HVAC Fan cycling, how frequent?

[paulbates]

Better news. I went back through the manual. I make the ECM blower change it's speeds by having the fan on, then, turn it off and back on within 1-3 seconds. Using that method showed that are there three speeds I can select.

 

It turns out both of mine were configured for number 2 speed @ 200 watts, and that number 1 speed runs @100 watts, using my DTE meter app. With this lower power consumption, I'm going to try something closer to what Bill described.

 

Thanks for the RTM reminder! 

Edited November 3, 2015 by paulbates

[bipto]

Virtually all consumer motors have been brushless (ECM) for decades. Clogged filters combined with closed vents can overheat both the motor and control board due to higher power consumption and lead to premature failure. Newer electronics can sometimes circumvent failure by decreasing the fan motor's effort.

 

 

Unfortunately a number of furnaces are still PSC motors. I looked up the specs for the furnace Michael had shown in his energy efficiency label and it's a PSC motor (if I read the model number correctly).

 

Carrier 59SC2A Series 100 Furnace: http://dms.hvacpartners.com/docs/1009/public/02/59sc2a-02pd.pdf

 

In shopping for our new system last summer, I found that while the majority of manufacturers' high-efficiency (~92%+ AFUE) models were likely to offer ECM technology, it seemed to me that most models in the 80% class (value / contractor lines) are still equipped with PSC motors, no doubt to hold cost down...   

[bipto]

Better news. I went back through the manual. I make the ECM blower change it's speeds by having the fan on, then, turn it off and back on within 1-3 seconds. Using that method showed that are there three speeds I can select.

 

It turns out both of mine were configured for number 2 speed @ 200 watts, and that number 1 speed runs @100 watts, using my DTE meter app. With this lower power consumption, I'm going to try something closer to what Bill described.

 

Thanks for the RTM reminder! 

That's great news Paul, anxious to hear how you make out...

[larryllix]

With the multispeed ECM motor there is usually an extra speed connection to use for HA without getting into complex "is it already running" logic.

 

Usually the highest speed selected rules and the lower speeds do not matter as you are only selecting logic inputs.

 

The PSC motors may not like this idea though as two windings being energised can produce some unexpected results. My past home had  a multispeed PSC motor with a few relays that switched to the fastest speed called. When I spec'ed my current furnace, about 7 years ago, the ECM motors were just coming on the scene and hard to find.

[MWareman]

Unfortunately a number of furnaces are still PSC motors. I looked up the specs for the furnace Michael had shown in his energy efficiency label and it's a PSC motor (if I read the model number correctly).

 

Carrier 59SC2A Series 100 Furnace: http://dms.hvacpartners.com/docs/1009/public/02/59sc2a-02pd.pdf

Thanks for this! Very helpful. Seems the motor is claimed to support multiple speeds, but I'm not sure how to use that. I'd like the furnace fan to run full speed for heating or cooling, but low speed for periodic air mixing. Not sure how to do that - I guess I need to RTFM...

[paulbates]

Thanks for this! Very helpful. Seems the motor is claimed to support multiple speeds, but I'm not sure how to use that. I'd like the furnace fan to run full speed for heating or cooling, but low speed for periodic air mixing. Not sure how to do that - I guess I need to RTFM...

 

FWIW, when i follow the path in Carrier for your furnace to the owners manual, the link showed that I had selected it, meaning its the same file as mine. The owner's manual is not bespoke to any particular model of their condensing gas furnace line. Which means you should be able to change the speed used for the "fan on" call by manipulating the fan control on the thermostat.

 

Started here

Clicked this link under the owners manual section

 

This is procedure I followed on page 8

"Comfort Fant - On all but the base series furnace, the blower speed can be increased or decreased if desired due to change of seasons, large gatherings in your home, etc. Simply change your FAN from ON to OFF for 1 to 3 seconds (or AUTO depending on your thermostat), and then return to ON. The blower will switch to the next higher speed. There are at least three speeds to choose from. If the blower is running on its highest speed, a request to change will direct the blower to return to its lowest speed"

[MWareman]

No fan control options on the thermostat. Besides, the fan is controlled by a separate Honeywell vent controller. I'm also confused - the document says both 'single speed' and lists that multiple speeds are possible.

[Scott847]

No fan control options on the thermostat. Besides, the fan is controlled by a separate Honeywell vent controller. I'm also confused - the document says both 'single speed' and lists that multiple speeds are possible.

 

I believe the single speed reference is saying that the furnace will only run at a single speed for heating or cooling. The multiple speed reference is to the PSC motor tap speed options that are set during installation for cooling and heating - the selected taps set the heating and cooling fan speeds, which results in the designed air delivery CFM.

 

The installation manual for the 59SC2A has an important note in all caps (see below). The cooling CFM is set to balance efficiency and dehumidification while avoiding freezing the evaporator coil. The heating CFM is set most importantly to avoid overheating due to too low CFM and also to keep the static pressure within design limits.

