Need advice on a couple things for heating my home.

Discussion in 'Boiler Forum' started by videopuppy, Sep 20, 2011.

  1. videopuppy

    videopuppy New Member

    Messages:
    10
    Location:
    southeastern pa
    I currently have a Weil Mclain boiler that is about 20+ years old, but unfortunately it has a leaky exchanger. I want to replace this OIL burner boiler, but not sure if I would stay OIL or go LP (no nat gas where I'm at). Here are a few questions I have.....

    First some info....

    House is roughly 900 square feet, 1.5 stories. Cast Rads on both floors (total of 5 rads), one zone. I think the boiler is currently 160,000 BTU's (which seems to be too much). Hot water boiler, not steam. Currently have domestic coil.

    1.) I have been kicking around going to LP but the cost of LP and Oil are about the same when you think of the BTU difference, however, in my case I think maybe I can get away with 100,000 BTU's or so instead of having 160,000 BTU's for this small house?

    2.) It's getting a bit late in the year for doing a complete swap out to LP as I would have to get a tank, or two, run the line, etc.... Should I just put in a used OIL boiler for now? I see a lot of used ones around (some being only 10years old) for around 500.00. They mostly were replaced because people went to NatGas or LP.

    3.) Can I exhaust an LP boiler out thru my chimney? The chimney is about 18 - 20 ft high.

    4.) Anyone know about any rebates going on for OIL or LP installs?

    I think that about does it for now.

    Thanks
    Dave
  2. Dana

    Dana In the trades

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    In SE PA, unless your house is a leaky uninsulated wreck with cracked single-pane windows I'd be shocked if your design day heat load was even 50,000BTUs on a 900' house. Using a crusty old-schooler's "Lessee, OK condition single panes, not so insulated walls, call it 25BTU/foot, 35BTU just in case" crude reckoning you'd come up with a heat load of 32K, and even that's likely to oversized by 2x if the place has some insulation and storm windows or double-panes.

    In any sort of reasonable shape I suspect you're looking at 25KBTU/hr tops. That would make even the smallest oil-boilers more than 2x oversized, but there are some tiny cast-iron LP units in the 30-35KBTU range, and some modulating condensing versions that go even lower.

    Big oversized high-mass radiators can be pretty good for getting better than 90%+ efficiency out of a modulating-condensing propane burner. (which would usually be side vented with plastic flue.) Your 160KBTU oil boiler in that small a house was probably only getting 55-60% efficiency out of the oil due to oversizing & age so a mod-con would be a reduction in annual heating cost. An 86% efficiency side-vented cast iron LP boiler can do OK if it's RIGHT SIZED for the actual heating load. IIRC you'd have to drop to ~82% efficiency to be able to run it up the flue, but the flue would be oversized for any right-sized boiler and would need a narrowing liner, so there's no incentive to go lower-efficiency/higher-cost.

    A smallest best-in-class triple-pass oil boiler would cost about the same to run as an LP mod-con, but that too would require adjustments to the exhaust venting from your 5x oversized pig of a boiler.

    It may not work for you (from a floor-plan or aesthetics perspective) but a 2 or 2.5 ton (24-30KBTU) ductless mini-split heat pump would likely cost less up front and cost less to run than any oil or LP burner, and would likely cover 100% of your heat load. A 2-head "multi-split" with one interior unit for each floor would still be cheaper than a mod-con or high-efficiency oil boiler, and at 15cent/kwh electricity would 40% cheaper to run than 90% LP or 87% oil at current prices. (And you'd get high-efficiency air conditioning out of it too.) Where you're off the gas-grid, ductless split systems tend to be the cheapest/best way to go, as long as it meets the design day heating requirements.

    Design temps in SE PA are in the low-teens F, which isn't a probably for any mini-split. Most still work at +4F, some are still able to put out 70%+ of rated heat at -13F outdoor temps. Any of them on this list will work at 0F, most are good for colder. Anything with an HSPF of 8.5 or greater will be pretty cheap to run.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    Rather than thinking, you need to know what your heat load is. If I had to guess, and don't take this as anywhere near a recommendation to use, I'd guess you actual heat load is closer to 20-30K BTU. Without knowing the type of construction, windows, insulation, orientation and other things, it is impossible to tell. But, that 160K unit definately IS too big unless your walls are swiss cheese.
  4. videopuppy

    videopuppy New Member

    Messages:
    10
    Location:
    southeastern pa
    The house is 1800's, stone, newly updated, plenty of insulation and all windows are no more than 10 years old (double pane).

