Variable speed air handler settings

ChrisJ

Ok something I'm not quite understanding and have had no luck finding information on is how a variable speed air handler is set up.

Not, how to change the speeds but why.

For example, I see units list different settings for different CFMs per ton.

Why would you run on anything but the lowest setting? In the TAM7's case 350 CFM per ton? Wouldn't this yield the best moisture removal, lowest power consumption and lowest noise?

I realize there must be a reason, otherwise none of the units would allow you to change the blower setting. But what is it?

I'm assuming this is directly related to blower speed, but maybe not?

Tinman

Without giving it a whole lot of thought, the first thing that comes to mind is the different static pressures you'll encounter in different duct systems. As the static changes the cfm changes. They are inversely proportional to one another.

ChrisJ

Stephen Minnich said:

Without giving it a whole lot of thought, the first thing that comes to mind is the different static pressures you'll encounter in different duct systems. As the static changes the cfm changes. They are inversely proportional to one another.

But it also shows a range of static pressures for each,

From .1 to .9.

This is what's so confusing.

Ironman

350 cfm per ton may cause the head pressure to go too high in some heat pump systems on a mild day in the heating mode. This is especially true with the new micro channel coils.

Techman

ChrisJ, look at the air as if it were water, only a certain amount of air will flow thru a duct at a given pressure(static).All of the "devices"in the duct system cause a "resistance to flow"of the air,including the duct itself, and the fan has to deliver the proper CFM to each zone/room after all of the resistances are figured in.Just like in hydronics, the speed and quantity (CFM/GPM) of the fluid(air) can/will make noise,if done wrong. Doing the Load Estimate will dictate the CFM needed and the ECM motor should follow suite. There are several "classes"ECM motors available,also.

Now, there are only two types of duct systems.
Type#1-Those duct systems put in by me or by my EXPERT PROFESSIONAL TinKnocker Buddy, or
Type #2-Those duct system put in by others.

HOW THE HELL AM I SUPPOSED TO KNOW HOW ANY OF THESES SYSTEMS WERE DESIGNED/INSTALLED. lol.

Interested in some reading material,there,Chris?

ChrisJ

Techman said:

ChrisJ, look at the air as if it were water, only a certain amount of air will flow thru a duct at a given pressure(static).All of the "devices"in the duct system cause a "resistance to flow"of the air,including the duct itself, and the fan has to deliver the proper CFM to each zone/room after all of the resistances are figured in.Just like in hydronics, the speed and quantity (CFM/GPM) of the fluid(air) can/will make noise,if done wrong. Doing the Load Estimate will dictate the CFM needed and the ECM motor should follow suite. There are several "classes"ECM motors available,also.

Now, there are only two types of duct systems.
Type#1-Those duct systems put in by me or by my EXPERT PROFESSIONAL TinKnocker Buddy, or
Type #2-Those duct system put in by others.

HOW THE HELL AM I SUPPOSED TO KNOW HOW ANY OF THESES SYSTEMS WERE DESIGNED/INSTALLED. lol.

Interested in some reading material,there,Chris?

Always am interested in reading material!
What do you recommend?

The static pressure isn't making sense, because American Standard's chart shows different ranges for static pressure at the same cfm per ton. That's what I'm not understanding.

I'm trying to size everything for less than 0.1 per 100'. Whether or not it's actually going to work out like that, who knows.

Ironman

ChrisJ said:

Techman said:

ChrisJ, look at the air as if it were water, only a certain amount of air will flow thru a duct at a given pressure(static).All of the "devices"in the duct system cause a "resistance to flow"of the air,including the duct itself, and the fan has to deliver the proper CFM to each zone/room after all of the resistances are figured in.Just like in hydronics, the speed and quantity (CFM/GPM) of the fluid(air) can/will make noise,if done wrong. Doing the Load Estimate will dictate the CFM needed and the ECM motor should follow suite. There are several "classes"ECM motors available,also.

Now, there are only two types of duct systems.
Type#1-Those duct systems put in by me or by my EXPERT PROFESSIONAL TinKnocker Buddy, or
Type #2-Those duct system put in by others.

HOW THE HELL AM I SUPPOSED TO KNOW HOW ANY OF THESES SYSTEMS WERE DESIGNED/INSTALLED. lol.

Interested in some reading material,there,Chris?

Always am interested in reading material!
What do you recommend?

The static pressure isn't making sense, because American Standard's chart shows different ranges for static pressure at the same cfm per ton. That's what I'm not understanding.

I'm trying to size everything for less than 0.1 per 100'. Whether or not it's actually going to work out like that, who knows.

Uh... Do you you use a manometer to check static after installation?

