[teddy bear]

Well understood - thank you. The question becomes whether my central AC unit on dehumidify (humidistat built into Tstat, runs fan at lower speed) is more or less efficient than a high efficiency stand alone unit ducted into central supply. Right now, they (AC and basement dehumidifier) work in tandem with basement unit running 100% duty cycle on hottest days and central AC cycling. I have not seen a definitive answer on this efficency question, do you have data?

Nonetheless, in my house I believe there will always be a need for a dedicated basement dehumidifier (AC is not ducted to basement and during shoulder seasons, basement can be significantly cooler and have much higher humidity than upstairs.)

Best to get you a/c to do as much dehumidification as possible. This means that the cooling coil should be cold enough to maintain your desired %RH, while operating at a high cooling load condition. Overnight and during the sholder season, the dehumidifier is needed to supplement the a/c. Basements are a different animal. Cool basements are seldom adequately dehumidified by the a/c. In other words, I agree with you. I suggest you use your current dehu until it fails mechanically or you are unable to control the basement. A free standing Santa Fe in the basement, recirulating air by your air handler may keep the home dry. If not ducting can be added to the Santa Fes. The Ultra-Aire is intended for full ducting throughout the home with remote low voltage controls.
Keep us posted.
I promised a post on the CO2meter.com data logger after I tested it. I have been using and it is good at temp/%RH/CO2. I am attaching the jpeg file. For 450 dollars including software, not bad. Good for long term monitoring for a HVAC contractors. Also will test the Honeywell which looks ok.
Regards TB

 

[pstu]

Well understood - thank you. The question becomes whether my central AC unit on dehumidify (humidistat built into Tstat, runs fan at lower speed) is more or less efficient than a high efficiency stand alone unit ducted into central supply. Right now, they (AC and basement dehumidifier) work in tandem with basement unit running 100% duty cycle on hottest days and central AC cycling. I have not seen a definitive answer on this efficency question, do you have data?

Nonetheless, in my house I believe there will always be a need for a dedicated basement dehumidifier (AC is not ducted to basement and during shoulder seasons, basement can be significantly cooler and have much higher humidity than upstairs.)

One of the interesting big variables IMO, is whether you desire the dehu to emit heat or take it away. If the former, all the standalone dehus have the advantage, they remove humidity at a certain kwh/pint and give you a certain number of heat BTUs in the bargain. This is due to the dehu not being a split system (which would be a form of AC, rejecting heat to the outdoors). For a basement I imagine this is great.

However for the main house in AC season, the cost is higher than the rated kwh/pint. That is because of the heat BTUs added to the house at the same time you are paying the AC system to remove it. A person should be able to estimate the amount of BTUs added, and with some research estimate the cost of the added AC load to get it back out of the house. Some people have wisely said that a dehu turns latent load into sensible load.

I do not really have the info to actually arrive at a number for efficiency. It is the kind of math problem which will seem really simple *after* someone shows how to solve it, but right now it seems hard<g>.

Best wishes -- Pstu

 

[pstu]

Best to get you a/c to do as much dehumidification as possible. This means that the cooling coil should be cold enough to maintain your desired %RH, while operating at a high cooling load condition...
Regards TB

TB, wouldn't it be interesting and useful to have an easy way to measure AC coil temperature? It's not the kind of idea which "strikes a chord" with the world of HVAC professionals. But neither is measuring the volume of condensate produced by an AC<g>.

Best wishes -- Pstu

 

[teddy bear]

One of the interesting big variables IMO, is whether you desire the dehu to emit heat or take it away. If the former, all the standalone dehus have the advantage, they remove humidity at a certain kwh/pint and give you a certain number of heat BTUs in the bargain. This is due to the dehu not being a split system (which would be a form of AC, rejecting heat to the outdoors). For a basement I imagine this is great.

However for the main house in AC season, the cost is higher than the rated kwh/pint. That is because of the heat BTUs added to the house at the same time you are paying the AC system to remove it. A person should be able to estimate the amount of BTUs added, and with some research estimate the cost of the added AC load to get it back out of the house. Some people have wisely said that a dehu turns latent load into sensible load.

I do not really have the info to actually arrive at a number for efficiency. It is the kind of math problem which will seem really simple *after* someone shows how to solve it, but right now it seems hard<g>.

