Just had my fall HVAC system checkup and the tech recommended this...

[spacejamz]

I have dual zone controller (upstairs/downstairs) to control a single HVAC unit...

He explained that when the upstairs thermostat makes a call, the damper restricts the flow to the downstairs (and vice versa when the downstairs thermostat makes a call)...this is common sense...He then added when the damper restricts the air flow to the other zone, it does make the hvac unit work harder than if there was no damper...which also kinda makes sense...

Since I have a new ecobee that I can add room sensors to, he suggested doing room sensors for upstairs and downstairs and just leaving both dampers open (so the whole house will get heated/cooled when the system runs) but the calls to the HVAC unit will only be based on the appropriate sensor (upstairs or downstairs) which can be controlled when setting the hvac schedule...

So when I am downstairs during the day (work from home and office is downstairs), it will only use the downstairs sensor to control the HVAC and then when I go to bed, (master bedroom is upstairs) it will only use the upstairs sensors to control the HVAC. This should make the HVAC work less harder (so it should last longer??) than using the dampers...

The downstair thermostat would not longer used...

Does any of this sound too crazy? It sounds about right to me but just wanted to get other opinions...

 

[RPD]

It's a band aid solution and neither is ideal really. The unit "works" harder in that the damper closing creates more resistance to the fan so less air goes that direction. This really doesn't affect the operational life of the unit, just inefficient because you are using more energy.

The problem with even the multiple sensors is given the mode (heating or cooling) other areas are going to be even further away from your set point. Eg you have a room that is usually colder than where the stat is. Well if your unit is trying to heat that colder space to set point, that means the other areas will be much warmer than your set point. Opposite for cooling mode.

You could just leave the dampers fully open and at the end of the day you aren't going to notice much of any change.

 

[pcgeek11]

Blocking damper reduces air flow, less air flow equals less work means less current draw on the blower motor.

 

[RPD]

Blocking damper reduces air flow, less air flow equals less work means less current draw on the blower motor.

Yea thats not how fans and motors work.

 

[pcgeek11]

Yea that's not how fans and motors work.

Yes, it is how they work. It is opposite of what your intuition tells you, but it is a fact. I didn't believe it either Many years ago when I was informed so I had to try it myself.

Get you a blower and put an amp-meter on it.
Then block off the air flow and witness the magic.
The blower gets unloaded as it isn't doing any work, the current drops and the rotational speed of the blower will increase.

Or you can look it up through the magic of the internet or some engineering web sites.

 

[RPD]

Yes, it is how they work. It is opposite of what your intuition tells you, but it is a fact. I didn't believe it either Many years ago when I was informed so I had to try it myself.

Get you a blower and put an amp-meter on it.
Then block off the air flow and witness the magic.
The blower gets unloaded as it isn't doing any work, the current drops and the rotational speed of the blower will increase.

Or you can look it up through the magic of the internet or some engineering web sites.

I'm convinced.
Oh wait, there's those pesky fan laws and also what type of motor the fan is being ran by matters as well.
Here's a quick read - https://foxfamilyhvac.com/psc-motor-vs-constant-torque-motor-vs-variable-speed-motor/
But please continue to educate me.

 

[Lost_in_the_HTTP]

"So when I am downstairs during the day (work from home and office is downstairs), it will only use the downstairs sensor to control the HVAC and then when I go to bed, (master bedroom is upstairs) it will only use the upstairs sensors to control the HVAC. This should make the HVAC work less harder (so it should last longer??) than using the dampers..."


So, you want to heat/cool the whole house based on the temperature you want in one room? Doesn't that kind of defeat the whole purpose of zoning? Why not set the Stat using a timer or something and forget all the added sensors?

This seems more like a situation for a couple of Mini-Splits that run independently of each other.

 

[pcgeek11]

It was obvious I was talking about a simple PSC Blower motor (off and on) and not a Variable speed or constant torque motor and controls configuration.

Sorry I wasn't more clear on this.

Good link in your post too.

 

[RPD]

It was obvious I was talking about a simple PSC Blower motor (off and on) and not a Variable speed or constant torque motor and controls configuration.

Sorry I wasn't more clear on this.

Good link in your post too.

Tell me you didn't read the link without saying you didn't read the link.

 

[pcgeek11]

Tell me you didn't read the link without saying you didn't read the link.

I did read your link.

Understanding Centrifugal Fan Motor Performance

Since air is what we work with it makes sense to insure that our students understand airflow and fan performance. Fan motor performance is o...

hvacrfundamentals.blogspot.com

The most convincing way to teach this concept is to have students figure it out for themselves using a centrifugal blower. Have them operate a centrifugal blower in free air with no restriction and measure both the amp draw and the fan RPM. Note that most centrifugal blowers cannot operate in free air for an extended time without overheating, so try and keep the free air operating time to a minimum. Next have them block one side of the air intake with a piece of cardboard and recheck the amp draw and RPM. Typically the increase in RPM is immediately obvious, but measurements prove the point. Have them slide the cardboard to block the intake only half way while watching the amp draw. A few minutes of experimentation will convince the students that blocking the intake actually causes an increase in RPM and a decrease in the motor amp draw. Next have them partially block the fan outlet while checking the amp draw. Once again, the amp draw will decrease.