Setting a constant torque motor to the correct cfm is like setting a 3 speed circulator on a hydronic loop. You use the pump curve based upon the circuit's head and your desired gpm. Same thing here: determine the static and select your desired cfm at that static.

Matt_67

It leads to better moisture removal and lower noise but not lowest energy usage. Energy usage increases when suction pressure lowers. Total system capacity also drops. I attached a performance chart for an old copeland a/c compressor that shows what happens.

ChrisJ

Matt said:

It leads to better moisture removal and lower noise but not lowest energy usage. Energy usage increases when suction pressure lowers. Total system capacity also drops. I attached a performance chart for an old copeland a/c compressor that shows what happens.

@Matt

Thank you for posting! That chart is awesome, it's really interesting to see what the EER does vs temperature.

I knew there was a relation but to see it in writing is nice.

ChrisJ

I found this PDF to be a good read.

Harvey Ramer

Also, most ECM blowers are constant cfm. They will automatically vary their speed in an attempt to maintain the cfm they are set for. This holds true up with the rpms ramping up to about .7 esp. After that the cfm setting will no longer be accurate and the motor will enter the danger zone of overheating. Likewise if you design your duct system for an esp of .5 or lower, the blower will automatically ramp down to maintain proper cfm. The energy consumption follows the blower speed. An esp of .5 is a good number to shoot for. You need manual-d to design your duct. What you want to shoot for is an equal resistance to flow from the airhandler unit to the termination of each duct run. Obviously each duct run may be carrying a different cfm and will need to be sized appropriately. Likewise the trunk line should be reduced no less than every time a 50% reduction in airflow occurs due to duct run outs. Don't forget to calculate every item in esp number. Including vents, return grills, filters is a biggie. Try to keep the face velocity on return grills and filters to 300 fpm or less. That reduces the pressure drop on the return and it will be so quiet you won't hear it. Guarranteed.

Also, be realistic with your duct runouts and try to not go more than around 125ish cfm per vent. That will help with air distrubution and duct balancing. That is with an assumption of normalcy in a residential application. Sometimes one must do what one must.

Oh, and don't forget about the coil. Normally if you get an airhandler the pd for the coil is already included in the esp charts. If you decide to get an individual coil and blower box, due to manual-s requirements, you will need to calculate the coil pd in the blower esp. And the coil pd numbers are with a dry coil. The pd increases some with a wet coil, so use some discretion. Same goes for a coil on a furnace.

Confused yet? Duct sizing done correctly is a bit more complex than hydronics.

I have attached an engineering chart for Hart Cooley return filter grills.

Harvey Ramer

Further more. If I remember your loads correctly and your Sensible Heat Ratio, You may find yourself closer to 425 - 450 cfm per ton for proper cooling.

I'm still not quite buying your load numbers.

ChrisJ

Harvey Ramer said:

Further more. If I remember your loads correctly and your Sensible Heat Ratio, You may find yourself closer to 425 - 450 cfm per ton for proper cooling.

I'm still not quite buying your load numbers.

Why such a high cfm per ton? I'm assuming you're looking at my sensible load and figuring a 3 ton unit won't keep up with it running lower?


Definitely not confused anymore, it's all starting to make sense.
Have also been looking into ways I can insulate the attic etc.

I've got a lot of learning to do. To be honest, I'm loving it.

Harvey Ramer

When you look at your sensible load versus your latent load, you select a coil that comes closest to matching that ratio. Then you adjust the airflow To dial it in closer. Higher cfm means a higher ratio of sensible to latent cooling. Your goal is to simultaneously satisfy the latent and sensible loads to your desired targets. If you can do that with a higher cfm you should. It increases system efficiency.

Still not buying your load numbers.

Harvey Ramer

Did you figure internal latent gains?

ChrisJ

Harvey Ramer said:

When you look at your sensible load versus your latent load, you select a coil that comes closest to matching that ratio. Then you adjust the airflow To dial it in closer. Higher cfm means a higher ratio of sensible to latent cooling. Your goal is to simultaneously satisfy the latent and sensible loads to your desired targets. If you can do that with a higher cfm you should. It increases system efficiency.

Still not buying your load numbers.

That's ok, you can have them for free.

You feel sensible is way too high, and latent was on the low side I'm assuming?

Harvey Ramer said:

Did you figure internal latent gains?

Yes, but only for people. Off the top of my head, I think we did for 4 people in the house which there's currently only 3, but that may change in the near future.

Cooking would be a big one, but I try to avoid cooking indoors during the summer. I suppose clothes washer, shower etc would also be big ones.

Harvey Ramer

ChrisJ said:

Harvey Ramer said:

When you look at your sensible load versus your latent load, you select a coil that comes closest to matching that ratio. Then you adjust the airflow To dial it in closer. Higher cfm means a higher ratio of sensible to latent cooling. Your goal is to simultaneously satisfy the latent and sensible loads to your desired targets. If you can do that with a higher cfm you should. It increases system efficiency.