    I have also thought about the Heat Pump (mini-split) scenario. What deters me is, 1.) How would I get the heat to all rooms? 2.) The life span of such units probably isn't near to that of a boiler. 3.) I think I would need several mini-splits to get the heat thru out the house as it's not one open space.

    Please advise.

    Best
    Dave
  5. videopuppy

    videopuppy New Member

    Messages:
    10
    Location:
    southeastern pa
    Probably a really stupid question.....

    If I'm WAY over BTU ing this house now at 160,000 BTUs and I reduce to around 40 - 50,000 BTU's Logically I should be using a LOT LESS oil, correct?

    Also, since I'm heating using cast-iron radiators do I need more output BTU's? I would need about 40,00 - 50,000 output BTU's?

    Thanks
    Dave
    Last edited: Sep 20, 2011
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,139
    Location:
    New England
    A 160K unit may only need to run 1/4 of the time. Assuming you needed that 40K (not likely if you've got new windows and tightened the house up - it would likely be less) and had a say 40K unit, it would run 100% of the time in the same situation. You lose a lot of energy initially bringing the boiler up to operating temperature. Running 100% of the time, it's constantly running at best efficiency. On a mild day, a good mod-con can reach the high 90's% efficiency. Let's say you really needed 41K and had a 40K unit...the house wouldn't immediately become an icebox, it would gradually cool off to a slightly lower temp. Say it could maintain 72 with 40K, but you needed 41K, it might cool off to 71-degrees. Now, with little reserve, if you used setback, it would take much longer to rewarm the house, but for a steady state, it's a minor decrease.

    CI radiators work as well at lower input temperatures as higher ones. Some of the baseboard heaters don't. So, say you ran the new boiler at 110-degrees on a mild day, they'd be on constantly at that low-fire situation (assuming a mod-con that can modulate its output) with the resultant lower losses, and higher efficiency while maintaining comfort. AN outside reset controller adjusts the boiler temp based on the return water temps and the outside temp. Colder out, it runs the boiler temp higher. Many of the mod-cons can modulate to 20% of its maximum.
  7. Dana

    Dana In the trades

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    Antique homes are often doored-off making them less suitable for heating with mini-splits, but even if one large zone can be handled that way it can be cost effective in cheap electricity areas. Keeping that zone on the warm side to reduce the load of the adjacent zones can cut the overall cost when the differences in operating costs are large. In some high-R home keeping the main zone warm with the mini-split is enough, with only 3-4F differences in temp between the room with/without the interior unit even on design-day. They make multisplits that can handle up to 8 zones, but that gets to be pretty pricey up-front.

    Given the price trends on oil & propane even if the heat pump craps out at age 10 (not likely, but possible), replacing components isn't as expensive as a new-install, and if you're saving 40% on day-1 odds are you'd still be money well-ahead.

    Is this a stone house with an interior studwall or some other means of insulating the exterior walls? If there's no wall insulation that would raise the heat load quite a bit, but it would still be less than 50K, most likely. How much oil did "the beast" use every year?

    If a new oil boiler is still 2x+ oversized for the load it'll be running a good 10 points off it's AFUE rating. But at 5x it's more like 20 points annualized, given the extreme low efficiency when in hot-water only mode. Assuming the beast is rated 85%, 65-70% is probably where it's running. If you buy a best-in-class 90% efficiency oil boiler and it's 2-3x oversized you'll hit ~80% efficiency and save maybe 20%. With a sufficiently small modulating LP boiler you'll be in the 90s on efficiency, but with somewhat more expensive source-fuel BTUs in most markets.

