Sizing for a new boiler in Watertown, MA

Discussion in 'Boiler Forum' started by mrjohneel, Mar 8, 2014.

  1. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    If you would rather size your boiler by anecdote than firm science. I installed a 50mtuh Lochinvar Knight in a 1924 1500 sq.ft story and half bungalow in St. Paul, MN. We confirmed there was NO insulation in the walls.

    Beyond lowering their fuel bill by 63%, we bathed 2 teenage boys, mom, dad and a dog with the 40 gallon indirect water heater. Yes I warned them that when it got below design conditions (-13°F) they might lose a degree or two on the thermostat overnight, but they are happy campers.

    ACCA Manual 'J' heat load analysis.
    Last edited: Mar 20, 2014
  2. Dana

    Dana In the trades

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    CLEARLY that family wasn't bathing enough! <LOL>

    I'm sure mrjohneel would also do fine with a boiler half the size (or even smaller) of the ALP-080, but with his radiation & zoning the oversizing factor isn't going to take a hit on efficiency or comfort. I certainly wouldn't advise paying MORE to downsize, even though that boiler arguably overkill. Almost all mod-cons out there are oversized for his loads, but with the high-mass radiation and a mid-mod output of ~15 KBTU/hr this one will perform just as well as a 50 KBTU/hr boiler. Were there an option that had a 28K max input that cost half as much as the ALP-080 the indirect would probably have have to grow slightly, offsetting some of the savings, but the installation labor would be the same. If it had the same 5:1 turn down ratio you'd get quite a bit more modulation out of it, but unless there is a local contractor supporting it it's not worth seeking it out.
  3. nhmaster3015

    nhmaster3015 Master Plumber

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    Thats what I've been saying all along. Just with a whole lot less verbage.
  4. Dana

    Dana In the trades

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    Without the verbiage it could be just the unsupported opinion of some nameless guy on the internet. It may be an informed opinion or maybe not- there's a wealth of ignorance out there. If you give 'em the napkin-math & a reference document, then hand 'em a crayon they have the tools to figure it out.
  5. JustAHomeOwner

    JustAHomeOwner Member

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    I realize this was a bit ago but could you please say which boiler rating to go by if not IBR?
  6. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    IBR --the former Institute of Boiler and Radiator Manufacturers--rated boiler for Gross and Net output assuming a 15% loss for jacket and piping. If you are using a modern condensing boiler installed in a conditioned space, the DOE number more accurately reflects the steady state output of any given boiler.

    Now if you are running design water temperatures below body temperature (condensing) the output will exceed the DOE number on the rating plated. If we add drive-by-sizing to the "never too big" mania we will get a boiler that never runs steady state and will rarely condense, wear out components and may not even meet the heat load its capacity would indicate. Then there is the over-pumping of the boiler and single-mined primary/secondary pumping and pretty soon Rube Goldberg could be your Daddy...

    But if you don't have an accurate Manual 'J' heat load, your wasting time on all this.
  7. JustAHomeOwner

    JustAHomeOwner Member

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    Location:
    NY
    That has me thinking. A year ago in early March I switched from fuel oil to NG. Since I wasn't going condensing, the plumber I used recommended a Burnham P205 but since I was buying it and it didn't matter to him what I had, I bought a P204 based on the IBR rating. Twenty years ago a comprehensive heat load in the summer and heat loss calc in the winter came out oddly enough just about the same at around 50k BTU. The single 4 ton central A/C unit I installed has done fine here for the last 14 years as did the small oil fired boiler I installed just before it. On the P204 the IBR is 70K, DOE is 80K. It's located in a heated basement with 6 zones and if the DOE is more accurate then maybe it's a bit too large? Thoughts appreciated.
  8. Dana

    Dana In the trades

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    If the boiler is located inside the conditioned space (say, a heated basement, or a not fully heated basement under fully heated space), it's inappropriate to size by IBR, since all of the heat loss from distribution plumbing & jacket losses is still inside the house.

    The only time IBR sort-of makes sense is when the boiler is located in an air-leaky uninsulated garage or outbuilding, with an insulated wall between the boiler and the space it is conditioning. In that scenario heat lost from the distribution plumbing & boiler jacket is truly lost, whereas in a heated basement the distribution & jacket losses aren't lost at all- they partially heat the basment zone.

    If your true heat load is 50K you could have safely dropped back to the P203, which has a DOE output of 52K. If the heat load calculations (like most) have some padding, even the P202 (DOE output of 40K) would probably have been fine.

    If the fuel-use/degree-day calculation is in close agreement with the heat load calculations you will still hit the AFUE ratings with the P204, since it's only 1.6x oversized. (That is, provided it isn't being short-cycled into lower efficiency by having cut up into six zones.) If a fuel use calc indicates that your true heat load is more like 40K or less, it'll come in a bit shy of it's AFUE efficiency, since it will be running significantly lower duty cycle than in an AFUE test.
  9. JustAHomeOwner

    JustAHomeOwner Member

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    TY for ringing in. I've been a bug with keeping track of the NG usage since switch from fuel oil. i have the record of usage entered 2 or 3 times per week for this past winter starting on the day I fired it up for the winter on 9/17. Heating degree day information is readily accessible from the net but how would I go about correlating that information with my heat loss calculation?
  10. Dana

    Dana In the trades

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    For higher accuracy it's best to use a period of 2 months when there was significant heat loading, and no cooling loads. Unless yours is a superinsulated house or purpose-designed for high passive solar gain, use 65F as the presumptive heating/cooling balance point for heating degree days. Degreedays.net is a reasonable source for nearby weather stations with good HDD data.

