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Question DetailsAsked on 5/2/2014

After rainfall, water/wetness appears around HVAC unit that sits on cinder block in basement room.How to fix?

We purchased home last year. Utility room in basement had water seepage issues. After moving to house, we got the basement waterproofing work done by doing exterior excavation - i.e., by digging to the footing of basement, applying 2 coats of concrete/waterproof cement (black rubberish). Back filled the dirt and re-sealed all the joints around the exterior wall. Last last month as spring started, we started noticing water signs/wetness around the 2 cinder block in that utility room. The HVAC unit rests on those 2 cinder blocks. After heavy rainfall, we noticed that the area around cinder blocks that holds HVAC unit gets wet and sometimes when rainfall is heavy, we can see water on the grout area too in between the floor tiles, which may have originated from cinder block area. Walls are completely dry. Only the floor area around cinder blocks is wet. Not sure why this would happen as we have not turned on AC. It only happens after heavy rainfall.

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2 Answers


I'm in Florida,so no basements here.I would guess that water may be getting in around the area where the refrigerant lines from the AC coil go outside(posssibly the drain as well),then run down to the floor.

Take a look at that area,both inside and outside.


Answered 6 years ago by BayAreaAC


When you say HVAC unit I am not sure what you are talking about - I assume you mean furnace and maybe A/C evaporator. As BayAreaAC says, an obvious cause is water coming in around the AC tubing, if it penetrates the wall below ground level to get to an outside compressor/condenser.

Not knowing how your A/C unit is set up, I guess it is remotely possible you are getting natural convection movement of the A/C fluid, causing air moisture condensation on the tubing, which then runs down the tubing to the evaporator coil and drips to the floor from there. Not likely, but easy way to tell is run your hand along the underside of the tubing when this is happening - if there is free water, that might be the issue. Another possibility is that your HVAC unit and evaporator are cold metal, so during humid period moisture is condensing on the metal fromthe moist air in the basement, then running down the legs to the blocks, then pooling under and around the blocks.

A second situation which might be your case, especially since you say this only happens after significant rains, takes a bit more thinking about to understand the mechanism. It may be that during rains your basement slab gets wet feet because the outside water level comes up outside the foundation, and seeps in under the slab. This is VERY common because the common basement waterproofing solution ignores common sense - they generally put the french drain that is supposed to keep your basement dry at the top of the foundation footer - which means the bottom of the drain pipe is at the same elevation as the bottom of your slab, so if there is water at the drain the bottom of the slab is wet also. Then, to provide drain slope, it is elevated a bit above that point towards the starting point - so much of the pipe is actually at or above the top of slab elevation. This results in the slab getting at least wet feet, and commonly a slight positive pressure on it from underneath whenever the outside water level comes up to the french drain elevation. Many people (contractors and homeowners both) aggravate the situation by putting their downspouts into the french drain, making the area wet every time it rains. If you did not have a french drain put in, then the water may be accumulating in the fill outside your waterproofed wall, and seeping in under the slab and getting it wet. SO, in your case, it might be that the ventilation in the basement is normally just enough to evaporate the moisture as fast as it comes up through the slab - which can total gallons of moisture per day over a full basement. However, at these two blocks, because they are sitting on the slab and preventing evaporation, with a rough surface between them so a bit of a gap, the moisture wicks up through the concrete, maybe combined with it condensing on the underside of the blocks, and then runs out from under it and makes a small puddle around it as it evaporates - or maybe just enough to dampen but not wet the surface but still form an evaporation stain, without visible water, depending on humidity and ventilation in that area. It is also possible that the floor was sealed after the HVAC unit was installed - so the only unsealed place is under the blocks, so that is where weepage and evaporation occurs in earnest. I wouldnot be surprised if the bottom of studs in any wallsdown there are getting damp too - which could eventually make for mold/rot problems, especailly if they are not treated.

It may be that, if you can do so without stressing lines and ducting, just small wedges driven under the cinder blocks or a thick washer at each corner might provide enough air gap to dry it out before it become seepage from under the blocks. Obviously, be careful not to dislocate the blocks and drop your HVAC unit on the floor.

A third possibility is very similar - that there is natural ground moisture under the slab constantly wicking through the slab (toward the drier side - the indoors) which normally evaporates, but when it rains the humidity in the ambient air goes up and the basement air humidity goes up with it too, so the moisture coming through the slab now forms free water at the protected underside of the blocks. If this is happening, I would expect the same thing to be happening at the junction of the slab and wall, where the colder wall keeps the slab cold, so it condenses moisture out of the air on the colder surface.

The grout moisture may be from that damp area, or may be another sign that the concrete slab is reaching 100% humidity, so free moisture appears at the surface of the tile through the grout.

I am betting that, the next time it rains, if you put down a square of saran wrap and tape it tightly to the floor around the edges, you will see free water condense on the underside of the saran wrap in short order.

You could also get a $15 humidity gage - not one of the rinky dink color change strips, but an actual dial or electronic humidity gage - also called a hygrometer. Available at box stores, hardware stores, Amazon, etc. See what your humidity is at floor level - likely reaching near 100% during humid, muggy weather.

Solutions - obviously next time is happens run dry paper towels around all metal items and floor to see where wetness is coming from for sure - track it back to its source. If generally high humidity in basement, get an automatic controlled dehumidifier, or provide a humidistat controlled fan to increase ventiltion at floor level and evaporate the moisture - but not you will then have to get rid of that moisture. If you have a full-house fan system or run the A/C then of course if the HVAC unit draws air from the basement it will take that moisture away, but otherwise a fan would just circulate it around and not remove it.

Since you say it happens only after heavy rains, look to getting moisture away from the outside of the foundation - though this should have been the first action before waterproofing the foundation. Make sure you have gutters, that the downspouts direct the water well away from the foundation through downspouts and then gutters or channels away from the house at least 3 feet even in clayey soils - more like up to 10 in free draining sands and gravels, and to such a place that the water cannot run back towards the house through the vegetation or topsoil. Also, correct any surface drainage that allows water to move to the foundation when it rains hard. Just make sure to leave at least 4 inches clearance between ground and siding and also between ground and top of foundation wall, for water and insect intrusion protection - more like 12-18 from siding is desireable in subterranean termite country.

Answered 6 years ago by LCD

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