Dehumidifier

[Mar3232]

Can't believe how much cooler my home is now after installing new windows and new roof with a ridge vent. Also increased the size of my soffit vents and although it's only May, generally by now I would have killed for an air conditioner but have been VERY comfortable. I do have fans in every room just to circulate the air and always keep the windows shut. Also, keep my basement door closed.

I do have a dehumidifier in the basement because I have some important stuff down there and the basement stays extremely cool, even with the heat from the dehumidifier. No windows at all in the basement, it's very tightly sealed. The house is also shaded for most of the day from the sun and I keep my south blinds closed.

As the summer progresses, I'll see if I can live without AC and thought of a couple of things --

(1) Creating a duct system that pulls air from the basement floor with a small fan to the upstairs. (It will be an experiment). The air in the basement is measurably cooler, maybe it will help the upstairs, maybe not.

But I was thinking -- why can't you have a small whole house dehumidifier in the basement that I can attach my own ductwork to that can suck the upstairs air and dehumidify it? (I don't have a furnace or ductwork anymore -- took it out thinking in terms mini splits last year).

Can't a dehumidifier simply suck the upstairs air and get rid of the moisture or do you have to have the dehumidifier outlet feeding upstairs also?
Does the dry air outlet need to displace the humid air or something for it to work?

Is the dry air coming out of the dehumidifier much warmer? or is that just the compressor heat?

And as far as the compressor creating heat in the basement, it isn't a problem (it isn't with the dehumidifier down there now) -- my floor between the basement and the upstairs is CONCRETE (kid you not). It's an industrial flooring system.

Also, if the basement air got too warm, why couldn't I vent it outside?

 

[Jadnashua]

A typical dehumidifier (well, anything with a fan) is unlikely to have enough capacity to pull air from a long ways away and maintain the required air flow across the coils unless it was designed for it. This would allow the coils to get colder, and ice up, potentially blocking all air flow. A dehumidifier is typically not designed to work with a ducted system.

If you ran the exhaust outside, that would mean you would need to bring in makeup air. That air might come from a duct, but more likely, from leaks in the house...that would bring in probably warmer air with higher moisture content, negating any benefit of the dehumidifier.

Warmer air wants to rise because it is literally lighter (the molecules are moving faster, and under normal circumstances, end up further apart, thus per volume, lighter). So any fan/duct system would have to have enough power to overcome that tendency for the warm air to rise. IOW, pushing colder, denser air up can be done (central air systems do it all the time), but they use a fairly big fan motor to do it. WHen the air temperatures are only a few degrees different, there is very little energy difference, and just compressing it with the fan action may make that air hotter, and also negate any benefit.

 

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[Mar3232]

didn't think about the fact that the whole house dehumidifiers are designed to work with an existing air handler -- wonder if they make a unit that will draw air bigtime by itself? (Not a portable unit but something I could attach ducts to?).

Anyway first things first, and I'm going to play with the idea of drawing air from the basement floor just for an experiment. My house is small, single story and the basement of course is right below.

I have a couple of 120v muffin fans that I thought I'd put in some round 6" pvc pipe -- put the fan on the ceiling blowing up and the pipe end a few inches off the floor.

Probably a waste of time. But I'm easily amused.

 

[Jadnashua]

An air handler, depending on the a/c size, may need in the order of 400+cuft/min per ton of a/c air flow to ensure the coil doesn't freeze up and you get the designed temperature drop across it...

Most muffin fans are only in the order of 50cuft/min or so...but, some certainly are more. If the air temp of the basement is only a couple of degrees below that of the upper story, the heat caused by running the fans may offset any cooling it might provide.

 

[Mar3232]

Makes sense -- my basement is so comfortable in the summer, dry and cool like it has AC down there. Wish I could utilize that but dream on I guess.

 

[Reach4]

Dehumidifiers take considerable power. That should figure into your plans.

For sharing air between basement and the rest of the house, I would add a cold air return to the basement. That may be as simple as adding a grill to an existing plenum. Make it so that you can cut that off. Then turn on the central furnace/AC fan, and the air will mix.

Note the warning shown here during heating season: http://www.nachi.org/forum/attachments/f20/26981d1235095972-basement-cold-air-return-0796-small-.jpg

 

[Dana]

Whole house dehumidifiers only make sense in low-gain houses in climates with high latent loads. For Indiana's latent loads they are extreme overkill- think "swatting flies with sledghammers".


If you have the air-leakiest house in Indiana leading to high latent loads combined with minimal sensible cooling loads you'd be better off spending the money on air-sealing. If that's the case, the band joist/foundation-sill, leakage between the top-floor-ceiling & attic, or any direct plumbing/electrical/flue chases that run between basement & attic would be on the air-sealing critical list. (Stack effect drives infiltration rates & latent loads more than wind.)

