I only run that 600 cfm fan on rare occasions when I am cooking. The fan I usually run is that Panasonic in the utility room. On sunny days I run it at 150 cfm. On cloudy days I run it at 50 cfm. 2 humans, no pets, LOTS of plants.

-Rosalinda

Since I am running my radiant heating system at a pretty low level - lowest speed on my 3 speed pump. hot water heater at 125 F (giving me 100 degree water running through the floor), and valves (flow meters) on my manifold set at half way on my GPMs (I don't feel like running downstairs to see how many GPMs I have it set at), I have lots of room to increase the heat if I decide low 60s is too cold at some future date. I lived for nearly 40 years without central heat in a poorly insulated very drafty house, so this very tight, draft free, well insulated house feels warm to me at 60, and too warm once it gets around 62 or 63. If for some reason I feel chilly, I have a little electric heater I can increase the room temp a degree or two.

-Rosalinda

Edited to add - I am running the GPMs in the slab at .75 GPM setting on the flow meters

Let's backup a little. Are any of these fans an HRV. Just how many plants are we talking about.What is the average height of the ceilings so volume of the space can be calculated.When you are exhausting all of this air ,where and how are you providing makeup air into the home.Your problem is a straight forward psychrometric problem. More information is needed.Bringing wet humid air into the space will only compound your problem no matter what the ventilation rate is.High humidity levels can cause health problems along with damage to the structure.The quantity of moisture presnt in your home is not only in the air but also has been absorbed into furniture, books,flooring materials just to name a few. With the dewpoint of 29F and low 30 F OSA the RH is nearly 80%. Before this air would be of any value for makeup air it must be heated and then dehumidified.

With the dewpoint of 29F and low 30 F OSA the RH is nearly 80%

What happens to the RH when it enters the house and becomes 61F air?

You can reduce the source of moisture (sounds like the plants) or run lots of CFM through a HRV. But when it's warm and humid outside, you will need a dehumidifier that can keep up with the plants.

Posted By ICFHybrid on 25 Feb 2013 09:23 AM

With the dewpoint of 29F and low 30 F OSA the RH is nearly 80%

What happens to the RH when it enters the house and becomes 61F air?

Air with a dew point of 29F becomes 29% RH @ 61F, uncorrected for air pressure/altitude, so you should be able to purge quite a bit of moisture ventilating with  outdoor air.  (Get used to using a psychometric calculator or chart for figuring this stuff out.)

In a very tight house a 600CFM exhaust fan may not be able to achieve anywhere near that ventilation rate unless a path is provided for air to enter (cracking a window on the other side of the house, so that it doesn't just take a short-circuit bypass in & out of the house.)  HRV/ERV systems provide the path by-design, and the cfm is pretty much guaranteed (as long as you keep the filters clean.)

Air that is 65%RH @ 61F has a dew point of 49F.   When it's 30F outside that 49F dew point indoor air has the potential to create real issues in timber-framed construction, since any wood in the assembly that is BELOW 49F would adsorb some of this moisture (via air leakage or vapor diffusion from the interior) and not be able release it until warmer weather.   Mold won't grow (but won't die) at 30F, and won't grow very quickly at 49F, (or even at 60F), but if you load up the sheathing or studs of wooden structure in winter it needs to be able to purge it quickly as the season warms up, because a 70-80F molds can really take off fast.

But IIRC  rosalinda has  ~5" of closed cell foam in the stud bays which is a decent air sealer and an even better vapor retarder, so even at 65%RH indoors any wood-mold/rot issues would be very localized at exfiltration points, and depending on how it's built it may be able to dry quickly enough to avoid damage.  But getting the interior dew point down to something like 35-40F (38-46% RH @ 61F) would be nicer to the house, and would still be in the comfort & health range for humans.

Rosalinda, it's your plants. Add up the number of plants by counting 4" pots once, 8" pots 4X and 12" pots 8X. How many is it?

ICF - I can't count that high I only have a few upstairs (11 in the main living area) but they are BIG and by your formula they come to 45 plants. All the downstairs ones (80 actual probably in the hundreds by your formula) go outside for the summer. No I am not going to get rid of any of my plants - I will just keep dehumidifying another way. Maybe one day I will have a greenhouse so most of those downstairs plants can overwinter out of the house, and that will probably help.

