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Gary
21-09-2002, 04:57 PM
The old 400 CFM/Ton rule (for A/C) comes from an era when evaporator coils were smaller and SEER was much lower. It no longer applies and is causing mold problems in humid areas. A better rule is needed.

At the techmethod website, we have been crunching the numbers in search of a better way. We have considered typical to heavy latent loads, with typical bypass factors from .05 to .20. This assumes a system which has no humidity controls, with a target RH of 50% at the thermostat.

The bottom line is:

Near the end of the run cycle, the airflow should be adjusted to achieve supply air DB temperature that is 20-25F below the thermostat setpoint.

Comments?

superheat
23-09-2002, 04:50 PM
Are you saying to adjust the airflow so that the coil temp before cycling off should give a minimum 20 degree split. That is not going to give the SEER the costumer bought.
I like to offer a dehumidistat on all installs. They can have high SEER under high ambient and still get good dehumidification when needed. I hope it saves my butt in case I get into a mold situation. I can say the costumer declined the option that would have prevented the mold infestation.
I have been putting relays on the fan for several years. Now VS drive will do that for you.

For minimum airflow, I like to run the temp down in the zone several degrees below the lowest expected setpoint then adjust airflow to get a 35 degree evap.

Gary
23-09-2002, 11:37 PM
The challenge here is to maintain 50% RH plus or minus 10%, without humidity controls.

Split refers to the difference between SA and RA. I am referring to the difference between SA and the temperature at the themostat, which is not necessarily the same as RA.

Without humidity controls, a 35F evaporator may in fact drop the RH below 40% under light latent load conditions.

twcpipes
28-09-2002, 10:34 PM
Hi Gary,
Something else has happened over the last years that has severely impacted mold and mildew. The manufacturers have, especially, on package equipment, reduced rows to two and three where they used to be 4 as a standard. Even York now has succumbed to the cost effective idea of saving rows. They were about the last to produce four row units. When it comes to de-humidification, there is no substitute for rows. I have a project in design right now and the Trane factory, LaCrosse not Oklahoma, wants to argue about primary and secondary cooiling coil apps. They are having a real problem with their new engineers understanding the need for de-humidification, rows and stages.
The new package "stuff", though, all seems to be three row and sometimes even two row.

Gary
29-09-2002, 12:50 AM
That's a good point, Tom. In formulating the suggested rule (20-25F SA below desired setpoint), I used bypass factors less than 0.20.

BPF this low is unlikely with less than 3 rows, (although possible at extremely low velocity). Otherwise, the supply air would need to be several degrees warmer to achieve the same RH results with a 2 row coil, because of the higher BPF.

Gary
30-09-2002, 01:13 AM
Perhaps a clearer statement of the proposed rule would help:

"The airflow should be adjusted to provide supply air temperature that is 20-25F below the room air temperature under stabilized conditions."

twcpipes
30-09-2002, 06:58 AM
Hi again Gary,

I don't know. 20-25 degrees below space conditions at the t-stat or just in the space. Could it not present a problem or two. If the space were at 73F and the return at 74F you would be at a 54-49F supply. What did you calculate your evaporator temp to be under this senario. I realize all equipment is different. Couldn't it be below freezing in some instances, say crowding 28-29F.
Also what would one do with those clients who don't like to change their filters and they run 60% dirty as a norm?
Also, where would you apply the new standard as regards to types of equipment. Sometimes coils (built-up systems) create water carry-over when there is an excessive amount of condensate and a high coil velocity for that condition which would contribute to moisture build-up downstream of the condensate pan.
Just questions.
Tom

Gary
30-09-2002, 09:37 AM
I don't know. 20-25 degrees below space conditions at the t-stat or just in the space. Could it not present a problem or two. If the space were at 73F and the return at 74F you would be at a 54-49F supply.

Actually, 53-48F supply. Temperature at the thermostat should be considered representative of the space temperature, or what's a thermostat for?


