Dehumidification - Thermostat or Furnace?

[joe1234]

Hello.

I am replacing our entire HVAC cental air & heating system.

I am looking at variable speed motor furnaces like the Goodman GMV95.

When it comes to dehumidification, do I need a thermostat that has a humidity control?

My understanding is that furnaces with variable speed motors run the blower motor slower for a while to remove the humidity in the summer time for example. So is this "automatic" on furnaces with the "DEHUM" option, or does the thermostat tells the furnace when do dehumidifiy?

I guess I am confused with the entire dehumidification process and who (thermostat or furnace) makes the call?

[Jay11J]

Originally Posted by joe1234

When it comes to dehumidification, do I need a thermostat that has a humidity control?

You can if you want. I have it and been pleased with it. The stat I would suggest is the Honeywell IAQ (TH9431), or if the budget don't allow you to do the IAQ, then a basic humidistat can work.

So is this "automatic" on furnaces with the "DEHUM" option, or does the thermostat tells the furnace when do dehumidifiy?

There's two way that it can be done.

-Furnace controls the blower ramping speed. (There are a few ways it can be set up by the dipswitch settings)

Say, it was set up the way I perfer. When there is a call for cooling, the blower comes on at 50% speed of full cooling speed for about 30 secs to get the coil cooled. Then the blower ramps up to 80% speed for about 7 min, then ramps up to full speed after 7 min, and it don't care of the humidity is high or not untill the t-stat no longer calls for cooling.

-Use of a T-stat like the IAQ. If the humidity is higher than set point (I have mine at 45%) the blower will stay at the 80% speed untill the humidity is removed. With this, you can get better comfort with this. I used to be a person who ran our A/C at 72˚, now with this, I run it 76˚, and I am comfortable!

[Jarredsdad]

Originally Posted by joe1234

I guess I am confused with the entire dehumidification process and who (thermostat or furnace) makes the call?

Both. Assuming a top notch IAQ stat, the stat directs the Furnace control board to run the Furnace Fan and A/C in Cooling Mode or DEHUM Mode.

First thing the A/C works on is humidity before working on sensible temp.

When Jay had His system at 72 the unit worked so much to get to set point humidity wasn't a problem by the time the stat was satisfied.

These days, Jay's system is looking at a Cooling call and a DEHUM call. Cooling gets handled first so his system starts in cooling until that is satisfied.

When that happens and there is still a call for DEHUM the system slows the fan down (Trying not to over cool the house) with the A/C (outdoor unit) still running.

Slowing the fan down also allows the air to stay in contact with the now colder coil longer and condense more moisture faster.

Make sense?

[joe1234]

Originally Posted by Jay11J

Say, it was set up the way I perfer. When there is a call for cooling, the blower comes on at 50% speed of full cooling speed for about 30 secs to get the coil cooled. Then the blower ramps up to 80% speed for about 7 min, then ramps up to full speed after 7 min, and it don't care of the humidity is high or not untill the t-stat no longer calls for cooling.

-Use of a T-stat like the IAQ. If the humidity is higher than set point (I have mine at 45%) the blower will stay at the 80% speed untill the humidity is removed. With this, you can get better comfort with this. I used to be a person who ran our A/C at 72˚, now with this, I run it 76˚, and I am comfortable!

Thanks Jay for the explanation. I am a bit confused however. In the first paragraph it sounds like you have no humidity "knowledge" - the furnance simply does a fixed pre-set ramp up to remove moisture and when the desired temperature is reached, the cooling stops. Humidity level may or may not be at the desired level when the cooling stopped.

On the second paragraph, a smart thermostat with knowledge of the humidity level inside the house makes the call when to stop cooling based on humidity level set at the thermostat.

You said your prefer method is paragraph #1 - no need for humidity level knowledge, but on your second paragraph, you say your have the thermostat humidity level set to 45% which you prefer because you are comfortable at 76% and having less humidity.

So which method do you have/prefer?

[joe1234]

Originally Posted by Jarredsdad

Both. Assuming a top notch IAQ stat, the stat directs the Furnace control board to run the Furnace Fan and A/C in Cooling Mode or DEHUM Mode.

First thing the A/C works on is humidity before working on sensible temp.

When Jay had His system at 72 the unit worked so much to get to set point humidity wasn't a problem by the time the stat was satisfied.

These days, Jay's system is looking at a Cooling call and a DEHUM call. Cooling gets handled first so his system starts in cooling until that is satisfied.

When that happens and there is still a call for DEHUM the system slows the fan down (Trying not to over cool the house) with the A/C (outdoor unit) still running.

Slowing the fan down also allows the air to stay in contact with the now colder coil longer and condense more moisture faster.

Make sense?

Yes and thank you so much. This is a very important topic which is not well understood.

So let's see if I have this correct:

An air handler (furnace) with a veriable speed blower has two calls a thermostat can request:

* COOLING

* DEHUM (dehumidify)

Cooling request simply runs the furnace blower at full speed* (with outside condensor on) until the thermostat reaches the desired temperature. Humdity level is not considered and may be at any level when the desired temperature is reached.

Dehumidify request runs the furnace blower at a slower speed (with the outside condenser on) to freez-up the coil causing faster condensation and thus removing humidity but not cooling the house faster. The temperature may go lower than set point in order to achive correct humidity level thus the house may be colder then desired.

So a smart thermostat like the Honeywell Pro IAQ that has a humidity level control and obviously a temperature level, can make the proper call to cool or to dehumidify? When both the humidity level and temperature levels are set on the thermostat, does the thermostat always start with the humidity call first followed by the temperature call, or can these two be switched around?

