S.W.A.G sizing units for loads

Discussion in 'General Discussions' started by saeheumsong, Feb 25, 2008.

  1. saeheumsong

    saeheumsong New Member

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  2. gabby

    gabby Member Industry Professional Forum Leader

    S.W.A.G---scientific wild ass guess ….definition provided by urthbuoy

    Most innovation starts here with a group of scientific principals thrown together in a mish-mash glob of unrelated ideas, and fine tuned into an unrelated product or a different application of a product or service. This is a good thing.

    Having HVAC units, boilers, compressors, loops (heat exchangers), and ductwork sized by this process is not a good thing.

    Today on “This Old House” they were demonstrating this principal in action. The man had a 1975 175K Btu boiler. He wanted to replace it with a more modern, energy efficient boil and got three quotes. The first quote was for a 175K unit (looked at the existing boiler size). The second quote was for a 150K Btu boiler based on a ratio of walls to windows, and basically seat of the pants “this is what we install for this type home in this area.” The third quote was an actual manual J heating and cooling load calculation for the home, balancing the window types, wall insulation, ceiling insulation and orientation of the home for solar gain. The program used was similar to Elite’s software….it could have been Raugeo’s for sizing boilers. The result was a 90K Btu boiler output, or a 110K Btu boiler. This was based on the worse winter temperature swing that was part of the program for his location. The 90% efficiency of the boiler set the closest size input to a 110K Btu system.

    The man has been paying nearly double for equipment usage that was oversized. If buying into quote 1 or quote 2 he wouldn’t have been any better off, or not much better off. The load would not have been balanced to the equipment needed.
  3. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    To add to the discussion, I've been told that if it ends in "000" it's a SWAG.
  4. gabby

    gabby Member Industry Professional Forum Leader

    What can you do to lower the cost of heating and cooling, meaning reducing a 6 ton load to a 5 ton or 4 ton load? If you have an existing structure you have to work within the confines of prebuilt walls and structure. However if you are building a new addition or adding a second floor, those constraints disappear.
    If building a new structure you have a blank slate to work from. Most people think more about appearance and architecture than they do about things that can't be readily seen, but it's these details that set the stage for future costs and comfort, thus elevating their importance to "dollars well spent."

    Let's open this up to the pros, those who install and design systems based on what the homeowner has chosen for windows, insulation, vapor barrier, sound proofing, air comfort levels for winter and summer for different regions of the country, and air circulation (flow). They have programs for inputting various choices, that can be replaced with better choices that in the long run will save you money and/or improve your comfort level. What could be worse than building a dream home that you are miserable living in either because of comfort, or cost of comfort.
  5. zach

    zach Member Forum Leader


    I posted this on another thread that had gone off topic some. I took the liberty of reposting here as it seems more appropriate to your discussion/questions.

    You and Engineer were having a discussion on ways to reduce loads. So, here is a repost.

    I live in upstate NY. Back in the spring I had a home energy audit done as my wife and I are planning some major renovations on our home. When our 1250 square foot cape was built by my wife's parents back in 1980 (remember expensive fuel back then) the first floor exterior walls were built as a pair of 2x4 walls with a thermal break between. The house was sited to maximize solar gain in winter with an overhang to assist with solar gain in the summer. Design temp here is -6*.

    Last year(max set point of 68*, 62* overnight) we used about 325 gallons of fuel oil. I tell professionals this and they do not believe me. It is due to the foresight of my in-laws.

    Anyway, my point is this: In preparing for our renovations (which consists mainly of a shed dormer on the rear second floor) the guy who did the audit for us recommended a book to me which I find interesting and helpful. The money spent on the audit was well worth it. He gave me some great tips and continues to do so. I am going to use spray foam in the second floor roof and walls carrying over the first floor r-values.

    Here is the book:
    "Builder's Guide to Cold Climates"
    Lstiburek, Joseph
    ISBN: 1-56158-374-X

    The book provides some great ideas. I highly recommend both the book and an audit by a reputable professional. There is also a book by the same author titled, "Builder's Guide to Mixed Climates". The info says this book is for climates with average winter temps above 45*

    Kevin aka Zach
  6. gabby

    gabby Member Industry Professional Forum Leader

    Yes, I saw this and I wrote down the book titles to check them out in the library.
    I did my own research over the years and have a pretty good handle on ways to save energy costs in a more conventional home; that does not mean certain practices can't be adapted from other schemes.
    I think you will see some similar techniques is this type construction compared to what your in-laws employed.

  7. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    All are swag

    Unfortunately all three are wrong, the proper way to install a retro fit boiler is based on the radiation load with no regard to manual J. If you replace all the radiation at the same time then use a manual J.
    Other wise a 100MBH boiler paired with 150MBH of radiation may fall short of heating the home under full load circumstances.
  8. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Joe that's an interesting point I hadn't considered but want to mull a bit.

