Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

Sizing a boiler in Watertown, MA

mrjohneel Member Posts: 6
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.


  • SWEI
    SWEI Member Posts: 7,356
    edited March 2014
    You are on the right track

    Since your load is so small, you are basically looking at a choice between the smallest mod/con boilers available from a number of manufacturers.  My current working hypothesis is that the design day heat loss should not be less than three time the boiler's minimum firing rate.  Most currently available mod/con boilers have a minimum firing rate of  16-17k on their smallest model.  Here are some of the lowest minimum modulation rates [in square brackets] of which I am currently aware, followed by their maximum output capacity (in parenthesis):

    Viessmann 200-W B2HA 19   [11,580]    (64,655)

    Lochinvar WHN055                [10,450]    (53,250)

    Lochinvar Cadet CDN040        [8,545]     (37,600)

    There may be other options -- I would certainly appreciate knowing about those if someone would share them.

    You can size an indirect water heater to your DHW demand -- the maximum firing rates above will come into play there.

    Don't give up -- a properly sized, installed, and commissioned mod/con is a thing of beauty that will make you smile each time you walk in the door to a quiet, comfortable house.  The low gas bills don't hurt either.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    the boilers modulate down

    "Everyone told me not to worry about the apparently large size because

    the boilers modulate down"

    I am a homeowner, not a heating contractor. My installing contractor did the same thing. But I read John Seigenthaler's big book (second edition) and the installation manual for the boiler the contractor recommended. So, since the contractor did not do a heat loss, I did one three different ways. The easiest was to look at the nozzle of the old oil burner and calculate 70,000 BTU/hour (input) that not only ALWAYS gave enough heat, but rapid cycled its entire life. It never leaked and never died. So I insisted on getting the smallest boiler in the product line. And that turns out to be about twice the size I calculated for my house in this location.

    It is true that a mod-con can modulate down, but based on only one boiler, the smallest in the product line, it does not modulate down anywhere near enough. In my small zone, it will not modulate down enough even when it is 14F below design temperature outside. Even the large zone, that is about 5 times greater heat load, it cycles more rapidly than I prefer when it gets somewhat over 50F outside. So I suggest getting the smallest mod-con you can find that is any good. I cannot say what boiler that would be.

    "And each told me that the DHW tank is the one

    that is driving their higher calculations."

    Perhaps. I just calculated what size DHW tank (an indirect) I thought I would need using the manufacturer's  little charts. And I have never run out of hot water. THe thing is that the boiler is set up to give priority to the domestic hot water, so if there is hot water demand, it shuts off the house and puts the entire output of the boiler into the indirect. And that runs about 10 minutes at a time, two or three times a day. That makes no difference. If I had 6 teen age daughters, and a wife who runs a coin-up laundry in the garage, that would not work out, but I do not have a giant hot water demand. And if I had such a demand, depending on what its profile is during the day, perhaps a larger tank would work. Say if I ran the dishwasher, the washing machine on sanitary cycle, and washing my car with hot water in the wintertime, Only the shower and the washing the car would draw water for a long time.  But how long is a shower? 72,000 BTU/hour going into the indirect and 2 gpm coming out the shower head for 15 minutes? A few gallons at the start for the washing machine. A couple of gallons for the dishwasher? I do not usually do that, and I have never washed my car with hot water. And my indirect is nominally 40 gallons, but 6 gallons are in the outer tank and 34 or so are the domestic hot water. Sometimes I do the dishes or take a shower and the indirect does not even call for heat.
  • Ironman
    Ironman Member Posts: 6,560

    I'll add my two cents worth in concurring with what's been said. Size the boiler to the btu loss of the house, not the domestic. If you feel you need more domestic, then increase the size of the indirect. You won't have any faster recovery, but the additional storage capacity will give you more initially.

    From what you've described, if you had a gas water heater for domestic, a 40 gallon with a 32k btu burner would be sufficient. If you go with a 40 gallon indirect with a 55k btu boiler, you'd have more domestic capacity than the 40 gallon gas water heater would give you.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Ironman
    Ironman Member Posts: 6,560
    A Couple of Others

    Utica/Dunkirk CSS 50k

    T.T. 60k.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • SWEI
    SWEI Member Posts: 7,356
    Utica SSC

    10k minimum firing rate - another one for the list.  Haven't installed any of those, but am always interested to learn.

    I am a big fan both of the Triangle Tube boilers and of the company.  Unfortunately, the minimum firing rate on the PTS60 is 16k.  This is an artifact of a design decision TT made years ago:  Utilize a single control module running the same firmware on all sizes of their boiler -- making it that much easier for installers and distributors to stock spares (an admirable thing in my book.)
  • Ironman
    Ironman Member Posts: 6,560

    I had some input with the design of this boiler and I'm impressed with what the engineers finally put into production. The ECR 97 GB is the same boiler with the added feature of being able to pipe out of it top or bottom and floor or wall mount.

    The HX is a high grade SS helix coil, water tube. They promote it as self cleaning and the ones I've serviced were in perfect shape after running up to three years with no maintenance.

    It has built in p/s with an on board UP15-42 circ on most sizes. Easy to install.

    Check it out on the "Product of the Month" link below.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mrjohneel
    mrjohneel Member Posts: 6
    variable speed circulator

    Interesting read. Thanks. But it bases its argument, I think, on the fact that water circulates through the boiler at the same speed no matter what the modulation. My contractors are quoting me variable speed circulators (so-called "smart pumps") that seem to address the concern, no? Specifically, they propose Wilo Stratus variable speed.
  • bob eck
    bob eck Member Posts: 928
    Burnham Condensing boilers

    I was told Burnham has a zoning control coming out soon that I believe will work with their Alpine and their new condensing boiler coming out soon that will run zone valves or circulator pumps for each zone and you will be able to tell the control the BTU of each zone and the boiler will only fire at the right BTU for each zone calling for heat. I was told the boiler will be able to fire at only 1,000 BTU. This control is being made by Taco just for Burnham and the control will only work with a Burnham condensing gas boiler.

