Fresh Air Ventilation's Effect on %RH in the home

[Carnak]

So are you saying you have fresh air piped directly into the return and have it configured in way that only opens the damper when A/C is running and closes when the A/C is not running? In other words you have restricted the fresh air damper ever from opening unless the A/C is on. And this is result of the hot humid climate you live in.

If this is true I assume your relying upon your the A/C to remove the RH and you have not incorporated a dehumdifier.

What I'm simply wanting to know is for a climate like the Twin Cities where dewpoints above 55 are only present for a small percentage of the time relative to rest of the year. Can a fresh air intake piped directly into the return be done without rasing the indoor humdidty to a level that would cause problems without the use of the rather expensive dehumdfier combo like the Santa Fe etc.

If the answer is yes but would need to be setup in way where the damper only opens where their is a call for Heat/AC. Is it the Hoyme product that accomplish this?

After reading TB's comments he leans more towards the combo that includeds the dehumdfier which negates having to worry about the outside dewpoint and can have a system that provides continus fresh air irrespective of the outside conditions. Obvioully there is fairly significant preumium to pay for this type of setup but very much respect his opinion on setting up a system this way

My present system maintains low RH, typically under 45% and good IAQ in probably one of the more humid climates on this forum, and it is do primarily to the fact that I have a tightly built home. I will be close to an 80 dewpoint here for the next six months.

I view dewpoints of air supplied to the space like this, when it is higher than 60 you run the risk of overwhelming what the AC can keep up with. When it is lower than 55, it is going to dry your house out.

When out door air has dew points is in between those values as long as the AC is cycling on and off you will be fine.

So if you use a fan like mine to always blow in fresh air when your outdoor dewpoints are high, you are going to elevate your indoor humidity level when it starts getting humid outside.

If you use your furnace fan to do this, you will elevate your humidity level from the fresh air, PLUS, you are going to be re-evaporating the moisture on your cooling coil and this will easily raise your RH by another 5 to 10 points.

If I ran my air handler all the time and just drew in 40 CFM of air constantly, it will drive mositure into all my furniture and my indoor RH is going to go up over 70% on the off cycle and drop down to 65% as the compressor runs. It is the stupidest thing I could do, but it sure says "hey I should buy one of those premium dehumidifiers teddy sells"

But given I have a couple brain cells to spare, I decided to give myself double the code required fresh air when my AC runs. Net result a home where the RH is usually in the low 40s and good air quality.

The best money spent in the quest for a constant supply of fresh air will always be the ERV as it reduces the mositure load of the fresh air in the first place. Nothing operates with less energy. Period.

When you still need to get rid of additional moisture in the home, it is really hard to justify the cost of a premium dehumidifier. The payback compared to a smaller stand alone unit is just not there, even if you burned out a couple kenmores because your ventialtion scheme was designed to constantly exercise a dehumidifer.

So I have an ERV sitting around now waiting to go in, I will try a few operating scenarios with it. Constant low speed, intermittent high speed and 'on demand'

I am just curious to see how fresh I can get the air without exceeding 50% RH else teddy will send a horde of ten million dust mites my way.

[Carnak]

If the answer is yes but depends on the dewpoint outside. Then is their such a system that a homeowner from the control planel simialr to a thermostat could simply setup up the system like the following.

Mode 1
-Only open damper when a call for A/C occurs (typically late summer when dewpoints are consistely above 55). This address the rasing of the indoor RH as the A/C coil would remove most of it.

Mode 2
- Damper opens based on pre-set sechedule (typically this mode would be set for early summer and the heating season where RH concerns would not be an issue

Seems simple enough for Northern Climates? What am I missing?

Summer and winter are two different things.

In winter your home needs more air.

A sign of a lack of air in the winter is you have a problem with condensation on your windows.

A sign of excessive fresh air in the winter is you have to run a humidifier.

So if I was suffering from not enough fresh air in the winter and I was doing this on the cheap but wanted a good control scheme I would do the following.

Have a de-humidistat open the damper. What is the setting? As high as your windows will permit. Up north of Duluth, the good starting point for someone with decent double pane windows was 35% when it got below freezing. You will figure out the setting easy enough.

In the winter, odds are the heating system runs a fair bit, and the colder it gets outside, the more air you will need to keep those windows clear as the inside surface of the glass will get cooler.

