We replaced our forced air system about a year ago with a Carrier Infinity system (2 stage, 17 SEER, variable fan, 80% nat gas).  The condensate line runs into a utility sink in the laundry area (as does the laundry drain).  The equipment is in an unfinished section of the basement - next to the washing machine and dryer - and has no vents but the aluminum ducting is exposed (uninsulated but aerosealed from the inside).

When the system runs in low stage (which is most of the time), the ducts sweat close to the air handler - the humidity level can climb up to 70% in there, while the rest of the house (including the finished part of the basement, which is vented - and has air passages to the unfinished section) is in the 30-45% range.

Since the A/C is so effective at removing humidity, when it is running it is constantly dripping water into the open utility sink.  My theory is that the dripping water is causing the humidity level to climb so high in this area, which is causing the duct sweating.  I'm running a dehumidifier in there now which is set to 55% and removing about a gallon / day (and running often).  I would prefer not to have to run the dehumidifier - not only because I have to keep emptying it (no floor drains), but it uses about 475w - and I don't feel like adding to my power bill.

Does it make sense that the dripping water is causing the humidity level to climb so high in this area?  I'd need a plumber to run the condensate line directly into the drain (am planning on having a plumber run the washing machine drain hose into its own stack so it doesn't just dump into the utility sink - the pipes are large enough.

Thanks.

I would insulate the duct and increase air flow through the room. Or if you want to minimize effort, raise the dehumidifier so that it is above sink level (and can dump into the sink too).

I would be surprised that dripping water into a sink could raise the humidity level that high since the dripping water is such a small amount and it is staying in liquid form.  I guess we will not know for sure until the plumber has re-routed the condensate line. 

Did this problem start only after replacing the old unit?  Have you made any other changes that would let ambient air into the space?  Ideally, I would try to keep the humidity below 50%.

I also doubt that the dripping water would raise humidity, but it is not an acceptable way to expel the water to the exterior

Thanks for the responses. This area of the basement is subterranean - although there is an old basement window next to the HVAC equipment. We didn't have this problem before the new system. Things we changed besides the system:
- Sealed the air ducts with aeroseal (the original 60 year old aluminum ducts leaked like a sieve and most of them are behind drywall between the basement and the main level of the house)
- I sealed up the basement window in the corner (metal frame, original 60 year old window) with silicone caulk around all the gaps in the frame and the window panes
- Added a return just outside the door to this area of the basement
- Added make-up air vents between this area of the basement and the finished section (to meet code for the nat gas furnace - didn't meet it before)
- closed off the gap at the top of the class B vent to the attic

The dryer vent is in good shape, cleaned of lint, and the flap on the outside shuts properly when not in use.

Before installing the system, in order to do the manual J, we had a pressure test of the house done - for a 60 year old house, we have it sealed up pretty well. As far as specific penetrations in the equipment area, I believe I have closed off the sources of outside air - unless humidity is leaking through the painted cinder block walls from the ground. We have very good drainage and have not had an issue with ground water penetration, though.

So no one thinks that the dripping condensate sitting in the sink is causing the high humidity? I definitely agree that the high humidity is causing the duct sweating (as well as the decreased airflow of low stage a/c) - since the dehumdifier has completely stopped the duct sweating. I could elevate the dehumidifer and have it dump into the utility sink also, but I feel like I might just be throwing more fuel on the fire by doing so. Plus slurping up a lot of power.

The "humidity is leaking through the painted cinder block walls from the ground." Sounds like you sealed up the basement, so with less air movement to vent it, the humidity coming in through the block is condensing on the ducts. The ideal interior solution is to fasten XPS foam - like Styrofoam against the block. Do not use fiberglass or a vapor barrier as the moisture will condense within it. Moisture coming through concrete is the reason in new construction we seal the outside of the concrete, put poly under the floor before pouring the floor, and one of the main reasons for insulating the walls with foam. ALso a continuing and very common problem with older homes. Otherwise you are left to deal with the moisture as you are doing.
Bob I

I think Bob is right on.  I wonder if the make-up (ambient) air is also contributing to the problem.  I know you have to have the make-up air.  Can the fresh air be vented directly to the gas furnace.

Posted By sastexan on 01 Aug 2010 11:38 AM

- Added a return just outside the door to this area of the basement

this is a potential red flag , possibly pulling moisture from basement to living area?

The "humidity is leaking through the painted cinder block walls from the ground." I don't have any reason to suspect that to be the case - the paint is in pretty good shape on the cinder blocks - not sure of the age of the paint, nor the type of paint used.

The make up air is coming from the finished part of the basement - not outside.

The humidity levels in the living areas are very comfortable thanks to the Infinity controller - below 50% (when it is hot outside, it runs enough to drop humidity level all the way down to 35%). Even when the humidity levels on the main level and the finished part of the basement are in the low 40s, it is still upper 60s or 70% in the equipment / laundry area - where the ducts are sweating.

The house is on a concrete slab. Not sure if that would make a difference.

You have high humidity on one area and low in another. Increase the ventilation between the areas.

You should be able to check whether you have moisture coming in through the concrete by taping a piece of poly to the wall and one to the floor. If it is well sealed and there is moisture it will show up on the poly.

Most basement humidity problems (except in chronic flooding situations) in the cooling season are caused by outdoor air infiltration, not groundwater diffusion through painted CMU. If the CMU and slab don't look actively WET, it takes a heluva long time to get a gallon of water through the masonry and into 55%RH room air.

The water from the sweating ducts is humidity drawn FROM the surrounding room air, not adding TO the humidity of the room. If the room is significantly cooler than the rest of the house, it's relative humidity will be higher. Same air, cooler temp==higher RH and conversely. Raising the temp of that room will lower the RH, but not the duct sweating problem, because the dew point won't change even if the RH does, and the duct is clearly below the dew point of the air mass in the room.

Pressurizing/depressurizing the house with a large window fan and running around finding & fixing as many air leaks as you can improve the situation. It's often the case that air-leaks into the attic depressurizes the place, leading to air infiltration into the basement in one large "stack effect". Air-sealing only at the basement or only at the attic is only half the problem- do both and the stack effect is broken. Sometimes plumbing stacks/chases and flue chases form their own stack effect, and are usually blockable. Even seemingly small mortar cracks in a block wall can form a significant air leak.

By sealing your ducts you've changed the room air flow, since you no longer have duct-leakage to drive air exchanges (a good thing, really). By running the AC blower on low it's also lowering the duct temp near the air handler, making it cooler & more prone to sweating. Insulating the ducts to ~R2 with closed-cell foam is usually sufficient to stop the sweating, but R6+ would be better from a system efficiency point of view. But slowing the whole-house infiltration of outdoor air would also likely reduce the RH of that room to where duct sweating would be minimal.