    If anyone from Burnham can give us more info that would be great.
  • mrjohneel
    mrjohneel Member Posts: 6
    follow up

    I asked the contractors how they reached their recommendations on boiler size. One guy said "Proper boiler sizing is not done to manual j but to existing radiation" and then 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 BTUss per Sq foot of cast iron x 328 Sq feet = 60,680) I told him I thought measuring radiators was only appropriate for steam heat not forced hot water. He recommended the smallest unit of the contractors -- an 80000 Burnham Alpine.

    The other guy said that my utility company -- National Grid -- no longer requires a heat loss analysis and the "reality is that heat loss is irrelevant" because of the new modulating technology. He added 17.9% to a basic heat loss number to account for "heating capacity lost through the piping" and he added a measure for my unheated basement (which I didn't do) to come to 68,512 BTU heat loss for my house. He said "you can oversize a boiler by 50 to 100%" and he recommended a 100,000BTU Bosch and reluctantly said he'd consider my recommendation of the 79,000 BOSCH model.

    The third guy recommended a 110,000BTU Lochinvar and hasn't gotten back in touch me as to why he chose that.

    Another guy said mod cons were BS and to go with a Burnham ES2.

    And the fifth guy hasn't got back to me yet. He's the guy that spent the most time measuring spaces for heat loss so I'm very curious to see the number he came up with.
  • mrjohneel
    mrjohneel Member Posts: 6
    one other thing

    I just have to add: my 1928 1510-sq.-ft. house has dense pack insulation in the walls, new high-end storms on all the windows, rigid foam and tape inserted into the weight wells of all the widows (the single pane main windows have 1970s-era springs to operate them), an air sealed garage ceiling; about 12-inches of batt insulation in the attic; an energy efficient back door. Sixty-plus thousand BTU heat loss? Really?
  • icesailor
    icesailor Member Posts: 7,265
    Bathing Pigs:

    I have never heard such hog wash as has come from that group of uninformed heating rookies.

    And they wonder why so many are going to FWA/AC or Hydro-Air.
  • mrjohneel
    mrjohneel Member Posts: 6
    Why 110BTUs

    The guy who suggested the 110,000 BTU Lochvinar just wrote me this:

    "The domestic water tank would be the largest load within home.

    My recommendation would be (WHN110) for this reason and the fact that the next model down (WHN085) is very close in its lowest rated output.

    WHN085 Range (17k – 79K BTU)

    WHN110 Range (22k – 102K BTU)

    As you can see they are close at the lowest output but (WHN110) will support better recovery for domestic water tank."

    Everything I've read says don't size the boiler to the indirect tank. I have no patience for this industry anymore.
  • Zman
    Zman Member Posts: 6,983
    You have won..

    Congratulations, you have made it.

    Insist on the correct size as you have calculated and enjoy a comfortable and affordable system.

    You might want to remind the gentleman that is sizing for DHW that:

    The 4500 watt electric heater he installed last week puts out 15,367 btu/hrs and the 40,000 btu 70% efficient gas heater he installed yesterday puts out 28,000 btu/hrs.

    Enjoy your new found knowledge.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Eastman
    Eastman Member Posts: 927
    cast iron rads

    One thing to keep in mind, if your system contains a lot of water a smaller boiler won't be as responsive to thermostat changes. For example, if you like a night setback and expect to wake up and crank the thermostat, your boiler must first bring the system up to temp and that takes longer with a smaller boiler.
  • icesailor
    icesailor Member Posts: 7,265
    Lots of water:

    It's also why modulation is a wonderous thing of beauty. On higher fire, the load pick-up is higher and as the system heats up, the fire goes down.

    Simply beautiful.

    Like a 80,000 BTU boiler on high fire, turning down to 20,000.

    Like what Richard Pryor said in Silver Streak.

    It is.
  • mrjohneel
    mrjohneel Member Posts: 6
    What we decided

    I decided to go with a Burnham Alpine 80 with a 45-gallon Super Stor indirect.
  • Jason_13
    Jason_13 Member Posts: 299
    Burnham Alpine

    The Alpine at least you can turn down the input rpm's to match the heat loss and leave the fan rpm's higher for the domestic if you want. Almost like two boilers in one. They do turn down to 16k input which may be about 14k output.

    The other thinkg I like about the newer version is the low fire hold for two minutes. Any Alpine produced since the beginning of this year has the option of choosing number one on the response speed, in the central heat menu, when the boiler fires it drives it into low fire for two minutes. this allows time for the water to circulate and hopefully coming back to the boiler. Then the boiler only modulates up to the required rpm's. The same can be for DHW but it is a one minute hold time.

    The new IQ control you will be able to input down to the 1000 btu heat loss but the boiler will still only fire at it's minimum, say 16k. It will total all zone demands and not release from minimum input until the total of zone demands is above the minimum firing rate.

    The biggest problem with oversized mod/cons is not going low enough. Sure they modulate back when firing but the most efficient operation is the bottom side of the operation. If you have a 16k zone and the boiler modulates to 16 that is great. But if the minikmum zone is 16k and the boiler only turns down to 30k that is where you are loosing the most efficiency. So the statement that over sizing is not as important could be no further from the truth.