Why do I like this approach. Beacuse when you breathe you exhale CO2 and water vapour. Controlling water vapour building up indirectly responds to how much you are breathing. It is so simple, and it is accurate.

When you are not there humidifying your home, the fresh air is off. It responds to you being there, it is the ulitmate low tech occupancy sensor.

When you are running cooling I would say open the damper when the compressor runs.

Now what I like about this is as follows. When you have the system off and no one is home, the place will heat up. When you turn it back on to cool the house off, it runs for an extended period of time and gives your house a massive purge of fresh air.

[teddy bear]

So are you saying you have fresh air piped directly into the return and have it configured in way that only opens the damper when A/C is running and closes when the A/C is not running? In other words you have restricted the fresh air damper ever from opening unless the A/C is on. And this is result of the hot humid climate you live in.

If this is true I assume your relying upon your the A/C to remove the RH and you have not incorporated a dehumdifier.

What I'm simply wanting to know is for a climate like the Twin Cities where dewpoints above 55 are only present for a small percentage of the time relative to rest of the year. Can a fresh air intake piped directly into the return be done without rasing the indoor humdidty to a level that would cause problems without the use of the rather expensive dehumdfier combo like the Santa Fe etc.

If the answer is yes but would need to be setup in way where the damper only opens where their is a call for Heat/AC. Is it the Hoyme product that accomplish this?

After reading TB's comments he leans more towards the combo that includeds the dehumdfier which negates having to worry about the outside dewpoint and can have a system that provides continus fresh air irrespective of the outside conditions. Obvioully there is fairly significant preumium to pay for this type of setup but very much respect his opinion on setting up a system this way

Fresh air ventilation only when heating and cooling sounds crude by most standards and inadequate for any quality installation for demanding customers. In all green grass climates, there are extended times when neither heating/cooling are needed. What about fresh air ventilation for health? This will not comply with code I know of.
Pretend that the duct fan sets on top of a deluxe dehumidifier. Add the damper/filters/timer. Connect it all to the a/c duts and you guess it, a crude ventilating dehumidifier. I came down the same path 1991. By the time you make all of the compenents high quality, design the most efficient unit, pay for the assembly, and add a profit, you have about the same cost we sell the Ultra-Aire dehu for. Fair enough! Simple, durable, efficient systems installed by a real servicing contractor are going to end up being more important to most than cobbing something crude together for that may work 50% less cost. I can live with this. Past all of the nit picking and insults, adequate fresh air when occupied, <50%RH, and good air filtering is what its all about. Regards TB

[he8833]

Past all of the nit picking and insults, adequate fresh air when occupied, <50%RH, and good air filtering is what its all about. Regards TB

TB
First these questions are not to call into question you or a product like the Santa Fe but rather to engage in a discussion to further mine and hopefully other homeowners who want to better understand the issues limitations and considerations when exploring the addition of a fresh air ventilation into a house. I don't want my questions to be misinterpreted. Thank you for your input and dialogue regarding the topic

Fresh air ventilation only when heating and cooling sounds crude by most standards and inadequate for any quality installation for demanding customers. In all green grass climates, there are extended times when neither heating/cooling are needed.

I agree and certainly if a homeowner has humidty concners, allegeries, or cannot open windows then "Fresh air ventilation only when heating and cooling" would be insufficent. But I keep reverting back to how "most" homes operate. In the cooling season the system would be setup only to open the damper when a call for cooling exists and this happens when its hot outside. When its not hot outside most people have windows open thereby getting fresh air ventilation. Again I understand this would not work for everyone.

When the dewpoint is <55 because the system is setup to only operate when a call for cooling exists much of the RH concern will be covered. However, the issue of RH seems like the biggest gray area. The folks over @ buildingscience seem to argue that concerns over rasing indoor RH by pullling outside air when the dewpoint is <55 is overblown. 50-75 CFM of air that contains high dewpoint will not have a significant impact on raising indoor RH. Most RH concerns they argue come from showers, cooking and people. I suppose they are correct to a certain degree. By that I mean how tight is the house, are there existing bathroom fans, and exactly how high is the actuall dewpoint outside and for how long is the damper open pulling in fresh air? 4 minutes of fresh air every 3 hours when the dewpoint is <65 doesnt seem like it would raise the indoor RH signficanlty. Bottom line: If I'm worried about RH concerns have the damper only open when a call for cooling occurs, this way the majority is handled by the coil. If extremely high dewpoints exist outside simply overide the settings and have the damper remain closed.

