Split AC: How to mitigate mold concerns? Or, other ways to cool leaky homes
Hello,
Our home is a very leaky 1961 home in Hawaii. Some experts here rightfully suggest controlling our indoor climate with air conditioning. We definately like the idea of lowering indoor humidity.
Our problem is that my family has a scary health history with exposure to mold, so we are now hyper sensitive.
Central AC — maybe possible with an exposed duct in high open beam great room, but challenging due to a very shallow attic space (24″ max, 2:12 pitch) over the bedrooms
Split AC – often a solution in Hawaii. However, almost everyone has stated problems with mold in the units — non-starter for us.
Mitigation includes frequent cleaning which can be quite expensive. A supplier at the local home show advised against normal chemical cleaning due to deteriation and damage.
I found one design from Bosch that seems to address mold concerns — “blue fin tech” that mitigates condensation remaining on the fins after running, and easy access cleaning. However, Bosch is not sold here and I cannot find much information.
Whole House Fans – sound interesting, but I am concerned about intentionally drawing in humid air from the outside, and then putting into my attic
Thank you for any thoughts,
Andrew
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Replies
My understanding is that a good drying out once a day will prevent mold growth in the units. So you could alternate between two mini-splits every 12-24 hours. Probably with the addition of dehumidifiers to deal with your leaky (you can't fix this?) building latent load. And positive building pressure to keep the walls dry and stop unfiltered infiltration.
Thanks Jon, I have inquired some if the "fan only" mode would actually blow air across the fins of the coil on the indoor unit -- it appears that this does not really happen.
Can maybe addresss leaks.
Andrew
I don't see how fan only mode for some period (1 hour?) wouldn't dry the coil. I've never seen any signs of an alternate airflow path in that mode - air should flow over the coil, vents, etc and dry them.
Evidently 15 min/day of dry conditions (< 55% RH?) is sufficient to stop the mold growth.
Hi Andrew -
I am attaching a screenshot of dry bulb and dewpoint temperatures for 12 months for Honolulum (Climate Consultant free online software). The yellow dots are dry bulb temperature, the green dots dewpoint temperatures, and the grey bar is the ASHRAE Standard 55 comfort zone.
You can see the issue: many many hours when the difference between air temperature and dewpoint is small but also in the comfort zone. This means many hours where you might be comfortable but dewpoint is so close that surfaces slipping just a couple of degrees get to either dewpoint (condensation) or very high vapor content at those cold surfaces, supporting mold growth.
You need a good temp/RH sensor (see this blog: https://www.greenbuildingadvisor.com/article/measuring-and-understanding-humidity) and then you need an HVAC system with extra latent capacity (low sensible heat ratio - SHR) or a central HVAC system integrated with whole house dehumidification. And then you need to use the temp/RH sensor to decide when you open up and when you close your home, not just for thermal comfort but for moisture control.
Peter
Peter - looking back to this. I have not been able to download the software yet. I cannot find historic dewpoint, but it is rarely above 71deg.
If I were to condition the air, but keep it above dewpoint, plus some pad... I would not need to be concerned with condensation, would I?
For instance, the home would be very comfortable at 78-80deg / 50-60% RH + ceiling fans.
Thank you,
Andrew
Start by air sealing the house- make the house less leaky, which will give you more ability to control the in indoor humidity levels & mold risk.
Daikin's "Quaternity" series mini-split heat pumps have the ability to dehumidify to a set point independently of the temperature set point, with minimal sensible cooling. The (patented, proprietry) coil in the indoor head is split into 2 sections, with an internal control valve that allows a portion of the coil to be in condensing mode for re-heat while simultaneously using the other half for cooling/dehumidification, adjusting the SHR dynamically as-needed to hit the humidity set point. This allows it to air condition/dehumidify to some degree even when the sensible cooling loads are very low without overcooling the place. Most other air conditioning units would need to over-cool the house to achieve the same level of humidity control. (Some commercial building scale AC units with reheat coils can get there too, but would be outlandishly oversized for a house. Do you have 3 -phase 480 V power? :-) )
Whole house fans do not dehumidify until/unless the outdoor dew points are lower than the indoor dew points. While nighttime ventilation strategies can be used for sensible cooling in Hawaii, it does next to nothing for the latent cooling/humidity issue.
Dana,
Thank you. I will check this out. I have not heard of this approach to cooling/dehumidifying, but will look into it. I will need to check with our electrician that has been working on our home. We have wiring in place for 3 head units, so perhaps.