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Heat Loss Calculation

KestrelKestrel Member Posts: 102
Hello all - fantastic site you guys have here - immensely informative, nice community - a really great find.  I'm a newbie to this, not a professional in the heating/comfort industry(a scientist/doctor type, actually), but I love learning and am pretty handy, and I'm trying my hand at designing a radiant hot water system for my house.  I've got Siegenthaler's book (and have been doing the problems!), and I've been reading the Idronics semiannual publications, reading the installations manuals for Triangle Tube and Viessmann modcons, essentially reading reading reading all I can.

I'm starting out with a heat loss calculation, as advised.  I'm wondering if any have come across this site:

It seems to actually calculate UxAxdT, and takes into account different building materials and windows and such, as well as air infiltration, and I'm getting numbers that seem to make sense.

I have a 105 year old frame house in Seattle, some insulation and window upgrades, but we still have some big singe-pane bay windows with that cool old wavy glass that I just don't have the heart to rip out - however they leak heat like a sieve.

(Presently we have gas forced air, the plant at the end of its life and needing replacing.)  The house has about 700 sq-ft on the first and second floors, and the same size heated basement. 

From the site calculator, I'm getting a total heat loss of 46870 BTU/hr.  When I look at my gas bill over the past year, the biggest monthly use was 7.5 therm/day this past February - or 31250 BTU/hr on average.  I went back and looked at daily temps around here that month, and on only two days did we get down to the design temp of 20'F.  On average we were 38.3'F for the lows - or about 18.3'F higher than design temp.  I took the dT at design (68-20) and the dT that month (68-38.3) and ratio'd them and got 0.62.  Dividing the ave BTU/H that month (31250) by this ratio (0.62) and I get 50400 - which agrees pretty closely with the prior calculation.

Am I on the right track?


  • KestrelKestrel Member Posts: 102

    That should be...700 sq ft on each the first and second floors, as well as the basement - for a total of 2100 sq ft.
  • Brad WhiteBrad White Member Posts: 2,393
    Seems to be

    within range, 22.3 BTUH per SF in your climate.

    If you have three years worth of gas usage tabulated monthly, we can back-calculate a bit (a form of triangulation if you will.)

    The first thing you do is find your three lowest months of gas usage. This will approximate your "base load" for cooking, domestic hot water, clothes drying and, yes, being outdoorsy, THE GRILLE. :)

    Subtract the average of those three numbers from all months in a given year, that will get you your net "heating only" therms.

    Do that for all three years.

    With a little algebra and data for your location from "", we can back-calculate an approximate heat loss (with some assumptions granted). Something to compare to your calculated value.

    This exercise can help you nail down less quantifiable heat loss segments such as infiltration (often a guess but a widely variable one).

    As for the wavy glass, I am with you! But good exterior storm windows if you can tolerate them, will do 80% of what a replacement window will do for one fifth the cost. Such a deal...
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • ChrisChris Member Posts: 3,056
    edited May 2011

    You are probably more qualified in the design of a heating system then the majority of contractors in the industry. Keep up the knowledge as it will lead you to the best decisions. Keep one thing in mind as you proceed. If your going to hire a contractor be prepared to be scared by what walks through the door. Be patient and you will find the right guy for the job. A good contractor will listen and work with you, not against you. At the same time listen and don't come across as knowing more. Use the knowledge to make decisions not to undermine.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Jean-David BeyerJean-David Beyer Member Posts: 2,530
    old wavy glass

    That glass can probably be saved. Two examples.

    Some years ago, the CEO of Sea Containers Corp (a major shipping company) wanted to restore The Orient Express train to its 1920s glory and coughed up about $16 million to do it. Some of the windows were hand made stain glass and they  wanted to preserve them. Fire code insisted that the windows be laminated safety glass. What they did was make a sandwich of safety glass, stained glass, and more safety glass that would pass inspection.

    On a less dramatic scale, the Quaker Meeting I attend has an almost 200 year old building. Not only do the windows leak heat as if they were almost not there, they lose panes because the wood is dry rotted so bad glaziers points will not stay in there long enough to get the putty on. Our building is classified historical, so there are some things we cannot do (like put in thermopane, vinyl windows, etc. We have an historical qualified contractor who fixed all the wood by impregnating it with an epoxy mixture.  These windows are much larger than standard. He also found a souce for old glass to replace missing pieces. He then custom made storm windows with wooden frames and clear plastic that looked historical, and it now costs somewhat less to heat the building.

