Problem: I have recently built a SIP house with moisture consistently around 70%.
Specs:
Home size: 2600 sf 2 story + 1200 unfinished basement (slab, insulated walls)
Location: northern Indiana, wooded
Insulation: 4″ polyurethane SIP panels (R26), blown-in on top of close cell foam ceiling (R50-ish)
Windows: 40% of exterior walls, hi-efficiency, 70% of windows face South, and 90% of those are under deep eaves or patio (Prairie style)
HVAC: central, 3.5 tons A/C, ERV on 24/7, fan on 24/7
We have one toddler, no pets. To get humidity to 65%, I have to set the temperature to 68F, which is too cool even for me. My own load calc shows a 2-2.5 ton A/C requirement, my HVAC guy says his supplier came up with 3.5 ton, so that was what’s installed. I had to get a dehumidifier in the basement to deal with growing mold. HVAC guy tried all kinds of tricks to reduce humidity, such as slowing the blower, to no avail.
Anyone with experience here? Everyone that looked at this problem never really done HVAC for a SIP home before. Thanks.
Replies
The sensible load is too low on your A/C system for it to remove the latent heat.
In other words, the house is cold before the system gets the humidity out. A smaller A/C system would help. However, by far the best answer is to install a whole house dehumidifier. http://thermastor.com/prod_90h.htm or similar
Any other choice you make will be a loosing battle.
3.5 tons certainly sounds too big for 2600 sq ft SIP house. The load calcs need to be adjusted for the sips.
Tom
Douglasville, GA
Sounds like a great house. Hang in there. Does your ERV have a dehumidifier built in? If not, could you have one installed on the intake side? Ask VaTom (here) for details on his setup.
I assume you have humid summers & dry winters like we do here in the Mid Atlantic. Winter should be no problem. Could you swap the A/C coil for a smaller setup?
My ERV does not have a dehumidifier built in. I've always wondered where does the condensed moisture go as the warm outside humid air passes the conditioned cold air in the exchanger. I assume that as the energy is transfered, the moisture must condense somewhere...Haven't time to track that answer down.But would it be wrong to assume the right size A/C unit would take care of humidity? It seems redundant to tack on a dehumidifier on top of the cooling system.
There are times during the year when you need to lower the humidity, but not the temperature.
An air conditioner optimized for removing humidity is going to be less efficient at removing heat and vice versa.
If the sensible load is not high enough (such as with a well insulated and sealed house), then the A/C will not run long enough to remove the latent (humidity) heat.
Building Science Corp did a couple of research studies on this and found that a separate dehumidifier + AC was the best (and least expensive to install, second-to-least expensive to operate) at keeping consistent humidity and temperatures in a home. They also found that a multistage AC system was NOT good at maintaining consistent humidity levels, to the point that it wasn't cost-effective.
Paul:"An air conditioner optimized for removing humidity is going to be less efficient at removing heat and vice versa."I see the logic here. But isn't there a sweet spot on this load vs. capacity curve that would match the load requirement of any home?
"But isn't there a sweet spot on this load vs. capacity curve that would match the load requirement of any home?"No. Part of the problem is that the load changes from day to day. There are days when no A/C is needed but dehumidification is.The other part of the problem is that with the amount of insulation in this SIP home, there is very little sensible load.
Paul:From what you are saying, would it be correct to say I can get close with the right size A/C, but optimal would be with a whole house dehumidifier?At the other end, are there any adverse effects with an oversized heating system? I have not used this yet, but I assume since (I think) the A/C is oversized, the heating capacity is also. I have a nat gas (variable speed) furnace.
"At the other end, are there any adverse effects with an oversized heating system? I have not used this yet, but I assume since (I think) the A/C is oversized, the heating capacity is also. I have a nat gas (variable speed) furnace."Yes, short cycling.But, in general, it is not as critical a problem as oversized AC.But I don't know how the dynamics of your house work.Just based on the run time during 0 degree days, I would say that the furance for the first floor is at least twice and maybe 3 times the right size.But the air is delivered by ducts under the slab and there is enough thermomass that the run air temp does not cycle much.
"An air conditioner optimized for removing humidity is going to be less efficient at removing heat and vice versa."
Please explain this statement. I'm not sure that I fully understand what you mean. I think I do, but instead of guessing....
I'm not an A/C expert, I've just been doing a lot of research before starting the construction on my own home.
