Sizing for a new boiler in Watertown, MA

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

  1. mrjohneel

    mrjohneel New Member

    Messages:
    6
    Location:
    Massachusetts
    Hello, I'm one of the many trying to take advantage of National Grid's incentives to convert my 30-plus-year-old, forced-hot-water oil boiler to gas. My posting may be similar to others posted here but I'd appreciate any advice. My house is 1510 square feet, 2-story built in 1928. Last year, I had dense pack insulation blown into the walls and I personally replaced my old storms with good Larson Gold series storms. I have cast iron radiators. I also heat my house primarily with a pellet stove and for the past few years have only used oil to heat my DHW. But I want to upgrade to gas if I decide to sell my house and also to get gas into my kitchen while I live here. I've started the process, met with four contractors, and have received bids back from two. I'm set on a high-efficency boiler and indirect water tank, not a combi.
    Here are my concerns:
    1) None of the contractors, even the National Grid Value contractors did a full heat loss analysis. I know, I know -- it's the most important thing. Some measured the gross space and some counted radiator fins, and one guy took a little more time.

    2) I did my own calculation using a series of online calculators and taking into consideration wall insulation, cold partition length, etc. I also used "old-school" methods of just relying on square feet or radiator size. I know, I know -- I'm not an expert but all of my numbers, including fudge factors, never came close to 40,000 BTUs for my house.

    3) Here's the real concern: the two bids I have and the 2 bids I expect based on my lengthy conversations with the guys, have suggested boilers ranging from 80K (National Grid's preferred Burnham Alpine) to 110K (Lochinvar Knight). Everyone told me not to worry about the apparently large size because the boilers modulate down. And each told me that the DHW tank is the one that is driving their higher calculations.

    So isn't this against everything I've read here. Shouldn't I go with a small boiler (say 50,000 BTUs if my calculations are right) and have it prioritize energy towards the water heater when I'm taking a shower. You're not supposed to size the boiler to the big BTU number on the 40-gallon DHW tank, correct?

    I should note it's just me and sometimes a girlfriend in the house. Two back-to-back showers sometimes. The dishwasher may (but probably isn't) running as I shower. It's one zone. Listen, I know each situation varies and I've only sketched out broad details, but it's a big investment and I think I'm being led down the wrong path. Who knew HVAC could be so complex? thank you.
  2. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    I couldn't have said it better myself and can't tell you how gratifying it is to know that someone is listening, even if the contractors are mostly deaf.

    We perform ACCA Manual 'J' heat loads everyday. We offer the service to architects, engineers, contractors and homeowners alike, but the vast majority of our work is for homeowners like yourself, as they have more skin in the game.

    Yes, a proper heat load is must.

    Yes, a properly sized condensing boiler is the perfect way to heat a home with old cast iron radiators.

    Yes, an indirect water heater coupled to a condensing boiler will make more hot water, cost less to operate and last longer than anything currently available anywhere.

    No, you do not have to up-size the "size' of any boiler, much less a condensing boiler, to accommodate an indirect fired water heater. Most of the manufacturers' installation manuals (we have installed 10 to date) point this fact out in writing. There are rare exceptions.

    No, sizing the radiation is not relevant in hydronic heating, only steam. If you have too much radiation your boiler can operate at lower water temperatures unless it is over-sized for the load or radiation, in which case all is lost.

    Yes, condensing boiler will modulate on outdoor reset, if the sensor is installed and the boiler programmed for you climate and radiation and the boiler is not grossly over-sized for the load, 150% is the accepted maximum.

    http://www.badgerboilerservice.com/boilerchoice.html

    We just consulted on a Triangle Tube Prestige 110 in Northern Minnesota. It seems the boiler would only "stay on" during our recent cold snap, -25°F for a few weeks. That's right, the boiler modulated down to 25% of its rated output and remained on 24/7 only when the house was at design conditions. The rest of the year the boiler "bounced off the bottom" short-cycling, driving down fuel efficiency and comfort. The local distributor, whom should know better, blamed it on the radiation. After we did a proper heat load (42mbtuh) the new TT60 specified and installed all worked well. This is how modcon boilers get a bad name.

    Believe it or not, the distributor also argued that they would "never less than a 110 when and indirect was installed". Naturally this flies in the face of common sense, since the typical water heater has a 65% 40mbtuh burner, energy wasting flue and paltry insulation. All indirects have more than a inch of solid quality insulation (Amtrol being the notable exception) and no flue.

