What mechanical system makes the most sense?
I am in a zone 4a Kansas City Missouri. In the heat of the summer it can get up to 95 with high humidity and it doesn’t cool down at night.
We are building a 3500sqft ICF house with R-24 walls. (We have the labor and resources for it to be the most affordable option) and a R-60 SIP roof, cathedral ceiling. We are thinking about building with a 4′ crawl space, unvented, 20 mil vapor barrier. Our windows and doors we are sourcing from company’s in the U.S. which build to the PassivHaus standard.
I have a few questions:
1. In the winter we are thinking of heating with a wood stove. We have access to tons of free fire-wood. I believe this should be sufficient especially since we tolerate 58 in the house. What effect will this have on the pressure in the house and will it be a large issue? What does this mean for ventilation needs?
2. Is the ERV system and a ductless mini-split (one downstairs and one upstairs) seem sufficient and will the ERV cause too much of a humidity problem for us. Also, what would we do to condition the crawl space. Would open vents in the floor be enough. (I have read a lot of information about this on the forums but can’t come up with a solid answer)
Or should e just install a conventional A/C unit, best efficiency possible, condition the crawl space and other areas and would this eliminate the need for a ERV?
Thanks!
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Replies
Jordan,
Q. "What effect will this [the use of a wood stove] have on the pressure in the house and will it be a large issue?"
A. You can find a thorough discussion of your question in this article: All About Wood Stoves.
Q. "What does this mean for ventilation needs?"
A. All tight homes need a mechanical ventilation system, regardless of the type of heating system that is installed. For more information on this issue, see Designing a Good Ventilation System.
Q. "Does the ERV system and a ductless minisplit (one downstairs and one upstairs) seem sufficient?"
A. A properly designed and installed ERV system will meet your ventilation needs. Whether or not two ductless minisplits will meet your heating and cooling needs depends on two factors: (a) Your heating and cooling load calculations (usually performed using Manual J), and (b) the layout of your house, which may or may not be conducive to the system you cite. For more information on this issue, see Rules of Thumb for Ductless Minisplits.
Q. "What would we do to condition the crawl space?"
A. All of your options are explained in this article: Building an Unvented Crawl Space.
Q. "Should we just install a conventional AC unit, best efficiency possible, condition the crawl space and other areas and would this eliminate the need for a ERV?"
A. You always need a mechanical ventilation system (for example, the ERV you propose). Installing a conventional air conditioner does not eliminate the need for a ventilation system, because a conventional air conditioner does not provide any fresh air to your house.
Q. "will the ERV cause too much of a humidity problem for us."
A. In the summer, the ERV will add a small amount of humidity to the house, but less than any other way of providing ventilation, short of a ventilation system with active dehumidification built in. One way to reduce the already-small humidity load the ERV introduces is to run it as low a speed as you can and still get adequate fresh air, or run it as few hours as you can and still get adequate fresh air.
In the winter, the ERV will keep the humidity higher than an HRV or other ventilation system would. Normally, that's likely to be nice in a cold climate, because the humidity can trend lower than you'd like in the middle of winter with an HRV.
In the shoulder seasons, where you aren't actively dehumidifying with the A/C inside, but the outside air isn't very dry, an ERV might land you with higher humidity than you'd like, depending on the number of people and their showering and cooking habits. This is not very often a problem, but if you are worried about it, an option would be an ERV with the ability to swap in an HRV core for those seasons. For example, that's an option on at least some Zhender units.
As others have mentioned, you'll want some type of mechanical ventilation regardless of heating/cooling system.
Regarding heating/cooling, it seems the best system for your climate would be appropriately sized/designed ductless or ducted mini splits. Especially if you plan to install solar PV.
I would skip the wood stove because it adds complication to the ventilation system and the cost of a good stove, chimney, and related building expenses are not cheap. The one benefit I see to a wood stove is heating during a power outage, but I would guess a "medium/high performance" home in your climate could coast through an outage?
Thanks for those articles Martin...I actually read them all before posting, and that is how I found this website. I guess I am still unclear on a few things and perhaps it is because I am a bit dense..
1. Does an ERV run all the time? I saw some articles about running them every 20 minutes, but then there is an issue that when it isn't running, a draft is coming through the vents anyway.
2. I don't have any programs to calculate energy loads. Who do I need to find to do this? When I call HVAC technicians, none seem to be interested. At what stage in the building-design process do I need to figure this out?
Thanks.
Jordan,
Q. "Does an ERV run all the time? I saw some articles about running them every 20 minutes, but then there is an issue that when it isn't running, a draft is coming through the vents anyway."
A. All ERVs have a variable ventilation rate. Some vary the ventilation rate by adjusting the fan speed; others vary the ventilation rate by the use of a timer (for example, one that operates the ERV for 30 minutes per hour or 20 minutes per hour). Ideally, you want to ventilate your house at the ASHRAE 62.2 rate or less. Most ERVs have backdraft dampers that limit infiltration and exfiltration when the ERV isn't operating.
Q. "I don't have any programs to calculate energy loads. Who do I need to find to do this?"
A. Here are a few articles for you to read:
Saving Energy With Manual J and Manual D
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
Q. "When I call HVAC technicians, none seem to be interested."
A. You're right. That's a big problem. You can consult an energy rater certified by RESNET or the Building Performance Institute. (To find a certified rater near your location, visit the RESNET and BPI websites.) Or you can hire an energy consultant or mechanical engineer. Or you can perform the calculations yourself using advice from this web site.
Q. "At what stage in the building design process do I need to figure this out?"
A. Before the design is finalized, so that you know the size of the necessary mechanical room and the location of the ducts. Some ductwork may require the use of open-web floor trusses, so you can't wait to the last moment to design your HVAC system.