This seems like a resonable approach while @ the same time being concious of the RH concerns.?

[Carnak]

Teddy

You pick and chose what you want to read in a code, and you only follow it when it suits the system you sell. Then you chose not to follow it when it is out of your product's range, or it makes your home too dry.

I pick and chose different parts of the same code as well, and I exceed it on many occasions. Then I turn a blind eye to a few things.

But if you want to stop flip flopping on a key point of what pressurization does, I will have a very civil debate with you, but you have to hold back on some of the scare tactics you like to use.

You stack the deck in your favour and you tend to get a little upset, when I have similar CO2 numbers and lower RH.

So like I said, you want to go civilly through the whole 62-2 code, let's do it.

[Shophound]

In all green grass climates, there are extended times when neither heating/cooling are needed.

I'd sure like to know what those "extended times" are for my particular slice of green grass climate.

I've lived in this particular slice going on 34 years...one sure fire way to know one does not understand our climate is to not understand the saying "If you don't like the weather in Texas, just give it a minute. It'll change."

[teddy bear]

I'd sure like to know what those "extended times" are for my particular slice of green grass climate.

I've lived in this particular slice going on 34 years...one sure fire way to know one does not understand our climate is to not understand the saying "If you don't like the weather in Texas, just give it a minute. It'll change."

Great, just ventilate your home with the correct amount of fresh air when you are in it. No dehumidification required unless the %RH excedes your "Standards"-50%RH. I looked at weather data for Denton TX. Looked like WI-maybe 3-4 weeks early for temperature.


CDT Max Temp Min Temp Max D P Min DP
4/11/2009 71 42 52 37 Rain-Thunderstorm
4/12/2009 63 51 54 49 Rain-Thunderstorm
4/13/2009 64 48 52 39 Fog
4/14/2009 73 37 47 36
4/15/2009 73 48 49 44
4/16/2009 75 55 54 47 Rain-Thunderstorm
4/17/2009 64 55 57 48 Rain-Thunderstorm
4/18/2009 70 57 63 55 Rain
4/19/2009 70 52 59 44
4/20/2009 80 44 47 37
4/21/2009 84 48 51 41
4/22/2009 91 52 54 46
4/23/2009 86 64 59 37
4/24/2009 84 66 65 59
4/25/2009 86 69 66 61
4/26/2009 79 72 67 58 Rain
4/27/2009 73 57 66 56 Rain-Thunderstorm
4/28/2009 75 55 64 54
4/29/2009 75 62 67 1 Rain
4/30/2009 82 59 75 67 Rain-Thunderstorm
5/1/2009 80 71 75 71 Rain
5/2/2009 73 59 73 57 Rain
5/3/2009 66 55 57 54
5/4/2009 69 51 59 50
5/5/2009 75 62 70 60 Rain
5/6/2009 80 64 72 62 Fog-Rain
5/7/2009 86 64 74 64 Fog
5/8/2009 90 71 75 68
5/9/2009 75 60 67 51 Rain
5/10/2009 78 60 68 57 Rain
5/11/2009 75 59 65 57 Rain-Thunderstorm
5/12/2009 87 60 68 59
5/13/2009 89 73 72 66
5/14/2009 82 69 69 60 Thunderstorm
5/15/2009 87 68 69 65
5/16/2009 75 55 66 49 Rain-Thunderstorm
5/17/2009 73 53 51 42
5/18/2009 77 52 51 44
5/19/2009 80 50 54 48
5/20/2009 81 50 55 49
5/21/2009 84 53 62 52
5/22/2009 84 64 65 62 Rain
5/23/2009 84 66 68 63 Rain
5/24/2009 84 62 68 61
5/25/2009 87 63 67 61
5/26/2009 91 68 72 57 Rain-Thunderstorm
5/27/2009 79 60 63 56 Rain-Thunderstorm
5/28/2009 84 55 61 55
5/29/2009 87 55 61 53
5/30/2009 89 57 62 54
5/31/2009 91 57 66 56
6/1/2009 88 68 64 59
6/2/2009 87 64 71 61 Rain-Thunderstorm
6/3/2009 79 66 69 58 Rain
6/4/2009 82 62 61 50
6/5/2009 86 55 61 55
6/6/2009 91 64 66 59
6/7/2009 91 72 66 62
6/8/2009 91 72 70 64
6/9/2009 91 73 72 64
6/10/2009 89 66 73 64 Fog-Rain-Thunderstorm
6/11/2009 75 66 70 66 Thunderstorm