If you already have a hot air furnace or AC air handler, a standalone dehumidifier in the basement and running the air handler on a duty cycle/timer to periodically mix the dehumidified basement air with the rest of the conditioned space usually works. (This is even true for very tight high-R/low-gain homes in the gulf coast states.) A crummy muffin fan won't cut it, but some 5-7" tubeaxial fans used for electronics racks might have sufficient CFM to cut it with a 6" PVC conduit. (But using 6" hard-piped duct would be more appropriate for this application.)

 

[Mikey]

My problem is humidity in the attic. I'm not sure how it gets there, because the attic is well sealed, but I do want to get rid of it. It's about 6000 cubic feet, 70' x 30' give or take. Low headroom, extremely well insulated.

First thought was to open a new attic return port and an attic supply port in the existing airconditioning system, which does a dandy job of dehumidifying. The air handler is located in the attic, so opening the ductwork to continuously circulate some quantity of attic air would be easy. I believe the AC system has enough excess capacity to handle the additional load, so something like a 6-8" inlet and a similar outlet would cool and dehumidify a little bit at a time, and the effective size of the new ducts could be adjusted to condition just enough air to get the humidity down to a reasonable level and maintain it. Condensation at the outlet port would have to be dealt with.

Second thought was a small dehumidifier, like the well-reviewed Frigidaire 70-pint system. Quick and dirty, but might not handle the large volume efficiently. Could run the drain hose out the AC condensate drain, so there'd be no hauling of water buckets.

Would appreciate your expert thoughts, especially in the area of how big a hole I'd need in the ductwork if I went that route.

 

[Reach4]

My problem is humidity in the attic. I'm not sure how it gets there, because the attic is well sealed, but I do want to get rid of it. It's about 6000 cubic feet, 70' x 30' give or take. Low headroom, extremely well insulated.

Non-expert thoughts:

Describe the insulation. Is there a vapor barrier? I would expect that the vapor barrier would be on the un-conditioned side in the warm climate. You would want to make sure that vapor barrier was in good shape, and repair if needed.

Are the ducts and air handler in the un-conditioned space, as I would presume? Those items would need to be insulated against the hot air.

I am suspecting for you that the suggestion would be to open up the existing eaves vents and/or add more eaves vents and to add roof vents if needed. The eaves vents must not be blocked with insulation.

 

[Mikey]

Sorry for the delay. The insulation was an experiment (see: http://greenstarpanels.com/) that I had high hopes for. Basically, the entire attic volume is sealed by the ceiling/insulation below, and 3" insulating panels on the underside of the rafters and at the gable ends. The rafter bays are vented from the soffits below to a ridge vent above. But I believe the installation was poorly planned and haphazardly executed, such that the attic volume was never properly sealed. The vendor has made repeated attempts to fix the problem without success. If we could hire some folks about 6" tall to rework the low end, we might have a chance, but even the smallest, most dedicated installer can't get anywhere near where he's got to be. An alternative is to remove the lower 4' of the roof deck and fix things from above, but that's a last resort.

The panels have an aluminized mylar facing on the lower side. Air handler and ducts are in the attic (unconditioned) space, but in theory (and in new construction) the temperature in the attic space is significantly lowered, so that they're only fighting 90° or so temperature, instead of 135°, which I used to see commonly. In any event, all the trunk ductwork is of 1 1/2" R-6 ductboard constructed on site. R-6 is the local code, which I think is borderline; the galvanized laterals are sealed and wrapped to R-8.

We've replaced all the old corroded soffit vent panels with new vinyl (ensuring that there was a clear path to the rafter bays), and installed continuous ridge venting, although there are some rafter bays that wind up unvented because of hips and valleys.

In new construction, the system works beautifully, but in my home getting the panels sealed at the low point was nearly impossible, contributing (I think) to an overall system failure. Also, the panels were designed for rafters 24" o.c., and mine vary from 22" to 26". The attic temperature has been lowered as hoped, but the humidity seems to be constant. It's almost as though a certain volume of water got into the attic space prior to sealing it all up, and with no air circulation it's going to stay there, probably absorbed into all the fiberglass. I'm looking at a dehumidifying approach to test that theory, hoping that after an initial major effort, a subsequent maintenance amount of dehumidification will keep the attic comfortable.

 

[Dana]

In a central FL climate attic humidity is almost always due to leakage from the outdoors. Moisture is not absorbed by fiberglass (it has effectively zero moisture buffering capacity), but it is absorbed by wood, but it would have to be pond-soaked wood to have enough stored humidity that it becomes the CAUSE of high attic air humidity.

If you find and fix all of the serious air leaks you won't have the problem. You may not be able to find the leakage points easily with a visual inspection- it often takes a blower-door test pressurizing/depressurizing the house to figure it out.