    Are you saying that oversized radiation may cause near rooms / zones (near to the boiler, plumbing-wise) to steal all the heat from the boiler such that remote rooms / zones are starved?

    That would seem to result in near areas being overheated.

    Could that not be mitigated by covering some of the baseboards in the near rooms / zones? I grew up in a house with baseboards and they had metal flaps that could be folded down over segments of the pipe fins, effectively stopping the convection of heat and returning it to the boiler.

    My thinking is that a room by room Man J coupled with knowledge of per foot btuh radiation would suggest selective reductions in the radiation then allowing a boiler sized for the whole house load - cheaper, smaller, and more efficient.

    Am I missing something? Boots on the ground considerations overriding engineering theory?

    Another idea - maybe over sized radiation could be corrected structure-wide by simply reducing the temperature of the circulating water, allowing the reduced btuh input to the system to be spread wider and more evenly.
  9. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    You could cover some radiation or de rate it in the hot water systems you are thinking about, but that works for you not a consumer. Can you imagine telling the homeowner, your radiators won't feel as hot and you have to cover them from time to time.....what about infloor? how do you cover a section of that or explain to the customer it may be cold on their feet but it'll get to 70 in here (eventually)......
    and that's only for hot water, as it is not specified, steam could be a possibile scenario and not sizing by radiation for a steamer means all your steam recondenses before reaching perimeter radiation.

    Of course one could always reduce the radiation to meet the load and charge the extra labor, but that is a difficult sale to make for a contractor particularly when the others have swagged and used their thumbulator or manual J.
    Fact is it is often difficult on boiler sales in mid MI as most of the guys around here did not cut their teeth on boilers and have little idea how to size or install them. I've made a second career out of following a large company around here and replacing their steamers after 8 or so years.
    Swag topic is a good one, but suggesting man J is the proper or complete way to size a boiler inadvertantly is swag itself.
  10. gabby

    gabby Member Industry Professional Forum Leader

    Richard Trethewey has been doing boilers for many years and is very knowledgeable in that field. He never mentioned the name of the software he was using or all the details he used to size the boiler, only that a manual J was used within the programming. I don't want to mislead you into thinking that was the only data input. It was a small segment of the show, thus a lot of details probably landed on the cutting room floor.
    A conventional boiler was replaced (3' x 4' x 4' guessing on size) with one that was the size of a tankless heater, if that makes any difference. The point the show was making was that swag results were not used for the final sizing but detailed software to figure the boiler size for the load of the home.

    You bring up valid points as does engineer, but don't let my lack of knowledge about radiant heat mislead you into thinking a manual J was the only criteria for sizing.
  11. gabby

    gabby Member Industry Professional Forum Leader

    Re: S.W.A.G sizing units for loads/hot humid climates

    Additional reading reference material for builders/engineers/installers for hot/humid locations.

    Manufactured Housing Research Alliance

    4.11.2 Data
    Air conditioners among the homes in the sample typically were oversized. Cooling capacity ranged
    from 1.5 to 3.8 tons per 1000 square feet of floor area, with an average of 2.3 tons per 1000 square
    feet. The maximum air conditioner capacity recommended by MHRA sizing charts for the hottest
    climates in Louisiana and Florida is 2.2 tons for a 1000 square foot home40.
    Data collected in the field show that homes with moisture problems had oversized cooling equipment.
    On average, the cooling capacity found in the 67 homes was 1.5 tons oversized compared to the
    recommendations of the MHRA sizing chart.
    Although the latent load fraction (the percent of moisture removal compared to the total cooling
    sensible and latent) was not examined, typically air conditioners with higher moisture removal
    capacity have an additional 3rd row of cooling coils. Out of the 36 homes for which the equipment
    was examined, 8 had 2-row coils, 23 had 3-row coils and 5 had 4-row coils (Figure 5-21).

    The prevailing theory is that oversized air conditioners run for shorter periods of time and that
    dehumidification of the indoor air depends more on equipment runtime than equipment capacity.
    Therefore, homes with oversized equipment tend to have higher indoor humidity and a
    correspondingly greater propensity to experience moisture problems. In this data set, however,
    moisture problems correlate with smaller capacity equipment – not oversized equipment.

    This apparent discrepancy may be explained by pressure differentials. If operating the air handler fan
    causes the house to experience negative pressures, then a home with oversized equipment will be
    subjected to shorter periods of negative pressure. If the effect of negative pressure is more
    problematic than high relative humidity of the indoor air, then oversized equipment that runs less
    frequently may protect some homes from certain types of moisture problems (particularly problems
    within concealed cavities).
    Another issue relates to whole house ventilation systems. Many of the homes in this study had
    ventilation systems that bring air into the home only when the air handler is operating. Oversized
    systems will run less and will bring in smaller amounts of humid outside air through the ventilation
    system. This counterintuitive result may be characteristic only of homes with ventilation systems of
    this type.
    This data set had a high percentage of difficult to diagnose and repair moisture problems located
    inside building cavities. There were moisture problems found in homes both with oversized
    equipment and properly sized equipment. Over-sizing cooling equipment cannot be recommended as
    a measure to reduce moisture problems. This result does suggest, however, that the duration over
    which a home experiences negative pressures may be a very important factor, thus air conditioners
    that have a long run time or homeowners who operate with the thermostat set for continuous fan
    operations are expected to have increased levels of moisture problems on average (assuming their air
    distribution systems generate negative pressure in the home).

