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Thread: Boiler help

  1. #46
    Retired Defense Industry Engineer jadnashua's Avatar
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    Are you more comfortable out on a sunny day with constant radiation, or when a cloud covers the sun? Having the boiler run constantly at the proper output means the radiators are a relatively constant temperature rather than getting super hot for a few minutes, cooling down, then getting hot again. An oversized boiler just can't run constantly to MATCH the heat lost, it MUST cycle. A right-sized boiler runs at the design temp, and with smart controls, can adjust its output to match the need...just enough heat in to match that lost - i.e., constant temp, constant comfort. Gross example...take a blow torch, momentarily point it at something, then turn it away (simulating turning off)...you can achieve a reasonable average temp, but it won't be particularly comfortable. Just like a dimmer on an electric light can get just the right illumination, something like a mod-con can achieve just the right heat input to get things 'just right'. This only can happen when the size is matched to the load.

    Not having enough radiation capability is a problem, but you have plenty. The only time a big boiler might be required is if you used a huge setback on the coldest night of the year. With today's better boilers, a small setback is okay, but your comfort level and fuel use is likely to be similar by just setting and forgetting, or a modest reduction if you like to sleep cooler. Personally, I just adjust what covers I use and leave the thermostat the same. Before I finished my remodeling, I used a hydro-air system...the boiler was and is more than adequate, but the heat exchanger I had was not able to provide enough heat to allow a big setback, but worked fine when using a modest one or keeping things constant. In this case, I was radiation limited. Then, I added in-floor radiation, and that problem went away. Constant temp allows it to be comfortable, regardless of the outside conditions. ANd, with the smaller boiler and running more efficiently, my fuel bills went down significantly.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  2. #47
    In the Trades Tom Sawyer's Avatar
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    Boilers ain't like *****es. Bigger isn't better
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  3. #48
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    Quote Originally Posted by Dana View Post
    Yeah.... RIIIIIGHT! :-)

    Even the worst implemented Man-J on an 1800' house with a design temp of +15F wouldn't come up with a 150K boiler with 125K of output, a whopping 70 BTU/hr per square foot of conditioned space- which would be the heat load of an 1800' tent @ 15F with a flap left open.

    The lazy rule of thumb guys in my neighborhood use 25BTU/ft for an older home with at least some insulation and storm windows, 35BTU/ft for uninsulated houses with single pane glass (twice what the 150K boiler recommendation implied). Design temps around here are 0-5F, and these rules of thumb reliably oversize the units by a substantial margin. Applying the 25BTU ft to this house would still only be 45K, and barely-legal efficiency 55K-input boiler would cover it, but it would in fact be about 2x oversized, given how generous the fuel-use calculation was on boiler efficiency. (There's simply NO WAY a 1932 vintage 5-6x oversized boiler with a retrofit oil burner is hitting anywhere NEAR 85% efficiency!)

    Typical heat loads @ +15F for 1930s houses in that area will be well under 15 BTU/ft, and many will be around 10. Odds are pretty good that the true heat load of this one between 20-25K, and cheap 2-plate atmospheric-drafted 50K cast iron boiler would handle his load down to inland-Alaska style design temperatures south of -25F. This worry about a 50K boiler being too small for the space heating is just nuts- most 1800' homes in northern Minnesota can be kept comfortable with a 50K boiler (but 60-70K wouldn't be an efficiency-disaster overkill, for those handful of days every 50-100 years when it hits the -50s.)
    I do not use rules of thumb, i do not use btu per square ft. I use manual J. Also i install mostly mod cons, and i do worry more about low firing rate than high. Mod cons properly set up can follow the load very closely.

  4. #49
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    Quote Originally Posted by Steve Lanham View Post
    Longer burns to me means longer time for your t-stat to be satisfied. Which means sitting there listening to your boiler run waiting for your room to warm to desired temp.
    you do not need thermostat with properly set up outdoor reset mod con. Anybody advising installation of thermostat or zoning with mod con installation does not understand principles of outdoor reset and constant flow systems, and mod con operation in general. None of my installations have a thermostat, some have TRVs to control temp room by room.

  5. #50
    In the trades Dana's Avatar
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    gennady: Just to be clear, that wasn't a knock on you or your methods- the saracasm was about very obvious LACK of the use of a Manual-J by his contractors, all of whom were recommending 100K+ burners for a sub-30K load.

