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HRV vs ERV and which one to buy?
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JeffInCO
New Member
Posts:24
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| 04 Feb 2011 10:38 AM |
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I am looking into installing an HRV or ERV. My house is in Northern Colorado, a couple of miles east of the Rockies (known as the "Front Range" around here).
A few questions:
1) In a dry climate, such as ours, should I go with an HRV or an ERV? I've seen conflicting information... In the winter while heating, I want to keep as much humidity in the house as possible as it is difficult to keep the humidity at 30%. Without any humidification, our heating season humidity would quickly drop to 20% or lower. In the summer, we commonly see temperatures in the mid to high 90's, but it is dry compared to much of the rest of the country. Still, our WaterFurnace is pulling some humidity out of the air during the summer when it is in cooling mode.
2) As far as sizing goes, the total conditioned space of the house is about 4000 square feet. My understanding is that the current ASHRAE recommendation is 3 CFM/100 square feet plus 7.5 CFM/person. We're a family of six, so 3*40 + 7.5*6 = 165 CFM. Should I look for a unit that can just barely exceed this, or a unit where 165 CFM is in its mid-range (with some "breathing room", pun intended)? In general, are there selection guidelines based on a CFM requirement?
3) Finally, which makes/models are recommended? I'm planning to do the install myself, so I am limited to those that I can purchase. It looks like venmar, lifebreath, and renewaire are some possibilities.
Thanks!
Jeff
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Dana1
Senior Member
Posts:6991
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| 04 Feb 2011 04:01 PM |
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ERVs don't return humidity super-efficiently in cool weather, but they return some, allowing you to run a higher ventilation rate without excessive drying. There's little down-side to going with ERV, and since you can run higher winter ventilation rates there is a slight health-factor benefit.
Dew points are low enough in summers that you don't benefit a whole lot from ERV during the cooling season though- your sensible cooling loads far outstrip your latent cooling loads. The fact that your WaterFurnace condenses a bit of water in the summer isn't an indication of a NEED to dehumidify. No matter how dry the summer air is, the dew point above the temp of the cooling coils of a compressor system, and you'll get some condensation out of it. But the volume of water condensed is much much lower than in places where the dew points are well north of 70F (where the latent load is more than 80% of the cooling energy use for the incoming ventilation air.)
If can air-seal the house sufficiently to get your air changes per hour @ 50 Pascals (ACH/50) in a blower door test down below 2, you would almost certainly be able to maintain 30% RH indoors all winter without active humidification. The fact that it's dropping below 20% unless you actively humidify it's an indication that your natural ventilation rates are relatively high, and the argument for active ventilation may be moot. Houses that are PassiveHouse tight (<0.6 ACH/50) always need active ventilation to bring the humidity DOWN to 30% in winter. Canadian R-2000 standard (<1.5 ACH/50) homes can also be in the same situation. With cooking/bathing/breathing for six the amount of moisture introduced by your family SHOULD be able to keep it at 30% and then some.
With "natural" ventilation the infiltration/exfiltration points are somewhat random, and don't guarantee that the ventilation air is distributed where it's actually needed. Ventilate with an ERV, yes, but DO try to get the natural ventilation rate down- it'll save heating/cooling energy, and you'll end up with better indoor air quality overall. Tightening up the house to the point that you can decommission the humidifier and run the ERV in winter under dehumidistat control would be ideal.
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JeffInCO
New Member
Posts:24
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| 04 Feb 2011 04:59 PM |
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Maybe I need to double-check on the humidity... It seems like I need to do some extra to keep the humidity around 30% (according to our thermostat), but for the last couple of weeks I've only been running a small room humidifier (a fan with a big wick, basically) which has been adding about 1/2 - 3/4 gallons to the air per day.
The house is very tight, according to an energy audit by the electric company. This matches my experience in running the Waterfurnace this winter. We have 4000 square feet. The calculated heat load for the house is around 51kBTU's per hour at design temp (-4 F). Our waterfurnace is a 4-ton synergy 3d. The first time that the unit kicked into second stage (except for times when I've changed the setpoint) was Tuesday morning at -18 F outside. Walls are 2x6 framed (on 16" centers -- if I'd known different before the project started, I might have done 24" centers). We have 1" of closed-cell foam and 4.5" of cellulose in the walls. Attic is R-38, basement walls R-10. In any case, the house has performed very well and is likely too tight to be without forced ventilation.