To optimize the A/C system for removing humidity, you decrease the airflow. Typically, this will put it at about 350 cfm per ton. To optimize the A/C system for cooling, you increase the airflow to something between 450 and 500 cfm.
Less airflow across the coil lets the coil get colder, which removes more humidity. More airflow across the coil, cools more air, which improves the cooling efficiency.
Another method to accomplish the same end result is to change the size of the evaporator coil. Going to a larger coil increases the cooling efficiency, a smaller coil (colder) increases its ability to remove humidity.
For the typical home, a decent compromise can usually be found that will give acceptable results for both. However, if you have a very well insulated and sealed structure, then the sensible load has been decreased a lot more than the latent load. Depending on what kind of humidity levels you are willing to accept, it may not be possible to get there with out going to a whole house dehumidifier.
Based on my research, using a whole house dehumidifier and an A/C system set up for the best cooling efficiency, I can keep the humidity between 40 and 50% and the temperature at what ever setting I pick.
Without the dehumidifier, in a well sealed and insulated home, you will have days where either the relative humidity exceeds 60%, or you lave to lower the thermostat to levels most people do not consider comfortable.
On some of their high end systems, Carrier has a control system that will vary the airflow (cfm) across the coil based on the requested temperature and humidity settings. However, in a well insulated home, the payback time for this type of system is longer than the expected life of the system, and a separate whole house dehumidifier will give you equal or better results with a smaller investment.
Edited 8/28/2006 2:59 pm ET by paul42
Some of your research seems sound and some is questionable. You did confirm what I thought you meant.
Manipulaing the air flow to tailor the performance of a cooiling system is a good idea, if done with full knowledge of the consequences and effects. 450 to 500 cfm/ton would be possible in very few residential systems and would result in a marginally comfortable system in very arid regions only. This is of course without a secondary system to dehumidify the air. Not a recommended practice. The high pressure drop and decreased coil effectiveness would most likey rule the adverse qualities of a system setup to operate like that. If a competent designer sized the ducts to accomodate and other features of the system were selected acheive this, it is possible. I don't design HVAC systems in a dry region, ( I do design HVAC systems, btw) so I cannot say that this not what they do in Vegas or Tucson, but in most of the country, good designers wouldn't go there.
The 350 cfm/ton is approaching the area that few publish performance data, though, in my professional opinion and experience, a good practice. This is the best way to get decent s/t ratios out of the cheap crap that is sold today as "standard".
Mis-matching evaporator coils, as described in your general statement, IMO, is also a poor practice. An oversized coil is more forgiving than an undersized one, which, BTW, does not improve moisture removal, all other parameters being the same. An undersized coil will result in high face velocities, poor dehumidification, poor or reduced heat transfer, high pressure drop, noise and in many cases moisture carry-over.
Some of the items you mention are used by HVAC designers to address the limitations of standard "cost effective" readily available equipment and components, and to select a system that is best suited for the application.
Some of your statements are correct but for the wrong reasons. For instance, a coil in a (we'll assume that we're discussing only properly selcted and charged systems) DX system does not "get colder" at lesser air flow rates. The temperature of the coil is based on the physical properties of the refrigerant, i.e. the boiling point at the pressure downstream of the expansion device. This is typically 42 to 45 degF. The air passing through the coil at a low flow rate is cooled more per pound than air passing through the coil at a higher flow rate, below its dewpoint and moisture condensed out of the air stream. As the air becomes dryer, more of the energy transfered from the air stream to the refrigerant is sensible.
In any building, regardless of the level of insulation and envelop tightness, a suitable system can be designed to meet most conditions comfortably and cost effectively.
Your comments on the coil size make sense.However, I think that the typical “recommended” practice does not make sense for the house I am building. I started out with the expectation that I would have a two speed heat pump and that would be the best answer for controlling the humidity and increasing efficiency. However, after running the numbers and doing a lot of research, I have found that will not always work for a very well sealed and insulated house. My house is being designed and will be sealed and tested to meet better than .25ACH50. It will be insulated and set up such that the A/C system will not need to run between 8a.m. and 8p.m. and it will still keep the house within two degrees of the normal set point on 100 degree days. At approximately 2,800 sq. ft. the “recommended” practice for my house would require something less than two tons of A/C or a small two stage system. And the A/C system, even with considerable tweaking and loss of cooling efficiency, would still not control the humidity especially on cool humid days. By far the best answer I have found is to put in the whole house dehumidifier. In my case, the dehumidifier will also include the necessary fresh air intake and control.Once you commit to adding the whole house dehumidifier, then it only makes sense to set up the A/C system for maximum sensible heat efficiency and that includes the 450 to 500 cfm/ton idea.The 350 cfm/ton approach does work well in most applications and makes the dehumidifier less critical. It just isn’t the right answer for every humidity problem.