    That said, in a house where they are not going to be changing up the radiation and clear fuel-use history indicating a low/very-low load a Manual-J is a waste of time. The napkin-math on fuel use is more than adequate for sizing the boiler. In the US the availability of boilers with sub-30K (max) output is extremely limited- any mod-con installed in this place would have to be the smallest-of-the-line from most vendors. And with sub-25K heat load the low-fire output of most mod-con is still too high to get much modulation & load tracking out of them. Given his radiation type & size outdoor reset wouldn't have sufficient added value in either efficiency or comfort over a hot-water heater based fixed-temp system to be worth paying much for.

  6. #51
    DIY Junior Member Steve Lanham's Avatar
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    Dana.. I just set an appointment with someone to do a manual J for tomorrow . Now your saying its a waste of money. I was thinking everyone recommended a manual J. Well, this contractor is supposed to be a expert in steam and water systems. I'll question him tomorrow and see if he thinks its nessesary . I'm glad this isn't a rush job..Thanks again !!

  7. #52
    In the trades Dana's Avatar
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    A properly done Manual-J would more than likely just cement the case for a smallest-in-class boiler. If the sole purpose of the exercise is to size the boiler, it's a waste of time. Show him the napkin math on the fuel use.

    But Manual-J can be useful for figuring out where envelope upgrades would add to comfort & efficiency too, if they're willing to run the numbers on the building as-is, and again with tighter air-leakage plus some insulation upgrades.

    Air leakage is almost always worth reducing (and unless they're out there with a blower door their numbers will just be a boilerplate WAG), and fixing all the gaps in the insulation is usually worth it too, eg:

    The biggest thermal gap in most pre-Y2K homes (as well as the biggest air leakage) is at the foundation walls, foundation sills, & band joists. A poured concrete foundation has an R-value of about R1, the band joists are about R2, and even if you let the basement run a 55F when it's 15F outside, that's a 40F delta on an R1/R2 surface area, losing 20-40BTU/hr per square foot. In an 1800' a 2-story house with a 900' footprint there's 120-150' of perimeter, and the band-joist alone is on the order of 1500-2000 BTU/hr of heat loss, and if you have ~18" of exposed foundation that'll be worth over 5000 BTU/hr when it's that cold out, making it well into double-digits as a percentage of the total heat loss, and that's with a less-than-cozy basement temp(!). Putting 1.5" of fire-rated rigid Thermax on the basement walls & band joist with edges taped & sealed reduces that part of the load by about 90%, and the basement would than run in the mid-to high 60s F, increasing the surface temp of the floor on the first-floor by a few degrees, with a noticeable improvement on barefoot comfort, and reducing the summertime relative humidity to boot.

    Spray foam does a better job of air sealing the band joist & foundation sill, and goes in quicker than cutting & cobbling rigid foam, but at the very least spray foam would be used to seal the crack at between the sill & foundation, the sill & band joist & subfloor, as well as any cut rigid foam at the band joist & sill.

    Air sealing floor/ceiling between the attic and conditioned space is the other biggie from an air infiltration point of view. Fixing the air leaks at at both the bottom & top of the house mitigates the stack effect pressures far more than any leakage that happens in-between, with a direct reduction of both heating and (primarily latent) cooling loads. Even a blower door test number doesn't tell you WHERE the air leaks are, and models that use a blower door number to come up with an infiltration rate are by nature extremely imprecise, if better than a WAG.

  8. #53
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    We do a lot of basement remodels, tear-outs, passive radon, insulation, radiant floors etc. But if they only have money for one HVAC improvement we use it to foam the rim "band joist and foundation sill" as it is the lowest part of the thermal "stack" and with all that Dana suggested including condensing warm humid air in and already wet basement.

    Well done Dana.

    PS Using the last fuel bill may tell you with certain accuracy what the heat load was for that particular month in that particular heating system but little else. The reason we do ACCA Manual 'J' is because the calculations predict the load over time, purposely ignoring mean extremes. Like blower door tests, fuel usage is just another tool.

    Residential HVAC contractors that are willing and able to produce a Manual 'J' are a small and elite group among an admittedly backward group of technicians. This is particularly true of the small minority that specialize in hydronic heating.