I'll get a second humidity monitor and see what happens if I turn off the little humidifier... maybe it's not doing much.
Jeff
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Lee Dodge
Advanced Member
Posts:714
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| 05 Feb 2011 04:54 PM |
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Jeff-
I am also in Colorado (Salida), and also have very dry air in the house, although I am the only inhabitant. Dana says it is because my house is too leaky (2.4 ACH at 50 Pa, which is tight relative to existing housing stock), but when the dew points are below zero F, it is hard to keep humidity in the house without adding some. Dana's reference point is Mass., and it was a LOT more humid in New England when I lived there than it is in Colorado. I tend to turn off my HRV in winter to reduce drying, except during the day when I put it on recirculate 67% / vent 33% of the time to help distribute the passive solar heating. Maybe an ERV would work better, assuming it wouldn't freeze up in the extreme cold. I would suggest a blower door test to see if you really need mechanical ventilation.
Lee
www.ResidentialEnergyLaboratory.com
in a net-zero energy house |
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Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
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JeffInCO
New Member
Posts:24
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| 05 Feb 2011 11:30 PM |
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Lee,
Thanks for your thoughts...
I looked back at the blower door portion of our energy audit. It is not completely clear, but I think we were at 814 CFM at 50 Pascals. The auditor used the finished portion of the house (2800 sq ft) to compute the air volume assuming a 9-foot ceiling height (we actually have 9 foot on the main level, 8 foot upstairs, and 7.5 feet in the basement). He wrote down 2 ACH's under these conditions. Under the auditors assumptions, I have 25200 ft^3 of volume, so:
ACH = ( 814 ft^3/min * 60 min/hour ) / 25200 ft^3 = 1.9 (pretty close to the auditor's ACH=2).
If I take the unfinished (but conditioned) basement into account, I have 814 CFM and about 34400 ft^3 of conditioned air volume. This equates to about 1.4 ACH's.
I don't know how to convert this "forced" air changes per hour (at 50 Pa) into an "unforced" number (at a theoretical 0 Pa).
In any case, the auditor told me I need ventilation air. I think he's right.
As for humidity, it looks like it's holding around 30% today without the little room humidifier. But, our low today was 25 F which isn't very cold.
Jeff
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Lee Dodge
Advanced Member
Posts:714
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| 06 Feb 2011 12:57 PM |
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Jeff,
I agree with you about using the total conditioned volume in ACH calculations, and the 1.4 ACH looks good. The rule of thumb for converting the 50 Pa number to natural ventilaion is to divide by 20. (See Max Sherman's "The use of blower door data" for details.) That would give 0.07 natural ACH, or 41 CFM. You computed that you need a total of 165 CFM, so forced ventilation needs to add 124 CFM. So I agree that according to ASHRAE recommendations you need to add ventilayion.
What is behind the ASHRAE standard? I am not sure. Maybe a topic for discussion on this forum. Getting rid of humidity is a major reason, and it sounds like you do not need that during the winter time. Getting rid of CO2 is another, and that is rarely measured, so an unknown. Getting rid of odors are another, but bathroom and kitchen fans probably take care of most of that. My thought would be not to go overboard on forced ventilation during the winter, since it dries the air, but you need to evaluate how odors build up with the house sealed up.
Lee Dodge
www.ResidentialEnergyLaboratory.com
in a net-zero energy house |
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Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
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Dana1
Senior Member
Posts:6991
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| 07 Feb 2011 05:48 PM |
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Posted By Lee Dodge on 05 Feb 2011 04:54 PM
Jeff-
I am also in Colorado (Salida), and also have very dry air in the house, although I am the only inhabitant. Dana says it is because my house is too leaky (2.4 ACH at 50 Pa, which is tight relative to existing housing stock), but when the dew points are below zero F, it is hard to keep humidity in the house without adding some. Dana's reference point is Mass., and it was a LOT more humid in New England when I lived there than it is in Colorado. I tend to turn off my HRV in winter to reduce drying, except during the day when I put it on recirculate 67% / vent 33% of the time to help distribute the passive solar heating. Maybe an ERV would work better, assuming it wouldn't freeze up in the extreme cold. I would suggest a blower door test to see if you really need mechanical ventilation.