    Since most condensing boilers prioritize domestic hot water the performance of this combination can't be matched. Some modcons even limit the time DHW can draw to eliminate concerns about the house going cold while you're filling the tub.

    The "big BTU number on the 40-gallon DHW tank" relates to the potential, not required, heat transfer capacity of the tanks' heat exchanger.

    ACCA Manual 'J' heat load analysis and read the installation manual to your installer if you must.
    Last edited: Mar 9, 2014
  3. mrjohneel

    mrjohneel New Member

    Messages:
    6
    Location:
    Massachusetts
    Thanks BadgerBoiler. One contractor's heat loss numbers were higher than the others because she said she added 17.9% for the heating capacity lost through the piping in my house. Also, roughly, when you're doing a heat loss calculation, how would you figure the basement the boiler is located in? That is, in many of the programs I've seen there isn't a place to factor in an unheated basement that does not have insulation on its ceiling. I know you can factor in how the floor is constructed/insulated in the heated space above a basement, but is there a component in heat loss calculations to factor in the unheated basement as a whole? These two factors apparently bumped up my heat loss to about 68,000.
  4. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Some still use the long obsolete "IBR" rating for boiler sizing. In the rare cases where it is valid the operators usually misapply it. We "account" for everything and service a lot of boilers. The combination of designing, installing AND servicing has proven invaluable in my understanding of heat loads, insulation and a raft of things I previously thought superfluous.
  5. Johnnyradiant

    Johnnyradiant New Member

    Messages:
    1
    Location:
    BC
    If part of your motivation includes selling the home don't tailor your job too close to your needs - 1 person maybe 2 showering, if the house could be sold to a family with 4 people. Unless you just want a system to look good but maybe not meet the need.
  6. mrjohneel

    mrjohneel New Member

    Messages:
    6
    Location:
    Massachusetts
    I decided to go with a Burnham Alipine 80 with a Super Stor 45-gallon indirect. (Wish we well.)
  7. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    50 btu/sq.ft. that should do it; twice.

    Make sure they install the boiler with primary/secondary piping adding the extra pump will burn another 88 watts to the first 88, every, time, the burner fires, which will be a lot since the minimum output on this boiler is 14mbuth and your "design" load is likely only double that.

    And don't forget to add another (that's 3) 88 watt circulator for the indirect water tank, minimum 1" copper pipe, also following the manufacturer's instructions.

    You can lead a horse to water...

    Wish I would have stayed in bed.
  8. mrjohneel

    mrjohneel New Member

    Messages:
    6
    Location:
    Massachusetts
    Thanks. The guy I chose showed me that his heat load calculations were higher than mine accounting for my cellar. And he included three circulators. I had 5 companies in my house, including ones certified by the utilities. The 80K boiler was the smallest one bid. The main takeaway from all of this is that you can be a saavy, committed homeowner, who does his homework and you're still at the mercy of an industry that can't agree on anything in terms of equipment types, equipment size, basic calculations to determine the types of equipment -- or price. No one agreed with my idea of getting a 50K BTU Bosch; in fact, they all told me they wouldn't do it. So I'm trusting that they are all not wrong, and that they know more about their business than me (or you).
  9. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Logical, but inaccurate.

    If the cellar (basement) is not conditioned or is insulated the loads should be ignored in most cases.

    I was being facetious about the circlulators. We used one on our last condensing boiler with indirect-fired water heater, heating over 5000sq.ft. in Minneapolis. In most single-boiler systems, one circulator will suffice.


    http://www.badgerboilerservice.com/boilerchoice.html

    http://www.terrylove.com/forums/showthread.php?46277-New-Boiler-and-Indirect-water-heater-question

    Matter of fact, of the 10 condensing boiler manufacturers we have used, zero, recommend up-sizing the boiler to enhance DHW capacity. The typical indirect water heater is the perfect storage vessel with less 1/2°F standby heat loss due to superior insulation, the lack of burner and flue. If you think about it, most people get by with a 40 or 50 gallon conventional low-efficiency water heater with a burner output below 40mbtuh. The lowest output condensing boiler exceeds this number. If you tie the "smallest" boiler with a 40 gallon indirect, you are ahead. If you need more hot water get a bigger tank. You use DHW for an hour a day and the boiler has 23 hours to make it up. Compared to the output of the typical electric water heater, a 50 boiler and 40 indirect are MONSTERS!

    Naturally every system should be sized first for the heating load, since radiation is limited and the DHW loads seriously considered. We design systems for 1000sq.ft. cabins and 10,000sq.ft mega homes with full-body showers and we always do the math.

