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Older ASHRAE handbooks only listed design dry bulbs and coincidental wet bulb temperatures. Most humid time of year did not necessarily coincide with the hottest time of year and was leading to problems. Now they list design humidity scenarios and the coincidental dry bulb at the 0.4%, 1% and 2% levels. 1/52 = 1.9% so if a system could handle a little more than the 2% design level it would be doing pretty good. A good book, more on commercial/institutional applications. http://www.masongrant.com/html/humid...ign_guide.html
I understand design load conditions. How does manual J estimate the senible/latent load for evening hours or wet cool weather? Like 75^F, 90%RH, very little sensible with a lot of latent load? These conditions may last a week. TB
A simple way to estimate the load on residential homes would be to use your manual J. The ACH calculation should give you the information you need. Broward county codes are heading your way for new homes. And supplying fresh air will be the requirement and not an option. If you are on a new-con job you will size your equipment to handle this load for both heating and cooling. However when you try to upgrade an older home move on it carefully. I would offer the customer the benefits of bringing in air in a controlled fashion. Then as part of that service I'd want to inspect the home. Maybe perform a blower door test. Identify ductwork deficiencies and repair those found, Identify other building problems like attic fans with stuck louvers, fireplaces with opened dampers and belive it or not improperly vented attics and crawl spaces. Problems in these areas could be greatly exagerated by installing fresh air to an existing system on an older home. And if you don't inspect the home for these problems and inform the customer prior to installation guess who takes the fall?
more on commercial buildings and mold in general. Free chapter from a good book http://www.masongrant.com/pdf/design...DG_C7_Mold.pdf
Found a FSEC report Found one report from FSEC on the subject. It describes a half dozen methods of providing mechanical ventilation, seems to point out extra humidity load due to a FA duct between outside and the air handler return plenum: http://www.fsec.ucf.edu/bldg/baihp/p...xperiments.pdf There certainly are low-AC-load times when I would want to avoid pulling in humid outside air. I'll have to do a lot of thinking about ventilation controllers, ERVs and the like. Thanks for all the help! -- Pstu
Found a FSEC report
Originally posted by jrbenny TEDDY: Why couldn't you unbalance the ERV? There are balancing dampers in the connecting ducts. You deal with these auxillary devices regularly. So, I'd like your opinion. The airhandler leaks 50-75 cfm on the supply side. This combined with the natural leakage of the rest of the structure is enough ventilation for pruging pollutants and renew oxygen. The objective is eliminate negative pressure without adding unecessary load. Adding an equal amount of fresh make-up air(50-75cfm) is the simplest, lowest load solution. During shoulder seasons, make-up air when occupied, without the air handler operating, provides positive pressure with a minimum operating cost. If you required 100 cfm of total fresh make-up air/50 cfm exhaust even with high energy cost, you will have difficulty justifing a unbalanced ERV. Occupancy based, 50-75 cfm of filtered make-up air ventilation and <50%RH will not have mold growing between carpet/concrete or dust mites in carpets/mattresses. Oxygen levels are high and pollutant levels are low. This is excellent indoor air quality. Supplement dehumidification is always required for the high outdoor humidity with low/no cooling loads conditions. Also you can maintain <50%RH without any a/c operation, allow temperature set-up during unoccupied times. This saves a lot of energy. These features sells dehumidifiers or a/c reheat. Sorry, TB.
Re: Stack Affect? Originally posted by Xavier "there *already* is a sizable infiltration" pstu, can you please explain "stack affect" in a home when the inside temperature is lower then the outside temperature? I am not aware of a home with AC operating going negative, unless there are exhaust fans running or leaking duct work in a non-conditioned space. Therefore, why do you think you require additional outside air to be brought into the home? Finally, if a home has positive pressure, summer or winter, it will increase the possibility for mold in the exterior cavities! Fix the problem, do not treat the symptoms. Not likely in a house but possible in a taller building, kind of an iverted stack. Cold air falls to lower levels, pressurizes low level, depressurizes upper level.