tufen.... you are MIA from the discussion.. i'd sure like to know if you have found a source of the moisture
i mean the air in your house should only be gaining humidity from respiration, showers , plants and air changes
this should be a piece of cake
what is your next step ?Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike-Since moving to the new house, I've been Net deprived due to Comcast's refusal to serve my house. That's another thread...Luckily our weekend place is still wired.Because I had the "right-size" A/C discussion with the HVAC guy before they system's installation, he was aware of my suspicion that 3.5 tons was over sized. So after trying everything to deal with the moisture, he changed the unit to a 2 ton. But it hasn't been hot enough to put the system through its paces. Preliminary use showed that it does much better at reducing humidity. It brought moisture down much further than before. But in normal use with current weather conditions, the humidity has creept back to around 65%. I think the next piece of this puzzle is the ERV. Been too busy working on the new shop to do moisture testing of the basement concrete. But I will put the ERV on a timer to limit its use. That and some warm weather, and we'll see.
What type of ERV do yu have? Mine has an intermittant setting that will run it at low speed for 20 in every 1 hour. Tom
Douglasville, GA
It's an Aprilaire sized for the house, without any sort of controls, just a plug. Until my home automation is set up, I'll have to find my old mechanical timer.
"Once you commit to adding the whole house dehumidifier, then it only makes sense to set up the A/C system for maximum sensible heat efficiency and that includes the 450 to 500 cfm/ton idea.
The 350 cfm/ton approach does work well in most applications and makes the dehumidifier less critical. It just isn’t the right answer for every humidity problem."
Paul- The dehumidifier is a good solution to your system needs; however, I would still set your HVAC system up to address your latent loads. Velocities between 350 and 400 cfm per ton are still appropriate. The dehumidifier is not an efficient way to remove moisture from an energy consumption standpoint. Even the DEC ultra-aire, which is very efficient, is not as economical as the AC at removing moisture. It will add sensible heat to the space and should only be used when there is minimal sensible load and the HVAC system just won't keep up with the moisture load on it's own. My Ultra-aire runs for only dozens of hours a year and mostly handles the fresh air and filtration issues. My HVAC is probably moving around 375 cfm per ton. Also unless you increase the duct and grill sizes(all else being equal) increased velocities mean increased noise.
Be sure and maintain the filters. An ECM motor will often be configured to maintain the desired air flow even when the static pressures are increased. Dirty filters will cause them to run harder as they ramp up to overcome the increased resistance. If the motor does not operate this way, you can freeze up the coil. You probably already know this. You have obviously done alot of homework.
"...set up the A/C system for maximum sensible heat efficiency and that includes the 450 to 500 cfm/ton idea."
I am interested in your source for such a recommendation.
"The 350 cfm/ton approach does work well in most applications ..."
This is based on your research? Personal experience?
It looks like I still have some more to learn – but then again, that is why I am on this forum – and others. Ground breaking on my new house is scheduled for January, so I still have plenty of time to improve the design.
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450 to 500 cfm / ton will make a system more efficient at removing sensible heat at the expense of not controlling latent heat.
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However, as Ray Moore pointed out, the A/C system can still be the more cost effective method of controlling humidity. It just won’t do the whole job by itself – which means the whole house dehumidifier is still needed.
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I don't think that I can come up with a computer simulation that would be realistic enough to prove it one way or the other, and I don't know of any existing research. Add a heat pump water heater into the mix and it gets even more complex. At this point, the engineer in me wants to design the house so I can try it either way - assuming that a variable speed blower has a wide enough speed range to make the test feasible.
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The 350 cfm / ton for controlling humidity is 95% research and 5% personal experience.
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The 5% personal experience is based on the fact that the duct work in my current house is way too restrictive and I suspect running 300 cfm / ton or less, but it does a great job of controlling the humidity! I have measured a 30 degree temperature difference between the return air grill and one of the vents half way across the house. If I had not insisted on the service people adding a TXV a few years ago, the coil would probably be freezing over. The relative humidity has been staying between 39 and 42% during the summer months when the A/C is running a lot.