    If money is no object, you could probably use one of these techniques. I would guess the custom storm windows would be the cheaper way to go.
  • KestrelKestrel Member Posts: 102
    Data from my gas company

    Brad - thanks so much!

    The data I can get from my gas company only goes back two years.  What I can get is total use, therms/day, ave temp and heating degree days.  It is as follows (I hope this stays in register by just typing in):

    Month     therm/day      Ave temp     HDD

    5/2009      2.97                 54             303

    6/2009      1.14                 64               39

    7/2009      1.13                 68               21

    8/2009      0.76                 69               14

    9/2009      2.36                 65               40

    10/2009    3.93                 53             345

    11/2009     4.85                48              486

    12/2009    6.76                 40             741

    1/2010      4.69                 46             626

    2/2010      4.87                 47              528

    3/2010      4.69                 49              462

    4/2010      4.92                 51              447

    5/2010     4.47                  54               294

    6/2010      2.50                 59              179

    7/2010       1.21                 65               59

    8/2010       1.14                67                21

    9/2010       2.36                62                83

    10/2010     4.02                55               264

    11/2010     6.25                 47              514

    12/2010    6.75                 44               666

    1/2011      7.26                 42               669

    2/2011      7.66                 40               723

    3/2011     6.69                  44               637

    4/2011    6.43                   47               522

    The baseline (ie summertime) use seems to have gone up some - which I attribute to my older child going from 13 to 14 and discovering hygiene!  Gawd those sowers can be long!

    Subtracting out about 30 therms/day for baseline (hot water and gas rangetop - alas the grill is propane, though in use all winter!), and then ratioing to HDD - one gets a pretty constant 0.264 therms/day/HDD, at least in winter - it's much more variable in summer, with less actual heating use.
  • KestrelKestrel Member Posts: 102
    trying to learn...

    Thanks for the vote of confidence, and the advice!

    I'll be back with my thoughts on design soon - hope your confidence is rewarded!
  • KestrelKestrel Member Posts: 102
    Glass options

    I'll head right down to the 7-11.  MegaMillions drawing is tomorrow, IIRC.  ;)
  • KestrelKestrel Member Posts: 102

    That last paragraph should read...

    Subtracting out about 30 therm/MONTH for a baseline...

    and ratio of therm/month/HDD yields 0.264 for the heating season.

  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    Which of the following weather stations

    best reflects where you live?

    On, I can (you too can) get a comma delimited Excel file for any range available, historically.  Good record keeping so far that you did, nothing to subtract from that! We can get you a reasonable number to compare, I believe.


