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Posted by David Jensen on May 10, 2007, 8:58 am
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>>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.
> 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.
Are you saying that most higher end residential A/C equipment uses ECM
motors?
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>> 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.
> Correct
>> 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.
> 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.
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
construction.
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>> 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 temperature.
> Ummmm......no....he missed the mark on that one.
I'm glad to see that humidity is controllable with the variable speed air
handler, beyond what I was being told.
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>> 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.
> 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.
>> FWIW, this is on a Lennox 18.6 SEER heat pump. Sorry that I don't have
>> the model info with me here.
> 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)
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.
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> FWIW, if you have humidity issues because of an oversized system, thats a
> whole different problem.
This house has 2581 square feet and has Low-E double pane windows and R-45
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.
David
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Posted by <kjpro on May 10, 2007, 9:20 am
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<snip>
> 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.
show/hide quoted text
> You see that this is a 4-ton air handler and a 3-ton condensor unit. It
> my understanding that by "mismatching" the two that you can effectively
> increase the efficiency. Is that correct?
<snip>
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.
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Posted by David Jensen on May 10, 2007, 9:49 am
show/hide quoted text
>> 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?
> 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.
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Posted by <kjpro on May 10, 2007, 10:16 am
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> >> 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.
> > is
> >> my understanding that by "mismatching" the two that you can effectively
> >> increase the efficiency. Is that correct?
> > 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
> > 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
show/hide quoted text
> to educate the HVAC guys on their own trade, but I've seen an unfortunate
> amount of ignorance out there.
<snip>
It's not just our trade, unfortunately, it's happening EVERYWHERE.
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Posted by gofish on May 10, 2007, 11:22 am
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>> 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?
>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 for one would love to see David post the ARI number for the
mismatched system......without such ARI certification number his
system is probably 10 seer at best.
What size is the lineset?
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>>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.
> 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.