New Retrofit IAQ Build Sanity Check / Ultra-Aire XT150H efficiency with constant fan

[teddy bear]

As far as amount of ventilation, 100% of the time is perhaps desirable, but not strictly necessary -- I found 60% ventilation eliminates all household odors, but at 40% you can still smell the sprayfoam insulation in the area where the smell was strongest. One of the "fine tuning" aspects that hasn't yet been complete (work being done today) is a fantech exhaust fan in the attic to try and create a slight negative pressure in the attic. I bought a unit capable of 300CFM, but plan on using a fan speed controller to have it set to 50CFM the majority of the time, and configured to run from a relay only when the fresh air ventilation fan is running. The higher levels may be nice when having future home improvements done -- eg if more foam needs to be sprayed, it will reduce the impact of the offgassing, or if contractors will be spending a long amount of time running wires, it will increase their comfort and therefore probably increase their work quality.

I am not a fan of any unneed exhaust fans. The idea is to put a positive pressure on the structure. There are plenty of imperfections for the stale air to leak out of. When a bath fan or the clothes drier is used, there is air available with causing a negative pressure which would suck in unfiltered air. No big problem, do not use the fan unless you have specific need for attic exhaust, Please.

Using a CO2 Meter, it seems that no matter the amount of ventilation, CO2 levels never fall below 500 ppm, even when the house is unoccupied for most of the day, but it seems easy to stabilize below 600PPM even when sleeping overnight with a door closed. I'm trying to figure out what the source of the CO2 is -- The house has all electric appliances, with the only combustion being the propane range, which is obviously not constantly in use (and never when unoccupied!), the only pet is a spider, and there are plants in the house which should generally reduce the amount of CO2 further (during the day anyway, but might elevate it slightly during the night). I think the ultimate answer may just be that with occupant CO2 gets diluted, and is therefore going to follow a logarithmic curve for elimination.

Outside air is usually about 450 ppm. Meters can be off a little or you may located near a heavy traffic area. The main source of CO2 is the occupants. Report your average CO2 levels after 7-8 hours of constant occupancy with mixing of the air and CO2 from the occupants. The following chart shows the cfm of 450 ppm of CO2 fresh air need to delute the CO2 to the level on your meter.
1 2 3 4
PPM CO2 Total CFM Fresh Air
450 0 0 0
500 212.0 424 636 848
550 106.0 212 318 424
600 70.7 141 212 283
650 53.0 106 159 212
700 42.4 85 127 170
750 35.3 71 106 141
800 30.3 61 91 121
850 26.5 53 80 106
900 23.6 47 71 94
950 21.2 42 64 85
1000 19.3 39 58 77

If you have 3 occupants with a 700 ppm CO2, you are getting 170 cfm of fresh air into the home. This assumes prefect mixing and 450 ppm outside air. The ideal amount of fresh air is an air change is 4-5 hours. Also the closer that the occupants are to the meter causes less perfect mixing of the CO@ with outside fresh air. A 4,000 sqft. home with 9 ft ceiling is 36,000 FT.^3 . 150 cfm provides an air change in 4 hours.

At this point, the main complaints about the system is actually the Honeywell VisionPro IAQ controller -- I'm not sure if there's anything better, but this leaves a lot to be desired in many ways. Eg: It only displays "Heat On" or "Cool On", but not "Vent On" or "Dehumidifier On" -- it would be nice to know when it wants to run those. Also, it seems that when you configure the Ventilation Limit (setting 404), it really only sets the limit, not the actual runtime -- so if it calculates that you only need 40% venting and you set the limit to 100%, it won't make any difference, though there does seem to be some logic there I can't figure out, as it will run the vent continuously during some hours, and rarely during others, but I can't find any documentation for it.

Another big question I have is -- does the dew point of the discharge air from the XT150H vary depending on the input? Specifically -- if the return air is mostly outside air at 85F / 70% RH or inside air at 78F / 46%RH, does the discharge air dewpoint change considerably, and just the temperature changes (IE: it's more efficient the more humid the air is?) or is there an ultimate limit on how much humidity it can remove at a certain time, and the dewpoint will gradually fall as the return air becomes drier?

Thanks!

The dew point of output of the Ultra-Aire XT150 H vaires with the dew point/temp of incoming air. Wetter air increases the amount of moisture and temperature. No limit really. Also as the input dew point drops, the output dew point drops. Not worry though. During the hottest times, your a/c should be able to dehumidify your home with the dehu not operating. That way you operate the dehu based on the inside %RH. During those hoursof high cooling loads, the %RH is below the set point of the dehumidistat. With extreme low %RH settings, the a/c may need adjusting to have a colder coil to contribute to better moisture removal. At 75^F, 45 %RH, you need <45^F a/c coil temp. You might be over thinking this all a little bit, but congrates!
Wow 45%RH with fresh air and occupants. First class indoor air quality and comfort!

Got to run.
Regards TB

[somedude]

So it's been about a month. I had the exhaust fan running in the attic for most of the time, interlocked with the fan in the XT150. Looks like it had a major effect in accelerating the curing of the foam in the attic, I'd say the smell has been reduced by 90% in the attic.

Overall, the system is great -- a few friends have commented on how the air quality is great in my house since then, and they didn't notice how poor it was at their house or office before spending some time at my house.

Over the last couple of months, Florida has had many wildfires, including two major wildfires adjacent to Miami-Dade, and the outside air conditions were very bad. On some days the smoke was so bad, you couldn't even see the clouds in the sky anymore. It's been about 30 days, and I decided to check out the condition of the filters after putting up with such poor quality air. The 20x24x4 pre-filter, which I had planned on replacing once every 6 months was pretty disgustingly dirty -- certainly dirtier looking than I would normally allow my HVAC filters to get, and I decided to change it out (very) early. It's finally started raining after 4 dry months, so I'm hoping we're done with wildfires for the year, and hopefully the next one lasts 6-12 months. The Pure-air merv16 filter still looks brand new, other than a few pieces of pollen that seem to have made it around the pre-filter somehow.

Interestingly, the wildfire that happened the week before the system was installed caused all the HVAC filters in the house to get very dirty despite only being 3 weeks old -- this time around, the HVAC filters still look brand new. Looks like the benefits of positive pressure are obvious.

[teddy bear]

Nice to get some positive feed-back. keep us posted.
Regards TB