Summers are NOT a problem - I have cool air circulating from outside all night, shut the house down in the morning, and it stays comfortable all day. I don't have humidity challenges in the summer except for a very few days each summer, less that 7.

On the rare occasion I run that range hood at 600 cfm, I open the sliding glass door on the southeast corner of the house to provide makeup air. Actually, whenever I use the range hood at any speed, I crack that door open, if the window on the south side is not already open.

The only CC foam is in the cathedral roof section (front 26 feet) and the band joist. The non cathedral part of the ceiling (back 20 feet) has blown in cellulose. All the walls have fiberglass batts. The ground floor sheathings is sealed with caulk at the seams and every place sheathing meets framing is caulked. The walls can dry to the inside and outside.

Today - dew point 7 degrees F, outside temps in the high 30s (38 at the moment) with a few glimpses of sun, but otherwise cloudy. Ran Panasonic vent fan for 4 hours at 50 cfm humidity went from 65 to 61.

AC Wizard - There is virtually no duct work in the house (by design) and the only combustion is the propane cook stove. The hot water heater that provides both DHW and heat, has its own air supply. There are no Ervs or Hrvs as they did not make sense given no duct work. Make up air comes via open windows - usually the upstairs window in the library on the south side of the house. On dry sunny winter days, I can get the inside humidity down to 50% or less with little or no affect on the house temp. On cloudy days - something I have just started to try based on Dana's dewpoint and RH advice, the humidity is dropping, but only a few percent, with a 2 to 3 degree drop in house temp over a 4 to 5 hour period. I DO turn my heating system off when I run the Panasonic ventilation fan (50 cfm). The thermal mass in my ground floor concrete slab seems to keep the house temps stable, despite the open window and fan exhausting warm air to the outside. The water temp in the slab has never dropped below 60 degrees, even with the heat source off.

So yes, I could use a source of dry warm air on cloudy winter days, which would solve all my issues with humidity - just haven't figured out a way to do that yet that appeals to me.

All the gory details of my house are at this link -
http://www.greenbuildingtalk.com/Forums/tabid/53/aff/21/aft/76819/afv/topic/Default.aspx

-Rosalinda

ICFHybrid, I just wanted to add to what Dana said. Dewpoint of 29 becomes 29RH at 61F only after energy is added in the form of sensible heating. Rosalinda you are a good candidate for an HRV and am I assuming your combustion air for the stove is from the surrounding area of the kitchen.

Dewpoint of 29 becomes 29RH at 61F only after energy is added in the form of sensible heating

Yeah, well, when it gets inside, it gets heated, right?

ACwizard - yes combustion for the stove is from the surrounding air. The kitchen is part of a great room(kitchen,LR,DR) - 26 X30 with cathedral ceiling.

I was under the impression that an HRV is not very useful without ducts taking the air from various parts of the house and adding it back to others.

ICF - LOL

Anyone have some outside the box ideas for warm dry air on cloudy winter days? Any new super efficient products out there (which is what made me think of a small mini split)?

-Rosalinda

ICF - I can't count that high

Ha Ha. Well, take that rough formula and figure for every 10 of those pot sizing "units", there is about 1 oz of water going into the air. Per hour. So, lets say you added up 300 units that gives you 300/10 = 30 * 1 oz per hour or a quart per hour. That's quite a bit. A separate greenhouse for the winter is about the only real solution.

You don't need a source of dry air. Even saturated winter air is dry once inside.

Didn't we talk about a solar chimney a while back? That would help on the sunny days, but nothing except energy input can do something about the gray days. An HRV might be the lowest energy using device that you could count on.

Not exactly.If a certain quantity of air from outside at 29F dewpoint @31F is allowed to mix with a certain quantity of air @61F @65RH within the space the end results would be the ratio difference of the two design conditions.As Dana pointed out if you heat this air from outside without adding any more water vapor the air would end up at 61F @ 29 RH.Without adding additional heating to the outside air the indoor space temperature would fall.