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One of the reasons that my current duct work is too restrictive is that I paid an HVAC contractor to fix it. The other reason is that I am too lazy to spend a few hours under the house fixing it myself. I really, really hate working in the crawl space, but at least I did patch the holes that they left behind.
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Would it be wrong to assume the right size A/C unit would take care of humidity? It seems redundant to tack on a dehumidifier on top of the cooling system.
Thats my first thought - track down your A/C installer & pursue that route first - If he was fully informed of the house design, than he is responsible for what he installed & should make it right. Any advanced heat loss calculation will include SIPs in their program - they are not that exotic.
But I'm just an informed opinion, not an HVAC consultant. Our ICF (similar to yours) home is cooled by 2 smaller window units (money ran out, so central air was an option that was dropped for now) they have no trouble keeping up & indoor humidity feels low.
Best wishes in fixing your humidity problem.Treat every person you meet like you will know them the rest of your life - you just might!
Wow, a house like mine being cooled by 2 smaller window units???!? I am really over-sized at 3.5 tons!Very interesting comment! I noticed when I did my load calc, that there was no way to account for a sheltered window. While I can spec their orientation for solar gain, I had to fudge most of my South facing windows to account for this. There is nothing in the program to handle a radiant barrier such as a layer of closed cell foam. And nothing for "unconventional" construction. Seems so rudimentary to me.
If you have the deep overhangs like a passive solar home, you will get the winter sun directly, but almost no summer sun. If the manual J missed this, AND SIPS I am certain your A/C is oversized.
I downloaded a heatloss calc program off the net - $50 for one time use. I never used it, but I remember it having an ICF option. Rescheck, a program from the govt (also online) had similar options - my house is 43% more efficient than code - not difficult with sips or icfs.
Your HVAC guy needs to get with the program. And fix the problem.
btw - I have no idea what the equivalent tonnage of my window units would be.
Treat every person you meet like you will know them the rest of your life - you just might!
To figure out btu to Ton
12000 BTU to 1 Ton
Jeff
2 tons it is! Humidity feels low, and $300 covered the whole system (but its not central - maybe when the bank account recovers)
Treat every person you meet like you will know them the rest of your life - you just might!
The manual J does't "miss" anything. It only uses the input given to compute loads. I don't often use software to analyze small buildings and I have never used the manual J. No matter what tools used, it is quite easy to accomodate any construction, conventional or otherwise. Your load of 2 to 2-1/2 tons seems reasonable, though. I have a similar home in a similar environment. Though not utilizing SIPs, it is a heavily shaded, very-well insulated and sealed home in N. IL. My air conditioning system works out to 1500 square feet/ton. Yours should be in that range (1200 -1500 sf/ton), too. Were my house 2600 sf above grade, I would have a 2 ton system.
It is very possible to correctly size a small split DX system to both dry and cool the air. The idea that you can only have one or the other is nonsense. A properly sized system for your house can cool it to 80 degrees when its 100 outside, and otherwise keep the inside (relative) humidity) in the 50 to 60% range while maintaining cooler temperatures. Only during long periods of relatively cool and highly humid weather would there be a slight problem. Unless you own and operate a natatorium, a dedicated humidifier is a waste of money. Any system will vary the ratio of the sensible capacity to the total cooling capacity, based on the "wet bulb" temperature of the air entering the coil, i.e. the air inside your house. When very humid air enters the evaporator, the S/T can be as low as 30%. As the air dries to the practical limit of affordable equipment, the ratio will increase to 90% or so.
If you had calculated loads of 2 or 2.5 tons, why did you ever allow the contractor to intall amost twice that? Regardless of how much fault is yours and how much is theirs (both you and they ARE at fault here), a propoerly sized system is necessary for it ever to work and be comfortable in your home.
Edited 8/28/2006 1:28 pm by Tim
tufenhundel,
I cool my 5000 sq foot home with three small window units.
However moisture seems to be an issue with me as well, I'm hoping my HRV will solve it because it's worst during the winter.
golly , tufen..
sounds like a well sealed house... it should be easy to control your humidiity..
my guess is you have a huge reservoir of moisture constantly replacing and overwhelming any effforts of the mechanical system to dehumidify
my first list of suspects would be the basement slab
have you tried any of the standard tests for vapor transmission.. like taping a sheet of plastic to the floor ?
what is the historical RH for your summer climate ?
have you tried measuring RH in your basement as opposed to the first floor ?
and yes... i would question the methodology of the heating "engineer " who sized your system... it's either oversized without regard to the true cooling load ( using averages instead of specifics )
or you have a huge source of moisture that is not being accounted for
Mike:"you have a huge source of moisture that is not being accounted for"This was what the HVAC's supplier was trying to sell to me. I not so sure on this one. My logic--if this is the case, I should at least see peaks in humidity when my A/C is off, and valleys when it is on (assuming my A/C is sized right). But I can't buy there's that much moisture in a new house to overwhelm my A/C.Maybe a concrete specialist can comment on the basement slab. It was poured over a plastic vapor barrier...over 8 months ago. I'd think it'd be (relatively) dry by now.