    Best of luck.
  10. Dana

    Dana In the trades

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    3,024
    Location:
    01609
    With mod-cons the min-fire output is at least as important as the max fire, since the min-fire and radiation mass & output on your smallest zones deterimins just how cool you can run it, and much condensing efficiency you can get out of the thing.

    The ALP-080 is the smallest of the Burnham mod-cons, with a min-fire input of ~16K, which isn't that different from most 50K mod-cons, and not a terrible choice here. What's the min-fire on the Bosch unit you were looking at?

    The output of the ALP-080 at high fire is sufficient to support a 24/365 shower at 2 gpm at eastern MA incoming water temps- wouldn't sweat that one too much unless you're into installing six side spray monster-puking-gusher type of showers designed for par-boiling the humans.
  11. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Min input on the HTP Contender 12.5, Bosch 57 is 12.9 NG, and my choice for this one. 40 gallons standing gives you 2 reasonable showers, if you want more buy a bigger tank.

    Most are still using warm air heat load programs that come out "fat" for load-lowering radiant floor heating systems. But hey, if the homeowner doesn't mind a bit of cycling and worries about running out of hot water, I would sell him the bigger boiler.
  12. Dana

    Dana In the trades

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    3,024
    Location:
    01609
    From the sounds of it they're not using warm air heat load programs, they're not using heat load programs at ALL, just pulling numbers out of their behinds or using square feet x BTU rules of thumb more appropriate for uninsulated buildings or something.

    The heat load of a 750' per story near-cube of a house at 1920s style glazing fractions just isn't very much at all if it's reasonably air tight and has storm windows over the antique double-hungs. Given the air retardency of the the dense-packed cellulose in the stud bays and the boost in both air tightness and window U-factor of the Larson Gold low-E storms it probably beats the real world performance of a 1500' IRC 2012 code-min ranch house. It's probably currently at about 25 KBTU/hr @ +10F (the approximate design temp in Watertown), and with air-sealed & insulated basement walls & band joist (still recommended) it would come in under 20K, if experience is any guide.
  13. mrjohneel

    mrjohneel New Member

    Messages:
    6
    Location:
    Massachusetts
    So, Dana, am I making a big mistake with the Alpine 80? Should I put the kibosh on the deal and insist on something smaller? I haven't paid the deposit yet as I am waiting for pre-approved 0% loan documents to clear. Thanks in advance.
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    For the small area you're trying to heat, it will end up short-cycling and never meet its rated efficiency with that boiler. Radiant floor heating is comfortable. WIth a boiler like that, you might also be able to run a snow-melt system, and never have to shovel your driveway again. It is overkill for your application. And, in that part of the country, you'd welcome the ability to run it as a/c in the summer, too.
  15. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
  16. mage182

    mage182 Member

    Messages:
    71
    Location:
    NY
    Are you going through a National Grid or something similar deal to get the Burnham unit at a steep discount. In that case you're probably pay around 1000 for the unit itself. I wish Burnham still made the 63k unit. Is that still the only brand offered in the promotion? It may be worth looking into other brands even though they may cost a little more, they might save you more money in the long run if you can find a unit that meets your output needs very closely. Coming from someone who got in on the National Grid deal and ended up with a GROSSLY oversized Alpine unit in a house about the same size as yours, getting the right sized unit will take first priority over cost when I do this the next time around.

    The Buderus GB142s smallest model is 84.8k with only 100-30% modulation. Size wise its a worse choice than the Alpine, unless you're going to start getting into the possibility of getting an Ergomax tank. Plus it costs 3-4x what the promotional rate is on the Alpine.


    This thread as a lot of good info on it from Badger and Dana on considerations for sizing and sizing when using an indirect water heater.
  17. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    The GB142 is slightly oversized but then again, if his loss is around 40,000 almost everything is oversized. You leave out a few things. One is the availability of the unit itself and repair/replacement parts. Two is whether or not there are qualified technicians in the area to service the unit. Three is that he has cast iron radiators in the house which are a huge advantage over baseboard in giving him more water volume and the ability to modulate to lower temperatures. Four is that a buffer tank will solve the cycling problems if needed.
  18. Dana

    Dana In the trades

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    3,024
    Location:
    01609
    Actually he has high-mass high-volume cast iron radiators in this place- it won't short cycle if pumped and programmed correctly, even if he has it cut up in to 2-3 zones. Running the math on the water volume of the zone with the least amount of radiator would be prudent, but I wouldn't sweat it too much. Changing over to radiant floor would likely LOWER the thermal mass in the zones.