You are right, its a perfect condition for fresh air ventilation without a dehumidifier. The heating/cooling should take care of everything. No more dehus needed for TX.
Regards TB

[pstu]

I think all those days which do not exceed 60F dewpoint, could be taken off that list. As well as those days which are a genuine summer.

In addition, one of the functions of 24/7 intermittent supply-only ventilation will be to inhibit infiltration. That is useful whether or not people are in the house. Or do people think otherwise? I am learning some valuable pros and cons for each method.

I am still a believer in the usefulness of dehus, just those who argue they can do it with AC may indeed be correct much of the time. We should not forget it will depend on the building structure (so many are mediocre) and the %RH one tries to achieve.

Regards -- Pstu

[teddy bear]

TB

When the dewpoint is <55 because the system is setup to only operate when a call for cooling exists much of the RH concern will be covered. However, the issue of RH seems like the biggest gray area. The folks over @ buildingscience seem to argue that concerns over rasing indoor RH by pullling outside air when the dewpoint is <55 is overblown. 50-75 CFM of air that contains high dewpoint will not have a significant impact on raising indoor RH. Most RH concerns they argue come from showers, cooking and people. I suppose they are correct to a certain degree. By that I mean how tight is the house, are there existing bathroom fans, and exactly how high is the actuall dewpoint outside and for how long is the damper open pulling in fresh air? 4 minutes of fresh air every 3 hours when the dewpoint is <65 doesnt seem like it would raise the indoor RH signficanlty. Bottom line: If I'm worried about RH concerns have the damper only open when a call for cooling occurs, this way the majority is handled by the coil. If extremely high dewpoints exist outside simply overide the settings and have the damper remain closed.

This seems like a resonable approach while @ the same time being concious of the RH concerns.?

Regarding the dew points effect on the home. The resulting inside %RH depends on the indoor temperature. At 75^F, a 55^F dew point is 50%RH. Under a rug on a 60^F concrete floor, a 55^F dew point results in 90%RH. At the surface of insulated concrete basement wall there is a similar situation. In home with t-stat setup/back the resulting inside temperature must be considered. Even though the dew point is seldom high, mold is a big problem in the Northwest because of turning off individual room heat. A room at 60^F with 55^F dew point is 90%RH. Also add the moisture from the occupants and diffusion of moisture through the outer surfaces of the structure to the moisture load. You will find resulting indoor dew point several degrees above the infiltrating/ventilation air. 70^F dew point air is reduced 55^F dp is about 45 grains/lb. of air or 3-4 lbs. per hour/100 cfm. A family of four generate 15-20 lbs. per day. This makes fresh air the high load with high dew points the big load.

4 minutes of fresh air every 3 hours- You must be kidding. Using Canadian logic, you need (total volume / ??) ??? cfm. May be 6,000 cfm for 3 min? That would be an air change in 6 hours.
I can not be responsible for no heat-no cooling- no ventilation. Keep us posted on how all this works. Regards TB

[Carnak]

Canadian logic has a home in the tropics with lower RH than a home in Wisconsin and good CO2 levels.

Canadian logic figures Wisconsin logic is a Mickey Mouse appraoch to drive up humidity to force people to buy a dehumidifier.

Sort of like selling global warming to an Eskimo so he buys a deep freeze

So are you ready to go through the whole 62.2 or not?

[pstu]

>>4 minutes of fresh air every 3 hours- You must be kidding. Using Canadian logic,
>>you need (total volume / ??) ??? cfm. May be 6,000 cfm for 3 min?

Honestly, did someone ever say that? If they did then that "4 minutes per 3 hours" statement is the ridiculous claim that should be knocked down. I have to believe you understand what Lstiburek, Carnak et al really mean when they say intermittent ventilation.

I'm still pondering if it is a virtue to include fresh air only when people are present. Assuming no Chinese toxic drywall of course<g>. My incomplete understanding sees some virtue on either side of this argument. At least there is a tad bit of energy saved by not ventilating...