 

The notes also seem to indicate the Continuous Fan setting can only be set to one speed, which is the fan speed used for heating. Sorry for the bad news....

 

****************

 

*** 52SC2A installation manual http://dms.hvacpartners.com/docs/1009/Public/00/59SC2A-01SI.pdf

 

-- Note on page 29:

 

"2. Blower speed taps are not always in the same order. Factory default blower connections are as follows:

a. Heating airflow --- BLUE (also used for Continuous Fan)

b. Cooling airflow --- BLACK (enabled when the Y terminal is energized)

ADJUST THE BLOWER SPEED TAPS AS NECESSARY FOR THE PROPER AIR TEMPERATURE RISE FOR EACH INSTALLATION"

 

-- Note on page 74:

 

"3. Continuous Blower Mode

 When the R--to--G circuit is closed by the thermostat, the blower motor BLWM will operate on HEAT speed. Terminal EAC--1 is energized as long as the blower motor BLWM is energized. During a call for heat, the blower BLWM will stop during igniter warm--up (17 sec), ignition (7 sec), and blower--ON delay (25 sec), allowing the furnace heat exchangers to heat up more quickly, then restarts at the end of the blower--ON delay period at HEAT speed.

 When the thermostat “calls for cooling”, the blower motor BLWM will operate at COOL speed. When the thermostat is satisfied, the blower motor BLWM will operate an additional 90 sec on COOL speed before reverting back to continuous--blower (HEAT speed).

 When the R--to--G circuit is opened, the blower motor BLWM will continue operating for an additional 5 sec, if no other function requires blower motor BLWM operation."

 

[MWareman]

Thanks for the information - even though it's not good news.

[paulbates]

Given the way the documentation reads, many models have variable speed ECM motors. The speed changes & and the procedure to change them applies to those models. There are other models with the PSC motor and those are single speed.

 

The manual could be a little clearer about that, especially when there is one version of manual for a variety of models.

[larryllix]

My ECM fan has multiple logic inputs to control speeds.

 

When the heating mode is selected it starts on a very slow speed and then ramps up to   medium speed after about 45 seconds. Then (if selected) ramps up to a high speed about 5 minutes later. This is using two 24vac logic inputs.

 

One of them is phase sensitive to get more logical inputs. originally I had the input from another 24 vac supply that was fed off the other phase of my panel and the fan would not ramp up properly. A call to their tech support identified I was feeding it the wrong phasing and a feature that wasn't fully supported on the ECM motor was being enabled. I moved the breaker over to the other phase of my panel and it functioned properly ramping through the speeds again. That just amazed me that a motor 24vac input could be phase sensitive. All selected in the ECM programming.

 

The same fan motor also has two A/C 24vac  logical inputs that control three different speed selections.

 

Then there is a humidity lowering input (jumper to live)  that just slows all speeds down a bit. easily takes a humidistat dry contact. Work with A/C only.

 

IIRC there is two fan on logical inputs that do not use a logic table and only provide two discreet speeds.

 

On top of all these speeds the ECM motor continuously varies speeds to compensate for filter clogging and other air restrictions.

 

Timing and speed logic is all programmed by the furnace supplier using this generic GE ECM fan motor. When it goes defective I have to go back to the furnace manufacturer to get the same secret programming setup.

[Teken]

I wanted to provide the forum members some visual aids and statistics which illustrates why I followed a different path and approach. Below are three screen captures and one of them shows how much power is consumed when the furnace is taken out of the loop.

 

As you see the image called Fan Only consumes a modest 81.XX watts.

 

The next image shows the furnace brought on line with the (inline) exhaust fan (Fan & Furnace) which consumes 414.XX watts. Lastly, this is what my furnace consumes at its maximum speed out put which shows 551.XX watts.

 

This is why I automate the HVAC system depending upon season, temperature, humidity using the inline exhaust fan option only. The energy savings in this mode simply can not be over stated.

 

This has been a great discussion and lots of insight for me along with many others.

 

BTW: Larry, much thanks for that one link regarding the three different types of motors in use today. I saw something similar two years ago but it never addressed the GE style it was a good read!

[jerlands]

I think a common problem with single speed blower installations is the installer leaves the factory settings and doesn't adjust for ESP. 

 

 

Jon...

[larryllix]

I think a common problem with single speed blower installations is the installer leaves the factory settings and doesn't adjust for ESP. 

 

 

Jon...

There was typically a shutter on the fan output to close down the volume. The installer had to do calculations and close down the damper to suit the CFM required.

 

On squirrel cage fans the less air you move the less power is drawn and the fan speed increases slightly due to less load. If the adjusted damper restriction was removed you could overheat  your fan motor with volume overload.

