joe.ami
Veteran Member
Posts:4377
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| 19 Mar 2010 12:32 PM |
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Not sure why it hasn't come up for me yet, but this week I had 2 different log homes I had to do some calcs on.
In the first I had to help a friend comply with the energy code to get his C/O, with the R values I finally gleened, and the help of Res check, I was able to get him a compliance certificate.
For the second, I'm bidding geo in a log home and need to calculate the load. Energy trade-off documentation is one thing, but sizing equipment requires more than guess work.
It would appear that logs have ~.7 to 1.5 R/inch of thickness, clarification of which thickness is a little more ambiguous. In other words if I have a 10 inch swedish coped log (with 6" coping) do I use the 10, the 6 or the difference.
When you discuss this with log home folks they talk about thermal mass to distraction and have hunches and studies but no easy to use number. Just give me an R-value!!!
Seller of the home kit suggested R-16 but I still wanted indepent confirmation.
When I called my old friends at HVAC calc, low and behold their update has logs!
So I ran the calc with the log choice and found the btu results within 3% of R-15.7 (the closest I could come to R-16).
So there's my confirmation, but does that mean logs are worth 1.6 times the log diameter?
Joe |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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tinoue
New Member
Posts:96
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| 19 Mar 2010 01:22 PM |
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They're going to get themselves into trouble during periods of extreme temperature as the thermal mass ceases to help, but in general, the thermal mass will certainly moderate the loads.
1.6x log diameter seems pretty generous. And if they're taking full credit for the diameter, they're being even more generous. I certainly wouldn't want to install a geo system in there sized with a straight manual J calc.
Don't get me wrong, seasonally adjusted energy usage is certainly much lower, but that doesn't have anything to do with peak load requirements.
Here's an interesting research report
http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal/results.html
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geome
Advanced Member
Posts:987
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| 19 Mar 2010 03:06 PM |
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Here are some select web sites (there are a lot of them). Hope this helps.
http://74.125.93.132/search?q=cache:4-nbFpikzDwJ:www.logassociation.org/resources/reprint_thermal.pdf+r-value+logs&cd=6&hl=en&ct=clnk&gl=us
http://www.energysavers.gov/your_home/designing_remodeling/index.cfm/mytopic=10170 |
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| Homeowner with WF Envision NDV038 (packaged) & NDZ026 (split), one 3000' 4 pipe closed horizontal ground loop, Prestige thermostats, desuperheaters, 85 gal. Marathon. |
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engineer
Veteran Member
Posts:2749
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| 19 Mar 2010 05:18 PM |
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I'd be inclined to split the difference between average thickness of 8" and what they claim. I suppose that as wood dries its R-value rises, but it is also somewhat species-dependent.
The thermal mass makes quite a difference for a typical cold night, but the effect wanes during a multiday cold snap.
My own experience with thermal mass is as follows: I use a design temp of 30, but even during the January snap where we had hard freezes 10 nights in a row, the average temp a high mass wall 'sees' is still above 30, so my 038 held the line even locked in Y1, though I had multiple 16+ hour daily runs and one over 20 hours. I further hobbled it by pinching back the open loop flow so as to fill a pool with 40 degree water for snowmaking - I ran well below 1 GPM / ton, EWT being 71.
We had no complaints from ICF / geo customers, some of which have up to 1500 SF / ton, and none have strips.
I don't think wood offers as much thermal mass as ICF, and ICF buries the mass in insulation on both sides, further delaying the 'soaking' of heat through the walls. It might be worth running the numbers.
For the log home I'd be a bit conservative as to R-value, but I think I'd cover myself for the rare event of multi-day cold snaps only with extra strips, not let the issue drive up the tonnage at all.
Do the construction methods guarantee a low and reliable infiltration even over time as logs dry shrink, twist and split?
There's a high end log home 10 doors down from me, and I'm glad its there as I expect it to serve the needs of the entire zip code's termites...
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Curt Kinder <br><br>
The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
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joe.ami
Veteran Member
Posts:4377
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| 19 Mar 2010 06:58 PM |
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The aux. coil always hedges the bet, the other helper was HVAC calc who's only horse in the race is liability if I entered the data correctly.
J |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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tinoue
New Member
Posts:96
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| 19 Mar 2010 08:01 PM |
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Posted By joe.ami on 19 Mar 2010 06:58 PM
The aux. coil always hedges the bet, the other helper was HVAC calc who's only horse in the race is liability if I entered the data correctly.
J
Absolutely, that's a very good practical solution that can work great if the load calc is pretty close. |
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jonr
Senior Member
Posts:5341
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| 20 Mar 2010 10:00 AM |
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One log house I'm familiar with was horribly leaky.
You could just measure the heat loss (vs trying to estimate it).
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joe.ami
Veteran Member
Posts:4377
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| 21 Mar 2010 09:21 AM |
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Posted By jonr on 20 Mar 2010 10:00 AM
You could just measure the heat loss (vs trying to estimate it).
I'll bite. How do you propose to do this (in a way that satisfies a code that specifically calls for a man. J calc.)? j |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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jonr
Senior Member
Posts:5341
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| 21 Mar 2010 10:17 AM |
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One could heat the house with electric heat for some period (a day?) and measure both indoor and outdoor temperatures and the heaters kwh usage. Or do something similar with an existing heating system. Then work the calculations backwards so that an accurate Manual J has been done.
More accurate - yes. Practical - maybe. |
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engineer
Veteran Member
Posts:2749
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| 21 Mar 2010 10:26 AM |
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I don't share jonr's confidence in the usefulness of measuring heat loss for a day. I agree that it could be done fairly accurately, but the day chosen for this exercise will have unique conditions of sun and wind that have significant impact on heat loss.
