Systems for maintaining proper RH in Boulder, CO
I’ve read a lot of content on this website about the dangers of adding humidification systems to homes, as this question has come up on a new home we’re designing in Boulder, CO. I have some specific questions about our situation —
It’s hard to tell if there’s a guarantee that a properly sealed house’s RH in the winter will be above 30%, which our client seems to want. If they do add the humidification, then it seems like it becomes a problem in the spring and non-winter months -I assume it’s possible to turn off the humidification in those months to avoid the mold issues?
Another question — If our primary heating is from radiant floor heating, then I wouldn’t think the forced air systems should be running very often – and if humidification/ventilation is tied to that system, then would they be running more than needed, or how would that affect the RH? It seems like excessive ventilation is one contributing factor to drying a house out.
Also, our mechanical drawings show that we have HRVs planned (even though the HERS worksheet says ERV) – it seems like we should have an ERV to keep the moisture inside the house in the winter, is that correct?
Thanks in advance for clarifying.
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A sealed house will have high humidity, a new sealed house will have really high humidity.
Humidifier: obviously you can turn it off, it’s not a requirement of any heating system.
Ventilation should be a fixed value, I wouldn't run ventilation based on humidity conditions. It should be ducted separately if a ducted system isn't the primary system.
I have a large(ish) passive tightness sealed house with forced air in zone 6. The HRV is ducted into the forced air system and runs continuously as does the furnace blower, the RH at this moment is 38% at the main level tstat. During winter when the house is more closed up it runs in the low 40’s. We still have a dehumidifier in the furnace room that is set at 40 and runs periodically. We have never had the need for humidification and with a tightly sealed house run normally you should never need humidification, rather you will need dehumidification.
Down south may differ, don't live down there, don't care bout down there.
The humidistat will control the humidifier to maintain the humidity level you want. Such as system can be easily controlled as needed.
I have in-laws in aurora, and I’m out there several times a year. I know the humidity gets uncomfortably low in that area. You’re less likely to have the same kinds of issues with excessive humidity levels as people in other climates. What you want to avoid is conditions that allow for moisture to condense inside your walls, or on window frames where things can rot. The issue is most severe with either or both of very high indoor humidity levels (over 35-40% or so in the winter), and very cold outdoor temperatures. You can minimize the possibility of condensation problems with good air sealing (air tight drywall, etc), and exterior rigid foam well above code minimum levels to help keep the sheathing warm enough to be above the dew point during all but the coldest of nights.
30% isn’t super high, so your project goal should be doable. Another thing to think about is that there are two main types of central humidifier: pad types, which use relatively little energy but waste a lot of water, and steam types, which are VERY efficient with water use but use a LOT more electrical energy. Which type you choose will depend on your relative water and electric rates. My brother in law went with a steam type to save on water (higher water rates in aurora), but it runs him around $100ish/month to run in electricity.
Bill
Or just go to your local drug store and buy yourself a $15 vaporizer and use it when needed......
How much humidity is safe depends on the current outdoor temperature. Consider a humidistat that accounts for this.
Yes, an ERV will retain more moisture than a HRV. Which is better is highly dependent on the specifics (building tightness, size, number of occupants, climate, etc).
In a cold climate, it doesn't matter much what the humidity is outside: even 100% humidity isn't much moisture when the air is sufficiently cold. The factors that matter are the air leakage rate (hopefully low in your new building), the ventilation rate and type, and the habits of the inhabitants. If they take long showers in open enclosures and cook by furiously boiling big pots of water, and work out on indoor exercise machines, they will introduce plenty of moisture into the air and a moderate-airflow HRV will maintain a good moisture level. If they take short showers in better enclosed shower stalls, shut off the water while soaping up, cook at a simmer in a covered pot (or order takeout) work out outdoors, and use a high ventilation rate with an HRV, it might get too dry. If it fits in the budget, a ventilation system with swappable HRV/ERV core is the nicest solution. I personally swap cores seasonally--HRV in the shoulder seasons when removing humidity is the priority and ERV in the summer and winter to maintain relatively low or high humidity levels (compared to outdoors).
I used to recommend monitoring winter humidity as a proxy for estimating whether you have enough ventilation, but now that you can get an air quality monitor and measure CO2 levels, I think that's a better indication of whether you have enough ventilation.
Nearly impossible to keep relative humidity in winter in even a very tight house with an HRV in a cold climate. That has been my experience, ACH50 of 1.25, 3,500 sf, 9000 hdd. Charlie has laid out all of the factors regarding winter RH. You want ultimate control of the ventilation system, therefore it should have it's own ductwork. Continuous ventilation levels will have to be measured closely to ensure enough fresh air but not over drying. I see nothing wrong with a humidistat control for times when RH levels get too high in winter.
It's been mentioned, but don't discount the amount structure size (in proportion to # of occupants) matters.
HRVs do dry out a house in the winter months. In fact, many HRVs used to (still do?) come with controls that shut the HRV off once humidity got to the lower desired set point.
As many have pointed out, personal habits, house size and house tightness all drive the indoor moisture level and the amount of dehumidification required to maintain indoor RH. ERVs do dehumidify in the winter (and humidify in the summer), they just do it far less than HRVs do.