Re: Stack Affect?
A few scattered thoughts From that blower door test, the report tells me I need 409 CFM to obtain 2.0 Pa pressure difference. With the two air handlers of 2600 CFM combined capacity, it would take over 15% leakage and simultaneous running to produce that. The CFMs for any reasonable FA inlet, will have less than 1.0 Pa consequence, perhaps far less. Although I will seal every leak I can find, I doubt this house will get to the level to ever really worry about mold or other problems in the structure, traceable to infiltration. In my opinion the problem is limited to IAQ. Thinking this through, makes me see the extra benefits of a ventilating whole-house dehumidifier such as Teddy Bear promotes. A more direct solution to the humidity issue, if not the pressure issue. Xavier: Excellent and interesting question, but I cannot explain "stack effect" in summer. Rather think it is academic. Take note there is far less difference between indoors and outdoors in summer, than there is in cold weather -- I submit it is a small phenomenon as a result. As stated earlier, I most certainly do have ducts and air handler in non-conditioned space, they can never be sealed 100% or even 97% actually, that would be the main source of pressure issues. And I believe your statement about positive pressurization is correct in cold climates where exfiltrated air meets cold objects below its dew point. In hot-humid climates that cannot happen with exfiltration, it can happen with infiltration. Researched who is near that can do Aeroseal. Remarkably, there is a franchise 65 miles away in "Aggie-Land" (College Station TX), but none whatsoever in the Houston metro region. Care to travel a bit, Dash? <g> Best wishes -- Pstu [Edited by pstu on 02-12-2006 at 03:06 PM]
A few scattered thoughts
Estimating "natural" infiltration I appreciate every one of the replies. Overall it seems there exists no plain and simple answer to my queries. It might help that a blower door test reported 69 CFM ventilation would be provided by building leakage -- in the context that 64 cfm more would be required to meet ASHRAE 62P ventilation level. The same report said air changes at 50 Pa = 5.3, and said Manual J air change rate is 0.24/hr or 149 CFM summer, 0.40/hr or 249 CFM winter (same section said C=261 and N=0.650, the meaning of those last two are completely unknown to me). One thing of note: the summer conditions are visited upon us day after day, reliably between June and September. We can count on that. The winter conditions will only exist very sporadically -- typical weather in winter months bears no resemblance to design temperatures. This is very unlike colder climates, but in this hot-humid climate I would mostly neglect the winter problem. I suppose that model contains a professional opinion that this house has approx. 70 CFM infiltration average? Under those assumptions I would guess that up to 35 CFM via fresh air inlet, would essentially displace existing infiltration and add very little humidity to the house. And a trivial amount of positive pressure. If fresh air were upped to 70 CFM the existing infiltration would be reduced but not eliminated, and I would guess the total humidity load would be increased slightly. But as 70 CFM of outside air (114 grains humidity at design conditions) would go directly to the AC, it would be substantially conditioned by the time it got to any room. I would expect the AC to do slightly more work, but the IAQ to be improved also. There remains the problem of air handler runtime being very high in summer, rather low in spring, fall and winter. I would think the correct design would size the duct small enough to avoid over-ventilation in the peak summer. That would leave the rest of the year well short of ASRAE standards, however those milder months are exactly the sort where opening a window makes sense. On average anyway. Tentatively, I think a fresh-air inlet would be endorsed by some FSEC papers, and if kept on the skinny side would not hurt the humidity problem. Are there any golden nuggets of info, perhaps from the blower door report, that I have overlooked? Thanks as always! -- Pstu [Edited by pstu on 02-12-2006 at 02:35 PM]
Estimating "natural" infiltration
Stack Affect? "there *already* is a sizable infiltration" pstu, can you please explain "stack affect" in a home when the inside temperature is lower then the outside temperature? I am not aware of a home with AC operating going negative, unless there are exhaust fans running or leaking duct work in a non-conditioned space. Therefore, why do you think you require additional outside air to be brought into the home? Finally, if a home has positive pressure, summer or winter, it will increase the possibility for mold in the exterior cavities! Fix the problem, do not treat the symptoms.