    KBFI (airport): Seattle Boeing, WA, US (122.30W,47.53N)KRNT (airport): Renton, WA, US (122.22W,47.49N)KSEA (airport): Seattle, WA, US (122.31W,47.45N)KPAE (airport): Everett, WA, US (122.28W,47.91N)MSEAOL: MesoWest SEATTLE - OLIVE AND BOREN WA US WAAQ, Seattle, WA, US (0mi/0km)MEBSW1: NOS_NWLON Seattle, WA, Seattle, WA, US (0mi/0km)MAU074: APRSWXNET Seattle WA US, Seattle, WA, US (0mi/0km)KWASEATT154: Herrera Environmental Consultants, Seattle, WA, US (0mi/0km)KWASEATT165: Capitol Hill, Seattle, WA, US (0mi/1km)KWASEATT169: Capitol Hill, Seattle, WA, US (0mi/1km)KWASEATT116: Queen Anne - Southern Exposure, Seattle, WA, US (1mi/2km)KWASEATT110: Volunteer Park, Seattle, WA, US (1mi/2km)KWASEATT179: Queen Anne, Seattle, WA, US (2mi/4km)MUWASH: MesoWest UNIVERSITY OF WASHINGTON WA US WAAQ, Seattle, WA, US (2mi/4km)KWASEATT155: West Seattle (Two Dogs Observatory), Seattle, WA, US (2mi/4km)KWASEATT109: University District, Seattle, WA, US (2mi/4km)KWASEATT167: Wallingford, Seattle, WA, US (2mi/4km)KWASEATT14: Fremont, Seattle, WA, US (3mi/5km)MSEADU: MesoWest SEATTLE DUWAMISH EAST MARGINAL WA US WAAQ, Seattle, WA, US (3mi/5km)KWASEATT57: Ballard, Seattle, WA, US (3mi/6km)KWASEATT142: Magnolia - MaxHome, Seattle, WA, US (3mi/6km)KWASEATT108: Magnolia, Seattle, Seattle, WA, US (3mi/6km)KWASEATT58: SE Green Lake, Seattle, WA, US (3mi/6km)KWASEATT119: University Park, Seattle, WA, US (3mi/6km)KWASEATT153: Ballard - West Woodland, Seattle, WA, US (3mi/6km)KWASEATT17: Phinney Ridge, Seattle, WA, US (4mi/6km)KWASEATT147: Georgetown, Seattle, WA, US (4mi/6km)KWASEATT107: Roosevelt - Tomster, Seattle, WA, US (4mi/6km)MAS437: APRSWXNET Seattle WA US, Seattle, WA, US (4mi/7km)KWASEATT148: Ballard, Seattle, WA, US (4mi/7km)KWASEATT96: Ballard, Seattle, WA, US (4mi/7km)KWASEATT152: Near Discovery Park, Seattle, WA, US (4mi/7km)MD6665: APRSWXNET Seattle WA US, Seattle, WA, US (4mi/7km)KWASEATT122: Beacon Hill, Seattle, WA, US (4mi/7km)KWASEATT180: Bryant, Seattle, WA, US (4mi/7km)KWASEATT12: Bryant, Seattle, WA, US (4mi/7km)

    EDIT- Sorry, when I first pasted them in, they were stacked line by line. I apologize for the wall of text!
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • Jean-David BeyerJean-David Beyer Member Posts: 2,530
    Therms per day...

    I have all the gas bills my house ever used (almost two years supply). There is just one of me, and I am not a teenager. Gas is used only to run my mod-con boiler that has an indirect fired hot water heater attached. Hence it runs all year. I shower almost every day, and do laundry on average every three days. Front loader washing machine. I have a diswasher that I use once or twice a week. So not all that much hot water. In the summer I use 4 to 5 therms per month. The peak this winter (in New Jersey) was 99.09 therms for January 19 through February 18. The peak the previous year was 125.72 therms from December 16 through January 19. I am not sure I had my outdoor reset curves set up all that well yet as it was my first heating season. It does not seem the 2009-2010 heating season was colder than the 2010-2011 heating season. So I guess the difference was that outdoor reset makes a lot of difference, if it is adjusted right.

    Degree days around here  52F setting for each month. I do not need much heat when it is hotter than that outside because my computers and other heat generated, or coming through the closed windows seem to provide most of what I need.

    10/01/09    81

    11/01/09    103

    12/01/09    522

    01/01/10    636

    02/01/10    593

    03/01/10    234

    04/01/10    77

    05/01/10    20

    06/01/10    0

    07/01/10    0

    08/01/10    0

    09/01/10    2

    10/01/10    54

    11/01/10    228

    12/01/10    684

    01/01/11    764

    02/01/11    486

    03/01/11    353

    04/01/11    110
  • Mark EathertonMark Eatherton Member Posts: 5,582
    Let me introduce you to my leetle frend......

    Paul Pollets.

    Based on what I've read so far, you want to do it RIGHT.

    Paul only knows HOW to do it one way, and that is RIGHT!

    You will not regret making contact.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • KestrelKestrel Member Posts: 102
    Lots of data!

    I'm nearest this one:

    KWASEATT116: Queen Anne - Southern Exposure, Seattle, WA, US (1mi/2km)

    The other Queen Anne site is closer, but apparently very new and has little data associated with it.

    I looked at several other sites in the vicinity and they all seem pretty close.  What's interesting is that the HDD data from the gas company is much more variable - I suppose they're averaging over a larger area - because their data seems close to my station's at 62-65' as the baseline, but their highs are higher and lows lower.

    Anyway, how do I translate the HDD from my local station into a heat loss estimate?

