Icynene AND HVAC
My wife and I are considering using whole house Icynene for our new home in Houston, but are concerned about the HVAC requirements with such a tightly insulated home. Our builder has not used icynene, but their HVA subcontractor has. The do a manual J on all their homes, but not a manual S or D ( would those be required ). My primary concern is how to achieve appropriate fresh air in a tightly sealed home and appropriate moisture control. The more I read the more confused I get. I have read options from an ERV to a whole house dehumidifier ( but is this necessary if the sizing is appropriate ) to a simply fresh air intake. One of the “green builders” and others I have spoken recommended a filter on the fresh air intake and definitely a 95% variable furnace? Are there any considerations for whole hose ventilation that I should consider? What are my options? How do the differ functionally and financially? What other HVAC ( rather broad question ) should I consider?
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Replies
Rahul,
1. Any tight house needs a mechanical ventilation system. The means a system that introduces fresh air into the house, or exhausts stale air from the house, or both.
2. A dehumidifier does not provide ventilation.
3. Since it sounds like you are unfamiliar with ventilation options, I suggest that you start by reading this article: Designing a Good Ventilation System.
Your concerns are all important, and the fact that you are aware of them gives you a fighting chance of getting it right. The Manual J calculation is important, but you need to have someone knowledgeable evaluate it to make sure it is done correctly. I have seen many that have significant errors in them including incorrect window U factors and overly high or low infiltration rates, all of which lead to incorrectly sized equipment. Regarding Martin's comment #2 above, I believe that some stand alone central dehumidifiers can be installed to bring in fresh air, removing humidity from it when necessary, as well as remove humidity from interior air when AC would otherwise not work. I think this is an interesting strategy that I am considering using on a house I am getting ready to build, although I have not heard reports, either positive or negative about their performance. The variable speed furnace will help with the humidity issues as well.
Martin,
I have reread the article you linked.
My gross understanding is that I need to accomplish several goals with a tightly sealed home.
1. The AC should be appropriately sized.
2. I need an appropriate ventilation system. To me, this is where it starts get confusing as I have read many things. Do do as you described a Central-Fan-Integrated Supply Ventilation with a fresh air intake. Speaking to my HVAC subcontractor for my builder the simply do a fresh air intake with no motor damper with the variable speed furnace. Or I can do an ERV in a climate such as Houston. My concern is appropriate installation, but one that will not blow my budget.
3. I may also need to address humidity control. If the AC is sized appropriately, what else do I need to do. It seems to be the article is saying that a dehumidifier is need with the ERV, but is it need with central fan integration supply.
Rahul,
1. Your decision to install a central-fan-integrated supply ventilation system is fine. To do it right, though, you'll need an AirCycler control, a motorized damper, and an HVAC installer who knows what he or she is doing.
If your HVAC installer does H and AC, but not V, you may have to educate him or her. You can start by suggesting that the contractor visit the AirCycler Web site.
2. If you want an ERV, you can certainly do that. If you go that route, it's best to include dedicated ventilation ductwork, separate from your heating and cooling ductwork. This option will cost more than a central-fan-integrated supply ventilation system.
3. Humidity control is a separate question from ventilation. If I were you, I wouldn't worry about dehumidification yet. Your AC unit may provide adequate humidity control. After you move into your new house, check your indoor humidity levels. If it is too humid indoors, you can always spend $250 for a stand-alone dehumidifer.
Rahul
I believe you and I exchanged emails back in the summer? I am currently doing Icynene on some homes and working through the same issues you are. I have some local contacts of some energy consultants I might recommend, if you want to email me. These companies also do testing of the house for air leakage as well as the ducts. I will also share some ideas on variable speed vs. single speed AC systems, air quality, and humidity issues, some of it is based on comments Joe Lstiburek made recently in Houston regarding our climate.
I can be reached at [email protected]
Allan
Did not mean to post as "anonymous".
I think we are currently in the VENTILATION DARK AGES
As a Texan who lives in a tight house I agree with Martin's advice for the near term.
I also like the standalone dehumidifier.
There are other controllers... honeywell for one
What is VERY important is that the ACTUAL flow is field measured and calibrated for your house.
I worked with one of the better HVAC contractors in town and....
Sad to say.....I had to read the controller directions myself and insist that the contractor come back and field measure the flow and reset the controller.
During this process it was discovered that a damper was stuck TOO.
Rahul, all of the advice given on this is good. However, the only thing left out is you need to make sure the HVAC contractor knows what they are doing with the ductwork installation. The biggest problem with HVAC systems is poorly designed and installed ductwork. Without good ductwork even the most expensive HVAC systems will not perform well. I assume with the house being in Houston that the ductwork will be in the attic and your are encapsulating the attic with foam (make sure you get a minimum of 6 inches on the roof deck) thus bringing it within the thermal control layer which is a very good thing. Just make sure that the ducts are sized correctly (this is where manual S and D come in) and installed in a free flowing layout with minimal lengths, and no sharp bends, turns or crimps. Once the system is installed it's performance should be verified with a complete commissioning process which includes air flow across the coil verification, flow hood measurements at each register to ensure each register is delivering the CFM required in the heat load (each supply runout should have a damper installed so the flow can be adjusted if required) and the refrigerant charge should be verified via proper subcooling and/or superheat measurements. Your HVAC contractor should do this as part of his normal design and installation processes. If not get another contractor. You have heard of the old adage of 'trust but verify'? My advice with HVAC contractors is 'don't trust and verify twice'... There are good ones out there but in my experience they are few and far between.
P.S. ACCA has a free HVAC installation standard you can refer to if need be. It is downloadable as an PDF from their web site ACCA.org
There are three main issues that need to be considered, aside from temperature control, in any new home in your climate. Ventilation/Humidity Control/Filtration. This is particularly important in a home designed with enhanced insulation systems such as those with spray foam. Spray foam is a great product but it does introduce challenges for ventilation and humidity control. The ventilation issue is easy to understand (these homes don’t naturally ventilate as well) and must be dealt with mechanically by one means or another. Humidity control on the other hand is not as well understood. Older HVAC systems in looser (less insulated) homes ran for long periods even in milder weather and the design of the A/C system removed more moisture than today’s equipment. So now, equipment runs less because of enhanced insulation and even when it does run, it removes less moisture than in the past (warmer coil, fan purge at end of call etc.). Current building science tells us that it is necessary to mechanically ventilate and provide dehumidification separate of the A/C system.
It is extremely important to maintain <50% RH throughout the home to avoid mold/mildew/dust mites and other biological growth. In green grass climates (such as yours), a whole house dehumidifier is important when the A/C is unable to maintain <50%RH. This is very comfortable and will allow the home to limit A/C operation when unoccupied during hot weather.
Fresh air change to purge indoor pollutants and renew oxygen is primary. This is very important during calm winds and moderate outside temperature when home will take 24 hours to naturally change the air. The home should change air in 5-6 hours when occupied. Mechanical ventilation is a must and supply ventilation (putting homes under a slight positive pressure) is the preferred method of ventilation in humid climates. An ERV will actually add moisture during periods of high humidity outdoors with no means of removing and may or may not properly pressurize the home.
And when introducing fresh air from outside it is important to provide good quality filtration of the outside air so we aren’t introducing particulate that will aggravate allergies and add unwanted dust.
Today’s Whole Home Ventilating Dehumidifiers are a perfect match for the issues associated with homes in your climate especially tight ones. Look for Energy Star qualifying products so you are assured of the lowest possible operating costs. Direct your browser to Ultra-Aire Dehumidifiers for more info.