PLC for Humidity Control

[gnconroy]

I am looking to control humidity in a crawlspace.

I will use a RH sensor outside and another one inside. If the RH inside is greater than the RH outside, trip a relay to turn on an exhaust fan. If the inside RH is less than or equal to the Outside RH, then do nothing.

Can anyone suggest an inexpensive why to accomplish this?

Thanks

 

[OkiePC]

Do you have sensors in mind? analog output? You just need two comparators, there is probably an analog signal conditioner on the market with that simple bit of logic built in.

Two analogs does make the price go up a bit for any type of programmable relay or PLC, but you could do this with an analog Click and an interposing relay for a couple hundred dollars (in addition to the sensors). That might be the cheapest option, even though the Click is tiny and cheap, it is way overkill for what you're doing.

 

[Tom Jenkins]

I've had good luck with humidity sensors from Omega Engineering - they also have temperature in the same unit.

I'm not aware of a humidity switch, so your best bet is to use a PLC with analog inputs.

 

[danw]

Relative humidity is the ability of air to hold moisture at a given temperature.

The only way the attic RH could be higher than outside air is

- if the air is cooler than the outside air, so it can't hold as much moisture, so the same absolute amount of moisture means a higher RH.

- if some how or other you're pumping moisture into the attic, like from a shower vent or from areas of the house that have no moisture barrier.

In the summer, when the attic is likely hotter than the outdoors because it traps the heat, for the same absolute moisture content, then the attic RH is likely to be lower than the outdoor RH because a higher temperature in the attic can hold more moisture than a lower temperature.

RH sensors tend to not be inexpensive, so I'm wondering what problem you're trying to solve by going through the exercise.

Dewpoint, on the other hand, provides the temperature when water will condense out. If you're trying to prevent condensing water, you need dewpoint, not RH.

Dan

 

[leitmotif]

I think two basic questions must be asked
1. Are you referring to the attic ie above the living space or the crawlspace ie below the living space?
2. Why do you think you need to do this?

Dan Bentler

 

[OkiePC]

I've had good luck with humidity sensors from Omega Engineering - they also have temperature in the same unit.

I'm not aware of a humidity switch, so your best bet is to use a PLC with analog inputs.

http://www.google.com/search?q=humi...sugexp=chrome,mod=11&sourceid=chrome&ie=UTF-8

+1 for Omega. I found some on their site with analog for as low as $170. There may be less expensive ones around. I forget the make/model of what we recently installed for a cold storage system. I will check in the am.

 

[shooter]

Even the cheap sensors (you know these sittingroom clocks are good enough for this)
put a potmeter on it and use the PLC.
Put the sensor outside in a tube with fan etc.
Check the humidity outside.
start the fan for one minute
if RH is lower stop fan if RH is higher keep running for 1 hour.
then repeat cycle.

 

[geniusintraining]

You don't need a PLC

I have a lot of information in this area, you can call me if you want to talk about it, you have a few options that may be better then a PLC

My first one was two vent fans and a simple humidity controller that turned on and off the fans as needed, up there (NY) you really should not need to know the outside, just if the inside reaches xx% then the fan(s) should kick on

Vent fan http://www.rewci.com/crspbave.html
Controllers http://www.rewci.com/conmon.html

But being in the south this did not work as well as I wanted... it just sucked in humid air and made things worse, so I sealed up the space also installed a french drain and then installed a industrial dehumidifier

http://industrialairtreatment.com/library/dehumidification/fral-horizon-dehumidifiers-oandm.pdf

Its been about two years and I could not be happier, its drop the wood moister content to almost nothing and stop all condensation

Moisture is very bad for several reasons and needs to be fixed if you have any...

Best of luck

 

[gnconroy]

Even the cheap sensors (you know these sittingroom clocks are good enough for this)
put a potmeter on it and use the PLC.
Put the sensor outside in a tube with fan etc.
Check the humidity outside.
start the fan for one minute
if RH is lower stop fan if RH is higher keep running for 1 hour.
then repeat cycle.

Your on to what I am try to accomplish. I do not want to draw in wet air, but when the air outside is dryer that the crawlspace air, I want to vent it and draw in the less humid air from the outside.
I am not an electronics guys, I just dabble. Can you elaborate on what you have in mind?

Thanks

 

[gnconroy]

I am looking to control humidity in a crawlspace.

I will use a RH sensor outside and another one inside. If the RH inside is greater than the RH outside, trip a relay to turn on an exhaust fan. If the inside RH is less than or equal to the Outside RH, then do nothing.

Can anyone suggest an inexpensive why to accomplish this?

Thanks

Sorry, I should add that this is a below grade Crawlspace. I am not looking to seal the crawlspace but rather vent it with fans which will activate only when the air outside is dryer than the air sitting in the crawlspace. If the humidity is the same or greater outside then the system will not turn on the fan.

 

[gnconroy]

I have a lot of information in this area, you can call me if you want to talk about it, you have a few options that may be better then a PLC

My first one was two vent fans and a simple humidity controller that turned on and off the fans as needed, up there (NY) you really should not need to know the outside, just if the inside reaches xx% then the fan(s) should kick on

Vent fan http://www.rewci.com/crspbave.html
Controllers http://www.rewci.com/conmon.html

But being in the south this did not work as well as I wanted... it just sucked in humid air and made things worse, so I sealed up the space also installed a french drain and then installed a industrial dehumidifier

http://industrialairtreatment.com/library/dehumidification/fral-horizon-dehumidifiers-oandm.pdf

Its been about two years and I could not be happier, its drop the wood moister content to almost nothing and stop all condensation

Moisture is very bad for several reasons and needs to be fixed if you have any...

Best of luck

The problem with only an inside sensor is that the system will turn on when the level reaches, say 75%. If the air outside is at say 85% or 90%, it will draw wetter air in and therefore increase the level in the crawlspace. the system will continue to run because it is well beyond the set point. Sampling outside air and comparing it to inside air will prevent this from happening.

 

[geniusintraining]

The problem with only an inside sensor is that the system will turn on when the level reaches, say 75%. If the air outside is at say 85% or 90%, it will draw wetter air in and therefore increase the level in the crawlspace. the system will continue to run because it is well beyond the set point. Sampling outside air and comparing it to inside air will prevent this from happening.

If you have a moisture issue in your crawlspace... then the crawlspace RH will NEVER be lower then outside RH, even when its raining outside, trust me it has been a issue for me for 8 years, the best way is seal it up and install a industrial dehumidifier

 

[geniusintraining]

Another thought... do you know where the water is coming from? you may not need any dehumidification and only need a good drain with a little air flow

 

[leitmotif]

Here in the Northwest we get a little rain. The dirt outside the house is quite wet. If the foundations are placed and built to keep the water out then the soil in the crawlspace is bone dry which is as it should be.

On existing houses it may require trenching around the house and setting drains. Horribly expensive and real hard on the shrubbery. But it is the only long term, least cost of operation, effective solution.

After that the only choice is one of two methods either ventilate and hope you can keep the RH of air above moist dirt low enough so that the humidity in dirt is higer (so to speak)
OR put in powered heat pump dehumidification equipment. You can use heat from the condensor (refrigerant) on the heat pump to heat hot water or the house and the water from the evaporator can be used for irrigation toilet flushing etc etc etc.

In all cases -- if you want dry rot and moisture problems seal it up tight and do not ventilate.

Dan Bentler