A couple of thoughts from a fellow homeowner, who also had to fight his AC guy not to oversize the heating/cooling plant 100%.
First of all, I would check to what extent you need to run the ERV 100% of the time, and at what speeds. Have a consultant perform a blower-door test on your home and find out how many ACH are occuring w/o the ERV in action, then adjust the ERV speed/frequency to match the actual need. You may be adding a lot of moisture by running the ERV 100% of the time. I would also verify that it's running/hooked up correctly.
I second the need to take a closer look at the basement slab/foundation walls. Where is the moisture coming from? Have you tested the floor and walls to see if the moisture is seeping in? See the Building Science Corp web site for excellent info on how to dry your basement. Another good book is the pros for pros series' Foundations book.
More importantly, I hope this is not a vented basement, bathrooms, etc. where little grilles let humid outside air in! Since you insulated it, I presume that you know better...
I would also consider going around the house with a moisture meter to figure out if you have a leak somewhere. IR cameras can sometimes be used to the same effect. Since you have a vented roof, the whole area up there ought to be scrutinized, particularly if your AC system resides up there. AC systems should not be installed in vented attics, IMO, there is too much leakage. Also check all penetrations, make sure that the place does not leak like a sieve in the upwards direction.
You can optimize your AC system for dehumidification by adjusting the air handler fan speeds (lower them as much as possible to boost latent heat removal). However, if that does not do the trick and the source of the problem is a oversized system (i.e. not a wet slab, open doors, etc.) then you have little choice but to opt for an AC swap-out. If it comes to that, opt for a right-sized system that offers 2-speed operation and watch the humidity plummet.
Edited 8/27/2006 9:24 am ET by Constantin
Good point about the ERV, since I've wondered about bringing in moist cool air at night, when the A/C is not running. This will be on the to-do list.As for the basement, can it be such a problem? Let's assume the moisture barriers on the walls and slab work, can I eliminate the moisture from the concrete itself at this point?The walls were painted with some type of vapor barrier on the outside, and the slab was poured with a plastic sheath over a gravel bed. I understand this to be standard practice. If this system is working properly, how much moisture can I expect to see infiltrating the basement? The slab's integrity looks intact. As for the walls, they are behind plastic sheathing and fiberglass batts. Not what I'd do, but inexplicably county required it.
Whoa... no insulation under the slab?As has been suggested by others, doing the test is easy. Just follow the instructions re: applying PE or whatever to the floors and foundation walls (not the drywall) and see if water will mist them from the underside. Along the same lines, check the water management around the house, make sure that rain and other surface water does not congregate around the house, etc.If you have drains for the gutters, make sure they're clean, no debris, etc. However, I would also concentrate on the ceiling and the roof system. If your air handler is up there, that could very well be your source of humidity in the home.
"As for the walls, they are behind plastic sheathing and fiberglass batts. Not what I'd do, but inexplicably county required it."
This is one of the worst building practices ever perpetrated on the American public. I have seen so many pictures of moldy walls as a result of this practice. Aaaarrrrgh!
Check out the Aprilaire whole house dehumidifier which is built to integrate into your HVAC system.
http://www.aprilaire.com/index.php?znfAction=ProductsCat&category=dehumid
http://www.aprilaire.com/index.php?znfAction=ProductDetails&category=17&item=1700
http://www.aprilaire.com/themes/aa/en/manuals/1700.pdf
Installation manual:
http://www.sylvane.com/images/productpages/aprilaire/1700/safety-installation.pdf#search=%22aprilaire%20dehumidifier%22
Cost?
http://www.alpinehomeair.com/viewproduct.cfm/productID/453055570/p/Aprilaire_1700_Whole_Home_Dehumidifier_
Billy
Maybe someone hit on this already, but have you tried shutting off the ERV for a few days?
That might be a good start & then put it on a timer if you still think you need it.