How Low Can You Go?

clammy

On a home i did a few years ago the HO advised me by mistake at night he lowered his t/stat to what he thought was 65 but was 55 (bad eyes) only to wake up at about 2 o clock with the bed covers on and a very cold nose ,so he went to his stat to see his mistake and find that the house was at55 degrees ,as always i set up my central a/c sysytem so the master bed is always a bit cooler then the rest of the house ,i was cued into this years ago my a fellow a/c designer installer he stated that you must always have a cooler master bed why i asked he stated because that where all the family decession are made it made sense so i set up my duct work to allow for the mb to be at least 2 to 4 degree cooler then all the rest of the house .This system was in a expanded cape 12 seer 4 ton R 22 system the home was a little over 1800 sq ft peace and good lucj clammy

TGO_54

How Low Can You Go?

Professor,

With a R-22 12SEER split CAC system, what is the lowest temp you can get in the house and expect normal operation?
Thanks!

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TGO_54

How Low Can You Go?

Professor,

With a R-22 12SEER split CAC system, what is the lowest temp you can get in the house and expect normal operation?

Thanks!

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Techman

Normal

Hi Guys,Any properly designed AC system should be able to go down to 70-72* and that is it!Any lower and the evap will frost over.Anything in the 60's or 50's is high temp refrigeration,frosting the coil. The minimum suction press should be 58 psi[r-22] which is equal to 32* saturated freon.Design temps are set for a 15* TD or a 20* TD,from outdoor ambients.Packaged RTU that have freezestats is an example of this "no lower then 70*",also Hi Velocity systems have factory installed freezestats.The hand full of customers that have INSISTED on hi 60's get a defrost clock or a field installed freezestat.I think if ANY room or total structor goes into the 60's or lower and then the system gets shut down or setback then there will be sweating{not like a can of cold beer} on the furniture, floors, etc.Thats been my experience!

Empire_2

Good morning Terry

Eugene Silberstein 3

How Low?

Well, your evaporator saturation temperature should be just about 35 degrees cooler than the temperature of the return air. This is referred to as the delta-t of the evaporator coil, not to be confused with the delta-t "across" the coil.

Your room temperature should be no lower than about 65 degrees to avoid having frost start to acuumulate on the coil.

If you are starting up a system and you lmow the temperature will be cool, I turn on the heating system to get some load on the a/c system.

TGO_54

Thanks for the reply

I have a customer who needs to cool a bedroom in the house to 55 degrees. Any way to modify a standard CAC system to do this?

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Eugene Silberstein 3

How big?

How big is the bedroom?

Is this going to be tied into the main system for the house or can this be a separate system?

Brad White

55F in the bedroom?

Sheesh.

When I hear things like that, I think of people who forget to bury their deceased parents or cannot let go...

Sounds like a 2-stage cooling event or a tremendous amount of air at a lower temperature... I'd like to know how this one turns out!

TGO_54

Master bedroom

This is the masterbedroom and is already on it's own zone, forced air with a hot water coil for the heat, 3-ton system for the cooling. I don't have the total square footage but I would guess it is around 900 to 1000 square feet.

It is a seperate living area from the main house so it includes a sitting area etc. It is built over a crawlspace, has large sliding glass doors and the roof is a combination roof, ceiling beam - no attic. Ductwork is located in the crawlspace with supplys on the floor.

Client requires a fast response from the thermostat and will switch from heating to cooling during the summer. In winter the doors are open to cool the space, and the heat is only run for a few hours a day. The heating side works fine, but they would like to be able to cool the room to 55 during the summer. Currently the system can get down to 64 but no lower.

I look forward to your input.

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Brad White

Seriously

Tom, this seems like a 2-stage deal, taking the temperature down as far as you can (64 degrees as you said) then having a separate low-temperature cooler such as in a cold-box refrigerator, to work the second stage, using a lower temperature range refrigerant which Dr. Silberstein is in a far better position to advise.

I say "stage" not that the airflow is in series, but it could be. I am thinking two systems in parallel for more simplicity.

Stage 1 takes the room down to 64 but also (this is critical) dehumidifies the air to as low a dewpoint as you can. What is the lowest discharge temperature you can get? Say it is 49 degrees saturated off the coil.
Straight-line introduction to the room extended to 64 degrees will get you to 58% RH, may be too muggy, especially if you hit the second evaporator. Get the RH down as far as you can with the first system.

(You do not want to get near 60% RH or mildew can form and be nourished from the moisture in the air itself.)

I would suggest your space target wants to be 55F and 50% RH. To do this you will have to cool your air down to about 36.5 degrees saturated. (55 degrees in the space at 50% RH has a dewpoint of 36.72 degrees -but you knew that!

The second stage would take space air from 64F down to the 36 degree range to wring it out, (rather than taking the evaporator air from the first stage).