Best wishes -- Pstu

The a/c cools/dehumidifiers until the the ^F setting is reached. A dehumidifier is not operated unless the %RH is above the setting. If your dehumidifier cooled, it is an a/c. Not good, you need reheat to avoid over cooling the home.
This points out the need to setup the a/c to remove enough moisture when operating for an extended period to maintain the desired %RH. It is not efficient to remove the moisture with the most efficient dehu and then remove the heat from the dehumidifier with the a/c verses maintaining humidity with a properly setup a/c.
Also maintaining low humidity when unoccupied, heat helps reduce the amount of dehumidification needed to maintain a specific %RH.
If your a/c is not removing enough moisture when in continuous cooling, reduce the air flow which inturn reduces the coil temperature, removing more latent and less sensible heat. If over dehumidification occurs, increase the air flow which raises the coil temperature, reducing the latent cooling and increasing the sensible removal. A/cs move more btus per watt with warmer coil temperatures. Therefore avoid over drying the air.

Regards TB

 

[dbbarron]

A practical comment from my own home (main floor AC Tstat set to dehumidify (AC on Fan low) to 50% setpoint and not overcool by more than 2F - basement standalone dehumid set to 50%; upstairs AC does not have dehumidify mode (but could with better tstat)):

1. Main floor AC almost never overcools, even 2F to reach humidity setpoint.

2. In summer, when majority of dehumid load is present, Dehumid generates substantial heat into basement which must then be removed at some point.

3. In shoulder seasons when basement dehumid runs less frequently, heat sent into basement is desirable.

Thus, in heavier (summer) seasons of dehumidification need (and the drastically more energy intensive and significant), avoiding heat generation into the building envelope would appear desirable from an energy standpoint. This would seem to favor the central AC dehumid solution.

 

[dbbarron]

Here is some data from today:

Measurement period : 6am - 6pm
Outside High temp 80F; low 63
Outside RH - 53%-86%

Indoor AC (main floor) set to 50%rh, 74F (5 ton carrier infinity w/Tstat humidity control which runs air handler at low speed)
Basement dehumidifier (Frigidaire 70 pint) set to 50%
Both maintained setpoint. (main floor Tstat currently reads 74F,45%RH; basement Dehum reads 50% and is currently running)
Basement temp, mid 70's

Dehumifier Pints Collected: 5
AC Pints Collected: 17

Dehumidifier KWh used 6.6
Duty cycle about 80% (uses 650watts when running)

Does this make sense? Why is dehumidifier only running about half of its rated energy efficiency? (due to rating at 80%RH and it running in a 50%RH ambient? Is such an efficiency drop normal?) What does this tell me about how my system is architected? How to improve if it should be improved?

If you recall, on the hot and humid days, I can maintain 50%RH, but dehmidifier runs flat out - suggests AC is removing most of the humidity. Any advantage to shifting dehumidification load to Dehumidifier rather than AC? etc.

db

 

[beenthere]

Not really.

The warmer your basement gets. The warmer the dehumidifiers coil is. So it removes less moisture.

 

[teddy bear]

Here is some data from today:

Measurement period : 6am - 6pm
Outside High temp 80F; low 63
Outside RH - 53%-86%

Indoor AC (main floor) set to 50%rh, 74F (5 ton carrier infinity w/Tstat humidity control which runs air handler at low speed)
Basement dehumidifier (Frigidaire 70 pint) set to 50%
Both maintained setpoint. (main floor Tstat currently reads 74F,45%RH; basement Dehum reads 50% and is currently running)
Basement temp, mid 70's

Dehumifier Pints Collected: 5
AC Pints Collected: 17

Dehumidifier KWh used 6.6
Duty cycle about 80% (uses 650watts when running)

Does this make sense? Why is dehumidifier only running about half of its rated energy efficiency? (due to rating at 80%RH and it running in a 50%RH ambient? Is such an efficiency drop normal?) What does this tell me about how my system is architected? How to improve if it should be improved?

If you recall, on the hot and humid days, I can maintain 50%RH, but dehmidifier runs flat out - suggests AC is removing most of the humidity. Any advantage to shifting dehumidification load to Dehumidifier rather than AC? etc.

db

Good info, prefer 24 data because the dehu does more at night and a/c more during the day. Do as much as you can with the a/c while avoiding over-cooling. Trying to do more with your current dehu would be a poor. Dehus are rated at 80^F, 60%RH without cycling. Cycling and reduced %RH decreases efficiency. We get 3-5 pints per kw verses the 4-7.9 rating at 80^F, 60%RH. How much fresh air are you introducing or estimated CO2 ppm? Tell me how many pints removed in 24 hours, the number of occupants, the outdoor dew point, and interior ^F/%RH, I can estimated the cfm of fresh air infiltrating or ventilaed. Wind makes a big difference in fresh air infiltration.
You have much better outdoor conditions than we have in WI.
Keep us posted as the weather cools and dehu does more. YOur ability to control your home will be dependent on the outdoor dew point. Less than 55^F outdoor dew point will make it easy. Higher than 55^F DP will require more dehu.
Regards TB

 

[dbbarron]

Teddy Bear:

To supplement - overnight, from 6PM to 6AM, looks like you are spot on,

Dehumidifier removed about the same 5 pints, while AC removed perhaps 1-2

So, over 24 hours, Dehumidifier removed 10 pints, AC 19 pints

Overnight temperature averaged about 65F and humidity about 80%.

