Two AC Units in the Same Room

We need to know a little more here like:

Where is the condensation forming?

What is the room temperature?

What is the room relative humidity? (If you don't know, just use the outdoors RH that the weather man gives you on a day that the condensation is bad and we can make the assumption that it is pretty close.)

Likely you are just getting the room cool enough that you are falling below the dew point.

Stephan

Hi,

Is That both Units are Ceiling return or Ducted return ? if any one of the unit Ducted return that unit both supply and return has to be insulated.

Say both units are ceiling return, then both units supply ducts has to be insulated.

Hope this help for you

Thanks

You haven't stated the nature of the condensation. It is condensation at the air outlets, is it condensation in the ducts, is it condensation along the pipes? it is condensation on the walls? Normally, condensation occurs when fresh air or outside air infiltrates the room, especially if it is negatively pressurized, or when the fresh air supply is either dumped inside the room or free blowing into a ceiling plenum, it depends. Where condensation occurs on non-insulated ducts or pipes, then the solution is to provide proper insulation. Investigate where and when condensation occurs. Maybe the drain pans are not properly insulated. Do some psychrometric analysis of air conditions. Plot them on the psychrometric chart and you would have an idea of whats happening with your systems.

Hi, the condensation is on the ductwork of the old system it is not insulated, the new system is insulated, i'm wondering the return air on the new system is directed through the t-bar ceiling into the room area . Maybe i should have in the ceiling space just installed more eggcrates in the ceiling and cut off the existing return duct. the old system still works but is directing air outside this room, it seems i might be robbing air from one space to another.

A SMALL CEILING FAN WILL ALLEVIATE THE condensation PROBLEM AND YOU'LL USE LESS AC ELECTRICITY BECAUSE THE increased air flow will make you feel about 10 degrees cooler than it really is.

Hi If the condensation is on the walls and windows then probably you have too much make up air. The original unit was probably sized to the room and by duplicating it you have doubled the fresh air volume. As TBL suggested the best cure is to turn off 1 unit and add a ceiling fan. We do this as a standard practice in the tropics if helps with condensation and also makes you feel a lot cooler.

It appears to me that all you need to do is insulate and vapour seal the old duct.

The condensation is forming because the supply air temperature for the old unit drops the return air/ceiling air to below its dewpoint temperature. Insulation and vapour sealing is the only answer.

Good Luck!

Small ceiling fan. Its cheap cure. Also make sure the room is getting enough fresh air. Alot of people in a small room raise the humidity above what it is on the outside because everytime the breath they exhale moisture. Same reason your car windows fog up with windows closed. Isn't that also why on most cars you can't use recirculate and defrost at same time? If cooling is a problem ..paint your roof white! Reduces cooling load by 20%

Sorry but a ceiling fan will make no difference! Condensation will still occur on the duct if it is below the dewpoint temperature of the air.

RE: The cheap fan idea:

For condensation to happen.. you must have a cold surface in relation to warmer moist air , when the moist air hits the cold surface the moisture in the air liquifies. If the cold surface is warmed to same temp as moist air, condensation doesn't occur. Thus the fan helps circulate the warmer air which in turn heats the pipe (or in some cases air duct) and helps all stay "near enough" to each other in temperature to prevent the condensation.

Yes condensation can still occur but I have seen this work in the past. Of course insulating the pipe from the warm air would work too. But that may not be easy to do. A fan is cheap and quick.

You will see if you try it that sometimes for an "inapparent" reason, the bumble bee who has no logical reason does indeed fly! Try it with a window fan first if you think a ceiling fan is a waste of energy. (Aim the window fan at the area of most condensation and see if it doesn't elimnate the problem). I'm not going to say its 100% guaranteed to work.. but I'd go with 99% sure. There is always extenuating circumstances that could prevent the fan from working well enough.

Again: make sure you are getting enough fresh air from outside too. Many people put out alot of moisture into the air they exhale.

I would still go for insulating the duct or that which is causing the condensation. Although a small fan would also reduce the condensation, still it would not solve the cause. Also, take a look at how much fresh air is being pumped into that room. Maybe reducing the amount, would eliminate the condensation problem. If the room is not not critical to maintaining IAQ (Indoor Air Quality), you can reduce it to barest minimum. Also conduct air balancing, maybe there are hot spots and cold spots in the room indicating air flows are not balanced.

Consider that now you may now have an oversized AC system that is short cycling. This is a common problem with AC systems that results in high humidity. Assuming it is a DX system, the system only removes moisture from the air when a compressor is running. If the fan runs continuously the moisture condensed on the coil will evaporate back into the space when the compressor cycles off. In fact the condensation may not build up enough to drain off the coil before the compressor cycles off. This problem will be worse if fresh air is being introduced continuously. Under light load, it may be better to run half your system to avoid short cycling.

If it is a draw through coil/fan arrangement, the P trap on the coil may not be big enough to drain off the condensate properly. This is another common problem.