    As long as they do something to properly size the boiler, I am OK.

  9. #54
    DIY Junior Member Steve Lanham's Avatar
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    Well here's what it is ...had the manual J done..the napkin math and the J were identical...46k would keep the house warm no mater what...i mentioned that I was also putting in a 50 gal indirect in previous post..the spec. on the indirect states that a TriangleTube Smart 50 should have a boiler cap of 121 MBH. The manufact. said a 100K out put would do the job...Does anybody have any knowledge of the Solaia boilers...and would it be best to stay with 2ea. 40gal gas HWH and forget the indirect so that I could size the boiler more for the heat load of the house instead of the indirect..Thanks !!

  10. #55
    Nuclear Engineer nukeman's Avatar
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    Generally, manual J also has some padding and tends to over predict the load by maybe 20% or so, so even at 46k, you would never have a problem heating the place. For the indirect, you normally don't include that in the load. You can set it as a priority zone and the boiler will heat the water (instead of the house) when needed. People worry about this, but you generally will never notice.

    I think that I would go for a boiler in the 50-60k range (or smaller, depending on what you can find) and set up the indirect. I'll let the other recommend the brand, etc. I don't have much experience there. The only time that you may want more burner than that is if you had a large tub that you had to fill or wanted endless showers or something like that.

  11. #56
    DIY Junior Member Steve Lanham's Avatar
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    No large tub other then the clothes washer...everyone generally showers...I just feel like I should stay with the man. spec...also if the installer recommends this then thats the only way to get the warranty...I'm still waiting for some quotes so we'll see how it all ends soon enough...Thanks !!

  12. #57
    Retired Defense Industry Engineer jadnashua's Avatar
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    It's not that you can't use a smaller boiler to heat an indirect, it's that the amount of time it takes will be longer. If you look at the IWH spec sheet, it will have things like first hour delivery, and a few other things. To meet those values, you have to have the specified input heat. You have several choices...either get a bigger tank, or get a bigger boiler. A bigger boiler will cost you more upfront, and because it won't be used in its efficient mode 99% of the time, will cost more all of the time. A bigger indirect will cost a bit more upfront, but not cost any more runtime, and the boiler can then heat the house at its best efficiency forever, saving money all of the time. The downside is that it will take a bit longer to reheat all of that water in the tank. Probably not a big deal.

    IOW, the indirect manufacturers want their tanks to look good in the amount of hot water they can supply, so to do that, they specify a significant boiler. FWIW, I've got a 60g indirect with a 60K BTU boiler, and I never run out of hot water or have the house cold. To meet the IWH specs about first hour delivery, I'd have to have a much bigger boiler. The tank is maybe bigger than I need (but I do have a 6' air tub that gets used at least daily), I've never run out of hot water, and the house is never cold as a result.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  13. #58
    In the trades Dana's Avatar
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    The right thing to think about with the indirect vs. the size of the boiler behind it when running the indirect as "priority" zone is to compare the boiler output with that of a standalone tank's burner. A 50 gallon gas-burner typically comes with a 35-45KBTU/hr 80% steady-state efficiency burner, and is delivering no more than 36,000BTU/hr to the water. A 50K mod-con will deliver 88-90% combustion efficiency (unless you're keeping the water at 150F+ storage temps) and about 10KBTU/hr MORE heat to the water than the standalone.

    The 50 gallon standalone tank's performance doesn't suck- plenty of hot water for more then 90% of all households, but a 50 gallon indirect paired with a 50KBTU/hr mod-con running as the priority zone still beats it on raw hot water delivery performance.

    If your primary hot water load is showers (not tubs), you'd get both more performance with much higher "apparent capacity" and much higher efficiency by adding a drainwater heat recovery heat exchanger.







    At a 2.5gpm shower flow adding a 50% efficiency unit (typically a 4" x 48" or a 3" x 60") like adding more than 25KBTU/hr of burner output, enough to turn a 50K mod-con + indirect into an "endless shower" situation.

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  15. #60
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Dana;

    A low efficiency tankless with recovery coil... I wouldn't believe the numbers even if they came from other than the manufacturer. Were it insulated, maybe.

    Poor guy has all that fancy equipment and can't afford sheet rock?

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