Right now (70F, 65% RH, Dewpoint is 57F)

CO2 is low (600-800), but my measurement may be innacurate.
No fresh air system.

Average occupancy is about 2-3 people over 24 hours.

House has 50000 cubic feet of air. 1980s construction, 2x6.

Dehumidifier seem to only collect about 1 pint per kwh.

 

[dbbarron]

24 Hour data:

High T=77F, Low T=65F, Rh max (night) 80%, Min (mid day) 42%; dewpoint (day) 53

AC Collected 37 pints (50% setpoint, 74F)
Dehumid collected 7 pints (50% setpoint)

Dehumid used 13kwh - thats about .25 liters/Kwh - this sounds grossly inefficient - double checking that I am properly collecting all discharge, but I suspect my dehumidifier might be malfunctioning - perhaps this is why is has been running near 100% last few months - not load, but not working properly! Filter is clean - can anyone think of any other potential problem or am I misreading efficiency?

db

 

[beenthere]

Set your A/C's dehumidify set point to 45%.

 

[dbbarron]

Set your A/C's dehumidify set point to 45%.

Was thinking about this, but risk is overcooling upstairs in shoulder seasons. In, march, apr, we need heat, but basement will be over 50% humidity. Difficult to use main ac for dehumidification as it is also the furnace.

Thus, in summer this approach works, but not in spring and fall.

It seems like I will definitely need a basement dehumidifier for the shoulder seasons. They just keep burning out after a few years. Time for an upgrade.

 

[beenthere]

Was thinking about this, but risk is overcooling upstairs in shoulder seasons. In, march, apr, we need heat, but basement will be over 50% humidity. Difficult to use main ac for dehumidification as it is also the furnace.

Thus, in summer this approach works, but not in spring and fall.

It seems like I will definitely need a basement dehumidifier for the shoulder seasons. They just keep burning out after a few years. Time for an upgrade.

Not too much risk if its a VS blower. It will slow down the blower during normal cooling calls also.

 

[dbbarron]

Turning back to the start of this long thread, you will all recall the attached spreadsheet to predict humidity levels and need for dehumidification.

I have now had a chance to collect actual condensate through the hottest months of the seasons.

From spreadsheet, prediction was about 10 gallons based on average daily temperature on hottest day.

I am actually observing perhaps 5-7 gallons on what appear to be average heat/humidity days (thoughts on reasons for difference?). Most of this water is being removed by the main floor air conditioner which is set for 45% humidity (has separate humidity control on Tstat which reduces fan speed and will overcall in dehumidify up to 2F). Basement dehumidifier is essentially broken, not removing moisture properly), but due to open floorplan to basement, house is averaging just under 50% - likely same even with basement dehumidifier turned off.

Putting aside fresh air arguments, which I may find persuasive anyway, would seem all I need is a big box type basement dehumidifier due to low usage and capacity requirements (unless I go for the fresh air system).

Since main floor AC is bearing this dehumidification and cooling load, perhaps better money is spent upgrading 14 seer 1 stage to 16 seer 2 stage (already has VS air handler). Would get energy efficiency benefit for majority of dehumidification load as well as main floor AC load (2nd floor has recently installed 16 seer 2 stage VS air handler system). I've been told the compressor is also oversized for duct capacity anyway (5 ton unit, ducts only sized for 1200 CFM - downgrade to 3 tons) - kill two birds with one stone.

Comments?

 

[teddy bear]

Turning back to the start of this long thread, you will all recall the attached spreadsheet to predict humidity levels and need for dehumidification.

I have now had a chance to collect actual condensate through the hottest months of the seasons.

From spreadsheet, prediction was about 10 gallons based on average daily temperature on hottest day.

I am actually observing perhaps 5-7 gallons on what appear to be average heat/humidity days (thoughts on reasons for difference?). Most of this water is being removed by the main floor air conditioner which is set for 45% humidity (has separate humidity control on Tstat which reduces fan speed and will overcall in dehumidify up to 2F). Basement dehumidifier is essentially broken, not removing moisture properly), but due to open floorplan to basement, house is averaging just under 50% - likely same even with basement dehumidifier turned off.

Putting aside fresh air arguments, which I may find persuasive anyway, would seem all I need is a big box type basement dehumidifier due to low usage and capacity requirements (unless I go for the fresh air system).