Rosalinda, By taking combustion air from inside of the home and by not having a mechanical makeup air source then you have no means of controlling infiltration into your structure.Good duct design is important in any type of air moving equipment.An HRV would be no different.ASHRAE 62.2 requires mechanical ventilation and exhaust with only a few exceptions.With that being said I would run that 50cfm exhaust fan 24 hours/day in the mean time until the humidity level drops into the 50% range.

Rosalinda,

Are you completely confident of your humidity instrument(s)? Mass market humidity gauges are often wildly inaccurate, off by 10-15% or more.

Plants aren't the problem - watering them is! How much water would you estimate goes into plant pots per day (or week)?

I would guess that interior humidity at 60% would make all but the most energy efficient triple pane windows condense water like Niagara Falls whenever outdoor temp is freezing ot below. Are you experiencing heavy window dew?

Given the extraordinarily high latent load imposed by the plants, I suspect the best solutions is a combination of rationing water to the plants, operating one or more high efficiency dehumidifiers, such as by Thermastor, and exhausting indoor air only when outdoor dewpoint is usefully low. An HRV may be in order, but conditions may be far enough outside an HRV's normal operating ranges so that icing or excessive condensate may be a problem.

I doubt any heat pump, minisplit or any other, operating in cooling mode is a good alternative given the already low interior air temperatures...coil icing is highly likely.

Get a green house before mold takes over your main home.

Curt,
I am using very very cheap hygrometers, but I have half a dozen of them, and they all are within a few % of each other. I use them more in a relative way than as accurate gauges of finite numbers. I do get some condensation on the windows, but definitely not Niagara Falls. It does not drip off the windows or pool on the sills (like it did when I lived in England). Also when it is very cold, it is also often sunny and dry, so I can get the house humidity below 50%. My windows are medium price range vinyl, double pane argon, energy star. In general if the % humidity is less than 60% on my hygrometers, I do not have condensation on the windows - so it is possible the humidity is lower than what the hygrometers are reading.

I really don't know how much water I am using on the plants, maybe 5 gallons a week. The dehumidifier I have is supposed to be energy efficient, and is eStar rated, for whatever that is worth. My electric usage is within normal range - around 15kWh/day.

Thanks all for your advice and information. It is much appreciated.

-Rosalinda

Five gallons/day is a lot of water ending up in the air, and probably your biggest humidity source. (Daily 30 minute showers might add that much to the air, but maybe not.)

Using room dehumidifiers to bring the humidity down converts the heat of vaporization in the water into sensible heat, which is what you want in the heating season, and might be more energy efficient than a ventilation approach, since it lowers rather than adds-to the heating load. During the cooling season it would add to the cooling load though.

The Daikin Quaternity would also be reducing the heating load while dehumidifying during heating mode, but during the cooling season it would be dumping the heat of vaporization outdoors, and would probably be more efficient than controlling humidity with an HRV/ERV ventilation strategy year-round.

Dana, that is 5 gals per WEEK but I take the point.

It is strange, but when I run the dehumidifier in the winter the air it puts out feels cold, and when I run it in the summer it feels warm - wish the opposite were true

Today - dew point 10 degrees F, outside temps in the 40s (43 at the moment) with about 6 hours of sun, but now cloudy. Inside temp 64 (just with the sun - heat is off) Ran Panasonic vent fan for 7 hours at 110 cfm (I was mistaken, thought the high setting on it was 150 but it is 110) humidity went from 65 to 54.

-Rosalinda

Week, day, hey it's less than a full order of magnitude! :-)

A 70 pint/day (that's 8.75 gallons!) standalone dehumidifier should be able to keep up with your load if there is good air circulation everywhere. Tucke in a closed in a doored off basement it won't quite cut it, but if it's in the more common/open space shared with the bulk of the flora it should be cycling, not running full-out, and will be raising the temp of the space in the process. Exhaust only ventilation just adds to the heat load.

The dehumidifier I have is a GE 50 pint standalone. My floor plan is very open, so no air flow problems anywhere. And it does cycle when I have it running, but is a heck of a lot noisier that that super quiet Panasonic vent fan.

So much for those few blessed hours of sun today - it is now sleeting and raining - sigh

-Rosalinda