A theatrical smoke machine in a pressurized attic will make the leakage dead-easy to spot from the exterior, but if you've installed the panel system on the underside of vented rafter bays you'll only be able to see which bays are leaking by location the smoke coming out of the soffit /ridge vents, not exactly a pin-point locator. In your case it will probably take infra-red imaging cameras to spot them, by depressurizing the house and seeing where the hotter/colder air is coming in. You probably have major air leaks at the bottom of your panel system as you suspect, or you may even have air leaking up from behind the siding getting in at the top of the walls. Buy a case of can-foam to seal up everything you think could possibly leak and you may be able to avoid the cost of the blower-door /IR imaging diagnostic.

 

[Mikey]

Thanks, Dana. I was afraid someone would say something like that. There's no siding -- it's a brick house; the only point of intrusion from the outside is at the bottom of the panels, mostly, and via the two stair-doors in the garage. We recently replaced the soffit vents, which were old, corroded aluminum, and at that time we did the best we could to peek up into the rafter bays and make sure they were clear of insulation and the panels were tight against the walls. But that wasn't terribly effective, I'm afraid.

I did some blower-door testing a few years ago, and found and fixed a ton of leaks between the attic and living space. I got into a p*ssing contest over a contract issue with the guy who did the work, so now he won't talk to me any more, and I haven't been able to find a replacement. I'm very close to buying the equipment and doing it myself; I'd like to check the ductwork for leakage as well, but I'm pretty confident that that's good. The problem with finding out that the panel-wall seal is in fact the problem remains: how do we fix it?

I may just admit defeat and install a gable vent fan, but that doesn't really help the humidit issue much.

 

[Dana]

A gable vent fan will suck conditioned air out of your house via the attic, drawing humid outdoor air into the conditioned space from other leak points- it's decidedly NOT the solution. In FL the humidity source IS the great outdoors, so active ventilation only increases the latent load. It'll only lower the humidity levels in the attic by pulling dry conditioned air into it, but raise the humidity of the conditioned space.'

Brick is still a siding, and is usually built with a vented air gap of 3/4-1" between the brick & structural wall (whether the structural wall is studs or CMU block.) The air gap is a necessary capillary break to keep the masonry from wicking rain/dew moisture into the house. It's pretty common to vent the top of the masonry cavity into a vented attic, and if you block the attic venting, the super-moist air from that cavity will end up in the attic. You can safely seal the top of the cavity if you drill in vents to the exterior along the top course of brick. It can be as simple as drilling 3/8" holes through the vertical mortar lines every third brick, or you can core 1" holes through the brick every couple feet and install a screened vent plug to keep the critters out. There is usually corresponding weep holes on the bottom that serve as the convection air inlet.

When sun hits dew/rain wetted brick the moisture drives are pretty intense as it "cooks" the moisture out of the brick, but the sun-warmed brick also heats the air in the cavity, creating a convective force to draw drier air in, purging the super-moist air out the top vents.

 

[Mikey]

No, I said "brick", not brick veneer. It's weird, but the wall is 5 1/4"" of brick, 3/4" of sprayed-on foam insulation, random air gap due to uneven foam, then the finish drywall. It's been up for 40 years, and when we tore off some drywall during our recent remodel, we saw no moisture or mold anywhere in the wall; God knows why not.

I agree with you re the gable vent fan.

Started up a small (70 pint) dehumidifier yesterday and the RH dropped from 75% to 45% in one day. This is not a good test, though, because the outside humidity has also dropped somewhat. But it's promising. I'll keep you posted.

When we started this extensive remodel about 10 years ago, an architect friend said we should start with a D-10 bulldozer. I wish I had listened to him.

 

[Dana]

Even all-brick construction from the 1970s and newer was usually built as a cavity wall, with or without rigid insulation on the outside of the interior wythes, with the gap between the insulation & exterior wythes. Single wythe structural brick is extremely rare, but apparently that's what you have(?). Even solid masonry walls are usually at least 2 wythes, often with the inn

With spray foam applied to the interior side of the brick there is no point to drawing outdoor air between the foam & wallboard, since that would drag outdoor moisture into that space. The foam (probably urea-formaldehyde foam if installed in the 1970s) is almost a class-III vapor retarder, about 10-15 perms @ 1" thickness and does not wick moisture, so it has been at least somewhat protective. If it's closed cell polyurethane (not likely if installed before 1990) it would be about 0.8-1.2 perms @ 1", an order of magnitude more vapor retardent. Unless you installed something extremely vapor retardent such as vinyl/foil wallpaper on the wallboard, the air conditioning would still be able to pull that water vapor through the wallboard, keeping the foam & furring at relatively low mold risk.