Still not buying your load numbers.

That's ok, you can have them for free.

You feel sensible is way too high, and latent was on the low side I'm assuming?

Yes

ChrisJ

Harvey Ramer said:

ChrisJ said:

Harvey Ramer said:

When you look at your sensible load versus your latent load, you select a coil that comes closest to matching that ratio. Then you adjust the airflow To dial it in closer. Higher cfm means a higher ratio of sensible to latent cooling. Your goal is to simultaneously satisfy the latent and sensible loads to your desired targets. If you can do that with a higher cfm you should. It increases system efficiency.

Still not buying your load numbers.

That's ok, you can have them for free.

You feel sensible is way too high, and latent was on the low side I'm assuming?

Yes

Do you feel sensible of 31K for 1600sqft is way too high considering a 70F indoor temp and 95F outdoor temp?

bob_46

Mr. Ramer is giving you excellent advise . Be careful with flex I personally hate it based on all the problems I've seen in the field.

ChrisJ

bob said:

Mr. Ramer is giving you excellent advise . Be careful with flex I personally hate it based on all the problems I've seen in the field.

Hi Bob,

Absolutely, I consider Mr Ramer's advise to be gold.
I certainly hope he never feels me asking questions is disagreeing with him or insulting, I'm just trying to understand what is being said, and why.

Harvey Ramer

ChrisJ said:

Harvey Ramer said:

ChrisJ said:

Harvey Ramer said:

When you look at your sensible load versus your latent load, you select a coil that comes closest to matching that ratio. Then you adjust the airflow To dial it in closer. Higher cfm means a higher ratio of sensible to latent cooling. Your goal is to simultaneously satisfy the latent and sensible loads to your desired targets. If you can do that with a higher cfm you should. It increases system efficiency.

Still not buying your load numbers.

That's ok, you can have them for free.

You feel sensible is way too high, and latent was on the low side I'm assuming?

Yes

Do you feel sensible of 31K for 1600sqft is way too high considering a 70F indoor temp and 95F outdoor temp?

Thats not how you size AC in this climate. You aren't in florida.

I think your ratio of sensible to latent is skewed. The latent looks low. Have you calculated the infiltration and internal latent loads properly?

ChrisJ

Harvey Ramer said:

ChrisJ said:

Harvey Ramer said:

ChrisJ said:

Harvey Ramer said:

When you look at your sensible load versus your latent load, you select a coil that comes closest to matching that ratio. Then you adjust the airflow To dial it in closer. Higher cfm means a higher ratio of sensible to latent cooling. Your goal is to simultaneously satisfy the latent and sensible loads to your desired targets. If you can do that with a higher cfm you should. It increases system efficiency.

Still not buying your load numbers.

That's ok, you can have them for free.

You feel sensible is way too high, and latent was on the low side I'm assuming?

Yes

Do you feel sensible of 31K for 1600sqft is way too high considering a 70F indoor temp and 95F outdoor temp?

Thats not how you size AC in this climate. You aren't in florida.

I think your ratio of sensible to latent is skewed. The latent looks low. Have you calculated the infiltration and internal latent loads properly?

I didn't calculate it, someone else on the forum did but I don't want to mention their name as I don't feel it's my place. It took us several hours for us to do.

As far as calculating it in this climate, what numbers do you recommend for our area?

ChrisJ

Here's all of the pages, though we missed the ceiling load in the east bedroom, that was added later but I don't have that report with me.

Harvey Ramer

I'm not asking for names. I don't want to know. I'm sure I don't have all the info but I'm just hitting on it because it looks different than what I'm used to seeing for this Mid Atlantic/North Eastern climate.

A 20° DT between indoor and outdoor temp, typically brings good results in our climate. That is assuming proper coil selection and cfm setting.

If you had to pick between being comfortable for 5 months or for 5 weeks, which would you choose?

Harvey Ramer

ChrisJ said:

Here's all of the pages, though we missed the ceiling load in the east bedroom, that was added later but I don't have that report with me.

Ok, I believe your loads but I would change the target parameters.

This is comfortable
https://goo.gl/photos/ePprsYcZQELpXbY47

70° @ 50% Humidity I wouldn't want.
I would also change to a 20° DT for my outdoor target.

ChrisJ

Harvey Ramer said:

ChrisJ said:

Here's all of the pages, though we missed the ceiling load in the east bedroom, that was added later but I don't have that report with me.

Ok, I believe your loads but I would change the target parameters.

This is comfortable
https://goo.gl/photos/ePprsYcZQELpXbY47

70° @ 50% Humidity I wouldn't want.
I would also change to a 20° DT for my outdoor target.

