Posted By Clark on 22 Jul 2010 04:14 PM

Posted By Dana1 on 22 Jul 2010 03:00 PM
...  In relatively dry heating dominated climates HRV is a net-winner over ERV, but that wouldn't include IL. ...

But, I think it includes the northern two-thirds of Illinois, including Peoria, based on these recommendations from FanTech.

Like I said, the accounting needs to take longer term and a sharper pencil for places like Peoria or Worcester (where I live)- not likely to pay back in 2 years (about the limit for the average 'merican), but would probably still pay in under 10, possibly much sooner in high-price electricity areas (like mine).   The FanTech map is a "...do you need..." map, not a "...is it cost-effective in a 10 year net present value analysis..." map.

Do you NEED to go ERV to provide comfort at reasonable upfront & operating cost in  Rockford, Peoria or Worcester? 

Probably not.

But will it cost you more in winter heat loss than it gains in lowered dehumidification costs in those areas? 

Nope- the increased heating costs are "in the noise", hardly measurable outside of a lab, but the lowered dehumidification costs are several 10s of percent (if still quite modest.)

If you're a long-termer willing to make 10 year investments to break even,  the extra costs of ERV over HRV are worth it (if marginally) in my location (and yours), but never in the drier climates of the Rocky Mountains or Pacific Northwest, where dew points stay below 50 most the summer. The monthly average for Boise this July has been a dew point of 46F, in  Portland OR the average has been a dry 30F(!). In Pueblo, the July average has been 53F but in June it averaged 48F all of which doesn't add up to a huge annual latent load for maintaining 50-55% indoors at mid-70s indoor temps.  Of those random samples only Pueblo had any latent-load at all to speak of, but for Rockford's 62F July dew point average, or 60.5F June dew point average, there's plenty of longer-term payback in an ERV, if those are typical summertime averages.

For me it's typically 3-4 months out of the year that dew points break the mid-50s (August is usually the most-humid here.)  If you have 3 months of 55F+ dew point per year and dehumidify your  place to under 50%RH (recommended for those with dust mite allergies) it'll pay to go ERV.  If you're only holding the line at 60% RH/78F indoors, you can relax the dew point average up to 60F before it starts to pay.   At sea level a 60F dew point translates to ~55%RH @ 78F, but if 60F is the average, it means that going mere HRV would be adding latent load much of the time, but still less than half if you allow 60% RH indoor temps. 

If you bump that up, say let it get to 65% RH/78F indoors (the ASHRAE recommended limits). At that point  an ERV doesn't pay back until there are significant annual hours with dewpoints in excess of 65F. This is may be where the FanTech recommendation map works reliably.  But it sounds like you're keeping it 50%RH indoors or lower, and your energy use for dehumidification is dramatically higher than if you were running it at 65%RH.  This is because the number of hours when humidity must be extracted from the ventilation stream to keep it 50%RH indoors are an order of magnitude higher than keeping it at 65%RH in your area.

I love this site. Dana, you are a gentleman (or gentlewoman?) and a scholar.

Posted By galnar on 23 Jul 2010 12:02 PM
I love this site. Dana, you are a gentleman (or gentlewoman?) and a scholar.

Last time I checked I was still of the masculine gender...

Don't know about that "scholar" business though.

Posted By Clark on 22 Jul 2010 09:29 PM

 If I had to do it over, I'd opt for the ERV. 

Hold the phone!  I think the decision to install either an HRV or ERV, when it's not obvious which is the better choice, is going to be determined by the cost of electricity.  I just got done reviewing my last two electric bills for my ICF house.  During these two billing periods I was maintaining an indoor RH at <50% and an indoor temperature of 72F.  It actually felt chilly in the house.  Both electric bills were under $70 for a 30 day period.  Needless to say, I was extremely pleased.  It got me thinking what the savings I'd see had I installed an ERV. 

I don't know how much of that cost is due to dehumidification, but a quick calculation gave me a ballpark answer.  My dehumidifier consumes 6.9A x 110V = 760 watts and runs about 15 minutes each hour.  So each day it consumes 24 x .25 x .76 = 4.56 kwh of electricity.  At an average cost of $0.07/kwh it's costing me $0.32 a day to run the dehumidifier.  That's $9.60 a month.   I don't know how much of this cost would be eliminated by having an ERV, but, conservatively, say 50%.  That's a savings of $4.80 a month for 6 months or $28.80 per year.  If I ignore the heating season savings by having an HRV, and assuming the ERV costs $1000 more than an HRV, the payback is 34 years!  I'll be lucky to live that long.  I think I made the right choice. Do you agree, Dana?

If your assumptions are all correct, and the cost of the upfront cost of the dehumidifier didn't change by going ERV, and you anticipate the cost of electricity to remain flat or fall over the lifetime of the dehumidifer & ERV, you're right.

But the upfront cost equipment difference between an ERV vs HRV is 10-15%, not 50-100%, and the installations costs are identical.  It would have to be a ridiculously oversized ventilation system to add up to a to a grand- figure on a couple hundred difference (tops) for a 200cfm system.  Shop around and I'll bet it's only $100 more upfront to go ERV for a similarly sized system.

