. The evaporator runs cooler and the air has more time around it.

The results don't lie.





This 2 stage system squeezes water of the air out just fine.

Rich_49

captainco said:

Based on field measurements, two stage cooling is not dehumidifying as good as single stage. What you say sounds good it just isn't what is happening.

Field measurements of what ? Properly designed systems or systems designed by folks who think they know it all ?

Rich_49

ChrisJ said:

So,

The results are in.
My system most certainly dehumidifies better in LOW stage.

In high stage I typically have an 18 to 20 degree drop across the coil, depending on outdoor conditions.

In low stage however, 21 to 22 degrees is more common. The evaporator runs cooler and the air has more time around it.

The results don't lie.





This 2 stage system squeezes water of the air out just fine.

Hard to argue with math and data on a system with right sized ductwork . We must not forget identical systems will perform differently in buildings with different types of envelopes . Something many of these EXPERTS forget to take into consideration , no matter who they work for . There is a reason that these trades got so screwed up , just sayin

Harvey Ramer

Dehumidification in a home has everything to do with the amount of time the air is exposed to a condition that causes it to condense.

If a two stage system is properly set up with the correct sizing and airflow, it will most certainly outperform a single stage system in the same scenario.

Not talking about efficiency. That's a different animal.

ChrisJ

Harvey Ramer said:

Dehumidification in a home has everything to do with the amount of time the air is exposed to a condition that causes it to condense.



If a two stage system is properly set up with the correct sizing and airflow, it will most certainly outperform a single stage system in the same scenario.



Not talking about efficiency. That's a different animal.

According to the numbers on my setup, they claim it's more efficient in low stage as does a study I had read on 2 stage setups a while back.

Can I prove it? Of course not.

NateEnergySmart

Thank you for actually measuring on your system! You ended the thread with it.

Right sized, multiple stage HVAC consistently delivers good dehumidification in our experience. We almost entirely use higher end HVAC. http://energysmartohio.com/case-studies/

Rich_49

By the deafening sound of crickets I would imagine you have your answer Confused .

GW

Two stage systems have more run time that one stage

Ac operation equates to drier air

Two stage systems are better at dehumidifying

ChrisJ

Dehumidification?
I've got your dehumidification. All in low stage.

Henry

My variable speed York furnace works great with the two stage AC. It runs mostly on the lower stage unless it gets very hot outside. We have very humid air here and the York does better than my previous single stage and two speed furnace. As a 2 stage AC it was slightly oversized but since it runs on the lower most of the time, I appreciate the second stage when it gets very hot.

ChrisJ

Henry said:

My variable speed York furnace works great with the two stage AC. It runs mostly on the lower stage unless it gets very hot outside. We have very humid air here and the York does better than my previous single stage and two speed furnace. As a 2 stage AC it was slightly oversized but since it runs on the lower most of the time, I appreciate the second stage when it gets very hot.

It seems the smallest 2 stage they make is 3 ton, is that correct?

It's a shame because many houses would benefit from a 1.5 or 2 ton 2 stage unit.

Jack

ChrisJ said:

Henry said:

My variable speed York furnace works great with the two stage AC. It runs mostly on the lower stage unless it gets very hot outside. We have very humid air here and the York does better than my previous single stage and two speed furnace. As a 2 stage AC it was slightly oversized but since it runs on the lower most of the time, I appreciate the second stage when it gets very hot.

It seems the smallest 2 stage they make is 3 ton, is that correct?

It's a shame because many houses would benefit from a 1.5 or 2 ton 2 stage unit.

Well, they make them. Seven stage modulation on burner and blower. Models available in 3-8kbtu. 5.5-11, 8.2-16.7, 8.2-21.5 & 13.2-38. Rinnai direct vent wall furnaces. Easy install and bomb proof.

ChrisJ

Jack said:

ChrisJ said:

Henry said:

My variable speed York furnace works great with the two stage AC. It runs mostly on the lower stage unless it gets very hot outside. We have very humid air here and the York does better than my previous single stage and two speed furnace. As a 2 stage AC it was slightly oversized but since it runs on the lower most of the time, I appreciate the second stage when it gets very hot.

It seems the smallest 2 stage they make is 3 ton, is that correct?

It's a shame because many houses would benefit from a 1.5 or 2 ton 2 stage unit.

Well, they make them. Seven stage modulation on burner and blower. Models available in 3-8kbtu. 5.5-11, 8.2-16.7, 8.2-21.5 & 13.2-38. Rinnai direct vent wall furnaces. Easy install and bomb proof.

