high indoor humidity in summer after replacing attic fiberglass batts with celluose insulation

tinu2walia

It is likely oversized. J load said 3.2 tons but Xc16 only comes in 3 or 4. So they put 4. But with an oversized unit, you would think the first stage should cool the evap coil enough to cause condensation and at least not create moisture... but that is not happening... My guess is that stage 1 is not cooling at all....

GreenGene

HOW DEHUMIDIFICATION MODE WORK — NO EXTERNAL
DEHUMIDIFICATION DEVICE
NOTE - OFF, MEDIUM and HIGH dehumidification modes are also a function of
the HVAC system with NO external dehumidification devices installed.
In OFF mode, dehumidification if off.
In MEDIUM mode, dehumidification occurs if these conditions are met and signals are present at specific terminals:
dehumidification has been enabled on installer settings, and
the unit is in COOL mode, and dehumidification demand exists (RH above setpoint), and cooling demand exists (Y1 energized).
In HIGH mode, dehumidification occurs if all BASIC conditions are true, except cooling demand may or may not be present. Also note that: Maximum overcool from cooling setpoint is 2ºF.
Deadband temperature is limited to a minimum of 5ºF (instead of 3ºF in DRY or MODERATE modes) because of 2ºF overcooling.
AUX

page 17 http://resources.lennox.com/FileUploads/cd838d86-d8b4-4011-9b6a-a08558e7bf5biComfort-WiFi-Installation-Manual.pdf

njtommy

Sounds like the fan speed is too high for first stage. I would investigate in that.

tinu2walia

njtommy : That is exactly what my AC guy said. I had to call him to short the Y1, Y2 on the compressor to force it to go on stage 2. He confirmed that due to equipment vendor mismatch, the installers had not set the blower to run on lower speed on stage 1 and that could be rendering the stage 1 operation counter productive. He is going to come back next week to rewire the blower -> EIM connections so that the blower works on lower speed in stage 1. Till then we are always going to run in stage 2 as a temp fix. The humidity has been under control ever since we made the switch to stage 2 all the time...

Harvey Ramer

So you have a 2 stage AC unit mounted to (any old furnace)? We need a bit more detail on the furnace. More than likely your situation can be resolved with a couple relays and some low voltage wiring.

Here's what I am most suspicious of. The blower in the furnace is likely set up with one speed for cooling and one speed for heating. So when the system turns on 1st stage cooling, the blower is running full speed blowing entirely to much air across the evaporator coil. When that happens the refrigerant in the coil will evaporate at a faster rate as compared to the volume of refrigerant the compressor is moving, resulting in an elevated evaporator core temperature. More than likely the volume of air moving across the evaporator is not experiencing enough of a temp drop to reach dew point. So all that happens is sensible cooling, or a decrease in air temperature with no humidity removal. When air decreases in temp with loosing moisture, the relative humidity becomes elevated. That is because the absolute humidity (total grains of moisture in a given volume of air) remains the same, while the same given volume of cooled air occupies less space while retaining all the grains of moisture, causing relative humidity to go up.

Makes sense?
Good.

So if you have a multi-speed blower on the furnace, you should be able to have it run at a slower speed while the compressor is in stage 1. This could probably be accomplished with a couple relays and some wiring.

To select the proper blower speed for 1st stage, you'll want to look at the tons of capacity the compressor has on 1st stage and set the blower for somewhere in the vicinity of 400 cfm per ton. You'll want to verify the temp drop across the coil, usually around 17 to 20 degrees drop is acceptable.

I will say, there are more nuances to the information I gave you, but I have to stop typing at some point in time. If you hire someone who knows thier stuff, they will take care of all this for you.

Gordy

Now there is a man with a deep understanding of a piece of machinery.

JUGHNE

That 2 stage AC was maybe designed to marry up to a furnace with the variable speed blower, which would slow air flow down as others above said. 2 stage gas heating furnaces change speeds for hi/low stages, they also have dehumidifier capability by running blower at lower speed for the first 10 minutes or so of cooling with 1 stage AC.

GreenGene

It would have been nice to have all the equipment info upfront, that they were mismatched and what they were.

njtommy

An evergreen motor would be a nice fit for the application. I believe they are run off of 24volts for speed section would be very easy to do from your Honeywell EIM board.

tinu2walia

Furnace is 90+ eff single stage heating HEIL model # h9mpd100j14c2. It however does support 2 blower speeds. AC is Lennox XC 16 (2 stage). Tstat is the Lennox iComfort. EIM is also a Lennox EIM.
My AC guy will be coming next week to hook up the Lennox EIM to the blower control board such that blower runs in low speed in AC stage 1. I think he plans to connect the blower G wire to the EIM (but not the Y1 of the EIM). He then wants to connect blower Y wire to the Y2 of the EIM....
Will see if that permanently fixes the high humidity issue created by stage 1. Last night we temporarily bypassed stage 1 by shorting Y1 and Y2 on the AC and the humidity issues was fixed. RH stayed below 50% at all times, proving my theory that stage 1 is at fault.
So, we at least have the root cause identified and a temp fix in place...