    I was also thinking that if my present furnace is very inefficient (about 20 years old and not well serviced - my bad, I've learned!), then the gas use data is going to overestimate what I would need with a more efficient system - right?
  • Brad WhiteBrad White Member Posts: 2,393
    OK you asked....

    "Anyway, how do I translate the HDD from my local station into a heat loss estimate?"

    OK, here it comes. You asked for it :)

    Seems you have already downloaded your data? Good. We want to match that as closely as we can with the therms you used for heating. I do this for each month then add them up per year. Make sense? Sort of evens-out the spikes too.

    The Heating Degree Day (HDD) formula is set up like this, normally used when you already have a known or calculated heat loss (BTUH) and knowing the temperature difference at which that was calculated. You will likely have to guess at your annual system efficiency. Once done, there is an empirical adjustment factor called the Cd factor. This accounts for internal gains, solar gains, setback savings and a host of other factors. A typical value for 5,000 annual HDD is about 0.60. Can vary from 0.55 to 0.75. I will use 0.60 in my example.

    Here is the formula:

    (BTUH x HDD x 24)/ (Delta-T x Eff. x Fuel Heat Value) x 0.6 = Net Fuel Usage

    With "plug-ins", for example only, here is a set-up for you, not necessarily using your numbers: The "24" in the numerator is for 24 hours in a day, by the way. (The 100,000 in the denominator is for "BTUs per Therm of Gas". Were you using propane you would use 91,800 BTUs per gallon or if #2 oil, 139,500 BTUs per gallon. You get the idea). The "55 is a presumed but not verified delta-T. So fill in your own numbers of course. The 0.65 is just a guesstimate of efficiency. I have no idea.

    (40,000 x 5,000 x 24) / (55 x 0.65 x 100,000) x 0.60 = 805.6 therms

    Now the fun part! You know your Heating Degree Days. You know your Therms. You still will guess at your efficiency (and can tweak it later) as you can for the Cd factor. But by using Algebra, you can back-calculate to solve for your unknown heat loss, what we presumed as the 40,000 number.

    So the set up is the same but in reverse. Multiply your therms by the 0.60 Cd factor, then by your fuel unit BTU number, your efficiency and your delta-T. Then divide these by your HDD number and your 24 hours number and see what you get.

    I have taught elemental engineering to architecture students but I am totally no math teacher.. but have at it, and see what you get.

    A spreadsheet can make this easier of course. Within that you can adjust the "what-if" factors of system efficiency and Cd factors. This way you can "tune" the outcome by those variables, the others being more or less sound.

    Does that all make sense?

    I need lunch.

    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • Brad WhiteBrad White Member Posts: 2,393
    I second the motion

    I love Paul's work and his systems.

    Almost Zen-Like in simplicity.

    Paul is a thinker and melds brain, science and heart.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • KestrelKestrel Member Posts: 102
    After you finish lunch...

    To clarify, because I'm getting some screwy numbers.

    The HDD is dependent on a temp chosen as a baseline, right?  So if I pick 68', I'll get values based on that, and the 68' reflects the interior temp that were burning BTUs to achieve.

    Also, the delta T is based on this interior temp, right?  A question, the delta is between interior temp and average exterior temp?

    So I totaled my therm use for the past year from the gas company.  I averaged the 12 monthly average temps over that year, also provided by gas company, subtracted this from 68 to get dT.  Or have I made a big mistake?

    I'm using 1715 therms for the past 12 months (from my bill).

    HDD from at 68' of 6294 for the past 12 months.

    Average daily temp of 52.17 (from gas bill), therefore dT=68-52.17=15.83

    Efficiency of 0.70 and Cd of 0.70.

    From these I get BTUH of 8806, which seems 'off' by a factor of about 5-6, which seems bigger than subtle tweaks to efficiency or Cd would correct - seems like something fundamental is off in my numbers.
  • Brad WhiteBrad White Member Posts: 2,393
    HDD Baseline

    The baseline for HDD in residences is typically 65F. This is actually a bit outdated as it reflects construction that is a generation old. It is based on the notion that you will be "spotted" between three and five degrees due to internal gains, before you will need to add heat.