What does that have to do with 2 stage outdoor condensing units?

Jack

You mentioned furnaces;) for the AC side I guess it would be hard to beat mini-splits in those sizes

Gordy

Chris thinks everything is a furnace

ChrisJ

Jack said:

You mentioned furnaces;) for the AC side I guess it would be hard to beat mini-splits in those sizes

Henry mentioned a furnace, I mentioned only tonnage.
@Gordy is still upset that he's stuck with someone else's furnace.

Gordy

But it is a furnace.

GW

No, it's a boiler

ChrisJ

I've got a headache.

SO! On the lighter side, I dropped my CFM from 1200 to around 1060 today. It should improve dehumdification even more, but that's not why I did it.

For some reason, this air handler in this configuration ends up pulling small amounts of water into the blower compartment in high stage. I say "in this configuration" because it's horizontal and the evaporator is installed backwards, with the tip of the A facing the air filter and the mouth facing the blower.


I'm hoping the slight drop will eliminate that issue. 353 cfm per ton may be more appropriate for my area anyway, who knows.


If this doesn't stop it, I have no clue what I'm going to do.

Gordy

Its all that humidity your pulling out of the air, and now you want to pull more. You'll be waking up with cotton mouth, psoriasis, and nose bleeds.

ChrisJ

Gordy said:

Its all that humidity your pulling out of the air, and now you want to pull more. You'll be waking up with cotton mouth, psoriasis, and nose bleeds.

Sigh.

One day you'll learn to read my friend.

Gordy

ChrisJ said:

I've got a headache.

SO! On the lighter side, I dropped my CFM from 1200 to around 1060 today. It should improve dehumdification even more

Your obsessed with the low stage humidity removal. You know you are. You want to see how low you can go.

ChrisJ

Gordy said:

ChrisJ said:

I've got a headache.

SO! On the lighter side, I dropped my CFM from 1200 to around 1060 today. It should improve dehumdification even more

Your obsessed with the low stage humidity removal. You know you are. You want to see how low you can go.

Normally, that does sound like me. I'll admit it.

But no, sadly, I'm so far beyond tired of messing with this thing and being in that attic I just want it to work. August of 2016 I started planning to install my own central air. I never intended on re-engineering an air handler that didn't work right from the manufacturer............ You have no idea how beyond unimpressed I am. I should've went with American Standard. Too late now.

Learning how to install and setup a steamer and balance vents was a piece of cake compared to making this POS work out of the box. Now I know why everyone makes fun of them.

ratio

Now that your idealism is dead and disgust & apathy have replaced it, I can truthfully say welcome to the trade! We're here for you, brother. I'll tilt the suds to you tonight.

ChrisJ

ratio said:

Now that your idealism is dead and disgust & apathy have replaced it, I can truthfully say welcome to the trade! We're here for you, brother. I'll tilt the suds to you tonight.

I don't know whether to give this an LOL or a thank.

GW

Chris are you one million percent sure you followed the instructions exactly? Sometimes there are "drip shields" that require the installers' attention

GW

And it's not a pos, it's just not what you're familiar with. If all of the air handlers dripped from any manufacturer that manufacture would cease to exist really quickly

ChrisJ

GW said:

Chris are you one million percent sure you followed the instructions exactly? Sometimes there are "drip shields" that require the installers' attention

I've read, and re-read the instructions over and over.
Is it possible I missed something? Of course. But I don't think I did.

It dripping isn't the reason I called it a POS.
That's a minor issue in a seldom used configuration.

Please see my PM for the whole story. I'm not here to bash any manufacturer.

HVACNUT

> @ChrisJ said:
>
> Please see my PM for the whole story. I'm not here to bash any manufacturer.
>
>
> I bet it starts with a "G".

ChrisJ

Gordy said:

But it is a furnace.

I think I'm going to start calling an evaporator a boiler.

Just to piss @Charlie from wmass off.

Charlie from wmass

Call it Pookie if you like won't bother me any.

GW

we all have our peeves. Mine is when you're cutting cake for ANYONE OTHER THAN YOURSELF and you lick the accumulated frosting off your fingers---THAT GROSS, why do some people do that????????????And, some add that lip smacking sound as they do it.

Charlie from wmass

If it is your significant other they may be flirting. Lol

ChrisJ

Mine is I can't stand when people are inconsiderate.
Though people arguing that the Earth is flat may be second to that lately.