    4.11.4 Interim Conclusions
    The analysis indicates that air conditioning system sizing does not have as big an effect on moisture
    problems as do pressure imbalances. Instead, it suggests that a properly sized air conditioning system
    coupled with a well-balanced air distribution system may reduce air conditioning system-induced
    moisture problems.

    Possible Contributor

    Pressure imbalances
    1. Imbalances in the distribution of conditioned air. Imbalances in air pressures within the
    home and between the home and the outside were found to be significantly associated with
    moisture problems, particularly when they were amplified by closed interior doors.
    2. Duct leakage to the outside. Despite the fact that specific duct leakage test data did not
    correlate strongly with moisture problems, results did indicate that air distribution
    imbalances (to which duct leakage contributes), manifested as pressure imbalances, have a
    significant impact on moisture problems.
    3. High rate of shell leakage. Shell leakage alone did not correlate strongly with moisture
    problems; however theory states that it will contribute to humidification of the home if
    negative pressures with respect to the outside exist within the living area.
    Non-continuous vapor retarder and air barrier
    1. No ground vapor retarder under the home. Findings indicate that ground vapor retarders
    are effective in protecting homes from moisture damage, but only where a sufficiently
    designed drainage system prevents the accumulation of water under the home.
    2. Damage to the bottom board. Data indicated that the bottom boards of many homes
    experiencing floor moisture problems were damaged, indicating a need to develop guidelines
    for manufacturers, installers and owners to prevent damage to bottom boards and to facilitate
    repairs of those already damaged.
    3. Interior wall vapor retarder ineffectiveness. The literature review indicated that there is
    merit to a waiver allowing vapor permeable interior wall surfaces that will allow wall
    cavities to dry toward the interior of the home, in conjunction with a vapor retarder on the
    exterior in hot, humid climates. Further testing should be conducted to verify this
    4. Lack of exterior air barrier. Analysis of the data and review of the pertinent literature
    Alternatives for Minimizing Moisture Problems in Homes Located in Hot, Humid Climates: Interim Report
    54 Manufactured Housing Research Alliance
    indicates that air barriers should be located on the exterior wall surface in hot, humid
    climates to prevent the introduction of hot, humid air into building cavities that can
    contribute to condensation. Measures to limit outside air movement into building cavities
    should be explored.
    5. Ventilated attic space. From the limited data collected in this study on the effects of attic
    ventilation on moisture problems , it does not appear that ventilating attics contributes to
    ceiling moisture problems in hot, humid climates.

    Occupant Comfort

    1. Low thermostat setting. As hypothesized at the beginning of the data collection process, low
    interior temperatures contribute to moisture problems. Interim findings indicate that
    thermostat set points should be no lower than 71°F.
    2. Local cold spots. Local cold spots such as those created by cold air registers directed on
    floor surfaces, though not specifically studied in the field, are hypothesized to contribute to
    certain moisture problems.
    3. Introduction of unconditioned outside air. Introduction of unconditioned outside air into
    the home through the whole house ventilation system was not specifically part of the data
    collection effort in the initial phase of the study.
    4. Oversized air conditioning equipment. The results indicated that proper sizing air
    conditioning equipment would be effective in addressing moisture problems only when
    combined with good air distribution design practices, i.e. a balanced system.

    http://www.google.com/search?q=Builder% ... =firefox-a

    Click the pdf file “Building America Document Database Bibliography

    A collection of Department of Energy affiliated “pdfs” from a clean house to engineering and designing HVAC systems to moisture problems in humid areas.
  12. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I go off to Valley City to start

    roughing the back up, cast iron boiler to do construction heat and you guys go wild.

    IBR works. I do not use it unless the IRS is involved.

    DOE works too, but same as above.

    Gabby has been in site at the house we started to rough in and knows my work and is learning how I think.

    Joe is correct on how to size a boiler to the existing radiation.

    Take a deep breath and think two things: comfort, and fuel economy.

    Comfort is about not knowing you are comfortable. There is a cost attached.

    Fuel economy is just like above.

    Ask Joe, it is not me that will have the lowest bid, but you will never know Steph and I did the work, you will just not be uncomfortable.

    Sizing is THE ISSUE and will be. What I do is put in the smallest system that will not make you uncomfortable.

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