    In modern construction, SIP or foam panel construction, this "balance point" can be in the 50's. I have seen it in the mid-high 30's sometimes. But 65F is the basis. I get from the same site 5132 HDD for 2009 and 4810 for 2010, an average of 4971.

    Keep in mind that this is an annual event, so use the full year.

    I will have to pick this up on the other side of a meeting I am now leaving to. Will check back later.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • KestrelKestrel Member Posts: 102
    I think I got it!

    Took a bit - I guess the neurons are stiffening - ugh!

    So, for Feb of this year


    HDD 680

    Ave T is 40, Ave low T is 33.5

    I calculated BTUH for dT of 25, 30 and 35, and furnace efficiencies of .75 and .7

    75% Efficient

    dT               Cd=.6           Cd=.7

    25              44041           37750

    30              52849            45298

    35              61657           52771

    70% Efficient

    dT               Cd=.6           Cd=.7

    25              41105            35232

    30              49325            42278

    35              57547            49325

    So, the data I derived from the site, using data I entered for all the house wall/roof areas, window area, respective R values and insulation, and basement slab dimensions, at design T of 20 degrees and 4700 HDD per year, gives 51825 BTU/H.   This seems pretty correct - yes?
  • KestrelKestrel Member Posts: 102

    I will have to look him up.
  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    Excellent work!

    I tend to work the numbers on an annual basis to even-out any anomalies over the year. Just a different way to think about it. The only down-side to focusing on monthly totals is that, for example, degree-days aside, November often shows higher solar gains versus temperature. (Moderate outside temperatures and more direct sunlight to vertical glass at a lower solar angle, as one example).

    Not a huge deal though. By your reckoning, you have a solid grasp of the numbers, the values and variables. 

    If you have ever learned navigation with a sextant (way before GPS! And years ago for me...), you take three fixes, plot them and accept your location as the center of that triangle. I think you can do that, grasshoppa.

    But DO try it over an entire year or two, just to see the effect of the total year. I would be interested in what you derive.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • KestrelKestrel Member Posts: 102
    Re: solar gains

    Direct sunlight?  What's that?  :)
  • Brad WhiteBrad White Member Posts: 2,393
    Direct Solar

    It is one of those quirks of geometry.  Solar gains to a surface and glass in particular are higher the closer to perpendicular the angle is. An oblique angle will reflect more, an angle approaching perpendicular will cause more to be absorbed through the glass.

    To illustrate:

    In the mid-day hours in June (the 21st to be extreme), the sun is at its highest in the northern hemisphere. A little bit of overhang will block much of it on the south side of a building.

    Come November, this angle is lower in the sky so the gains are higher. (We actually have to air condition, cool, some spaces especially in commercial buildings because of this.) Now, in December, the angle is lower still, but the outdoor temperatures tend to be colder then, to cancel out some of the gains.

    Consider that at the 40th parallel, raw solar on glass can be over 200 BTUs per Hour per SF of glass and with absorption and reflection, shading taken into account, is still often over 100 BTUH/SF. A room with a lot of glass can meet or exceed its heating requirement.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • KestrelKestrel Member Posts: 102
    Re solar gains

    I was making a little joke about 'Huh, solar?' as I live in Seattle, where we have no sunshine, direct or otherwise.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,530
    No sun in Seattle.

    I was taken on a tour of a house just outside of Olympia about 35 years ago  that was almost built. It was designed for solar heating. The roof was two layers. The outer layer was some kind of transparent plastic, then an airspace, then some black stuff (tar paper?), then the stuff like plywood. Air was the transfer medium so there would be no problem with freezing. Little blowers moved the air and when there was excess heat, it was blown down into the sub basement. There was a big sub basement that was all crushed rock. The architect calculated how much rock would be needed based on all the solar data he could get from the weather bureau. He then put in 4x as much. This was not intended to store up enough for an entire winter, but just enough to get over the nights and very obscured times. When there was not enough heat coming in, the blowers blew the air from the house through the crushed rocks. He was pretty sure (his reputation depended on this) this would work. It did not depend on visible sunlight; he just needed enough infra-red to come through the clouds. The zoning types demanded supplementary heat, so he stuck a dedicated electric water heater in to some baseboard heater units. He did not think it would be needed, but it was there. I never met him again, so I do not know how it worked out.
  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    Solar Gains -Duh on my part!

