I posted a question here recently about thermostats with humidity control,
since we had speced out one for a client but instead received one without
separate humidity control.
I called the A/C subcontractor today to ask them to please replace the
client's thermostat with one that had independent humidity control. His
response was that he had never seen a residential heat pump with a true
variable speed air handler. He said that they have to set the dip switches
for a certain fan speed, but that after that, it doesn't vary the speed.
My understanding was that his description was correct for lower or middle
end units but that a true variable speed air handler would start our very
slowly and ramp up speed over time to help pull more humidity from the home.
If you had a thermostat with separate humidity control and it was calling
for a reduction in humidity, but not temperature, that it would kick the
unit on with a speed just high enough to prevent freezing the coil in order
to pull out the greatest possible moisture. The A/C subcontractors point was
that having separate humidity control on the thermostat wouldn't really
accomplish anything beyond what would be accomplished by turning down the
So what is the reality of variable speed air handlers in this situation? Am
I wrong in my understanding? If they don't dynamically changes the speed, I
am curious what their advantage is.
FWIW, this is on a Lennox 18.6 SEER heat pump. Sorry that I don't have the
model info with me here.
Air handlers with ECM motors are about as close to "true" variable speed as
you're gonna get with resi equipment. ECM motors (if their set up correctly)
will vary their speed to a point to help compensate for poor duct design,
but that isn't their designated purpose.
Yes and no.... it will kick the system on, but it will be in a "normal" mode
with the blower ramp up, etc. Depending on how you program the stat, to drop
the humidity, it may lower the temp as much as 3 degrees. In my area the
target indoor design temp/RH is 75degrees/50%RH. I have no idea where you
are, or what your design temps are so I can't do a lot for you on that.
FWIW, some systems are set up so that the heat strips will energize while in
cooling mode for reheat to help with humidity removal.
Ummmm......no....he missed the mark on that one.
The ECM motor has several advantages...one is being more energy efficient to
run, another is that they will ramp up to speed over a period of time,
another is they will (to a point) compensate for improper ductwork design
and air flow problems.
Its not just the heat pump, but the particular hair handler it is coupled
with that will make the difference. Without actually *seeing* what you have
and how its installed, my best recomendation is to get the Honeywell
VisionPRO 8000 stat (TH-8321U1006)
FWIW, if you have humidity issues because of an oversized system, thats a
whole different problem.
Lets see... for starters, it has built in humidity controls, will support
most any configuration with 3 heat and 2 cool capabilities, its easy to
program, it doesn't have to be on the wall and connected to program it, it
has auto change-over, it has intellegent recovery, and a whole lot of other
features. The IAQ version only requires 3 wires to connect the control head
on the wall to the control module that you mount on the side of the furnace
or air handler.
Yes, you forgot to mention it offers an intermittant fan function
that operates the air handler blower motor for approx 40% of the time
in a 24 hr period.
Luckily, its touch screen is under the minimum size stated in
California's new law regarding hazardous waste disposal of LCD
Are you saying that most higher end residential A/C equipment uses ECM
We are in Central Florida. We usually use Lakeland, Florida as the closest
well-known city, when using software such as Energy Gauge, which I use for
determining eligibility for contractor federal tax credits on new
I'm glad to see that humidity is controllable with the variable speed air
handler, beyond what I was being told.
In this case, the Air Handler is a Lennox CBX-32-MV-048-2306 and the
Condensor is a Lennox XP19-036-230-02 with an HSPF of 9.3 and SEER of 18.6.
You see that this is a 4-ton air handler and a 3-ton condensor unit. It is
my understanding that by "mismatching" the two that you can effectively
increase the efficiency. Is that correct?
After talking with the A/C subcontractor yesterday (when he said that he had
not seen a true variable speed air handler in a residential unit and that
the only variableness was via dip switches) I called the Lennox support
department about this. A guy named Ron there confirmed that, when calling
for humidity reduction but not cooling reduction, that the thermostat sends
a DS signal to the air handler to slow it down. I asked him if other
thermostats, like the Honeywell TH-8321, sent the same DS signal to the air
handler in the same scenario. He replied that he knew that Honeywell was
working on that, but that he didn't think that their products (or any others
presumably) were sending that signal, but were rather just running the unit
longer to bring down humidity at the expense of lower temperature. Do you
know if the Honeywell TH-8321 does indeed send the same DS signal to the air
handler that the Lennox Signature Stat does? I suspect that you know that
true answer to this question.
fiberglass in the attic. The local power company, Progress Energy,
conducted a blower door test and the technician said it was the tightest
house he'd ever tested. Before construction, the A/C subcontractor
recommended installing a 5-ton unit! We instead used the services of
www.calcs-plus.com here in Florida to help size the ductwork and equipment
properly, since we wanted to "get it right". I hope that we did. Since we
have a 4-ton air handler and a 3-ton condensor, I don't know if is is more
accurate to say that we installed a 3-ton 3.5-ton or 4-ton system here but
we tried to do our homework to get it sized correctly.
One other related issue that I'd like to clarify. . . The A/C subcontractor
said that they don't usually change the factory default settings on the DIP
switches. Ron at Lennox said that it is best to meaure the temperature
differential across the coil and adjust the fan speed DIP switches until you
acheive a TD of 18 - 22 degrees. Since the default factory setting is
apparently at the highest speed, it would seem that, to the extent that you
could lower it to the higher end of the 18 - 22 degree TD range, you would
be helping the humidity issue there as well.