Stack Affect?
Originally posted by pstu There is a HVAC tech who seems otherwise very good, tells me that a fresh air inlet duct would allow a lot of humidity into the house. I live in S.Texas and the climate is indeed hot and humid. The FA duct I imagine, would be probably 5-inch flex from the AC return plenum, to a pickup point under the eaves on the north side of the house. There would be a filter, a damper and I would expect airflow to be under 100 CFM when the AC is running full speed (possibly 2-stage system). The counter argument that I have heard is: the extra humidity load will be less than you think, in most houses. That there *already* is a sizable infiltration and that is worse because of being unfiltered, untreated air. The argument says that having a FA inlet increases the pressure inside the house (or at least makes it suck less) and there will be a reduction in infiltration. Certainly the FA inlet provides at least one pass thru the AC system to dehumidify. And it will be filtered whereas the infiltration air won't. Is there a common sense way to measure what's going on? Possibly to estimate the additional humidity load on the AC system? And last, is there any literature which attempts to give answers? Something from Lstiburek would be ideal, or Florida's FSEC maybe. Thanks very much -- Pstu First off pstudent, decent indoor air quality can be maintained with intermittent ventilation. Others argue for a constant lower level of fresh air. Keeping it intermittent and coinciding with the compressor cycling on and off then allows a system to draw in, cool and dehumidify this outdoor air and in the process pressurizes the home with dry air. As you will see on the Florida Solar Site, ASHRAE's humidity design and control manual and other published reports, 2 pascals of depressurization or being depressurized continuosly by 0.008" WC with respect to the outdoors has caused millions of dollars of moisture damage in the hotel/motel industry alone. Florida Solar documents how being pressurized by a mere 2 Pascal tends to reverse the process so that dry air tends to leak out. If your ducts were not leaking in the attic, a slight pressurization could tend to stop the humid air from infiltrating in during the cooling season when the system was running. So when the system cycles off you are prone to natural wind driven infiltration but it is not a constant problem. Running continuous exhaust fans(even attic power venters) or leaking supply ducts in the attic is a problem. The net difference between supply duct leaks in the attic and a fresh air intake could be too small as well. The leaking ducts are the problem. I do not like a constant lower level of fresh air in a humid climate as it pumps in the humidity when the system is off, drives up humidity, then you are getting forced to run a dehumidifier to control it else deal with higher than required indoor RH. So promoting this approach sells more dehumidifiers. If you are under 60% RH you are doing fine. Saying that you must always be under 50% is a good pitch to mandate selling a dehumidifier. The only time I see it recommended to keep it under 50% at all times is for archives and libraries, paper loves to absorb mositure and hates to let go of it. Paper is predigested mold food as well.
In a commercial application and an economy of scale, you can run the ERVs unblanced and maintain a slight positive pressure in a humid climate.
From a mathematical perspective, the additional latent load can be calculated. H(L) = CFM * 0.68 * (W2-W1) where H(L) = Latent Heat Btuh CFM = air flow W2 - W1 = change in Grains of water per lb. of dry air TEDDY: Why couldn't you unbalance the ERV? There are balancing dampers in the connecting ducts. You deal with these auxillary devices regularly. So, I'd like your opinion.
