Finding a heating system designer in southern NH?

Discussion in 'Boiler Forum' started by NH Homesteader, Sep 1, 2009.

  1. NH Homesteader

    NH Homesteader New Member

    Messages:
    4
    Hello,

    I was going to post a request for recommendations for a replacement oil furnace for the 1951 1,900 pound beast I just removed from my house. The post would have incuded a lot of details about the 3,300 sq. ft. house and the baseboard system etc. But then I started reading a lot of good posts here...

    How do I go about finding a competant heating system designer in southern NH who can perform an "ACCA Manual-J type heat loss analysis" or whatever is needed before I bug you regulars about boiler recommendations?

    Also, what might I expect to pay for such an analysis? I need to get after this because winter is coming, and the Missus is already freezing at the mere thought of it.

    Thanks, NH Homesteader
  2. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Did it still run? I've heard of one that made it to 80 yrs.

    http://www.amazon.com/s/?ie=UTF8&ke...=aps&hvadid=3027521901&ref=pd_sl_5y3mve3in0_b

    Without the Manual J calc., very roughly you'd need 20,000 BTU per Heating Degree Day for a house your size.

    http://www.degreedays.net/
    For Jan, '09 for Nashua there were 47 HDD each day, so you'd need 9.3 therms per day from your heating system. 1 Therm of energy can be had from approx. 0.71 gal of fuel oil and if your furnace is 80% efficient you'd need 9.3(0.71)1.25 = 8.3 gals/day.
    Last edited: Sep 1, 2009
  3. Dana

    Dana In the trades

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    These are exactly the kind of rule of thumb estimates that Manual-J and other heat-loss calculations were created to avoid. You could easily be off by more than 50-100%. The particulars count, and square footage of the house mean NOTHING (or not much.) Exterior surface area & U-values, air leakage, all trump square feet of living space. While guesstimates based on square footage can be within 50% of reality most of the time, the first standard deviation is very wide.

    A far closer estimate can be made comparing historical fuel consumption against degree-day data with some educated estimates of the efficiency of beastly antique. There's a pretty-good calculator based on Brookhaven Nat'l Labs boiler testing & modeling downloadable here:

    http://www.nora-oilheat.org/site20/index.mv?screen=home

    (click on the link roughly mid-page to download it.)

    The models were based on the testing behind this report:

    http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf

    With good fuel use and degree-day record over a couple of years, as well as the burner rating on boiler, the true heat load of the house can be calculated to within ~10% based on the model. If you have only fuel use data and pick the city nearest you on the pull-down list in the tool you'll still usually hit it closer than Manual-J. Play around with it a bit using different guesstimates of the efficiency until you're comfortable with the range of true heat-load, and use that as a point of departure when working with a heating contractor's estimates & recommendations. The lifecycle cost of being 3x+ oversized is considerable. (Look at some of the 2x & 3x oversizing tables in the test report document.)

    As for finding the right contractor, call a few, ask what heat load calculators they use for sizing the system, and if they include a printed report of the heat load analysis. Many will use a boiler manufacturer's proprietary software, most of which are pretty good, others will use something else. If they give you a pregnant pause, and a fuzzy answer, move on to the next one on your list. Manual-J will often overestimate by ~25-35% (a built-in safety fudge-factor), as will most proprietary heat load calculators. If you use the Broohaven-derived calculator expect it to 20-25% below what the contractor comes up with. If the boiler size options fall between those two heat load numbers, great! But don't go any bigger than 50% over (or 5% under!) what the Brookhaven model comes up with, or 10% over Manual-J/other.
  4. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    I'm just downloading and going to test the FSA calculator myself but as an alternative one I use in the UK is this http://www.stelrad.com/UK/stars.html

    I'd be interested to see how anyone willing to try it finds it. If you don't see the wall construction used in the USA in the list you can always add your own as long as you know the uValue for its thermal efficiency.

    Of course I am assuming your J factor is our u factor the output is given in btu and kW for the boiler sizing and measurements are in metric but we still use feet and pounds here as a legacy.
    Last edited: Sep 5, 2009
  5. NH Homesteader

    NH Homesteader New Member

    Messages:
    4
    Thank you for the info. It is very much appreciated.

