Comments on: This Thermostat Setting Can Cost You Money and Make You Sick

By: David Butler

David Butler — Fri, 14 Aug 2015 23:00:47 +0000

@Bridgid, setting the DH @ 45
@Bridgid, setting the DH @ 45% in cooling mode will waste a lot of energy. I recommend 55%, and certainly not below 50%. As long as you keep it below about 60%, there's no risk for any adverse effect on your home. Beyond that's it's just a matter of comfort.

By: Bridgid Courtney

Bridgid Courtney — Fri, 14 Aug 2015 05:09:46 +0000

I have both the Humidify and
I have both the Humidify and Dehumidify features in my Lennox. I have questions as to the percentage to keep them at. Both can be set between 45 to 60%. For the dehumidification (during the warm months)should it be 45%? I keep the temp at 80 or 78.

By: Joe

Joe — Sun, 04 Jan 2015 22:08:02 +0000

I have experienced this issue
I have experienced this issue in our split-level home. The partially below ground lower level suffers from high humidity levels when the fan is on (low speed-continuous) during the AC period. The solution is to only use the low speed fan during the heating cycle, which is unfortunate because of air stratification in a split level home.

The fan coil is in the lower level/basement for our heat pump system, so it's upward air pull with blower above the coil above the filter/return duct from upstairs/downstairs return opening. When the fan turns off, the condensate comes streaming out of the pan to the floor drain.

This coming Spring, I will check and see if the fan delay is set to off when the compressor is off. This article got me to re-check the settings of our thermidistadt, and it seems that the HVAC tech didn't re-enable the "cool to dehumidify" option when he replaced a defective circuit board a few years ago.

I'm not too confident in average tech in this industry when it comes to calculating heat loss, sizing equipment properly, and understanding heat pumps. For example, our system is 30K BTUs, as designed and sold by the HVAC company. Using a heat loss calculator on builditsolar, I came up with 24K BTUs. Using our highest electric bill ever for this past January, and subtracting out our base electric load, I calculate 16K BTUs/hr using the published HSPF of 8.5.

I have learned a great deal from the posters on this blog and from related research studies on AC/heat pumps. Thanks to all of you for sharing your knowledge.

By: Tim

Tim — Fri, 05 Dec 2014 04:57:17 +0000

This may be a little late in
This may be a little late in the game, but according to Alison’s article it looks like a Chicago utility company is incentivizing higher relative humidity inside their customers home. Check it out:
https://www.comed.com/home-savings/rebates-tips/Pages/central-ac-cycling.aspx 
Please correct me if I’m wrong. Love the articles and comments

By: John Proctor

John Proctor — Tue, 30 Sep 2014 23:44:43 +0000

@Leo I strongly suspect it
@Leo I strongly suspect it is not due to physics, but rather (in the old days) what would fit and what it would cost.

By: Bobby Rhett

Bobby Rhett — Fri, 26 Sep 2014 04:22:50 +0000

I think we can all agree that
I think we can all agree that running the fan doesn't remove any moisture, and it does apparently reintroduce moisture from the coil into the living space. My premise is that running the fan in certain homes redistributes heat from the extremeties towards the thermostat, causing the compressor to run more. Whether these "certain homes" share common problems is the question. I can definitely say that in my experience, window covering is entirely cosmetic in the mid east region. People want daylight streaming through big open windows. This is a totally different mentality than the southwest.

By: Roy

Roy — Thu, 25 Sep 2014 16:02:20 +0000

I have trouble imagining that
I have trouble imagining that continuous fan operation during mild weather conditions can reduce whole-house humidity, but I can definitely see it reducing high humidity in some indoor locations by redistributing it to other drier locations, e.g., move humidity from a bathroom or kitchen to the rest of the house. I doubt if this lowered the humidity in the rest of the house too, but it might feel like it since people are more senstive to excessively high humidity and than low humidity.

By: Roy

Roy — Thu, 25 Sep 2014 15:59:01 +0000

As I mentioned before, we
As I mentioned before, we couldn't find any direct correlation between coil water retention and fan operation. The fan running does not seem to cause the coil to "hold" more water and turning it off doesn't seem to cause it to suddenly drain some water.

In some cases we have seen a sudden surge of water out of the condensate drain when the fan turns off, but we suspect that is due the water level in the condensate trap or drain pan changing due to the air pressure change. I don't think that this would be much water compared to what is held on a saturated coil.

John, is your observation about the difference in behaviour with up- and down-flow furnaces based on observations of the condensate drain or direct airflow measurements?

By: John Proctor

John Proctor — Thu, 25 Sep 2014 03:32:25 +0000

@ David: Shutting off the fan
@ David: Shutting off the fan at the immediate end of the compressor run has a big effect for upflow units and almost no effect on downflow units (combination of gravity and surface tension). However since the purpose of turning the fan on is only to get mixing in the house. The fan should only be on as often and as much as much as necessary to obtain that mixing, no longer.

By: David Butler

David Butler — Thu, 25 Sep 2014 02:46:11 +0000

Bobby wrote: "Maybe a
Bobby wrote: "Maybe a properly configured BPM/ECM motor has enough of an advantage to overcome this"

I understand your theory why continuous fan might lower RH in a poorly built home. But this isn't so much an issue of poor insulation, but rather poor mechanical design (grossly oversized and poorly balanced ducts). A properly sized and balanced AC system will achieve decent temperature consistency even if insulation is sub-standard, as long as window coverings are used to block direct sunlight in cooling season, which most folks figure out on their own.

That said, I think the overall impact on RH could go either way in that scenario, depending on all the factors previously discussed (e.g., air handler configuration, how much water remains on the coil, CFM of fan-only mode, and how badly the AC is oversized and out of balance).

@John: I agree that a 'stand-down' fan control like the one Armin designed would help a lot. But given all the variables, plus what Roy said about surface tension, how can we even guess at a useful stand-down time?