Originally posted by pstu and I would expect airflow to be under 100 CFM when the AC is running full speed (possibly 2-stage system). Is there a common sense way to measure what's going on? Possibly to estimate the additional humidity load on the AC system? And last, is there any literature which attempts to give answers? Something from Lstiburek would be ideal, or Florida's FSEC maybe. I looked around for the ready made formula but didnt find it. Dont know how to accurately predict what will happen. it shouldnt be too hard measure after the fact. use your psychrometric chart,and compare what your system does with and without the 100 cfm of unconditioned air. the results will vary with outdoor conditions as for predicting ,I havent figured that out yet,but I am close to a breakthrough.Math isnt kind to me.
have you tried get the honeywell "gray manual" it was available at their site to download. its a big 300 or so pages of this type stuff ,and the formula you need might be in there
Re: Why Originally posted by pstu It is my understanding that the central air handler is a source of de-pressurization and I cannot change its nature. The ductwork is in a vented attic. If the ductwork leaks only 5% the EPA will pat me on the back with Energy Star rating, but still with a 1400 cfm air handler I will be leaking 70 cfm every minute it is on (assuming non-VS for this logic). Having a passive FA inlet is the simplest way I can think of, that will have the inlet air supplied when needed and only then. Unless I get kinda Rube Goldberg, it seems the only way. I get the impression that one can only expect a modest amount of humidity transfer from an ERV. This is from reading a few specs, am I in error here? Pstu,I agree with your logic and this is a move in the right direction. The main problem with negative pressure is outside high dew point air sucked into the wall/insutlation. High dew point air condenses on the cool drywall/insulation, growing mold and peeling paint. ERV is a balanced air flow device. An ERV takes out 70 cfm, adds 70 cfm and transfers half of moisture from the wet air to the dry air. If you add the ERV ventilation to the home, the airhandler leakage/infiltration negative pressure problem continues. The home is over ventilated by the combination with additional humidity load. This not a solution to the negative pressure problem. The make-up air suggestion solves the negative air pressure problem. To get fresh air without cooling/heating load, you could operate the airhandler when the home is occupied on a timer. This would also help the bath exhaust and clothes drier operation. Maintaining <50%RH is last important point avoid growing mold/dust mites and to providing indoor air quality. Supplemental dehumidification is a must in any greed grass climate for high humidity during low/no cooling load. There are ventilating dehumidifiers that bring make-up air, blend the make-up with house air, filter both, and dehumidify when needed. Enough! TB
Re: Why
Why It is my understanding that the central air handler is a source of de-pressurization and I cannot change its nature. The ductwork is in a vented attic. If the ductwork leaks only 5% the EPA will pat me on the back with Energy Star rating, but still with a 1400 cfm air handler I will be leaking 70 cfm every minute it is on (assuming non-VS for this logic). Having a passive FA inlet is the simplest way I can think of, that will have the inlet air supplied when needed and only then. Unless I get kinda Rube Goldberg, it seems the only way. If you are concerned, then I am concerned. Just trying to figure out what is knowable here. I get the impression that one can only expect a modest amount of humidity transfer from an ERV. This is from reading a few specs, am I in error here? Best wishes -- Pstu
Why
24/7 or long run times I'd be concerned.Why not use an ERV??
There is a HVAC tech who seems otherwise very good, tells me that a fresh air inlet duct would allow a lot of humidity into the house. I live in S.Texas and the climate is indeed hot and humid. The FA duct I imagine, would be probably 5-inch flex from the AC return plenum, to a pickup point under the eaves on the north side of the house. There would be a filter, a damper and I would expect airflow to be under 100 CFM when the AC is running full speed (possibly 2-stage system). The counter argument that I have heard is: the extra humidity load will be less than you think, in most houses. That there *already* is a sizable infiltration and that is worse because of being unfiltered, untreated air. The argument says that having a FA inlet increases the pressure inside the house (or at least makes it suck less) and there will be a reduction in infiltration. Certainly the FA inlet provides at least one pass thru the AC system to dehumidify. And it will be filtered whereas the infiltration air won't. Is there a common sense way to measure what's going on? Possibly to estimate the additional humidity load on the AC system? And last, is there any literature which attempts to give answers? Something from Lstiburek would be ideal, or Florida's FSEC maybe. Thanks very much -- Pstu
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