    NH Homesteader
  6. Dana

    Dana In the trades

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    Location:
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    There IS no wall construction or other building-envelope specific data in the FSA calculator, in fact those details are completely irrelevant(!). The FSA calculator is based on a known heating-climate and determines the design-day heat load based on a very good model of boilers, the rated output of the specified boiler, and the seasonal (or fuel use per heating degree-day) experienced in the specified climate area.

    Heat loss calculations based on the construction details are guaranteed to have large built-in errors- an error factor larger than measuring the fuel consumption of a boiler in a specific location/climate for a season or more. Heat loss calculations may be the best method to determine the heat load for new construction, but for a replacement unit historical performance of the prior system is more precise (even with simple arithmetic models far less sophisticated than that used by the FSA calculator.) It takes good record keeping though- both fuel use and degree day data need to be known with reasonable precision, as well as the output & steady-state efficiency of the boiler.

    The "gold standard" for heat loss calculation in the US is the the ACCA Manual-J: http://www.acca.org/store/product.php?pid=172 There are several software packages availbable relying on Manual-J (or similar) methodology. Most come up with numbers that are within 10-15% of each other. For some (unfathomable) reason, in the US they tend to work with R value data instead of (the more direct & sensible for a heat loss calculation) U values, but the principles are the same. Heat loss calculators are more important/relevant/useful for determining radiator or duct sizing than they are for getting the highest efficiency out of a boiler, IMHO.

    It's somewhat shocking how many installed boilers have 2-3-even 4x higher output than even the Manual-J derived design-day heat load numbers, resulting in predictably abyssmal efficiency. The regression curves of part-load performance are pretty steep in the sub-15% of full load for most boilers, and if on design-day you're only at 25% of full load, (and for a few hours!), the average as-used performance is going to be far below that of a perfectly-sized boiler, yet 4x oversizing is far more commonly found than perfect-sizing. Actual as-used efficiency is only as good as the system designer...
  7. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    Yes I saw that on the FSA program.

    But whats a homeowner to do in your country if they had just moved in and hadn't got all the previous history on the boiler use age?

    In the past a lot of boiler and radiator sizing was carried out with what is know in the trade here as a Mears (http://www.mhmear.com/products.htm) calculator, this has recently been updated as it was found to over size by 25% if I remember correctly.

    I don't follow why the envelope of the building isn't relevant when calculating heatloss?
    When you know the heat loss you can calculate the heat input required for a given load. Yes pipe sizing as also relevant which isn't covered in the calculators. I also gave the link to the software as it is free might be useful, and it'd be interesting to hear how it compares.

    The book you linked to certainly looks the tomb of information :) We have a book which is often referred to not sure it's as comprehensive as yours as its 25% of the pages. http://www.cibse.org/index.cfm?go=publications.view&item=162

    I've found the Stars program useful myself Recently used it to spec out a 1950's house which underwent renovation. It was fitted out with Jaga (http://www.jaga.be/products.aspx?CLID=3&IMID=1&SET=0) radiator system low temperature low H2O content radiators, Viessmann 200 system boiler with weather compensated heating and solar hot water.
    Client has run it 12 months now and heat output has been spot on.

    Sometimes performing heat calculations seem nonsensical for instance: the heatload is calculated as requiring 15kW heat input but (a majority of homes have combination boilers) to get the flow rates for hot water anything up to a 35kW boiler may be specified to provide that flow rate (for power showers). Or the owner won't replace the heat emitters affecting the system performance.

    We also have building codes to limit the shower flow rates but these seem to go unheeded because ultimately the customer is paying and wants what they want.

    Still it's really interesting seeing things from your experiences compared to ours there's a lot to learn. So what if some of it isn't relevant its interesting nonetheless.
    Last edited: Sep 8, 2009
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,006
    Location:
    New England
    FWIW, a modulating boiler is more efficient at its lower outputs, so as long as you have one that can meet your needs at the max, on those days when it doesn't need as much, it can modulate. Some have a larger range of modulation than others. Mine can go from 20% to 100% (a Viessman unit). This gives you a little more leaway when matching to the house's needs (but the unit cost is higher). It works especially well with radiant heat. Efficiency is quite good, but depending on the system, may or may not pay you back depending on the fuel costs (likely to only go up).
  9. Dana

    Dana In the trades

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    Most mod-cons have an efficiency sweet spot somewhat above the lowest fire. At lowest fire the transfer efficiency on the heat exchanger begins to fall due to insulating laminar flow of the gases on the fire side. Turbulence is necessary to scrub away the insulating boundary layer, and that only happens at higher burn rates. This is why most mod cons are limited to ~1/4 or 1/5 turndown ratios. If they go much lower they rapidly fall off an efficiency cliff (if dropping from 98% combustion efficiency to ~80% is considered a disaster, which it might well be from a stack temperature point of view when using a PVC vent.)

