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do high-end residential heat pumps with variable speed air handlers truly vary the speed to control humidity?

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do high-end residential heat pumps with variable speed air handlers truly vary the speed to control humidity? David Jensen 05-09-2007
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Posted by David Jensen on May 9, 2007, 9:24 am
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
temperature.

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.

David



Posted by Noon-Air on May 9, 2007, 10:15 am

>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.

> 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.

> 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.

> 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)

FWIW, if you have humidity issues because of an oversized system, thats a
whole different problem.



Posted by on May 9, 2007, 11:06 am

>
>>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.
>
>> 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.
>
>> 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.
>
>> 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)

please state your reasons WHY the vision pro 8000.


>
>FWIW, if you have humidity issues because of an oversized system, thats a
>whole different problem.
>


Posted by Noon-Air on May 9, 2007, 11:12 am

>
>>
>>>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.
>>
>>> 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.
>>
>>> 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.
>>
>>> 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)
>
> please state your reasons WHY the vision pro 8000.

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.
anything else??

>>FWIW, if you have humidity issues because of an oversized system, thats a
>>whole different problem.
>>
>



Posted by on May 9, 2007, 6:43 pm

>
>>
>>>
>>>>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.
>>>
>>>> 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.
>>>
>>>> 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.
>>>
>>>> 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)
>>
>> please state your reasons WHY the vision pro 8000.
>
>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.
>anything else??

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
screens.


>
>>>FWIW, if you have humidity issues because of an oversized system, thats a
>>>whole different problem.
>>>
>>
>


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