Note that what I have been discussing assumes a "regular" PSC blower motor. ECM blower motors behave differently because they are programmed to adjust their output according to the resistance they are working against, but that is an entirely new discussion which I will save for another article.

I should also state that a regular bladed fan functions are different.... Before you call me out on that too.

 

[RPD]

I did read your link.

Understanding Centrifugal Fan Motor Performance

Since air is what we work with it makes sense to insure that our students understand airflow and fan performance. Fan motor performance is o...

hvacrfundamentals.blogspot.com

The most convincing way to teach this concept is to have students figure it out for themselves using a centrifugal blower. Have them operate a centrifugal blower in free air with no restriction and measure both the amp draw and the fan RPM. Note that most centrifugal blowers cannot operate in free air for an extended time without overheating, so try and keep the free air operating time to a minimum. Next have them block one side of the air intake with a piece of cardboard and recheck the amp draw and RPM. Typically the increase in RPM is immediately obvious, but measurements prove the point. Have them slide the cardboard to block the intake only half way while watching the amp draw. A few minutes of experimentation will convince the students that blocking the intake actually causes an increase in RPM and a decrease in the motor amp draw. Next have them partially block the fan outlet while checking the amp draw. Once again, the amp draw will decrease.

Note that what I have been discussing assumes a "regular" PSC blower motor. ECM blower motors behave differently because they are programmed to adjust their output according to the resistance they are working against, but that is an entirely new discussion which I will save for another article.

I should also state that a regular bladed fan functions are different.... Before you call me out on that too.

From the article for PSC motors:
If there are restrictions in the airflow, like undersized ductwork, dirty filters, or dirty evaporator coils, PSC motors start to lose performance. They deliver less air because they tend to bog down, drawing higher amps. The system’s static pressure (or blood pressure) builds up and fights back against the PSC motor wearing it and the capacitor down through overheating.
It's a gross simplification but when you add additional static pressure (blockages) the motor has to work harder to over come said pressure.

 

[RPD]

Since you don't have a grasp on how HVAC systems work I'll just get you to the end and the point.
You add more static (blockage), motor speeds up to overcome this. In your "example" your motor isn't keeping up the same airflow and/or static. As I try to understand what you are claiming, in your example that motor drawing less amps is NOT overcoming the static and thus that air isn't going to make it through the duct and to the spaces anymore. So congratulations you played yourself, but hey you are saving energy!

 

[Lost_in_the_HTTP]

Can you two take your fish slapping dance out on the dock?

 

[PowerEngineer]

It seems to me that the real concern should be how the change in air flow affects the efficient delivery of heating produced by the energy intensive furnace and cooling by the energy intensive air conditioner. In general higher air flows help efficiency. Note that the heat exchangers in these systems are rated to operate down to some minimum air flow. I would be reluctant to be closing dampers if doing so would drop air flow below the design minimum. IMHO any change in the energy consumed by the system blowers is likely too small to worry about.

 

[pcgeek11]

From the article for PSC motors:
If there are restrictions in the airflow, like undersized ductwork, dirty filters, or dirty evaporator coils, PSC motors start to lose performance. They deliver less air because they tend to bog down, drawing higher amps. The system’s static pressure (or blood pressure) builds up and fights back against the PSC motor wearing it and the capacitor down through overheating.
It's a gross simplification but when you add additional static pressure (blockages) the motor has to work harder to over come said pressure.

So I just tried my experiment I suggested to you above with an amp-meter and a blower motor (centrifugal) and a simple on / off switch.

Running with no blockage 7.18 amps.
Block the inlet it drops down to 4.30 amps.
Block the outlet and it drops also to 4.30 amps.

Try it with the blower in your shop vacuum and an amp-meter.

Did you read my link?

Anyway I'm done...

 

[RPD]

So I just tried my experiment I suggested to you above with an amp-meter and a blower motor (centrifugal) and a simple on / off switch.

Running with no blockage 7.18 amps.
Block the inlet it drops down to 4.30 amps.
Block the outlet and it drops also to 4.30 amps.

Try it with the blower in your shop vacuum and an amp-meter.

Did you read my link?

Anyway I'm done...

Yes you don't understand how airflow and external static works. You are done. It takes energy (aka fan motor horse power) to over come resistance in the duct. If you don't do this, guess what air doesn't make it to the end of the duct. Would you like to continue so boldly proclaiming your ignorance in this area?

 

[RPD]

Can you two take your fish slapping dance out on the dock?

No, everyone must know my dick is the widest.

 

[skyking]

this is going to be sooo much fun when I design my HVAC ducting next year.
We can take this show to Pike Place market.