And you might only average a 35-40% reduction in total dehumidification costs.  (Kinda depends how much you run your shower or boil your food when it's sticky outside.)

It might knock a fifty to hundred off the dehumidifer though if you can bump it down a single size within a vendor's model offerings.

Or a HECK of a lot less if it means you can then use a tiny standalone dehumidifier in the coldest room of the house (typically a basement, in summer) to manage the RH of the entire house, skipping an in-line "whole house" version on the ventilation system (which works for us, dehumidifying ~3000 sf of conditioned & semi-conditioned space within the pressure boundary of the house in a roughly comparable climate.)

Electricity prices aren't likely to stay flat or fall for the next two decades either, IMHO, but I've been wrong on long term energy pricing several times since the early 1970s- could be wrong again.  (If carbon gets taxed or we have to build nukes or large scale wind/solar to support the grid, anticipate at least a doubling of inflation adjusted cost over then next 2 decades- none of that stuff is cheap.)
 

Posted By Dana1 on 25 Jul 2010 07:59 PM

... But the upfront cost equipment difference between an ERV vs HRV is 10-15%, not 50-100%, and the installations costs are identical.  It would have to be a ridiculously oversized ventilation system to add up to a to a grand- figure on a couple hundred difference (tops) for a 200cfm system.  Shop around and I'll bet it's only $100 more upfront to go ERV for a similarly sized system...
 

I have shopped around and what I've found is that for a reasonably well-built unit, a DIYer is looking at from $800 to $1100 for an HRV and from $1100 to $2000 for an ERV.  A $300 differential within a product line is common.  HVAC contractors can get better prices, but they typically mark them up in the job price.  The reason I said that the price differential is around $1000 is that most, if not all, manufacturers will not offer a limited lifetime warranty on the ERV's enthalpic core as they do for an HRV's core.  I think the reason has to do with the durability of the materials used in the respective core's manufacture.  An enthalpic core is made from a permeable membrane (plastic? or paper?), while the HRV core is usually aluminum. That tells me that over an extended period, I'm probably going to have to replace the enthalpic core one or more times, especially in a freezing climate where the membrane deteriorates over time due to hundreds of freeze-thaw cycles.  When the core breaks, you have to replace it.  The cost of a new enthalpic core is around $300-$500, again, depending on the brand.  A couple of core replacements over the years and payback becomes an elusive dream.

There's another matter that should be considered when deciding between an HRV and ERV.  In my ultra-tightly built house, the accumulation of indoor humidity in the winter was a real problem.  The HRV does a good job of maintaining the RH at the desired setpoint.  The ERV feature of reclaiming moisture from the exhaust air makes it less effective in reducing wintertime indoor humidity.  I'm assuming an ERV will have to run longer to acheive the same winter time dehumidification performance as the HRV, further reducing the heat recovery efficiency of the ERV.  IIRC there's at least one ERV on the market which features a bypass mode that vents exhaust air around the enthalpic core to mitigate this problem, but, probably, at a higher equipment cost.

VanEE makes an HRV that can be converted into an ERV by swapping cores.  You can swap the cores to coincide with the core cleaning schedule and get the best of both worlds, but again you're talking a higher initial equipment cost.

I think that the above referenced FanTech chart that recommends an HRV for the northern 2/3rds of Illinois takes all of these factors into consideration and, in my (wavering?) opinion, one would be wise to follow their recommendation.

This whole discussion has been extremely helpful to me in learning about IAQ and the means to acheive it.  I'm sure the debate will go on as there is no one right answer.  The important thing is to make an informed decision and go from there.

Even assuming a $300 differential, within the lifetime of the core (assuming most of them last at least the typical 5 year warranty), you're still money ahead at national-average electricity rates when you're maintaining 50%RH in place with average summer dew points in the high 50s or higher.

ERVs are not SO effective at returning moisture during the winter that keeping RH under control via ERV alone is difficult. It would come at the cost of a slightly higher ventilation rate, with an associated increase in heat lost, but the cost of that extra heat loss is "in the noise"- measurable if you instrument it carefully, but not a relevant boost in monetary terms. Winter dew points are quite low, and the wintertime moisture sources are primarily from human activity, unlike the summer when a large fraction is from the outdoor ventilation air (that increases linearly with ventilation rate.) High wintertime ventilation rates with HRV can make indoor air excessively dry (<30%), but in your climate that won't be as much as of a problem at minimum-recommended ventilation rates as it can be in Manitoba or Saskatchewan.

We don't all live in labs, but actually measuring the power use of your dehumidifier (with an in-line watt-hour meter such as a Kill-a-Watt or similar) over a few months, and see how it correlates with outdoor dew points can be useful. I still think the FanTech map makes sense at 60 or 65% RH limits on interior air, not so much at 50%

Dana:

Are you familiar with hygroscopic icf materials that naturally regulate humidity and eliminate/compensate for humidity fluctuations? I have built a few and find the home owners never have humidity control issues with or without ERV/HRV.