Lee
www.ResidentialEnergyLaboratory.com
in a net-zero energy house
My reference is the psychrometric chart, and the moisture content of the air at wintertime dew points. Yes, it's different in CO vs. New England, but it matters less than you might think. The difference in moisture content of the air when the dew points are +15F vs. 0F (or even -15F) aren't dramatically different with each other compared to the moisture content of 40F dew point (70F/35% RH) air- it's all at the thin edge of the wedge on the left side of the chart: At 0F saturated (dew point) air has ~8 grains/lb At 15F dew points (typical for a New England teens & 20s temps) it's ~13gr/lb Both of which are WELL below the ~38 gr/lb of 40F dew point air (70F/35% RH) Ventilating with 0F dew point air the "makeup" moisture is 30 grains, with 15F dew point ventilation air it's 25 grains- a difference, sure, but it's a 1.17x factor, not 2x. As you go even lower in dew point the "makeup" moisture requirement differences shrink even further. Even ventilating with -150F or -250F dew point air the makeup-moisture difference is less than 1.5x what it is at +15F dew point air. Below 15F dew points, it becomes basically "who cares?"- it's all about the rate of interior humidity sources relative to the (natural + active) ventilation rate, since the differences in moisture content on the incoming air stream really aren't very different. It's fair to say that New England experiences more wintertime spells with much higher dew points (like over the weekend, when it was RAINING in southern New England, or now, when it's above freezing), but that's not what's making the difference when it's colder & drier, with average daily dew points in the teens or lower (like most of last month.) The real difference here is likely to be the number of occupants, fractional time of occupancy, plus active ventilation in pretty tight but not super-tight house. A single person who spends 10 hours/day out of the house will be generating far less interior humidity than a 3-4-person house where it may be empty only 4-5 hours/day, with 3-4x the amount of bathing/breathing/washing/cooking humidity emitted. Ventilating above & beyond an arguably-adequate natural ventilation rates makes it even drier. While 2.4 ACH/50 is pretty good, it's still 0.9ACH above Canadian R-2000 max standard. (That's more than an entire PassiveHouse difference in ventilation rate) In a PassiveHouse, even a workaholic commuting single-guy in Antarctica would not have to add humidity unless actively ventilating, but he might be on the edge with a minimally tight R-2000 house. |
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Brawler
Basic Member
Posts:229
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| 12 Feb 2011 11:08 AM |
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Jeff, I bought the Renewaire, i think its the smallest one, my house is tiny, and i am impressed with the construction. It has a 110 plug so all you do is attach the control wires and your vents. It was easy. We use insulated duck for insurance from condensation and to quite any noise down. House isnt finished but when we tested the unit it was very quite. More so than i expected. Good luck. |
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Brawler
Basic Member
Posts:229
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| 12 Feb 2011 11:13 AM |
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Photo of install. |
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Brawler
Basic Member
Posts:229
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| 12 Feb 2011 11:16 AM |
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Photo of install.javascript:amaf_insertHTML('%3Cimg%20src=/DesktopModules/ActiveForums/viewer.aspx?portalid=0&moduleid=369&attachid=2409%20border=0%20/%3E');amaf_toggleInline(2409,7093,0); |
 Attachment: zones_and_erv.jpg
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zehboss
Basic Member
Posts:216
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| 12 Feb 2011 12:06 PM |
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Hey,
The real question is, are you going to humidify the inside of the home? The HRV will help maintain the humidity in the home. HRVs are not required in non humid areas. Dry areas usually use ERVs. A well-built home should have an ACH/50 of less than .8 in my opinion. I recently build a zero energy home in Park City it tested at .3. I would hook up the fan again and finish sealing the house so it can work right.
Brian
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ICF Solutions
Engineering, Designing, and Building Passive, Net Zero, Self-Heated, Self-Cooled, Self-Electrified, Low Cost Homes
Basic shell starting at R-50 Walls, R-80 Roof structures. for $30/square foot
(360) 529-9339
[email protected] |
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Dana1
Senior Member
Posts:6991
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| 14 Feb 2011 01:42 PM |
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Posted By zehboss on 12 Feb 2011 12:06 PM
Hey,
The real question is, are you going to humidify the inside of the home? The HRV will help maintain the humidity in the home. HRVs are not required in non humid areas. Dry areas usually use ERVs. A well-built home should have an ACH/50 of less than .8 in my opinion. I recently build a zero energy home in Park City it tested at .3. I would hook up the fan again and finish sealing the house so it can work right.