Thanks so much for your time in replying.
If you want to control humidity, you sure as hell don't over size the
evaporator! That's going in the WRONG direction.
After reading the rest of your post, it's unfortunate, but your HVAC
contractor is out in left field. I would suggest that you find a competent,
reliable tech that is familiar with your Lennox equipment and have them go
over the complete system and set it up properly and locate any other
problems that may not have been diagnosed.
I would completely agree that it should not be the building contractors job
to educate the HVAC guys on their own trade, but I've seen an unfortunate
amount of ignorance out there. The old 500 SF per ton rule still seems to
dominate around here. That is why I question everything and look for
consensus on forums such as this.
increase in efficiency by having condenser of 4 ton and air handler of 3 ton
NOT in MILLION years! you contractor maybe good janitor if that.
not only will you have poor efficiency but you RH in house will be
I am not AC person but you better believe what I just told you.
OK, I finally got the ARI certification reference number for this system and
it is: 551503. The ARI directory shows the pair with a SEER of 18.6. I do
not know the lineset size, however, as I am not at the site location.
One other interesting point to note. regarding the ARI certification of this
unit. As installed with a 3 ton condensor and 4 ton air handler (ARI
reference # 551503) the unit is rated at 18.6 SEER. I looked for and found
the certification number for the same 3 ton condensor matched with the same
model air handler, except at 3 tons instead of 4 (ARI reference # 551667)
and the SEER rating for that matched set is only 17.75. Still good and
efficient, but less than the 18.6 obtained by oversizing the air handler.
As long as you slowed the fan down on the four ton unit to match the
condensor, it seems like it would even do better at lowering humidity. Am I
thinking straight on that one?
Do a FanHandler, David. Google it. These units are more reliable and
less expensive than OEM solutions like ECM motors.
If you're in the biz, you can buy one wholesale. Otherwise, you'll have
to find a contractor that will sell you one.
I've installed over 60 now, and all my customers are happy. We even have
them on our McQuay units in our own new building.
Just my 2 cents.
David are you having issues with humidity removal or was your question
directed more to a t-stat with buillt in humidity controls?
according to a sizing chart I recently picked up, lennox requires 7/8
suction for both 3 & 4 tons.
what cfm was the duct system sized for, and to refresh my memory didnt
you say the home size was around 2600 sf?
My original post was meant to confirm what I had thought; that despite what
my HVAC subcontractor was saying, these high end residential units do have
variable speed control beyond the "hard coded" DIP switch settings.
Clearly, the installer was out in left field on that one. I think that we
all agree with that.
Having confirmed that, I have been trying to determine if any of the
Honeywell thermostats offer the same features as the Lennox Signature Stat
(the subcontractor installed a Honeywell thermostat that does NOT offer
humidity control). Specifically, do the Honeywell units such as the
VisionPro IAQ control humidity by slowing down the air handler to remove a
greater latent load when humidity reduction is called for and do they
properly control the dual-stage compressor? I know that the Lennox
Signature Stat thermostat use the full capabilities of the equipment, but I
know that my clients would prefer the display on the Honeywell's. I don't
want to even consider a thermostat that wouldn't take maximum advantage of
the high-end equipment installed. I still have not received a definitive
answer on this question from this thread.
We live in Florida and so it seems like, to the extent that we can drop
humidity, the client will feel comfortable at a higher temperature and thus
the system will be more efficient. We are trying to achieve a balance of
maximum comfort and maximum efficiency.
I know that some on this thread were indicating that a bad decision was made
in using a 3-ton condenser and 4-ton air handler. It seems that the ARI
certification reference numbers that I posted here uphold that decision from
an efficiency perspective (someone please tell me where I am wrong here, if
I am). I talked Friday afternoon with a Lennox field rep who seemed quite
versed on their equipment. He said that you still want to slow the blower
down on the 4-ton air handler down to about 1,200 CFM to approximate volume
from a 3 ton unit, and that it is the larger coil on the 4-ton air handler
where the efficiency gains come from. He was not as much in favor of
setting the fan speed to a level that would achieve the 18 - 22 degree
temperature differential that the that Lennox factory technician suggested.
His argument was that the proper TD depends on the latent load which the
18 - 22 degree rule doesn't take into account.
Yes the house is 2,600 SF but it has low-E glass throughout, R-45 in the
ceiling and achieved the best blower door tightness results the technician
had ever seen. The duct work and equipment were not sized by the local A/C
subcontractor (they wanted to use 5 tons of cooling and told us we were
crazy to be using 3). The duct work and equipment sizing were done by
www.calcs-plus.com. This is their core business and they seem competent at
it. Unfortunately, I'm writing from home and don't have their specific
numbers for the house with me here.
In closing, it is not that we were getting poor performance out of the unit
as far as being uncomfortable in the home. It is just that we realized that
the system was not optimized with a thermostat that could best control
further humidity reductions. I am also quite suspicious that the air
handler DIP switches are still set at the 4-ton default factory setting.
Again, it is working, but it does not appear optimized. The way that I see
it, if you (or your client) pay for top of the line equipment, you want to
make sure that the equipment is optimized. Of course the same holds true
for low end equipment also. If is it a Camry or a Corvette, you still don't
want to be running with 10 PSI in the tires!
Thanks for your time in responding.
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