Since main floor AC is bearing this dehumidification and cooling load, perhaps better money is spent upgrading 14 seer 1 stage to 16 seer 2 stage (already has VS air handler). Would get energy efficiency benefit for majority of dehumidification load as well as main floor AC load (2nd floor has recently installed 16 seer 2 stage VS air handler system). I've been told the compressor is also oversized for duct capacity anyway (5 ton unit, ducts only sized for 1200 CFM - downgrade to 3 tons) - kill two birds with one stone.

Comments?

I calculate moist in grains per cubic foot of air. I assume your calcs are right. Getting 75-100 cfm of fresh air when occupied is the key. You are fortunate right now that you do not have that much fresh air. Also you have enough a/c cooling load to the remove your moisture. Your cooling load is soon to decline to near zero while the outdoor dew gradually subside over the next 3 months. A better a/c is not the answer. Avoiding fresh air and a cheap dehu may keep you dry. Correct fresh air when occupied and a couple cheap dehus will also work.
I am currently using CO2 control to determine occupancy and natural infiltration. I am removing 3-4 gallons per day via dehu and <1 gallon per gallon via a/c. Will post CO2 levels/%RH.
Best to have a high efficiency dehu with a good fresh air ventilation strategy.
Do the best you can and keep us posted.
Regards TB
PS An easy way to calculate the moisture load is using the free iphone app "Phoenix psychrometric calculator"

 

[teddy bear]

My calculations show 100 cfm of fresh air outside air with a 65^F dew point needs 2 Lbs. of dehumidification to get to a 55^F inside dew point which equals 75^F, 50%RH. If 70^F, 50%RH should be a 50^F dew point inside which is a 3 lb. per hour dehumidification.
Check the PDF.
Regards TB

 

[dbbarron]

So the error in my calculation is essentially in the infiltration factor (0.3); suggests should be more like 0.2. The 0.3 was completely a ballpark given the age of home (1986) and construction (seems very well insulated, 2x6 exterior, very deep insulated cathedral ceiling bays - so I assume tight for the age). Lower infiltration may be result of environmental conditions and/or tightness of house.

Nonetheless, TB, your calcs suggest a max 75 pint/day capacity (in the indoor condition) dehumidification to handle nascent dehumidification load plus fresh air load (with mechanical fresh air system) along with fresh air capacity.

As my AC can likely handle this load during hot months, I am more interested in the load given mechanical fresh air introduction during the shoulder months when the AC does not run. Will have to calculate.

This was a good exercise as it appears I do not need a larger (150 pint/day) capacity unit as long as I am relying on my AC for the bulk of the dehumidication load. Such a larger unit would normally be suggested just by the size of my home, but does not account for use of the AC or other factors.

 

[dbbarron]

My calculations show 100 cfm of fresh air outside air with a 65^F dew point needs 2 Lbs. of dehumidification to get to a 55^F inside dew point which equals 75^F, 50%RH. If 70^F, 50%RH should be a 50^F dew point inside which is a 3 lb. per hour dehumidification.
Check the PDF.
Regards TB

That's about 50-75lb per day of water or roughly same number of pints. As I am currently extracting at the lower end of that, summer infiltration is providing the lower end of what I need. Agree, a guaranteed minimum fresh air through a mechanical system is a better approach.

As I'd likely install a whole house type dehumid in order to get dehumidification and fresh air control, but right now it is a basement application, I'd like to place the dehumid in an unfinished utility room, draw air through a duct from finished basement space, and discharge into utility room, no ducts. However, if I bring in fresh air, will this be sufficient to get benefits of such throughout house? I'd rather not duct into first floor central AC if I did not have to.

db

 

[teddy bear]

That's about 50-75lb per day of water or roughly same number of pints. As I am currently extracting at the lower end of that, summer infiltration is providing the lower end of what I need. Agree, a guaranteed minimum fresh air through a mechanical system is a better approach.

As I'd likely install a whole house type dehumid in order to get dehumidification and fresh air control, but right now it is a basement application, I'd like to place the dehumid in an unfinished utility room, draw air through a duct from finished basement space, and discharge into utility room, no ducts. However, if I bring in fresh air, will this be sufficient to get benefits of such throughout house? I'd rather not duct into first floor central AC if I did not have to.

db

Taking 100 cfm of fresh air from outside, mixing it with basement air, and suppling the mix to the finished space will slowly force the mix up through the home. It works for me. Dehu operates when needed. If your home is routinely unoccupied, a timer in the outlet could timeout the dehu-fresh air intro to the time the space is occupied. Neat.
If occupied most of the time, operate fresh fresh to match.
Ultra-Aire 90H if fresh air is less than 100 cfm. Go with XT150H if more fresh air is desired.
Regards TB