    The min-mod output of the ALP-080 is pretty close to the average whole house load at his binned hourly mid-winter temperature averages, wouldn't sweat that one too much either. If there is enough radiator (and there most likely is, if they were able to keep up with the load in it's 1928 level of insulation, air-tightness, & windows), it would be hard for it NOT to hit it's AFUE numbers with the outdoor reset dialed in, even if it isn't modulating a heluva lot.

    I'm sure if you micro-zoned the hell out of the place with a radiant zone per room you could force the thing into short cycling and low performance, but I doubt that's in the plan.

    In this part of the country you can't run radiant cooling without running into condensation/dripping and potential mold issues. The outdoor summertime dew points in Watertown average in the mid to high 60s and will often enough spike into the 70s- to deal with the latent loads requires something other than radiant-cooled floors. (In fact, in homes with few west facing windows or significant shading factors the seasonal latent load will usually exceed the seasonal sensible load in this region.)
  19. mrjohneel

    mrjohneel New Member

    Messages:
    6
    Location:
    Massachusetts
    Thanks all. Just so you know one contractor based his heat load numbers and his bid by calculating my radiators (which I thought you were only supposed to do for steam heat not hot water). He said my "existing radiation" is 328 sq feet of net IBR or MBH. He said if I wanted a BTU number I should multiply (185 BTUs per Sq foot of cast iron x 328 Sq feet = 60,680). I didn't choose him but he also suggested the 80000 Burnham Alpine. And the house is only one zone with no radiant heat on the floors. I have a call into the contractor I chose just to see if he can allay my concerns and what his explanation is.
  20. Dana

    Dana In the trades

    Messages:
    3,024
    Location:
    01609
    That is not heat load calculation, that's a radiation calculation, which is useful for knowing how low you can take the water temperature and still balance the boiler output to the radiation output. That 185BTU per square foot EDR number corresponds to a water temp of about 190F, if you look at the graph on page 2 of this document. It's also useful for knowing how big a cast-iron boiler needs to be in order to pump-direct (no bypass plumbing or other boiler-protection methods) without ruining the boiler with condensation. But with a mod-con you WANT the condensation, since the condensed exhaust is heat recovered from the exhaust stream.

    It is remotely conceivable that the heat load was almost as high as 60K in 1928 when it had single pane windows and no cavity insulation, but it was probably significantly over-built for the load even for the house in that condition. (I'm guessing reality was never as much 50K, but it was probably at least 40K back in the day.) In it's newer-tighter-better-insulated condition I'm assuming you're looking at a true design heat load no more than 25K, that's about (25,000/328'=) 76 BTU/per square-foot EDR which means that even at your 99% outside design temperature condition you can run with water temps of about 130F and still meet the heating load, and the return-water temps coming back to the boiler will still be in the condensing range. It's hard to do any better than that!

    The min-fire output of the ALP-080 in ~95% efficiency condensing mode is about 15,000 BTU/hr. To balance perfectly with 328' EDR of radiation with no rise in water temp and absolutely no cycling that would be *15,000/328'= )46 BTU/hr-ft, which happens at a water temp of 115F, at temp at which the ALP-080 actually DOES hit the 95% range. Since there is significant water & iron thermal-mass in the radiation + distribution plumbing you would be able set up the differential/hysteresis and reset curve to run as low as 100F without any significant cycling issues, and be in the 98% combustion efficiency range during low load conditions, which from the boilers point of view is most of the time. The ALP-080 is rated 96% AFUE- but with some careful adjustment you'd be able to beat that by a point or two for an average.

    Going to a smaller boiler with only 12K min-fire doesn't appreciably change things at the margins, so unless it's a significantly cheaper boiler just go with the ALP-080. You are in absolutely no danger of short-cycling the -080, unless they set it up primary / secondary then pump the living-hell out of it, which happens sometimes with contractors who failed 6th grade math, but the same idiots would make the same mistake with somewhat smaller boilers too.

    I hope this allays any concerns you may have had about using this boiler. If the min-fire output were 30K instead of 15K I'd strongly advise down-sizing, but you have more than enough radiation output to manage 15K of min-fire boiler, and enough to stay in the condensing temperature range more than 99% of the time. Read the manual, learn how to adjust the outdoor reset and other programmable factors and don't be afraid to play around with it a bit to get it right. Don't leave that up to the contractor- it's in their interest to take a WAG on the high side for water temps to avoid the call-back when it doesn't keep up or it starts short-cycling. But it's in YOUR interest to dial it in, and save the last 2-4% on the fuel use, ending up with much more stable room temperatures to boot.
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