    My solar point was over-stated and ill-advised given your location!

    Agreed, Seattle is not the Solar Capital of the Universe, no more than Tucson is known for excellent coffee!
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • KestrelKestrel Member Posts: 102
    Annual Calculation

    So, thermal master, these are my ciphers....

    month         T ave        Therm-corrected         HDD        delta T           BTUH

    April '11         47                  156.5                   572           20                 26308

    March            44                  177.3                   592           23                 33117

    Feb               40                   199.8                   680           27                 38140

    Jan               42                   187.9                   685           25                 32969

    Dec 2010      44                   193                      647           23                 32985

    Nov               47                   151.3                   601           20                 24206

    Oct                55                    86.6                    351           12                 14234

    Sept              62                    45.5                    162             5                   6751

    Aug               67                      4.3                     130            1                     159

    July               65                      8.9                     145            2                     590

    June              59                     44.9                    247            8                   6991

    May               54                    99.7                     382           13                16312

    April               51                   127.6                    456           16                21525

    What this is telling me, I think, is that the 20'F design day BTUH of 51825 is probably pretty close to the mark (calculated on the website)

    Also, that my inclination to use a TriangleTube Solo 110 or a Viessmann 200W-26 would likely be appropriately sized.

  • KestrelKestrel Member Posts: 102

    Not at all, sir.  Your pronouncements on things thermal are always welcome to this neophyte!
  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    Honing in

    That said, you are finding out as many of us do, that your typical boiler, even a ModCon, is too large for the average house! Putting in a TT 110 with an output of say 99 MBH will be nearly double the size it needs to be on the coldest day.

    Speaking of heating alone, your heat loss numbers have climbed a bit in actual use. Now, that could be reflected in your having a higher Cd factor (maybe 0.75 in reality?), a lower system efficiency (65%? I do not know), or both!

    OR, it could mean that your house has more air leakage, infiltration, than you have estimated. The conduction portion ("A * u * Delta-T") is fairly linear and "stable". Unless you have a blower door test or tracer gas test (blower doors being much more common and available), you will not really know. More to the point, a blower door test can reveal some "cheap wow!" air leaks. I have found old drain spout fittings disconnected, open chases from basement to attic, missing stones in a crawlspace, things that were fixed in four hours and yielded immediate -and measurable- improvement.

    So why am I prattling on like this?

    Because by testing and air sealing you MAY be in line to drop your heat loss by enough to allow use of the nice SMALLER boilers, a Lochinvar Knight 80 or even a 50, or a TT 60 or 80 (I think they have an 80). Ideal for heating only, tightly sized to the house heat loss and modulation tailored to the heat loss of the moment. How cool.

    Issue Remaining: Domestic Hot Water Production:

    It may be a longer haul to heat domestic water with a smaller mod-con. It is common practice NOT to add capacity for DHW production, especially when you have a 90 to 110 MBH input boiler and your heat loss is in the 40-60 MBH range.

    See, you always had plenty of reserve even on the coldest day, for DHW production.This was because there were few if any smaller boilers in the US market. Now that you have the 50-60 MBH input range, you can heat with high efficiency and modulate down as you need to, as a heating-only device.

    When you had a roughly 100 MBH input boiler, that all went to heating your tank in 20 minutes or less without your heat feeling a blip. This is three times the pick-up of a conventional tank type heater (33-35 MBH in a 40 gallon tank).

    So now, if you have a 50-60 MBH boiler, it may take 40 minutes to heat that tank and it will not be heating your house during that time.

    So by this I am asking if you might think of a separate means to generate domestic hot water and get a smaller boiler which will modulate down to 33% (or with a Lochinvar Knight, 20%) and enjoy your ideal heating efficiency.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • KestrelKestrel Member Posts: 102
    Indeed, good questions

    I had planned on an indirect DHW system, as seems typical in these parts, but you raise very good questions.