Best wishes -- Pstu

[Carnak]

maybe someone can refresh my memory on the 62.2, but I think, based on some assumptions, that they are recommending 7.5 CFM per person, plus 1 CFM for every 100 square feet.

But a big assumption this makes is that you are also getting some infiltration of 2 cfm per every 100 square feet.


So then for good IAQ in a 4000 sqaure foot home with two occupants it would mean you should roughly have 2 x 7.5 + 4000/100 x 3 = 135 CFM but it is probably good enough if you can only pump in 60 CFM.


Let's try a smaller house, 1000 sqaure feet two people live in it.

2 x 7.5 +1000/100 x 3 = 45 CFM

I wonder how it works when the guy needing 45 CFM pumps in 80 CFM?

I bet it works pretty good even if the guy does not pump the air in steady

So I am wondering what really works better, pumping in half the air you really need about half of the time, or pumping in double the air you need half of the time

[Carnak]

The grass must not been green enough in St. Louis because you can count on one hand (and have fingers to spare) how many days the dewpoints were high and it was not really hot enough to be running AC

http://www.wunderground.com/history/...q_statename=NA

You want to tell me how you could not get as much fresh air ventialting at a higher rate when there is a call for cooling and or heating?

Do you want to explain the advantage of constantly pumping in that fresh air without having a jumper on your dehumidifier to force it to run all the time like it was a 100% outisde air unit? Because that is how you portray it it but it is not. I view it as you are pumping in the fresh air and that humidity all the time and the dehumidifier is cycling on and off based on the humidity level where the unit is or a single remote sensor.

It cycles off, dries out its coil, it dires out its cross flow HX, puts moisture right back into the air that you just paid to remove, along with injecting moisture into the home, then as the RH differential rises again, it turns on, pulls out water vapour to compensate for what it just re-evaporated, then catches up for what it let build up into the home, then it cycles off, revaporates water and the process repeats itself.

For the money you charge for those things they should be designed as a 100% outside air unit, you should sell them with an optional remote condenser like heat pipe does, except go a step farther, give an option to only dump half of the heat to the outdoors. Bard has an option like that and so does york

If I was going to invest that kind of money to ventilate all the time ( assuming an intermittent strategy did not work) it hands down has to be an ERV. It is cutting your moisture load. If it gets overwhelemed still, a small portable unit can easily handle it.

[Carnak]

And if the blowers had a little more power you could have a ducted fresh air supply that could be independent of the furnace or air handler blower.

Then you could have the fresh air directly where it was needed and not worry about revaporating all the moisture on the cooling coil. The one thing all your tests show is what makes the biggest difference in the bedrooms is when the furnace fan is running all the time, and this is independent on whether or not you have the vent function turned on or off.

So while in my anything but humble opinion, I think your experiments that try and stack a deck to show you have to have a ventilating dehumidifier have failed, BUT, I think that your experiments clearly show how important it is to make sure you are getting sufficient ventialtion to bedrooms because that is where people breathe the majoirty of the air they breathe in their lifetime. So I think that this is a commendable point that your research has shown.

[teddy bear]

I am attaching a chart from the current ventilation recommendations. This ASHRAE's chart is showing minimal fresh air flow for continuous ventilation. I have added the moisture impact of dry/wet fresh air. Four bedroom homes use five occupants for the 7.5 cfm X 5, not the number of current occupants. ASHRAE assumed that the homes had typical air infiltration. I make no claims or recommendations for actual amounts of fresh air or strategies. I suggest following recommendation of ASHRAE, American Lung ASS. American Medical ASS & other experts.
My personal experience is that 60 cfm of mechanical fresh make-up air when the home is occupied improved my home compared to a partially open window. I ventilate when the my home is occupied and do not limit my fresh air to when the heat/cooling system operates. As my data has shown I have very limited cooling cycle. My CO2 levels and formaldehyde levels indicate an acceptable air change rate. It may be better or poorer than others have achieved. I do recommend fresh air ventilation and humidity control others at every oppurtunity. I need to operate my dehumidifier whenever the outside dew point is +50^F. I am encourage everyday by our customer's reports of comfort and good IAQ. Regards TB

[Carnak]