Would an oversized coil running low CFM's drop that RH number?

I agree, 73F with RH in the 40s is nice, but what about when it's really hot out? How does your system do when it's 90+ out?

Harvey Ramer

It has no problem maintaining setpoint at all. There is typically enough fat built into load calcs along with the flywheel effect from the cooler nights that it doesn't become a problem. Even if your system would not quite reach setpoint by 1 or 2 degrees for a couple hours, the extended runtime will lower the humidity and you will still achieve comfort.

ChrisJ

Hatterasguy said:

The propensity to oversize...............alive and well.

I can attest that 75F with 50% RH is plenty comfortable. You're not looking for a refrigerator.

Gordy

The humidity point is crucial. 75-78 with low to mid 40's humidity level is plenty comfortable.

I work with a guy who finds it comfortable to have his house 62 year around......that's nuts!

The fly wheel as Harvey stated, and load calc swag is more than sufficient to carry through hot spells.

No different than winter heating.

Tinman

"Feels like" 98 in the Chicago area right now. My office is 68. No such thing as too cold for me.

ChrisJ

Gordy said:

The humidity point is crucial. 75-78 with low to mid 40's humidity level is plenty comfortable.

I work with a guy who finds it comfortable to have his house 62 year around......that's nuts!

The fly wheel as Harvey stated, and load calc swag is more than sufficient to carry through hot spells.

No different than winter heating.

Going off of my current radiation and boiler size.............
I should size my A\C for when it's 120F out.


I've had plenty of 78-80F this year with RH in the low 40s and it stunk.

If that's what you guys like in your home, so be it.

I'm with the guy liking 62 all year.

Gordy

Then you will be the guy posting on why condensation is running down my windows rotting them out. Unintended consequences

Gordy

And why my electric bill is so high......

ChrisJ

Gordy said:

Then you will be the guy posting on why condensation is running down my windows rotting them out. Unintended consequences

Gordy said:

And why my electric bill is so high......

Wow.

I really have no response to this.

JUGHNE

Feels like 67 here in NE, my office is 68, with windows open.

ChrisJ

@Gordy @Hatterasguy

ChrisJ

Hatterasguy said:

That would be a DT of 57F in the winter (6F design) and 15F in the summer (90F design), sonny.

And, Gordy is right. You have no clue how high your electric bill is going to get if you run 25F all summer with the 3 ton unit.

Here, I'll do it for you:

4KW for 720 hours in July with an estimated duty cycle of 65%.

1872 KWH @ $0.15 = $280.

Just wait until your wife sees that bill.

6F design?
The record low for my area is -19F.
I've already seen -9F and we had several nights of -5F in 2014. A design temp of 6F may be the case, but I wouldn't go for it, not in my house.


I'm aiming for a 25 degree DT, which is why I put it.

As far as electric bill, I'm currently running 2.5 tons of window units all of which have EER's of 9.2 and they run continuously very often in July and August. I also have a 50 pint dehumidifier running non-stop.


The meme is accurate.

ChrisJ

Sigh.

Don't you have someone else to haunt?

Harvey Ramer

I try to make everything a learning experience. What I have learned here is that, come hell or high water, you are going to oversize your AC unit. That's your business.

The question I would be asking now is which equipment has the best features to compensate for the shortened run times and still is able to reasonably maintain comfortable humidity levels. Most manufacturers have some degree of dehumidification control built into their higher end variable speed airhandlers, as rudimentary as they may be. Johnson Controls has 4 different dehumidification profiles you can select. From arid climate to humid climate. What it will do is on a call for cooling, it will run the fan just fast enough to keep the coil from freezing for a certain amount of time. Then it steps a bit higher and runs there for a while till finally ramping up to full speed. Different manufacturers do the same thing same slightly differently. But that is a good way. Understand that this comes at a sacrifice to system efficiency.

And don't get tricked into selecting a 3.5 or 4 ton coil just for a higher seer rating, even if it is a direct match up. You will already be oversized and that would throw your sensible versus latent ratio even further off track. That might be swell for dryer areas, but not here in the east.

ChrisJ

It's also not for the record high and I want 70-72, not 68. Yes, I do 68 sometimes now in one room but I won't be pushing for that throughout the entire house.

We get into the 100s in my area from time to time.

Harvey Ramer

See, if I were shooting for 68 degrees sensible, I would be shooting for 40% relative humidity. At 68 degrees with 50% RH, you could probably keep a head of lettuce on the counter top for like a day before it starts to wilt. Jk.

You don't select equipment based on the total of the sensible and latent and corresponding tonnage. You take the sensible heat ratio of the load and pick a coil that is closest to matching both targets. Every coil has a SHR when paired with a specific condenser unit. You can usually find that in the engineering data.