    When you end up with a therms per HDD ratio, first convert that to output-BTU/HDD using the nameplate efficiency of the boiler (depending on vintage P204 will be anywhere from 78% to 82%. If you simply divide the D.0.E. output by the BTU-input numbers on the nameplate, that's the efficiency to use.)

    With that number in hand, divide by the number of hours in a day (usually 24 :) ) to end up with a BTU per degree-hour number.

    Estimate your 99% outside design temperature based on that of nearby listed cities, and comparing mid-winter daily low temps using Weatherspark or similar weather history sites with those of nearby listed cities.

    Subtract your 99% outside design temperature from the 65F heating base temperature for the number of heating-degrees.

    Multiply your BTU/degree-hour constant by the heating degrees number- that's going to be within 10% of your actual heat load (usually within 5%.)

    This method fails if you are using auxilliary heating, leave the house unoccupied for days/weeks with the thermostats set to 50F, etc. but works pretty well for occupied buildings that are heated to 68-72F during at least good fraction of the day, and keeps it of 60F or greater the rest of the time.

    eg: Say your therms/HDD number works out to 0.3 therms per HDD, and you determine than your 99% outside design temp is +7F.

    In an 80% boiler that's 0.8 x 100,000 BTU/therm x 0.3 therms = 24,000 BTU/HDD, or 24,000/24 hours= 1000 BTU/degree-hour.

    You have 65F-7F= 58F heating degrees.

    Your heat load is then about 1000 BTU/degree-hour x 58F degrees = 58,000 BTU/hr.

    If your other heat load calculations used a design temperature different from the 99% outside design temp, use that to compare apples-to-apples, but using the 99% number is really the only "proper" temp to use for sizing the boiler, since the load rarely comes out EXACTLY at the output figure for the boiler, and the next size up delivers plenty of margin.
  11. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    The smaller GB142 is the 24, that's kW, 22mbuth minimum fire at low temp. You probably were not paying attention to the numbers above.

    OP, don't buy from people who don't do heat loads, or understand them for that matter.

    Your loads are so low that a gas-fired water heater, either combi or dedicated would be a better fit. But without a proper heat load, all is conjecture.
  12. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    Location:
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    The idea that the "industry" doesn't know how to heat a building is just silly. It may be an embarrassing fact that many in the field don't know a btu from their elbow, but the industry is doing fine, thank you.

    The HVAC industry uses the US standard ACCA Manual 'J' for residential and Manual 'N' commercial to determine, with amazing accuracy, the heating and cooling loads for any particular structure at hand. They are also quite comfortable using the reasonably accurate DOE ratings (adjusted for design water temperature) found on every rating plate of every gas or oil fired space heating appliance on the market.

    One should choose his equipment and his installing contractor by this simple and well accepted standard. No Manual 'J' no job order. The odds of getting what you pay for will be greatly enhanced without the futile effort of trying to gain 30 years experience via a few threads on the net.
    Last edited: Jun 5, 2014
  13. JustAHomeOwner

    JustAHomeOwner Member

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    Location:
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    Dana,

    Thank you for that detailed reply. Please read on as I'm still in need of advice. Working off of a 97 day intervile of some of the coldest weather this past winter,
    with good HDD information (3131) and detailed records of NG usage for that period (846 Therms) I arrive at .27 therms per HDD, and 925
    BTU's per HD/hour. The 99 percent temp. is +15F. That's 925 X 50 = 46,250 BTU's per hour. This, from a very cold winter here in the north east.
    However, roughly 25 - 30 percent of my home this past winter was heated to only 50 - 55 degrees. Adding that back in would still leave my 80k boiler
    oversized and short cycling. There's no indirect or DHW coil, should I drop the water temp from 180 to 170?
  14. Dana

    Dana In the trades

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    That 46K heat load number is credible, and heating the other 25-30% of conditioned space floor area to something more comfortable than 50-55F will not represent a 25-30% increase in fuel use or heat load, but it'll probably be on the order of a 10-15% increase. I'd be surprised if the true heat load with the other rooms fully heated would be anything like 60K, and is likely to be in the 50K range, as you previously calculated by other means.

    That makes an 80K output boiler only ~1.6x oversized, and it should hit it's AFUE numbers unless it's short-cycling on zone calls.