    I has struck me that designing the heat output to the temperature that is experienced a tiny fraction of the time is wasteful.  I had me thinking about climate - I had seen at various calculators using 15, or more commonly, 20'F as the design temp for Seattle, for example.  This past winter we got down to that on two days at the low for the day.  much more commonly, and seemingly for months on end (please please please come back sun!!) we have daily swings of 40-50'F, grey and rainy from October to May.  What I'm saying is that is seem inherent that the designs call for overcapacity.  Obviously you want that capacity for the rare times you need it, but the vast majority of the time is wasted.  In other climates, like the midwest or East coast, do the climate curves take you near the maximum design temp more often, or for longer periods?  Certainly the seasonal swings from hot to cold seem steeper elsewhere.

    It seems like a more efficient system would be to design to be maximally efficient for the maximal time, and have some sort of adjunct heat source for the rare but regular deviations colder.

    As for hot water, I really hate getting in the shower after the teenager - getting done quickly before tepid turns to cold is really no way to start the day.  The thought of 100K BTU backing up my supply of long, hot showers is really delightful!  Don't burst my bubble, Brad!

    Ahem...I do see your point, that yoking the DHW to the space heat source forces compromise when trying to make these appliances work in the most efficient points in their behavior.  Looking at comparative BTUs, the DHW, as best I could tell, was about 20-30 therms/month, while the heat was many times that most of the year, so we're talking 10-15% of total load.  But I suppose that DHW potion would represent the boiler going full bore - does that necessarily take it out of condensing mode, and therefore away from efficiency?

    BTW, I've been working on some calculations for the design of the under-floor radiant part of my project - should I run them by you guys here, or another thread?
  • Brad WhiteBrad White Member Posts: 2,393
    edited May 2011
    Some more thoughts-

    Regarding daily temperature swings and your ability to withstand them: If your building enclosure is insulated, tight and forgiving (a lower outdoor temperature balance point, specifically),  you can withstand a DHW call better than a loose house. (Seems obvious and it is.)

    See, a sudden drop in outside temperature will not show up immediately inside, not for at least one hour (in an unimproved older house absent holes in the walls), and five hours to overnight in a tighter house.  That is your floatation device.

    If your home mass, even if wood frame, but being warmed, also acts as a thermal battery. Add to that, if the house is "at temperature or near temperature" and not "recovering from a deep set-back", you are also in a better position to negotiate with physics. :)

    The slow pulsing of heat to maintain temperature at the rate it is lost (meaning good anticipatory control) is an asset you should have.

    From what is being discussed here, my inclination falls in three camps, each of which will require you to think about how you live and work with each set-up.

    Camp A says that you should get a separate source of DHW and let heating be "pure and tightly sized" -a 50 to 60 MBH boiler with a 5:1 turn-down. Let your DHW be a tankless type with a separate storage tank to be charged from it- abundant generation and volume to draw upon between charging periods. You may ALSO integrate an indirect in this as a back-up/assist or for summer loads, not to complicate things, but as an option.  (The storage tank can also be an electric DHW heater. Hook up the elements if you need the boost but it is hard to beat the cost of these as an insulated tank. More on this later.)

    Camp B says that you want to be more cost conscious and get as much benefit per dollar as you can. (Nothing ever wrong with that! But in this case you have a budget to adhere to.). To achieve that, I would say you go with a compromise 80 MBH boiler, maybe a 90-92 such as the Vitodens. Set up your DHW to charge in the AM before you warm up the house. Take advantage of "no heating demand" and be ready for AM showers while the house by then is warming up.  With this setup, you lose the finer turn-down (your minimum if 80 MBH will be 16 MBH with a 5:1 turn-down, still not too bad). But if you need to top off your DHW on the coldest day, it will take maybe 30-40 minutes from a cold tank (unlikely), but 10 minutes during a shower.

    A word on generating capacity relative to a shower: If you have low flow shower heads of 2.5 to 3.0 gpm (high by today's standards, but just for discussion), you will draw 2.0 gpm of hot water from the tank. To heat that steady state and keep up with it, will require between 40 and 45 MBH input at the boiler, roughly half the capacity we are talking about. What this means is that you can still charge the tank while taking a shower when all of the boiler capacity is being delivered.  Not a bad place to be. Could you do this with a 60 MBH or 50 MBH boiler? Maybe, just less assurance.