What did you calculate your evaporator temp to be under this senario. I realize all equipment is different. Couldn't it be below freezing in some instances, say crowding 28-29F.

This could occur, although it is unlikely on today's higher SEER systems. However, a temperature that would freeze the coil should be considered a lower limit.


Also what would one do with those clients who don't like to change their filters and they run 60% dirty as a norm?


Tell them to change their filters?


Also, where would you apply the new standard as regards to types of equipment.

It would apply to A/C equipment in areas where de-humidification is needed.


Sometimes coils (built-up systems) create water carry-over when there is an excessive amount of condensate and a high coil velocity for that condition which would contribute to moisture build-up downstream of the condensate pan.


I am trying to picture a scenario where the proposed rule would result in an increase in coil velocity.

twcpipes
30-09-2002, 07:28 PM
Gary,

Evaporator coils are rated in 300, 500, and 700fpm by variuos manufacturers of built-up systems.

To further discuss this subject, given the fact I am still invited to the party, I have another question:

1. Is your new rule to apply to existing installations or new equipment and installations. Manufacturers would have to accept this new rule, yes?

Also, in passing, you are predicating this rule on the fact that installers know where to put the thermostat. I find that is not the case in many installations. They also don't know how to treat the variuos loads in spaces as regards air flow direction and cfm.

I am certainly not arguing your new concept at this period in time but curiosity has peaked. You see, Gary, in Southern California the WB has been steadily increasing since 1973. At first for various ecological reasons (crop and grass planting) but of late due mostly to weather patterns (i.e. el ninos and el ninas) although they are still building homes and planting lawns in the last of the land available. Just to inform you how bad the situation is, water towers that were designed around ASHRAE 68wb just won't work anymore when you need them most.
So something definitely needs to be done in a hurry about mold and mildew if clients have the money to invest and upgrade. What they have now just doesn't de-humidify.
Thanks, Tom

Gary
30-09-2002, 09:54 PM
Evaporator coils are rated in 300, 500, and 700fpm by variuos manufacturers of built-up systems.


In a situation where de-humidification is needed, if a coil spits whenever it is removing moisture, then curtailing it's ability to remove moisture would seem a poor cure.


1. Is your new rule to apply to existing installations or new equipment and installations. Manufacturers would have to accept this new rule, yes?


It is not up to me to formulate rules in any authoritative sense. I write trouble shooting books. Should this prove out, it will be added to the procedures in my books as part of fine tuning a system. Manufacturers will do whatever they will do.


Also, in passing, you are predicating this rule on the fact that installers know where to put the thermostat. I find that is not the case in many installations. They also don't know how to treat the variuos loads in spaces as regards air flow direction and cfm.


I am not assuming anything. Poor installation practices are not going to go away. In the meantime, I look for ways to improve my customer's service skills, and maybe sell a few books. :)


I am certainly not arguing your new concept at this period in time but curiosity has peaked. You see, Gary, in Southern California the WB has been steadily increasing since 1973. At first for various ecological reasons (crop and grass planting) but of late due mostly to weather patterns (i.e. el ninos and el ninas) although they are still building homes and planting lawns in the last of the land available. Just to inform you how bad the situation is, water towers that were designed around ASHRAE 68wb just won't work anymore when you need them most.


I have family in Irvine (Lawson Refrigeration). Wave in that direction for me. :)


So something definitely needs to be done in a hurry about mold and mildew if clients have the money to invest and upgrade. What they have now just doesn't de-humidify.


What they have now can be made to de-humidify. Fortunately, the newer high efficiency systems can be made to de-humidify better than the older systems.

A cold coil removes moisture, but stops removing moisture at it's dewpoint. You can add whatever gadgets you like, but if the coil isn't cold enough, it can't do the job. Period.

Lowering the coil temperature lowers the SEER, and higher SEER is what sells, especially in California. The manufacturers are not going to recommend reducing evaporator airflow for that reason.

