Replacement wall AC


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Old 04-12-15, 02:05 PM
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Replacement wall AC

Replacing a dead 20 year old 25,000 BTU Emerson which I always felt was overkill for the room. 13x25x12 (large vaulted ceilings, 2 sky lights, and three windows facing south). Thinking about a 15,000 BTU Friedrich. Can't find anything bigger. Any thoughts?
 

Last edited by davebosox1967; 04-12-15 at 02:06 PM. Reason: Photos
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Old 04-12-15, 02:41 PM
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Please complete your profile so we know the approximate area where you live.

Normally, in a room of only 325 square feet a 5,000 BTUs/hour window unit would be sufficient. The south facing windows (how large?) and the skylights (again, how large?) would tend to skew the heat gain higher, maybe significantly higher depending on the geographical area. Still, I can't imagine that room ever needing 25,000 BTUs/hour of cooling. Remember that one ton of cooling is 12,000 BTUs/hour so the existing A/C was rated slightly more than two tons capacity.

Knowing the construction details is also important. How much ceiling and wall insulation (R value) is present? Are the skylights and windows single, double or triple pane construction? Do either the skylights or windows have low-E glass? How "tight" is the construction from the standpoint of air leakage?

Do you have any experience with the existing A/C? When it did operate did it cycle on and off? Did you often feel "clammy" when it was being used? For best humidity control you need size the A/C to run continuously on the hottest days without cycling. Bigger is NOT better when it comes to sizing A/C because it is continuous operation that lowers the relative humidity and that makes even higher temperatures much more comfortable.
 
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Old 04-12-15, 06:20 PM
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Thanks! I've added photos for context. The two skylights actually face more north than south (can't read the R factor but they were installed in 2005 so not that old). Each of the three windows are 18"x36" double pane (can't see the R factor). These are permanent windows and do not open and close).

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Last edited by PJmax; 04-13-15 at 08:01 PM. Reason: enhanced/reoriented pictures
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Old 04-12-15, 10:54 PM
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As I understand (no personal experience) New Jersey has fairly high humidity during the summer months so humidity control is more important (in my opinion) that actual cooling temperature achieved. Using the ceiling fan on low and blowing upward will keep the air from stratifying and also minimize annoying drafts. Some will have the opinion that the fan should be blowing downward but in my opinion the draft on one's body is very annoying.

I find that skylights are wonderful during the day to bring in light but they can also be a huge source of what is called "solar gain" when trying to cool a building. That yours are more north facing is a plus and as long as they do not allow a direct stream of sunlight into the house they are probably fairly benign. I have a southwesterly facing skylight in my living room and during sunny days it is the cause of the greatest heat gain in my house. During the night it is the largest cause of losing heat as it allows radiation back to the clear night sky. But in between those extremes it is also the principle source of light into my living room.

You did not state what experience you have had with this A/C unit, if any. As I stated, it should run continuously during the hottest days for humidity control. If it is cycling then it is most definitely too large. Honestly, I would think that even a 10,000 BTUs/hour unit would be too large for that room. There ARE some tricks that too often need to be used to keep a window (or through-wall) A/C operating at peak efficiency and I can discuss those if you like.
 
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Old 04-13-15, 06:07 AM
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Thanks again. This is a master bedroom and since we are not home during the day we would turn it on in the evening (7 or 8pm and run it until the morning). Yes it would cycle on and off on Lo but I guess I never realized it).

Yes would appreciate any tips or tricks!

Dave
 
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Old 04-13-15, 01:15 PM
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I've not forgotten you but I need to go out for a few hours so I won't reply until later this evening.
 
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Old 04-13-15, 08:08 PM
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Nice setup.

We sure do get some awful humidity in NJ and I'm sure those fans are very helpful.
I only run my central A/C when the humidity is high and the heat is up there. Maybe twenty or so days of the summer. I rely heavily on my ceiling fans.

I was thinking a 15k unit for your area as the air movement is a little stronger in a larger unit like that. You'll need to find something compatible with the existing sleeve unless your plans are to replace it.
 
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Old 04-13-15, 09:21 PM
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Just wonder if the cheaper route would be to use a dual hose portable A/C and fill in the wall A/C opening except for connections for the portable A/C's hoses to the outside. Example: Whynter 14000 BTU Dual Hose Portable Air Conditioner with Remote & Reviews | Wayfair
 
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Old 04-14-15, 01:20 AM
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Dave, it is now quarter after one in the morning and my nerve damaged hands and wrists are acting up something fierce after spending the last hour and a half typing responses to others. I make a mistake with about every seven keystrokes and I'm about shot. Please keep checking back as I WILL respond within a day or two.
 
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Old 04-14-15, 05:37 AM
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No worries Furd and hope you feel better. Thanks to the others for your thoughtful responses.
 
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Old 04-15-15, 10:25 AM
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I've been replying to numerous other threads instead of answering the ones I have placed on hold. It really is past time that I respond to you but... I need to go out and do several tasks today, not the least is to buy some groceries. I expect I will be gone the better share of the day but I promise I WILL respond tonight.
 
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Old 04-16-15, 01:09 AM
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First I want to comment on Ray's suggestion of a dual hose/duct portable cooler. While the dual hose is far superior to the single hose models ALL portables are less desirable than a window or wall unit. They will be noisier than the same size window or wall unit and their efficacy is lower because all the heat produced by the compressor and fan motors is inside the room as well as the heat radiated by both ducts if they are not insulated.