*( I put an asterik above on the blower running full speed to keep it simple, but I undersand that the furnace can be set as Jay explained to ramp up at set points to help remove moisutre first, but the actual moisture level is not being considered)

[Jay11J]

Originally Posted by joe1234

In the first paragraph it sounds like you have no humidity "knowledge" - the furnance simply does a fixed pre-set ramp up to remove moisture and when the desired temperature is reached, the cooling stops. Humidity level may or may not be at the desired level when the cooling stopped.

Correct.

On the second paragraph, a smart thermostat with knowledge of the humidity level inside the house makes the call when to stop cooling based on humidity level set at the thermostat.

Correct.

On the IAQ, it may "overcool" the home up to 3˚ below set point of the humidity is high. I've only seen this happen a few times on my system on a humid rainy day when the cooling load was low.

You said your prefer method is paragraph #1 - no need for humidity level knowledge, but on your second paragraph, you say your have the thermostat humidity level set to 45% which you prefer because you are comfortable at 76% and having less humidity.

Sorry wasn't clear on that. I perfer this "Profile of blower set up" if you did not get the IAQ or any type humidity control.

Goodman, I think has 5 blower profile set up, and the one I mention about is the one I perfer

So which method do you have/prefer?

I prefer the IAQ control vs a fixed ramp up time.

[Jay11J]

Originally Posted by joe1234

So a smart thermostat like the Honeywell Pro IAQ that has a humidity level control and obviously a temperature level, can make the proper call to cool or to dehumidify? When both the humidity level and temperature levels are set on the thermostat, does the thermostat always start with the humidity call first followed by the temperature call, or can these two be switched around?

It can be either way around.

On the IAQ, a few things it does.

Say for an example on mine. I have it set at 45% humidity, and cooling set at 76˚.

It's a hot dry day, house is 43% humidity, the house gets to temp, the IAQ will turn on the A/C to cool the house, and the blower will run at full speed.

Then you have company in the house, or doing laundry or dishes, the humidity in the home climbs up to 46%, the IAQ then tells the blower to slow down and remove the humidity.

The house temps reach the IAQ set point for temp, the humidity is still at 46% **, the IAQ shuts down the A/C. and waits for the next call for cooling or dehumd.

You still got laundry going, house is still at temp, but the humidity rise up to 51%, now the IAQ will start the A/C and run in Dhumid mode, the blower stays at a slower speed untill the humidity is below 50%, or if room temps falls below 3˚ of set point.

**= I did not mention in my last post that the IAQ will start the "over cooling" if the humidity is 5% over set point.

[joe1234]

Originally Posted by Jay11J

It can be either way around.

Thanks Jay.

If it can bet set both ways on the thermostat, unless I am missing something, you will want to set it to first call to Dehumidify and then follow by Temperature call.

Having it Dehumidifying first has several benefits:

* Less energy spent ( removing moisture first makes for faster cooling later ).
* Less changes of over cooling the house.
* More comfortable feeling?

I really don't see any benefits of Cooling first followed by Dehumidifying second, or am I missing something?

[Jay11J]

There is no set up to do one or the other first.

It all depends on what the weather is, or what's going on inside of the home.

The IAQ will either cool or dhumid as needed in the home.

[joe1234]

Does the HoneyWell IAQ shows on the display panel if it is "Dehumidifying" or "Cooling"? Or does it just show cooling when running the a/c?

Also, with a variable speed air handler motor and with this thermostat, can you tell the thermostat to run furnace fan only at say 30% motor speed?

I like the idea of keeping the fan blower running 24/7 for filtration and to reduce air stagnation, but how slow can you go, or is that a fuction of the furnace?

[Jay11J]

Originally Posted by joe1234

Does the HoneyWell IAQ shows on the display panel if it is "Dehumidifying" or "Cooling"? Or does it just show cooling when running the a/c?

It will only show "COOL ON" any time system calls for cooling or dhumd.

Also, with a variable speed air handler motor and with this thermostat, can you tell the thermostat to run furnace fan only at say 30% motor speed?

No, the only one I am aware that can do this a Carrier system with a Carrier Infifnity t-stat.

I like the idea of keeping the fan blower running 24/7 for filtration and to reduce air stagnation, but how slow can you go, or is that a fuction of the furnace?

Most brand runs at 50% speed of cooling speed.

For an example, I have a two stage Trane 60k furnace with a Trane XR11 2 Ton A/C, my dip switch is set at 400 CFM per ton, so it wil run 800 CFM at full speed, and fan on it only runs 400 CFM.

[Jarredsdad]

Originally Posted by Jay11J

There is no set up to do one or the other first.

It all depends on what the weather is, or what's going on inside of the home.

The IAQ will either cool or dhumid as needed in the home.

Hate to say it Joe. But here, you really need to think of Microsoft inside your tstat deciding what is best for you.

For now, but changes are coming.

Honeywell now has wireless tstat systems that have a remote control in addition to the "on the wall" stat.

[joe1234]

Originally Posted by Jarredsdad

Hate to say it Joe. But here, you really need to think of Microsoft inside your tstat deciding what is best for you.

Now THAT is a scary thought

Microsoft running my home temperature.

[Jay11J]

Originally Posted by joe1234

Now THAT is a scary thought

Microsoft running my home temperature.

LOL

There is not Microsoft running in the t-stat.. Maybe it's a Mac, cuz it's white!

[Jarredsdad]

Why do you think we call it an electric bill?

LOL

[Radiorara]

You mention the 9431 IAQ Honeywell. I do not see any references to this model. Is this the newer second revision of the t-stat? The only one I have seen was the YTH9421C1002.

[Jay11J]

Originally Posted by Radiorara

You mention the 9431 IAQ Honeywell. The only one I have seen was the YTH9421C1002.

My bad. My finger hit the wrong key.. Yes, it's the 9421.