It is up to the service techs to make the systems work properly. SEER is what sells, but when it comes time to fine tune the system, comfort is top priority.

Gary
01-10-2002, 04:17 PM
The airflow should be adjusted to provide supply air temperature that is 20-25F below the room air temperature under stabilized conditions.

I would stress that this rule is not something to be used all by itself. Once proven, it will be added to the TECH Method diagnostic procedure, which checks all aspects of system operation. And in fact it will be added near the end of the list. It has limited value if there are other problems in the system.

Everything is connected to everything else, and everything effects everything else.

Gary
01-10-2002, 06:02 PM
Evaporator coils are rated in 300, 500, and 700fpm by variuos manufacturers of built-up systems.


I assume this is based upon 400 CFM/Ton. In most, if not all cases, the proposed rule would reduce the CFM and therefore the velocity. While this may or may not cure the spitting problem, I would venture to say that it will not add to the problem.

herefishy
07-02-2003, 02:17 AM
TO: Gary

FR: The Fish

RE: subject

Gary,

This thread needs to be revived! Maybe I have my head in the freezers (as opposed to enviromental A/C). Is this mindset still prominent today? REF: recent issues of the "NEWS"


see ya".......

:)

Gary
07-02-2003, 07:47 AM
This thread needs to be revived! Maybe I have my head in the freezers (as opposed to enviromental A/C). Is this mindset still prominent today? REF: recent issues of the "NEWS"


Forgive me if I'm not overly impressed with what I read in the "News". When it comes to trouble shooting, the powers that be have their figurative heads up their collective butts. I formulate my own theories, do my own research, come up with my own procedures, and wait for the industry to catch up. They never do. Trouble shooting is the ugly stepchild of the industry.

I don't much care what the prominent mindset is.

That said, the 'SA 20-25F below average room temp' for humidity control seems to be holding true. It works, and is now included in my A/C trouble shooting procedures. :)

superheat
07-02-2003, 06:52 PM
I have seen rooftop units with less than good condensers. Maybe somebody used a pressure washer on them and bent the fins. I straighten the fins and drop the blower speed a little and now it is not tripping on high pressure everyother afternoon. Does it have the right fan speed? Well it is cooling the air and probably dehumidifying better. In a few years, mom and pop will be able to get their money togather and buy a new unit.

The problem with rules of thumb is: you gotta know when they don't work the best. It is better to keep that old beat-up peice of crap working for another summer than to lose the contract to somebody who can patch it up. Those rules of thumb usually assume everything is working pretty well.

Dan
09-02-2003, 06:06 AM
The old 400 CFM/Ton rule (for A/C) comes from an era when evaporator coils were smaller and SEER was much lower. It no longer applies and is causing mold problems in humid areas. A better rule is needed.

How about 350 CFM/Ton?:)

Gary
11-02-2003, 02:37 PM
De-humidification is about coil temperature and run time. The coil temperature must be low enough to achieve the desired humidity level.

Coil temperature can vary considerably with any given CFM/ton, being effected by such variables as relative coil size and return locations.

For example, if the returns are high and/or running through a warm area the coil temperature can be several degrees higher. Conversely, if the returns are low and/or run through a cool area the coil temperature can be several degrees lower. This will make a big difference in humidity control.

Using CFM/ton is a mistake, and has always been a mistake, but its effects were less damaging back when smaller coils were used and coil temperatures ran closer to freezing.

It is far more accurate to use room temperature and supply air temperature, adjusting the blower speed accordingly. Using room temperature compensates for variations in returns, and in using SA temperature we balance the coil BPF against the latent load. On the one hand, the actual coil temperature is going to be several degrees colder than the SA temperature due to BPF. On the other hand, it needs to be several degrees colder in order to handle the latent load.

Once we have adjusted the blower to give us the proper coil temperature, if this does not achieve the humidity target, then and only then should we consider extending the run time.