Every single window or wall mount A/C I have ever seen has the cool discharge air louvers at or near the top of the face frame with the air inlet immediately below. this is perhaps the single biggest failing of these machines. The cool air, being more dense, has a natural tendency to fall as soon as it is discharged and when it does it is then sucked into the inlet to run through the machine again. This has the tendency of fooling the thermostat (which is right inside the inlet grille) into thinking the room is cooler than it really is and will cause the machine to cycle off too soon. Of course, when it cycles off the discharge temperature will then rise, the inlet air temperature will rise and the machine will start again. The effect is heightened when the fan speed is set low.

These units, unless a very high end "inverter technology" unit have only an on/off thermostatic control for the compressor, the "high-medium-low switch" ONLY controls the fan speed. When the high fan speed is selected the noise level is often unbearable although it is on high that you will get the highest efficiency and efficacy from the unit. Running at a lower speed will, under some circumstances, wring a greater amount of moisture from the air (lowering the relative humidity in the room) but that only works while the compressor is running. Once the compressor cycles off, due to inlet air temperature, the condensed moisture on the evaporator (cooling coil) will again evaporate into the room and raise the relative humidity back to the former level. This re-evaporation can be slowed IF the "energy saving" mode is selected which shuts down the fan as well as the compressor when the thermostat senses the lower temperature.

The problem with using the energy saver mode is that once the airflow through the unit has been stopped the internal thermostat becomes much less sensitive to room air temperature and the off cycle will often become artificially long allowing the room to reheat to an uncomfortable temperature as well as the relative humidity also rising. This can continue in a "freeze or fry" cycle until you get so sick of it you leave the fan running continuously. This cycling is far more prevalent when the A/C unit is oversized for the room.

Yet another problem is that the sensing bulb for the thermostat is usually mounted in close proximity to the evaporator coil, often less than a quarter of an inch from the coil by means of a plastic clip. This means that the thermostat is affected by not only the incoming room air temperature but also by the temperature of the evaporator which can be in the mid thirties (degrees F.). For best dehumidification the evaporator needs to be as cold as possible but not so cold that the condensed water will freeze on the coil. This proximity to the coil will skew the thermostat and if you have a digital readout on the control panel it will read anywhere from a couple of degrees lower than the actual room temperature to several degrees low. This effect is increased with the fan on low speed and moving less air.

Yet, there IS some sound thinking behind this apparent madness in that IF the evaporator temperature does fall to below the freezing point of water rather than icing up and totally blocking all airflow through the coil the machine will shut down. Some higher end machines will have two thermostats, one for system operation and another monitoring the evaporator temperature and shutting down the machine before it freezes.


So the question is...what can be done to increase the efficacy of the A/C unit without also causing problems in its operation. In my opinion the first thing to do is to implement measures to minimize the amount of discharged cool air that is immediately sucked back into the intake. There are several ways to do this, all of them ugly You can start by making a baffle that goes across the top of the inlet louvers in a manner that baffles the discharge air from dropping directly to the inlet. A piece of cardboard as wide (or wider) than the inlet grille and that sticks out a minimum of about eight to ten inches is good. Even better if you add side panels so that all the inlet air is being drawn from a lower level. This baffle can also be made from sheet metal for a slightly less ugly appearance. You need to be careful in not making the baffle too small, you need a lot of open space to allow the room air to enter the inlet grille so don't try to just come out a couple of inches and then down with the baffle.

The next step is to make a transition piece for the discharge grille that will direct the discharge air up and out into the room away from the inlet grille. This can also be fabricated with stiff cardboard although sheet metal is preferred for rigidity. Again, do not try to make this too small or you will severely affect the airflow and the effort will be self defeating. If you make this discharge "duct" out of sheet metal then you will likely get condensation on it so you may need to use some foam insulation on the outside surfaces. I warned you it would be ugly.

These two modifications will greatly enhance the operation of any window or through-wall A/C unit. More often than not you will be able to use all three fan speeds without problems, the high speed for quick cooling and the low speed for constant running with dehumidification and lower noise factor. There is one more modification and that is to add some distance between the thermostat and the evaporator coil to reduce the effect the cold evaporator has on the thermostat. To do this you will need to remove the front panel/grille assembly, find the thermostat sensor (it will be obvious near the center of the evaporator) and determine how it is held in place, often just a plastic clip. Carefully remove the clip from the coil or sensing bulb from the clip and move it out of the way. You will probably need to cut a notch in the cover to allow the sending bulb outside of the cabinet (yes, this modification will void any warranty so best to not do it until the warranty is up) put the front panel/grill back in place and then mount the sensing bulb in one of the airflow slots of the grille. The result will be a thermostat that is much more responsive to room temperature and still offers a small protection to an iced up evaporator coil.

I've used these modifications for several years and not only have I had no problems but the greatly enhanced operation has made it well worth the effort.

But it IS ugly.
 
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Old 04-16-15, 04:55 AM
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Even 10k is on the large size for that room. 8 or 9k would probable cool and dehumidify very nicely.
 
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Old 04-17-15, 02:21 PM
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Furd thank you so much for the thorough response!
 
 

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