At what outdoor dew point is a dehumidifier not needed in a basement?

[teddy bear]

I have been waiting all summer to answer this question. This is 1,700 sqft. finished basement that I monitor. There are two occupants in the home 12 hours per day. No fresh air ventilation or dehumidification this week. The basement was <53%RH for the week. The outdoor dew point average <40^F for the week. This is a cool basement without much space heating yet. At higher outdoor dew points, I needed dehumidification.
I posted the data from my Hobo loggers.
Regards TB

WI 50RH Bsmnt No dehu.pdf

[tedkidd]

so coooool! (I just got 4 Ecobee's - put 2 in my rental - there is so much to learn from knowing how stuff actually performs.)

I see you are targeting a 5f air temp delta but maintaining 15-20f. How did you arrive at those decisions?

How do you guesstimate the temperature of the coldest surface?

[teddy bear]

so coooool! (I just got 4 Ecobee's - put 2 in my rental - there is so much to learn from knowing how stuff actually performs.)

I see you are targeting a 5f air temp delta but maintaining 15-20f. How did you arrive at those decisions?

How do you guesstimate the temperature of the coldest surface?

I delayed turning the heat on till this week. The smell of the basement is a good indication of excess dew point. Usually 50%RH in the basement is enough for carpet on the concrete.
I am attaching a illistration of the balancing effect of air moving through the home with a given amount of moisture load.
Changing any of the inputs will change the %RH in the home. Controlling the %RH requires adding or removing moisture from the air in the home. Dehumidifier or humififier!
Regards TB

Microsoft Word - Moisture balance in the house.pdf
WI 50RH Bsmnt No dehu.pdf

[amd]

Interesting data - thanks.

The type of moisture protection around the exterior side of foundation, as well has precipitation has a huge impact from my experience.

A basement can still be damp even when the outdoor dewpoint is well below 50F due to moist soil and/or precipitation.

With no damp/waterproofing - typical of older foundations, it might be necessary to occasionally run a dehumidifier or ventilate when it rains well until december or so.

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One thing I'm curious about is how much latent heat consumer grade dehumidifiers convert to sensible heat relative to total energy consumption.

Say a 50 pint unit consumes 500 watts when the compressor is running - about 1700 BTUs/hr.

How many extra BTUs/hr would be released at 75F/50% humidity?

What's the effective cop when it comes to heat production? Example: 0.2 units of latent heat converted into sensible heat would be 1.2

[teddy bear]

End of Oct in WI shows low dew points and then switch to +60^F. Interest effect on a ERV home with .4 ach CO2 control. This a foamed walls home.
The inside %RH tracks the outdoor dew point to the point where the dehu is activated.
Regards TB
Data by Hobo Loggers

10 23 12 erv home 2-789823-16-160.pdf

[tedkidd]

I've started an album of chart screenshots from my Ecobee portal. Look what can be learned by tracking how a home and it's equipment interact with each other and the environment.

This thing tracks indoor, outdoor temps, run time, and RH.

Each vertical red line represents up to 5 minutes of run time.
Green line is indoor temp
Black is outdoor temp.
Dotted red is set temp
Grey/light blue is RH

Really interesting to see cycle time and impact. TB - interested in your thoughts about rh line.

[teddy bear]

I could not blow up the picture enough to see the detail. Any chance you could post a PDF of the graph.
WI has +60^F outdoor dew point this week, it headed your way.
I have data on effect on a basement without dehumidifier. The house is getting about 40-70 natural infiltration this week. Basement zooming up.
You can see the outdoor dew point drive the indoor dew point/%RH. Aggresive air change would cause higher indoor %RH. OK for a day or two. The lag is the material of the house absorbing moisture. Mechanical ventilation and a good dehu is one of the near perfect solutions. This WI's high dew points last gasp.
Attaching PDF of data.
Regards TB

[madhat]

It's not the Outdoor Dew Point you need to worry about. It's the Indoor Dew Point, you need to find your coldest indoor basement surface Temperature. It could be a window pane or frame. Masonry wall or floor. It could also switch places over the seasons. I.E. Floor in Summer, Window in Winter. My Basement is much dryer since I put deeper sump pits in. Went from 24" to 36" Also put a Watch Dog system in so I don't flood during storm power outages.

[tedkidd]

So even pics at google + are too low? Hmmm, good to know.

I can get XML. Please send me your email.