    Camp C is similar to Camp B, except you would favor a smaller boiler (50-60 MBH) and add a buffer tank or tanks, (thermal storage) to the heating side to carry you over the heating troughs when DHW is calling on the coldest day or the system is topping off the DHW. As a sub-option, you could also tie in an electric DHW heater both for storage and to trim those few high demand hours. Can be economical in the long run.

    To the question, "does the boiler go out of condensing mode during DHW generation"?

    Yes, for a time. If a cold tank start (really only once when you think about it!), your boiler water enters fairly cool because it too is just starting. It hits room temperature or colder tank water via the heat exchanger and returns to the boiler somewhere near the temperature of a Wiccan's mammary. Condensation reigns.

    As the tank warms up as does the boiler, the boiler water being supplied crosses over the tank temperature and runs to limit (maybe 160-170, varies) or until the tank is satisfied, whichever comes first. During this time, yes, the return water may come back below the gas dewpoint (return water at or about 122F). But then it will continue to rise above condensing as the temperatures of tank and boiler begin to narrow. This phase does not last that long. Remember we are talking a 10-20 minute burn most times with the last half maybe being above condensing. Not a huge loss really.

    As for radiant design, yes, I would start a new thread. There is a glaze-over point especially where the focus goes astray and you may miss responses you desire. My take anyway!
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • KestrelKestrel Member Posts: 102
    A stunning wealth of information!

    I REALLY appreciate your time and insights.

    I've got to run off to work (ugh), but I'll try to digest this later this evening.

    Talk to you then!
  • KestrelKestrel Member Posts: 102
    gret thoughts

    I'm thinking that camp 'A' will be inherently more expensive on the front side - simply due to the need for two heat-generating appliances.  That and the fairly cramped space limitations that I envision for the downstairs mechanical room (I'll get to the BIG PLAN in a later post) make it harder to implement given money/space budget.

    Camp 'B' sounds more like what I've envisioned up to now - boiler with two radiant panel zones (2nd floor and newly finished basement, single ECM pump, TRVs) and one underfloor (main floor) radiant, and an indirect tank.

    I had thought some recently about a buffer tank, in place of a LLH or other smaller hydraulic separator, in addition to the indirect, to avoid short cycling and provide some thermal mass. 

    The overall scheme is 2 radiators on the second floor for 2 bedrooms (bath up there already has electric floor), PEX tubes in the floor joists of the main floor (now accessible as the basement is being remodeled for the teenager), and 4 rads in the basement (two bedrooms, project area and TV room).

    I want to keep it as simple as possible, but no simpler, and do it on as frugal a budget as possible, obviously.

    I had been thinking 80-ish mod-con boiler, LLH, indirect;  but possibly a similar boiler, boiler buddy tank and indirect.
  • Brad WhiteBrad White Member Posts: 2,393
    I think you have a plan!

    With what you said, I agree, a Boiler Buddy as a LLH and buffer tank will take you over the humps. ECM pump, no question, but with TRV's or other "Back Pressure Devices" to impose a Delta-P response.

    The 80-ish selection sounds good. The Lochinvar Knight WBN-081 (wall mount) or KBN-081 floor mount would be ideal in my opinion. Turndown will get you to 16 MBH input which is as low as I could find in this category.

    The TT line is also good but you have a 60 (one of the first of the smallest!) and then the 110. So the 80 MBH range at least is an option for you. Play it well!

    I have gone back and forth on wall vs. floor mount. Everyone loves the wall mount for floor space saving but in some cases there is no wall so that has to be built, at least a frame flat enough to install it. That adds cost to an installer and their price, so the floor model seems to work just fine. What gets me back to the wall mount is if you are in a flood-prone area. After last spring, I was glad we had a wall mount in the old house. (No flooding to speak of but some neighbors did not fare so well.)
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • Jean-David BeyerJean-David Beyer Member Posts: 2,530
    My boiler could have been mounted to the wall,

    but my contractor put it on the floor. Since it was in my garage, he should have mounted it 18" off the floor, but he did not, and the inspector passed it. The wall was right there. Since then, I realized that putting it on the wall may not have been the best idea anyway because the other side of that wall was my bedroom, and if the wall acted as a sounding board, I would have had to listen to it at night. The boiler is not all that loud, but it is something to think about.

    But the contractor should have put 3 concrete blocks there as a frame and filled it with concrete, or something. But he did not.
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