Brian
Methinks you've got that backwards (the "return" pinky is faster than the proofread, mayhaps?  ) ERV has the cores that swap humidity between air streams, HRV transfers sensible-heat only. I'm with you on air-sealing to sub- 1ACH/50 levels on new construction. The difference between 0.6 (PassiveHouse spec) and 0.8 or 1.0 or 1.5 (R-2000 spec) is somewhat academic unless you're going for PassiveHouse levels of energy use. (Hitting 0.3 ACH/50is impressively tight though!) Even at R-2000 levels of tightness most dwellings would not require active humidification unless (over)ventilating mechanically. |
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zehboss
Basic Member
Posts:216
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| 14 Feb 2011 02:43 PM |
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Sorry about the typo, middle of night, train of thought, did not proof it well. I have been building zero energy homes for 30 plus years, if I can help.
Thanks
Brian |
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ICF Solutions
Engineering, Designing, and Building Passive, Net Zero, Self-Heated, Self-Cooled, Self-Electrified, Low Cost Homes
Basic shell starting at R-50 Walls, R-80 Roof structures. for $30/square foot
(360) 529-9339
[email protected] |
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Garth Sproule
New Member
Posts:25
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| 14 Feb 2011 06:58 PM |
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The 'Factor 9" home was built very tight and is super insulated. It is located in Regina SK (very cold and dry in winter). I know that I read somewhere that they swapped out the HRV core to an ERV core in the winter months to help maintain indoor humidity. Don't know how well this works, but might be worth a try? You would have to monitor the humidity levels to make sure you don't get too high in winter. |
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bigskylivn
New Member
Posts:2
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| 01 Mar 2011 03:41 PM |
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Jeff,
I just got on this website. We are also in the front range building a tight IPS and ICF home. We'll have a ranch walkout with about 6,200 sqaure feet occupiable (not including large storage, closets, etc) and will assume 5 occupants.
After reading ALL the blog posts to your initial question of:
which do I select ERV or HRV, I honestly cannot see what answer you were given and I have the very same question.
Did you select HRV or ERV and if so why.
Did you select a Vendor and Model, and if so who and what size CFM for your situation.
Thanks
still confused in the front range
Jen
PS I am not HVAC super saavy so the simpler the better. |
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bigskylivn
New Member
Posts:2
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| 01 Mar 2011 03:42 PM |
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From Jen in the front range,
i made a mistake-the house is SIPS panels on the exposed areas above grade and ICF below grade walls.
Thanks again for help on HRV vs ERV in dry Colorado.
Jen |
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JeffInCO
New Member
Posts:24
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| 03 Mar 2011 08:52 AM |
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Jen,
I haven't made a selection yet. Based on the feedback in this thread, I think I'm leaning towards an ERV. As for models, my uncle (who also lives in this area) installed a Lifebreath HRV and is happy with it. He pointed out, though, that many of the Venmar models are higher efficiency.
As for sizing, my original post has what I think is the standard:
"... the current ASHRAE
recommendation is 3 CFM/100 square feet plus 7.5 CFM/person."
You mentioned a 6000 square foot ranch... is that 3000 main floor and 3000 basement (finished or not)? Or do you have more than 6000 when counting the basement? I think the sizing should be based on conditioned square footage, which may not be the same as finished square footage.
If you have 6000 conditioned square feet and 5 people, I guess you need 218 CFM of ventilation. I think Dana1 mentioned earlier in the thread that leakage counts towards this number, but you may need to purchase your HRV/ERV before your building envelope can be tested.
I'll post back to this thread once I eventually make a decision and do the install. Of course, it's warming up now, so it will be hard to tell how well the unit is working at least until summer when it gets hot.
Good luck with your construction project!
Jeff
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Dana1
Senior Member
Posts:6991
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| 03 Mar 2011 03:53 PM |
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Sizing your HRV/ERV for the ASHRAE recommended minimums is the right thing to do, but that doesn't mean you should always have to run it there. Of course the natural ventilation rate counts, but it's a squishy number that can't be directly calculated from an ACH/50 measurement though some try to estimate it by the 1/20 rule of thumb, it can easily be half or twice that. But the lower your ACH/50 numbers the less the ACHnat matters, and that's a GOOD thing, since the location of the natural ventilation is somewhat random, not necessarily where the ventilation is needed or best-applied.