If the log home is leaky, wind differences between the day chosen for measurement and other days will have an even greater impact.
Then there's the fact that it is unclear to me whether log house #2 has yet been built - one needs a Man J or something similar before a building permit is issued in some jurisdictions.
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Curt Kinder <br><br>
The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
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joe.ami
Veteran Member
Posts:4377
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| 21 Mar 2010 11:33 AM |
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Posted By jonr on 21 Mar 2010 10:17 AM
One could heat the house with electric heat for some period (a day?) and measure both indoor and outdoor temperatures and the heaters kwh usage. Or do something similar with an existing heating system. Then work the calculations backwards so that an accurate Manual J has been done.
More accurate - yes. Practical - maybe. Practical- no. I am bidding from blue prints, where shall I put the heater? On the floor plan or the elevation page? ;) More accurate- no. Someone as well researched as you should see the benefit of a 20 year weather average employed as part of a calculation vs a one day test. FWIW What you propose is already done to some degree. When I bid on an existing home, I have clients share their utility bills with me (after calculations are performed). The accuracy of both my load and operating cost software is demonstrated time and again. j |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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jonr
Senior Member
Posts:5341
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| 21 Mar 2010 11:35 AM |
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You are missing that it measures and accounts for the temperature aspect of weather (the primary factor) - ie, it ends up with btu per degree day from the house, not btu/day and still uses manual J for historical data - more data would have very little effect on accuracy. Wind (or more accurately interior pressure differentials) isn't a big effect but can also be easily measured (or just make sure that the measurement period was close to average). Agree on solar gain - just using night data would eliminate that issue (and some human activity issues). Agree on houses that aren't built yet (obviously - those are "plans", not "homes").
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joe.ami
Veteran Member
Posts:4377
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| 21 Mar 2010 12:29 PM |
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"ie, it ends up with heat loss per degree day from the house, not btu/day - more data would have very little effect on accuracy."
Your test shows a specific requirement for a specific parameters. Extrapolation is not as easy as you think. Thermal mass comes into play at night (when you suggest your test be performed), which is very much affected by sunshine (or lack therof) in the preceeding day.
" Wind isn't a big effect but can also be easily measured..." Really? Weren't you the one who said that (paraphrased) the only log house you were in was very drafty? How do you propse to measure the forced ventilation (required by code)?
"(or just make sure that the measurement period was close to average)."
So in sum;
More data adds little accuracy
Guess what average wind would be
Calculate the impact of ventilation
Disregard solar benefit and thermal mass
Extrapolate into peak load based on X
Or:
KW/heating degrees + calculated ventilation load + calculated thermal mass and solar benefit X weather average
Is that right? I'm having a hard time wrapping my mind around this "simple" test.
Wouldn't it be easier to use time tested and proven calculation standards based on 20+ year weather averages?
Don't pick this hill to die on my friend. Your proposed test begs more questions than it answers.
You may have been typing during my previous response, but in existing homes we are able to compare our calculations to existing appliance consumption. So to a limited degree we at least ratify our calculations with operating equipment.
J |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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engineer
Veteran Member
Posts:2749
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| 21 Mar 2010 09:32 PM |
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You and I are on the same page, at least. I correctly guessed that one of the log homes does not yet exist. The Feds have carefully compiled decades of weather data which the load software boffins have equally carefully buried in their Man J load calculators, why should any of us attempt to reinvent that wheel?
I too make a point to ask potential customers, friends, relatives, neighbors in need of energy aid to produce a couple years' utility bills. Some folks look at me oddly and sometimes i can feel the privacy vibe at work, but utility bills are crucial toward assessing the circumstances of each customer's situation.
The metaphor of a hill to die upon is priceless - In the spirit of engineering I shall salt it away and trot it back out at appropriate moments, assigning credit where it is due if I think of it...thank you! |
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Curt Kinder <br><br>
The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
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Paul Auerbach
New Member
Posts:88
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| 22 Mar 2010 09:32 PM |
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Joe, log homes are leaky (chinking has a tendency to shrink over time) and you need to account for that in your calculations.
Paul
Total Green |
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joe.ami
Veteran Member
Posts:4377
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| 23 Mar 2010 09:00 AM |
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Most envelopes deteriorate in time. I cover that in my contracts, not my calculations.
IOW......"assumes proper maintenance of structure to ensure........"
j |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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jonr
Senior Member
Posts:5341
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| 24 Mar 2010 08:42 AM |
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Solar heat storage of exposed logs (no glass involved) is a very minor (couple % max) factor in the midnight to dawn time frame.
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joe.ami
Veteran Member
Posts:4377
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| 24 Mar 2010 10:08 PM |
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Posted By jonr on 24 Mar 2010 08:42 AM
Solar heat storage of exposed logs (no glass involved) is a very minor (couple % max) factor in the midnight to dawn time frame.
I'm not sure if you are still chasing parked cars (re your "test") or commenting on thermal mass. I make no assertions regarding thermal mass of log homes (or any for that matter), my calcs are based on manual J. j |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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engineer
Veteran Member
Posts:2749
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| 25 Mar 2010 11:26 PM |
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Your Man J software may account for thermal mass for you when you tell it wall composition.
It has been awhile, but I recall substituting sticks for ICF walls keeping same R-value and infiltration rates and the load on the subject house promptly jumped up a ton according to the software.
A 2x6 studded stick wall assembly can match ICF for R-value and infiltration if really well detailed, but it does NOT perform anywhere near the same in real world, day-in, night-out operation. Thermal mass spreads the load from the peak hours to the non-peak hours. |
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Curt Kinder <br><br>
The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
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