    But a short-cycling mod-con is still a short-cycling boiler- oversizing them only to have them short cycle at low fire is not a recipe for maximum efficiency. With high mass low temp radiation (like radiant slabs) you can still get excellent performance out of them even 3x oversized, but it takes buffer tanks & design skill to keep an oversized mod-con running a true 90%+ with low-mass radiation in micro-zoned systems. Sizing a mod-con so that the average load during the heating system is well-aligned with the efficiency sweet-spot is ideal- better than oversizing. Most don't go much below 4-5kw (14-17kbtu/hour), which is already over my heating-season average, which means buffering or high-mass radiation would be required to keep the true efficiency up to snuff.

    Because the load on the boiler as measured by the fuel use per degree day tells you the heat load at any outdoor temperature far more accurately than any wild-assed guesses as to the true U-values of all of the as-built structures, or the actual vs. presumed rate of air leakage, etc. The boiler is completely agnostic as to the construction details, responding only to the state of the thermostat. Yet it's fuel use & modeled partial load efficiency curve tells the entire heat-load story with significant precision, quite independently of ANY construction detail.

    Heating systems designed for the heat load a minimum outdoor temperature that falls in the 95th-99th percentile of historical binned hourly weather data over several heating seasons are deemed "perfectly sized" for delivering adequate comfort without losing too much by way of efficiency. When the full output of the boiler matches the "design day" (~97th percentile temperature) heat load, the duty cycle & efficiency of the boiler are maximised.

    Manual-J is the standard here, and many software packages are based on it, as are some building-codes. IIRC California Title 24 now disallows oversizing of heating/cooling systems by more than 10% per Manual-J. Of course, the calculation is easily skewed by the whims & prejudices of the software data-entry person, so the 10% limit isn't perfect. But it probably keeps most systems from being more than 1.5-1.7x oversized, in which case they're still at least somewhat close to optimal. (Among it's many shortcomings, the US seasonal efficiency "AFUE" rating for heating equipment presumes 1.7x oversizing relative to the design-day heat load.)

    That is an issue here as well- the instantaneous load of a sustained hot water draw such as a shower can be several times the design-day heat-only load. The better-efficiency solution is to design in sufficient mass to the hot water storage rather than increase the size of the burner. (This is of course easier said than done.) There is a standby loss associated with the high mass storage, but it's a small loss relative to the efficiency loss of oversizing a boiler by a factor of two or three.

    eg: The peak heating load at my home is ~8-9kw, with an average mid-winter daily load of 3-6kw, but the instantaneous load of a winter shower adds something like 30-32kw. The solution to this mismatch (and that of the micro loads of separate room-by-room zoning switching in and out) is ~400lbs of boiler water in a buffer tank with an internal heat exchanger for the water. The load from the shower has also been cut in half (to ~15kw peak) by using a drainwater heat-recovery heat exchanger to pre-heat the incoming flow from the city water. With that much mass and a reasonable temperature hysteresis to the tank controls it's fairly insensitive to boiler oversizing (the mass establishes a pretty good minimum burn time), but even with a "perfectly sized" boiler for heating there is plenty of hot water available for multiple successive showers. If there's an Achilles heel to the system, it would be that I can't fill an 80 gallon spa in one go with a "right-sized" boiler driving the combined system, since tub-filling capacity limited by the mass of the storage- the heat-recovery capacity is only available for simultaneous drain/hot-water flows, like showers, not batch draws.

    Fortunately I don't have such a spa in my house. :) But if I did, and put in a 30kw boiler in to be able to support that load, the efficiency of the combined system wouldn't be TOO atrocious, since the thermal mass of the buffer keeps it from short-cycling, but it's still less than ideal.
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