Dan
15-02-2003, 01:43 AM
Once we have adjusted the blower to give us the proper coil temperature, if this does not achieve the humidity target, then and only then should we consider extending the run time.

This made eminent sense when I first read it, yet upon rereading it I am not sure I understand it. What qualifies the proper coil temperature? I think when I first read it I was assuming the coil temperature that provides the desired humidity, but clearly that is a misunderstanding on my initial reading.

Prof Sporlan
15-02-2003, 03:49 PM
Desired coil temperature seems to be the million-dollar question... :) For humid areas, a lower coil temperature would be desirable, even though it would lower the system’s efficiency. For a desert climate. a higher temperature coil would be desirable, as you would have less need to dehumidify, and it provides greater system efficiencies as a bonus. Perhaps a 40°F design evaporating temperature would be appropriate for a humid environment, and 45°F for a dry climate?

Gary’s suggestion of a 20-25°F between SA and set point would seem to be a good rule to follow.

Gary
15-02-2003, 06:47 PM
The comfort zone for humidity is 40-60% RH. I set the lower limit at 'SA 25F below room temp' because lower than this can result in dropping the humidity below 40%. This is not only detrimental to humans and their furnishings, but is a waste of energy. And even in the most humid areas, the humidity load varies. On a less humid day, we don't want to drop the humidity below 40%.

The other lower limit would be coil freezing. This can be difficult to judge. The coil temperature is going to be somewhere between SST (saturated suction temp) and SA (supply air temp). If the SST is at or above 32F, we needn't worry about the coil freezing. I have seen systems running as low as 26F SST without freezing, but this is cutting it far too close. I like to see SST at or above 32F.

Dan
15-02-2003, 07:46 PM
I have seen systems running as low as 26F SST without freezing, but this is cutting it far too close.

The effect of secondary surface - fins - can permit this although I don't understand the intricacies.

I am interested about run-time versus coil temperature regarding dehumidification.

Which would be the best? A 45 deg evaporator coil with an 80% run time to achieve a 50% RH, versus a 40 deg evaporator coil with a run time of 30%?

As I ask this question I wonder if there is a way to use data to ask the question more accurately.

Prof Sporlan
15-02-2003, 09:20 PM
Which would be the best? A 45 deg evaporator coil with an 80% run time to achieve a 50% RH, versus a 40 deg evaporator coil with a run time of 30%?
One can safely conclude the former to be the better situation... greater system efficiencies, and less compressor cycling which should prolong life. But run time, however, will be a function of outdoor ambient. A 30 percent duty cycle at a 95°F ambient in St. Louis would suggest the unit to be a bit oversized.. :)

Gary
15-02-2003, 09:59 PM
I am interested about run-time versus coil temperature regarding dehumidification.

It must be stressed that if the coil is not cold enough to achieve the desired humidity, then the run time is irrelevant. The coil stops removing moisture at its dewpoint and it can do no more, even if it runs 24/7.

For example, let's imagine that we have a reheat system where the room temperature is maintained at 72F and never shuts off. The temperature of the coil is 57F. If we follow the 57F dewpoint line horizontally to the 72F DB line on a psych chart, we find that the RH cannot possibly drop below 60%, regardless of run time. It can't get there. Period.

To my mind, it would make more sense to maintain a coil temperature of 46F, where it could not drop below 40% RH. The humidity load would then keep it somewhat above 40% RH. If we drop the coil temp first, we may very well find that the reheat is not needed. Thus, we should drop the coil temp before we consider such strategies as adding reheat to extend the run time.

Depending upon BPF, at 46F coil temp, the SA temp prior to reheat would be roughly in the 47-52F range, or in other words 20-25F below the 72F room temp.

superheat
17-02-2003, 04:50 PM
I would just like to add to Gary's words of wisdom.
Sometime the humidity and RAT are such that you can't get 20 split over the evap. Rules of thumb are handy to get you started, but you need to have an understanding of how it works to know for sure it is working right.