SWEI

Not familiar with the Heil line so I looked up your H9MPD. Newest spec doc I could find was from 2007 -- most were older. The 100J14C blower is spec'd for 2.5-3.0 tons (the 100J20C for 3.5-5.0 tons.) Fan performance differs quite a bit for the two motors.

I'm with @njtommy on this one -- you should give serious consideration to a motor upgrade.

njtommy

Hopefully you can get the CFM range for 1st stage down to 700-800cfms and no higher. I would rather see around 700cfm so you get a nice cold discharge air temp hopefully around 55ish so you can dehumidify without any problems.

GreenGene

That's where your high end communicating variable speed equipment really shines, they can vary the blower speed as you reach set point but still have a high humidity level then monitor coil temp and keep it above freezing.

Switching speeds on a standard system can give you some control but cfm should be measured and pressures/temps checked to ensure you aren't creating another problem.

tinu2walia

In hindsight getting the 4 ton XC16 was a mistake. I should have stuck with a 3.5 ton Xc13. But I wanted the 16 seers and Lennox does not make a 3.5 ton XC16 and I thought that the 1st stage of a 4 ton Xc 16 would be very good for me energy saving wise. The sales and installation staff of C&C air should have known better and should have adviced against using Xc16 in this setup upfront. I am really disappointed that they sold me a wrong unit. They are nice folks otherwise and are working with me to fix the stage one by selecting lower motor speeds on the existing blower but this seems like a risky track... They should have just told me to use the single stage Xc13 upfront, they knew what furnace I had... Also, eventually after ordering I found out that I was only going to get 14 seers from the XC 16... Me certainly not happy and am certainly not going to spend more money on the blower when the mistake really was the sales & installation people... Hope their trick to lower stage 1 fan speed works, if not I am going to use it in stage 2 all the time and ask for a partial refund...

JUGHNE

I would believe that anyone selling High End equipment should have known that 2 stage AC needs the fan speeds to match the tonnage...or realize that your blower has max air flow of 3 tons?

Your existing fan blower could have 3 to maybe 5 different fixed speeds.

I would make the play for the variable speed motor, hopefully the blower wheel is large enough.

I put in the standard 13 SEER and with the variable speed motor it will pop the system up to a 14 SEER.

GreenGene

There's only one way to tell.

Test the CFM.

This still has to be done, switching speeds is blind, you still don't know your actual CFM.

tinu2walia

Agree about CFM checks, hopefully the tech will bring the tools to measure CFM and external static pressure. Also seems like from page 2 of the manual that the blower can do upto 3.5 tons AC. They put a 4 ton. Not sure what issues can happen due to that... Perhaps the evap coil can freeze up?? But I think they put a device to prevent that...

tinu2walia

Should I just ask my AC guys to own up to their initial selling time mistake and replace the compressor with a 3.5 Ton single stage Lennox XC 14 or should I let him experiment with the lower fan speed on stage 1 of the XC 16 with my existing furnace and blower??

SWEI

A calculated load of 3.2 tons would lead me to re-visit the load calc. It's possible that a 3 ton unit might do the job, but you really have to know what you're doing with both Manual J and Manual S.

tinu2walia

The sq footage is 1925sq ft. Lot of glass. Attic Insulation is now R49. The time AC J load was calculated it was calculated with R19 and the guys came up with 3.2 Ton. My old unit used to be 3.0 ton and could never cool below 73 even after running all day (it did not have an A frame coil actually).
So a 3.5 Ton unit at this point should be enough. A 3 ton might work too... However, changing the compressor would require changing the evap coil again... I don't think the AC guys will do all that even though they speced it all wrong...
So, I am thinking that at this point I'll let him try the stage 1 fix with this 4 ton XC 16 compressor. Nothing bad has happened to it with the existing blower till now... If they can't get the stage 1 fixed, I'll then push for changing both the evap coil and compressor to a 3/3.5 ton single stage system...

Jamie Hall

A bit of a mess. However, I imagine that with some reworking of the controls -- which may require some real ingenuity -- you can get this thing to work. It may not be the most efficient solution, though. What needs to happen is that the air circulating through your system -- AC and furnace -- has to get cooled to 55 Fahrenheit (the dew point of your target air at 75 and 50% RH). Then it needs to be rewarmed to your target temperature.

However you can arrange your controls to do that will work. A sensible heat heat exchanger would be the most efficient approach -- but I've run into systems which did it by running the furnace!

tinu2walia

Problem fixed. My AC guy came and hooked up the y1 (cooling stage 1 signal) from the Lennox EIM to the Heat motor speed on the furnace (which was at Med-Hi) instead of being tied to the cool motor speed (Hi) and that has fixed the problem. The rooms have also been cooling all right in stage 1. Indoor humidity no longer increases with the AC in stage 1. Since the AC is oversized, I changed the lennox iComfort settings to let it run in stage 1 for longer periods now, that way I will be able to save some electricity too.
So much headache for 1 mistake by the installers. At least the new guy they sent was very patient stayed there for several hours and made sure the problem was fully corrected before he left.
Thanks for all you support folks!

ratio

Fantastic. I'm glad your issue was corrected, and thanks for letting us know how it turned out.