Moisture on Interior walls

Joannie_14

Maybe pull just one more string, if you haven't already, and test the space for Carbon Monoxide, just in case. If flue gases are entering the space, that can leave moisture on walls and windows, and it would be important to rule that out before assuming it's humidity as a result of everything else that's going on (although, that sounds like what you're dealing with).

Jay W

moisture on walls

Our client has a very small tenant property that has been renovated. I think it was an old chicken coop back in the day. We installed a new furnace with A/C in a closet and it works great. It is the smallest system we could design it is over sized for cooling a little. The return is an open plenium through transfer grills. The room that it is located in has water / moisture physically running down the interior walls in the cooling months. The structure is very loose and it has a wet dirt crawl space. Has anyone had this problem before or know what I might have done wrong. (Doylestown ,Pa)
Thanks,
Jay

Brad White

Any number of things

For one -and especially if the furnace is atmospheric and not sealed combustion- do NOT use the mechanical closet as a return plenum. Flue gasses always move to the lower pressure and the return fan in this case will always win. All flue gasses will be forced into the living space with the potential for disaster. That means dead occupants. If not for being leaky, I have no idea how one could survive that for long, unless I am missing something in your description of the situation.

Check that out immediately before heating season and duct that return. Even if sealed combustion, I have serious reservations in principle.

As for the original question, your furnace enclosure is under a negative pressure. Air will flow in by the path of least resistance. The wet dirt crawl space below is an ideal obvious source. The ductwork may not be that well sealed on the supply side and is jetting to cool the walls perhaps? Regardless, the walls are obviously below the dewpoint.

The fact that the AC system is over-sized also means that it is not dehumidifying the living space as well as it should, especially when it is not so hot out but humid too.

Paul Fredricks_3

Hey Brad

Aside from the sources of moisture and the walls being below dew point, is it possible to install a hot gas bypass in order to reduce capacity? I'd think that would help with dehumidification.

Brad White

Maybe a Rawal device, Paul

I am all for reducing load and dehumidification but how many techs install HGB these days on residential systems? Just asking.

The Rawal device is something a step up from traditional HGB but I have yet to specify it.

I was more in shock from the idea of CO in return air I kinda lost site of humidity

Paul Fredricks_3

I hear ya. But for all we know it could be oil fired with a fresh air boot.

I'm not familiar with the Rawal device. I'll have to go looking around.

I've only seen hot gas on one or two units in 27 years, but for the occasional time you need an 8,500 btu central air system....

Jay W

HEY THANK FOR THE RESPONSE. YOU ALL MADE VERY VALID POINTS. AS FAR AS THE CO PROBLEM IT IS LP GAS FURNACE WITH SEALED COMBUSTION THROUGH A CONCENTRIC VENT KIT. THE MOISTURE ISSUE IS VERY STRANGE I WOULD LIKE TO FIND OUT WHAT THE RELITIVE HUMIDITY IN THE NEXT 24 HRS. I AM SUGGESTING TO THE HOME OWNER TO INSTALL A VAPOR BARRIOR AND INSULATION IN THE BASEMENT/CRAWL SPACE. IT SEEMS THAT THE SYSTEM NEVER SHUTS DOWN DUE TO THE STRUCTURE BEING SO LOOSE AND DAMP. PERSONALLY I THINK THE CUSTOMER HAS TO MUCH GOING ON IN THE STRUCTURE DURING THE RENOVATION(OPEN FLOOR TO CRAWL SPACE, SPACKLING, WINDOW REPAIRS, AND PLUMBING)AND IS JUST NOT BEING PATIENT WITH THE SYSTEM. WELL, I THINK I'M JUST PULLING STRINGS AT THIS POINT.
THANKS AGAIN
Jay