    In general cutting the water temp back reduces distribution losses and jacket losses, but lower temp water means the radiation emits less heat, which may cause even more short-cycling issues. The more zones you have, the worse the ratio of boiler output to radiation output there is to any single zone. When there is a heat load on, try timing some of the burns. If you're not getting at least 3 minutes/burn out of it your as-used AFUE is going to be seriously hurting (5 minutes/burn would be OK, 10 minute/burn would be great.) If it's not short-cycling it's fine to cut the temp back to as low as ~150F for most gas-burners- as long as the return water entering the boiler doesn't drop below 130F for extended periods there is zero chance of destructive condensation on the boiler plates.
    Last edited: Jun 5, 2014
  15. JustAHomeOwner

    JustAHomeOwner Member

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    Location:
    NY
    I just shut down the boiler and water feed to it yesterday so burn time checks will have to wait for next winter. This will be something I won't forget to do, not knowing will bug me the entire summer. I'm unhappy that this boiler is 60 percent larger then I need, an extra 10 -15 K would have been fine. How's this sound. What if I plug the gas feed to one of the (I think there's 4) burners not where the roll-out sensor is? Gas-wise it would then be the same as a P-203. Sure it would void the warranty and maybe even kill the neighbors dog... yada, yada, yada. Like with oil fired boilers, changing the tip size is allowed, these NG boilers would be nice IF something to cut then down a bit would be viable. FWIW, I was already all over HDD and therm per, taking it out further was something I never though of and I thank you for that. Five days of cold weather would let me know if that was working as intended. While I fully realize this would be far outside of professional advice.. but. Thoughts?
    Last edited: Jun 6, 2014
  16. Dana

    Dana In the trades

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    Derating larger boilers by pulling burners and re-adjusting the draft is sometime a reasonable thing to do, but not for a 4-plater like the P204. A typical derating would be ~15% of the total, (say knocking a 6-plater down to the output of a 5-plate boiler) but more than that never really works to advantage- it becomes a true engineering project to go beyond that. At only 1.6x oversizing don't worry about it- it's far easier to screw it up than it is to improve it, and it should be able hit it's AFUE numbers if you can keep the numbers of burn cycles down (= burn lengths up.)

    It's not as if knocking off 25% of it's capacity is likely to change it's as-used AFUE, or it's propensity for short-cycling on zone-calls, and derating it could make it's efficiency even lower if the combustion air volume isn't dialed in to perfection. Oil burners are designed to be re-jetted with a wide range of air adjustment, but like most gas boilers the P204's air/gas mixture is fixed by design, and not adjustable. If the thing is short cycling like crazy in the shoulder seasons with 10+ burns per hour you can think about re-plumbing to combine some zones, or if it's not too bad, adding a retrofit smart boiler control like an Intellicon HW+ or Beckett Aquasmart or similar, and adjust the "differential" (temperature swings of the boiler during calls for heat) to get the most out of the thermal mass of the boiler and thus extending burn times, reducing the number of burn cycles, etc., and heat-purges the residual boiler heat to the its preset low temp at the beginning of a new call for heat, and near the end of a call for heat as it "learns" your heating system based on the recent system behavior, parking the boiler at a lower temp between burn cycles for lower standby loss.

    Setting up the Intellicon is simpler than some of the others, a DIY project for those with electrician skills and the ability to figure out your boiler's controls- it has fewer configuration options, but good "learning" algorithms. In a many-zone system retrofit heat purging controls can sometimes reduce the number of burns by half, and the standby losses by more than a third.

    If your radiation is all high-mass column radiators it may be just fine as-is, no system or control adjustments required, but if it's nothing but fin-tube baseboard the potential of short cycling on a 6-zone system is pretty high, and not necessarily soluble at the boiler end, even with a right-sized modulating condensing boiler. Some of the newer Burnham cast iron beasties come with with smart-controls already installed. Mod-con boiler fans like BadgerBoilerMN consider it akin to putting lipstick on pigs, but it does make a difference in the operational efficiency, even when 4x oversized or micro-zoned to the nth degree. The ES2-4 is essentially your boiler with fancier controls, with the air/fuel mixture tweaked to a higher efficiency (high enough efficiency that it shouldn't be used in a masonry chimney due to increased risk of exhaust condensation in side the chimney resulting corrosion/erosion of the mortar.) .

    If you go around and measure up the heat emitters zone-by-zone it's napkin-math to figure out if it's going to short cycle on zone calls.
  17. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    I have de-rated boilers using a combustion analyzer and a few decades of experience. The manufacturers of atmospheric boilers absolutely forbid the practice and it certainly should not be attempted by the novice or professional without the benefit of experience and training with gas burners using calibrated analyzers.
  18. JustAHomeOwner

    JustAHomeOwner Member

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    Location:
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    Good info here. TY. I've abandoned the idea of toying with the boiler for obvious reasons but am still wondering what to do with the system I have. I can easily join 3 zone valves to the output of any of the 3 thermostats on my ground floor where I suspect the most frequent instances of short cycling occurs. That's my plan going forward, I have much boiler watching to do next winter. Again, TY gentlemen.
  19. nhmaster3015

    nhmaster3015 Master Plumber

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    Buffer tank and hydraulic separator.
  20. JustAHomeOwner

    JustAHomeOwner Member

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    I'm reading about that on the Lochinvar site. Do they work as intended?
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