You will likely need reheat for that last stage so that you do not over cool the space. Perhaps a dehumdifier type unit with "free" hot gas reheat may be an option? Hot gas would divert to the reheat coil to maintain the lower space temperature and the balance is rejected to an outside condenser.

The second stage can have less airflow than the first which may give you better control. (Less super-cold air means less reheat but will still do the cooling job for that last 9 degrees.)

Random drive-by thinking.

Brad

EDIT: I have a concern regarding the vapor pressure in this space versus the adjacent spaces and the construction of the partitions with regard to moisture transfer and condensation. A properly applied vapor retarder should be considered on the higher vapor pressure side.

BEW

TGO_54

Thanks for the reply

> Tom, this seems like a 2-stage deal, taking the

> temperature down as far as you can (64 degrees as

> you said) then having a separate low-temperature

> cooler such as in a cold-box refrigerator, to

> work the second stage, using a lower temperature

> range refrigerant which Dr. Silberstein is in a

> far better position to advise.

>

> I say "stage"

> not that the airflow is in series, but it could

> be. I am thinking two systems in parallel for

> more simplicity.

>

> Stage 1 takes the room down

> to 64 but also (this is critical) dehumidifies

> the air to as low a dewpoint as you can. What is

> the lowest discharge temperature you can get? Say

> it is 49 degrees saturated off the

> coil. Straight-line introduction to the room

> extended to 64 degrees will get you to 58% RH,

> may be too muggy, especially if you hit the

> second evaporator. Get the RH down as far as you

> can with the first system.

>

> (You do not want to

> get near 60% RH or mildew can form and be

> nourished from the moisture in the air

> itself.)

>

> I would suggest your space target

> wants to be 55F and 50% RH. To do this you will

> have to cool your air down to about 36.5 degrees

> saturated. (55 degrees in the space at 50% RH has

> a dewpoint of 36.72 degrees -but you knew that!

>

>

> The second stage would take space air from

> 64F down to the 36 degree range to wring it out,

> (rather than taking the evaporator air from the

> first stage).

>

> You will likely need reheat for

> that last stage so that you do not over cool the

> space. Perhaps a dehumdifier type unit with

> "free" hot gas reheat may be an option? Hot gas

> would divert to the reheat coil to maintain the

> lower space temperature and the balance is

> rejected to an outside condenser.

>

> The second

> stage can have less airflow than the first which

> may give you better control. (Less super-cold air

> means less reheat but will still do the cooling

> job for that last 9 degrees.)

>

> Random drive-by

> thinking.

>

> Brad

>

> EDIT: I have a concern

> regarding the vapor pressure in this space versus

> the adjacent spaces and the construction of the

> partitions with regard to moisture transfer and

> condensation. A properly applied vapor retarder

> should be considered on the higher vapor pressure

> side.

>

> BEW



Brad,
Thanks for the reply, I knew there would be a lot to consider when designing a large walk in box (that's really what this will be) you bring up some important considerations about humidity and mold - something I hadn't thought about. If they go through with this, I'll keep you posted.

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TGO_54

Thanks for the reply

Brad,

Thanks for the reply.

I knew there would be a lot to consider when designing a large walk in box (that's really what this will be) you bring up some important considerations about humidity and mold - something I hadn't thought about. If they go through with this, I'll keep you posted.

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Edward A. (Ed) Carey_3

WHY

Why does he want the MBR at 55 degrees???


Ed Carey

A.J.

What is up with some people

I just lost an account after checking three times an install we did last August. They knew I had a problem with the install becuase it would not go down to 60 deg. were the stat. was set. I tried to explain several times but they knew more than me.

Brad White

I bet those same people

eye the 140 on their odometer with the same fascination...

Steve Ebels_3

Regardless of the 55* degree thing...........

I'd be very concerned with dewpoint in the walls and ceiling given the type of construction you've described. Running a roof ceiling combination at a delta T of 60*+ with any kind of humidity is a recipe for MOLD MOLD MOLD. You'd have to have that roof/ceiling combo 100% air tight to avoid the problem.

AFA reaching the 55* interior temp is concerned, I think you'd have to go to a commercial medium temp coil cabinet which is separate from the "normal AC system. As others have said, it could be set up as a two stage system in order to achieve the lowest possible RH in the house.

Doesn't sound like one I'd like to tangle with.

Empire_2

My .02

You can use on a SEPERATE system a fan cycling control, but this is not only impractical, but absolutely nonsense. There is no medical condition that requires a 55* room temp to live or anything else like that. The morgue has a nice room,...Might be a little small,but would be perfect...:=)

Mike T.

Paul Fredricks_3

Did I miss this?

Why do they need to get the bedroom to 55? And why do they need a quick change over?