[jerlands]

There was typically a shutter on the fan output to close down the volume. The installer had to do calculations and close down the damper to suit the CFM required.

 

On squirrel cage fans the less air you move the less power is drawn and the fan speed increases slightly due to less load. If the adjusted damper restriction was removed you could overheat  your fan motor with volume overload.

 

Similar to centrifigual pumps if you start without a load they can overspeed..  opening/closing dampers, dirty filters and timing of fresh air intake can all affect static pressure.

 

edit.. not to mention use of high efficiency filters.

 

 

Jon...

Edited November 4, 2015 by jerlands

[Teken]

Similar to centrifigual pumps if you start without a load they can overspeed..  opening/closing dampers, dirty filters and timing of fresh air intake can all affect static pressure.

 

edit.. not to mention use of high efficiency filters.

 

 

Jon...

 

Jon,

 

That is a huge thing I have seen first hand in several homes. Lots of folks were buying the very high (MERV) rated filters aka Hepa filters. Lots of people soon found out when the furnace had issues or declared a fault code why it was.

 

Every furnace has a maximum static pressure it can work in and what the MERV rated filters it can accommodate.

[jerlands]

Another common problem seen is use of flex ducting...

It's hard to overstate the impact of good design and installation on efficiency.

 

Jon...

Edited November 4, 2015 by jerlands

[Teken]

Another common problem seen is use of flex ducting...

 

 

Jon...

 

That isn't very common here in Canada but after watching lots of home renovation shows it seems to be very pervasive in America. I find it very interesting to see different techniques and methods used to accomplish the same end goal.

 

Then again seeing HWT, AC units in a persons attic is really a head shaker for me . . . I guess also seeing people with out basements is also odd to for some of us. I can understand if the area is prone to flooding or where the soil is not agreeable to build a basement but it seems like a wasted opportunity, no?

[jerlands]

That isn't very common here in Canada but after watching lots of home renovation shows it seems to be very pervasive in America. I find it very interesting to see different techniques and methods used to accomplish the same end goal.

 

Then again seeing HWT, AC units in a persons attic is really a head shaker for me . . . I guess also seeing people with out basements is also odd to for some of us. I can understand if the area is prone to flooding or where the soil is not agreeable to build a basement but it seems like a wasted opportunity, no?

 

I love basements   Here in Houston (a place I never thought I'd find myself) basements aren't practical and the norm is slab with attic installations.

 

(expansion in the South began after the advent of A/C)

 

Jon...

Edited November 4, 2015 by jerlands

[larryllix]

That isn't very common here in Canada but after watching lots of home renovation shows it seems to be very pervasive in America. I find it very interesting to see different techniques and methods used to accomplish the same end goal.

 

Then again seeing HWT, AC units in a persons attic is really a head shaker for me . . . I guess also seeing people with out basements is also odd to for some of us. I can understand if the area is prone to flooding or where the soil is not agreeable to build a basement but it seems like a wasted opportunity, no?

From friends that have A/C in the attic they love it and claim they would never change that. Mind you if you put the compressor up there it will be a head shaker too. so just the evaporator coils.

 

They claim the cool air drops down on the rooms from ceiling vents and has a better distribution. makes sense for economy of air pumping but the heat in an attic may be a detriment for keeping the air cold during transit.

 

Turbulence is not your friend!

 

I have a pipe run to my MBR that howls after a certain fan speed. I changed the run from 4" to 5" and changed the furnace boot size up too but it didn't work. Unfortunately, the only way to supply this run was right out of the plenum.  grrrr...

 

Right now I have a short piece of flex to accommodate an offset in the run which hasn't done jack skeet for the problem. Some day I am making a double walled pipe muffler that is supposed to work fairly well for stopping noise transmission. Some day I will be deaf anyway.

[paulbates]

To finish off what I did:

• Alternating fans 10 mins on and off - keeps temps even across the living spaces without 24x7 operation
• If the fan is run right after an AC cycle completes, it forces the moisture from the condenser back into the living space. Give it time to drain before fan cycling (22 minutes)
   

HVAC Cycle US Fan - [ID 0030][Parent 0017]

If
        'HVAC / Upstairs' Heat/Cool State is Idle
    And 'HVAC / Upstairs' Humidity Active is False
    And 'Attic / Attic-Damper Control' Status is Off
    And 'Kitchen / Doorwall Sensor-Opened' Status is Off
 
Then
        Wait  22 minutes 
        Repeat While $True is $True
           Set 'HVAC / Upstairs' Fan Mode On
           $V_US_Fan_Cycle_Sequence += 1
           Wait  10 minutes 
           Set 'HVAC / Upstairs' Fan Mode Auto
           Wait  10 minutes 
 
Else
        Set 'HVAC / Upstairs' Fan Mode Auto
        $V_US_Fan_Cycle_Sequence  = 0