Similarly the ASHRAE recommendations shouldn't be treated as gospel- a lot depends on the sources of indoor air pollution. If you're roasting a chicken over an open fire in you living room or playing poker with 5 stogie-puffing card sharks you probably want to triple or quintuple it. In a non-smoking household in a house built with low VOC materials/content, moisture can become the predominant indoor air pollutant, and if you're above 30% and below 50% RH, you're good. In homes with sub-1 ACH/50 numbers localized exhaust venting for bath & kitchen and ERV/HRV under dehumidistat control in winter can be a realistic and healthy option but probably not if you're prone to using aerosol spray products (cleaners/paints/adhesives, etc.), or if you are using combustion appliances that utilize conditioned space air as combustion air (atmospheric drafted hot water heaters or similar). Local active exhaust ventilation should be running whenever running gas or propane ranges/ovens are in use in any house, but critical for truly tight houses, but that's an intermittent use that shouldn't much affect your HRV/ERV duty cycle.
Don't OVERventilate to the point of dryness just to meet a spec designed to guarantee that the humidity isn't too high. About 0.25ACH/actual would be more than sufficient for most no-smoking households. See:
http://www.passivhaustagung.de/Passive_House_E/ventilation_and_humidity.htm
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FBBP
Veteran Member
Posts:1215
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| 04 Mar 2011 10:56 PM |
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Posted By Dana1 on 14 Feb 2011 01:42 PM
Posted By zehboss on 12 Feb 2011 12:06 PM
Hey,
The real question is, are you going to humidify the inside of the home? The HRV will help maintain the humidity in the home. HRVs are not required in non humid areas. Dry areas usually use ERVs. A well-built home should have an ACH/50 of less than .8 in my opinion. I recently build a zero energy home in Park City it tested at .3. I would hook up the fan again and finish sealing the house so it can work right.
Brian
Methinks you've got that backwards (the "return" pinky is faster than the proofread, mayhaps? ) ERV has the cores that swap humidity between air streams, HRV transfers sensible-heat only.
I'm with you on air-sealing to sub- 1ACH/50 levels on new construction. The difference between 0.6 (PassiveHouse spec) and 0.8 or 1.0 or 1.5 (R-2000 spec) is somewhat academic unless you're going for PassiveHouse levels of energy use. (Hitting 0.3 ACH/50is impressively tight though!) Even at R-2000 levels of tightness most dwellings would not require active humidification unless (over)ventilating mechanically.
Now I'm really confused!
The following from Venmar site:
http://www.venmar.ca/faq-hrv-erv.aspx
The ERV process is as follows:
In humid climates and air conditioned homes, when it is more humid outside than inside, the ERV limits the amount of moisture coming into your home.
In dry climates and humidified homes, when the humidity level is reversed, the ERV limits the amount of moisture expelled from your home.
From Passivhaus:
http://www.passivhaustagung.de/Passive_House_E/ventilation_and_humidity.htm
At the Passive house conference in working group II, Ruedi Krisi presented Enthalpy recovery ventilators, which recover humidity in addion to heat.
Those of us living in dry climates have trouble sustaining r.h. over 35%. Why would we not want to use a ERV so that we can ventilate at least a little to remove other pollutants without drying the air even more? |
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ecobuilder
Basic Member
Posts:102
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| 06 Mar 2011 08:49 PM |
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I have installed both types of systems in different houmes, here in MA. Here is what I have found from monitoring the homes performance of the last several years. The house with the ERV had high humidity, to the point were we needed to de-humidify to bring this down below 60%. The homes with the HRV's have been maintaining 30-50% RH through out the year. Both homes were less than 1 ACH/Hr @50. The thing I found was most important was control over the system, the ability to control the ventilation rate. While HRV's are typically less efficient than ERV's in their ability to recoup energy. This to me is a moot point. In tight homes I have never found them to be to dry, unless you are over ventilating. Turn the system down(back to controls) The ability to control the ventilation rate of the system will allow you to control the humidity to a point. I personally like the fantech units for the simple controls they offer. You can adjust fan speed(low,med,high) as well as set time of operation(15/Hr, 20/Hr,30/hr, cont). This gives you a large range of ventilation rates to work from. Of course this is just one mans opinion but one I have based on actual data. As with all of this stuff the wild card in the mix, are the occupants. How well do they understand the system and are they using it as designed. You can only hope! Tom Pittsley [email protected] www.eebt.org |
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