Correcting Oversized AC

[bucholtz-gmail-com]

Basics:

Have an oversized AC unit causing a humidity problem. I have been given many suggestions for fixing, but would like feedback on the different options.

Solutions:

After talking with different HVAC contractors about the problem, I have been given three suggestions. Before throwing good money after bad, I would like input on the validity of the suggested fixes. In addition, if there are any other fixes that could work, please let me know.

Suggestion 1: Replace unit with proper sized unit

This solution is "the proper way" to fix the problem, and was how it should have been handled in the first place. However, I am being told that everything needs to be replaced, including ductwork to account for differences in blower motor output of the smaller unit. I have been told that the duct work pressures will be different and that everything needs to be sized according to the system as a whole. Is this true?

Since all our new drywall will need to be ripped up to install new duct work, I would hate to have to go down this path.

Suggestion 2: Change size of compressor

Is this really an option??? One contractor said I could just put in a lower rated compressor, but I am skeptical of this solution. I have been reading other posts on the news group, and it sounds to me like the blower will still be moving air at the same rate over the coils, and that this may still leave my house humid since the moving air will have less contact with the coils. Is this true?

Suggestion 3: Install central dehumidification

This seems logical; with the only issues being the unit will run less efficiently since it would still short cycle, causing stress to controls, motor, etc. Additionally, I will be paying electricity for a dehumidifier to run on top of my AC. On the plus side, we will get humidity control year round, even when too cool for AC. Are there any other caveats to this approach?

[SQLit]

Yes

Not the approach I would take. Turtle says just the opposite bigger condenser smaller air handler. It is ok that I say that right mate?

First how was it determined that the unit is to big? What is the temp differential? return and supply air temps My unit is running between 20-25 degrees. What is the set temperature?

WAG for the home owner. Change the fan speed to one setting lower. See if that lengthens the run times. Which should increase the dehumidification

Your right about the issue being addressed in the beginning.

[udarrell]

There is a room TH that has a "wide variable differential temp 'manual' setting." Kick-on at say 78 kick-off at say, 74-F. Also, slow the fan CFM to close to 350-cfm per ton of cooling. Use a large 20" fan to circulate the air to keep within the "Human Comfort Zone."

What is that the Brand and Model number of that RM TH?

Install a dehumidistat wired in parallel to the RM TH; might get a bit cold if you set the dehumidistat too low.

- Darrell - udarrell

[Stretch]

several suggestions:

1) Install a ducted dehumidifier, like Therma-Stor or AprilAire. Tie it into your existing supply duct system and install a separate return for it.

2) Install the proper size system. No, you don't need new ducts for the lower air flow. Most duct systems are undersized anyway! Install dampers on all the branch ducts, do a room by room load calculation and do a proper air balance using a flow hood, This requires a competent contractor, with the proper tools and training; only about 1 in 10 can actually do this!

3) Measure the air flow with a flow hood or another accurate air measuring instrument. In high humidity environment, you should have around 350 CFM per ton. This does not mean measuring with the contractor's hand, that IS NOT accurate.

4) If the system is recent vintage, you may be able to replace just the outdoor unit if you can get a manufacturer's certified match. The outdoor and indoor units should be the same brand unless the manufacturer says otherwise. Then adjust the overall air flow to 350 CFM per ton. Also do an air balance!

5) Do not block parts of the coil, that will cause all kinds of problems. Adjust overall air flow with the blower speed and dampers. Blocking parts of the coil with metal can cause uneven evaporation of refrigerant and may cause liquid refrigerant to slug the compressor. BIG DAMAGE!!!

6) Seal the ducts, close the windows, provide return paths for all supply grilles. Opening windows will just allow more moisture to come in from outside. Installing exhaust fans will do the same thing by pulling the house into a negative pressure and sucking in outside air!

Get someone to measure the airflow and humidity and quit guessing. And get rid of contractors who guess!!!!!!

Stretch

[Travis Jordan]

bucholtz-gmail-com wrote: /snip/

Since you are going smaller in condenser and coil size (and therefore lower in velocity) there is need for new duct work....and this is the right solution to the problem. .

[PipeDown]

Increase the size of the area being cooled to better match the system, install a duct to the garage. The amount of energy you waste cooling the garage may be less than the cost to correct the problem. Not ideal but perhaps you have another uncooled space that is not a garage like an enclosed porch. You can close off this area during heating season.

Opening a window would have a similar effect of enlarging the cooling load but that is an obvious waste and least preferred but it may relieve some of your humidity problems short term. Not all the way, just a crack.

Increase the cooling load in other ways: Cut down a shade tree, open some curtains, install less efficient windows (not). have the kids leave the door open too often. etc.

[DiDo]

Since you are doing this for you self I think this would be you simplest solution to you problem of humidity? get you self some type of thin sheet metal and pair of sheer so you can cut sheet metal to size you may need, and start blocking air flow through the cooling coil, start from the bottom end, and make sure that you are putting this metal on the side where the air inter in to coil do it couple inches at time this will reduce temperature on the coil and increase moisture, (do not freeze coil) removal. start the unit and see the results if you have instrument that you can check Humidity before and after that would definitely help you but first check it before you make change log it down and check after the change so you can see the results immediately if you do not have instrument to make immediate check then you will had to gave some time to see the results This will work but I can't say how good and how much of air block you may need to do? I would like to hear from you if you follow suggestions Good luck from Dido!

[Vic Dura]

On Tue, 23 Aug 2005 20:14:18 GMT, "PipeDown" wrote Re Re: Correcting Oversized AC:

Also, you can try keeping doors & windows closed, but leave a couple of 100w lights on in several rooms or run a small electric heater near the air intake.

[Pagan]

Why would you need to rip out all the ducts? If you have loose ducts in the crawlspace above the ceiling, why not just replace those with smaller ducts, leaving everything that's encased in walls and such alone? Or, put some kind of flow restrictors behind the registers.

I wouldn't monkey with the actual design of the unit. What if, 10 years down the line, you need to have work done on it, and the contractor doesn't know about the modifications?

If you have humidity problems year round, this is probably a smart move regardless.

Pagan

[scott21230]

Just get a dehumidifier and leave the fan blower on all the time. Place the dehumidifer near one of the registers. Cheap and easy. When unit needs to be replaced next, then get the proper size. I would not change ducts, you should be able to do what you want by changing the fan motor speed.

[Carolina Breeze HVAC]

Yes. The ductwork is not a seperate system, its PART of the existing.

You mean a lower rated condensor? Some of the smaller, high SEER units use larger blowers and evap coils than the rated capacity of the condensor. The metering will have to be changed, and it possibly could be done....if you have done some reading about higher SEERS, you may also continue to have a humidity problem. I dont get your comment about less contact with the coils...you mean less contact time?

Sounds nice, but you will about crap when you see how much say..an AprilAire central dehumidifier costs.

[PipeDown]

Reducing a duct in place is easy, all you need to do is put solid foam insulation in the duct to reduce the cross sectional area. This may require disassembly of certain sections to gain access.

Increasing the duct size would require replacement

Just try the slower fan first and see if it works berore you get too worried about air velocity vs volume. Another posters suggestion about reducing the airflow at the condensor also sounds reasonable (inexpensive and reversable if it dosen't work)

Reducing the size of the registers or closing some dampers would have some effect as well.