Take the chart and add another 2 CFM of air for every 100 square feet and that is the air volume those charts base the ventialtion rates on. Going by that chart for one bedroom in the 3000 to 4500 square foot place would assume a total of 120 CFM to 150 CFM of fresh air, and there is someone here who has been saying that a 160 CFM ERV is perfect for you

When you pressurize, you are robbing Peter to pay Paul, by reducing infiltration. I have challenged you to run an exhaust fan at the same time as your system brings in the fresh air in the winter. You refused, I predicted your CO2 levels would plummet if you would. You said your house was fine without doing this. The exhaust fan would make you a balanced pressure ventilation scheme and you would then have your mechanical fresh air additive to the natural fresh air and this is EXACTLY WHAT THE CODE IS BASED ON.

Except, I know it would be so dry there in the winter you would be quitting thermastor and going to work for aprilaire so you could get a deal on a humidifier.

So you take the letter of the code and at best, you figure ventialting at half the recommended rate is okay.

Now the guy with the intermittent ventilation follows a lot closer to what ASHRAE says, but of course his method that also yields some of the lowest RH in the forum is unacceptable to you. I use close to double the code required rate when my system runs and this time of year I get a lot of run time.

The Code also has allowances for when your system runs 50% of the time, 33% of the time and you ventilate with more air than the continuous rate. This is what the "air cycler" is all about. I am just fortunate that nature and internal loads kick my AC on to at least run a little everynow and then that I do not need timers to kick on my ventilation.

[Shophound]

The grass must not been green enough in St. Louis because you can count on one hand (and have fingers to spare) how many days the dewpoints were high and it was not really hot enough to be running AC

http://www.wunderground.com/history/...q_statename=NA

You want to tell me how you could not get as much fresh air ventialting at a higher rate when there is a call for cooling and or heating?

Do you want to explain the advantage of constantly pumping in that fresh air without having a jumper on your dehumidifier to force it to run all the time like it was a 100% outisde air unit? Because that is how you portray it it but it is not. I view it as you are pumping in the fresh air and that humidity all the time and the dehumidifier is cycling on and off based on the humidity level where the unit is or a single remote sensor.

It cycles off, dries out its coil, it dires out its cross flow HX, puts moisture right back into the air that you just paid to remove, along with injecting moisture into the home, then as the RH differential rises again, it turns on, pulls out water vapour to compensate for what it just re-evaporated, then catches up for what it let build up into the home, then it cycles off, revaporates water and the process repeats itself.

For the money you charge for those things they should be designed as a 100% outside air unit, you should sell them with an optional remote condenser like heat pipe does, except go a step farther, give an option to only dump half of the heat to the outdoors. Bard has an option like that and so does york

If I was going to invest that kind of money to ventilate all the time ( assuming an intermittent strategy did not work) it hands down has to be an ERV. It is cutting your moisture load. If it gets overwhelemed still, a small portable unit can easily handle it.

Well said, sir. I fail to see the stumbling block that intermittent ventilation strategies appear to present to a certain audience. Bottom line in hot, humid climates is the incoming air must be dehumidified and filtered. Why then, must it be a constant feed?

By the way, we're enjoying some lovely subtropical weather today. Mid 90's with dew points 72 -75 degrees and wet bulbs spiking 78...real workout for cooling towers and leaky building envelopes!

[teddy bear]

Well now, everybody is happy. Regards TB

[Carnak]

et tu teddy?

[pmeunier]

...
It cycles off, dries out its coil, it dires out its cross flow HX, puts moisture right back into the air that you just paid to remove, along with injecting moisture into the home, then as the RH differential rises again, it turns on, pulls out water vapour to compensate for what it just re-evaporated, then catches up for what it let build up into the home, then it cycles off, revaporates water and the process repeats itself.

Nice, I had not thought about it. So bringing the outside air with a separate filter unit (with a "T" duct connection) is a little more efficient than using the dehu's fan because it avoids drying the dehu's coil.

For the money you charge for those things they should be designed as a 100% outside air unit, you should sell them with an optional remote condenser like heat pipe does, except go a step farther, give an option to only dump half of the heat to the outdoors. Bard has an option like that and so does york
...

That would rock if it was available in a residential capacity (90 pints/day). While you're at it put the noisy compressor outside as well. Any company that does that? I believe Lennox has something that comes close but it doesn't have the option of putting all the heat back in when so desired?