    Oversizing of the radiators doesn't demand more BTUs to deliver the heat, but rather can deliver more heat to the room at lower water temps, which increases condensing efficiency with a modulating LP boiler, but increases the efficiency with cast iron only slightly. With oil the water returning from the radiators entering the boiler can't stay below ~140F in normal operation without exhaust condensation destroying the boiler (or chimney), and with cast-iron LP boiler you'd need to keep return water above 135F. This is what limits the efficiency of cast iron boilers to the mid to high-80s. With oversized radiation and a small cast iron boiler there are some near-boiler plumbing to be done to protect the boiler from extended slugs of tepid water coming back from the radiators.
  8. videopuppy

    videopuppy New Member

    Messages:
    10
    Location:
    southeastern pa

    Please see answers in the quote. Thanks Dave
    Last edited: Sep 20, 2011
  9. videopuppy

    videopuppy New Member

    Messages:
    10
    Location:
    southeastern pa
    So it sounds to me, and I'm holding anyone to anything mentioned here, that if I stayed with oil and reduced my boiler to one that has an output around 40,000 - 45,000 BTU's (guess I'd have to hire someone to figure out exact BTU's) I would probably burn about the same amount of oil, but would be running the boiler more efficiently therefore also extending the boilers life.

    Any recommendations on a boiler in this BTU range?
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    Not quite right...a right-sized boiler would potentially run longer for any heat call, but because it would run more efficiently without the startup losses, you'd save money on oil, too. An oversized boiler loses a lot each time it starts. So, a smaller one that runs longer has fewer starts.
  11. videopuppy

    videopuppy New Member

    Messages:
    10
    Location:
    southeastern pa
    Can I figure out what BTU's I need to purchase?

    I'm ready to buy now, I've decided to stay with OIL for now. Can anyone tell me a way to figure out my BTU's?

    Is there a decent online Calculator? Manual J load calculator?

    Thanks
    Dave
  12. Dana

    Dana In the trades

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    At 350 gallons/year the 160KBTU beast is running well-under 80% efficiency, wasting a large fraction in off-season idling for the HW fraction.

    At your heat load there is no such thing as a right-sized oil fired boiler. The very smallest oil burners out there are ~60KBTU/output, and your hoped-for 45KBTU dream boiler is more than 2x and maybe as much as 3x your actual design condition heat load:

    At 138000 BTU/gallon, and a heating season of about 5200 HDD (Harrisburg PA), 350 gallons is 138,000 x 350/5200= ~9300 source-fuel BTUs per heating degree-day. Dividing by 24 hours that's ~390 BTUs per heating degree hour. Using +10F as a design temp (+11F is the 97.5th percentile for Harrisburg, +14F for Philly) that's 65-10=55 heating degrees. 55F x 390BTU/F-hr= 21,450BTU/hr, and that's SOURCE FUEL BTUs.

    That's a hard upper-bound- the heat load cannot exceed that implied by the heat content of the source fuel, unless you're also running a wood stove or some others supplemental heat source you're not telling us about (such as keeping 28 dogs indoors or something. :) )

    Assuming 85% combustion efficiency that works out to about 18,250BTU/hr. If you discount it 20% for the hot water fraction (could be more, could be less) the actual heat load is on the order of 15,000 BTU/hr.

    That's a credible number for a very tight house with good windows & decent attic insulation, even with R1.5-R2.5 stone or masonry walls. I have 2200' of fully conditioned above grade space, and my +10F heat load (including 1500' of semi conditioned basement) is under 30K, with known gaps in the insulation, and circa 1923 windows + so-so storm windows.


    Even the very smallest oil boilers out there have outputs 4x that 15K number. Even if you assumed 20KBTU/hr, the smallest oil boiler would be ~3x oversized.

    Going forward with oil, take the VERY SMALLEST of any of the available options, and protect the boiler from sub-140F cold water return from radiation with a boiler bypass with a thermostatic valve. There's probably sufficient mass in the big radiators to keep it from short-cycling, and you can probably do well with running the radiation at temps LESS than 140F. A Buderus Logano 115WS/3 or similar would be about as good as you can do for a chimney vented unit, and you'll probably have to put in a narrowing stainless steel liner in the chimney to have sufficient draft.

    Taco has a very decent load tool downloadable for the low "free" price of giving them your email address & name:

    http://www.taco-hvac.com/en/embargo_form.html?soft=TacoLoadInstall1.3.5.exe

    The instruction vidi lives here:

    http://www.taco-hvac.com/products.html?current_category=370&show_vid=1

    If you don't already have air-conditioning, putting in a 1 ton mini-split, and running it in heating mode in the shoulder seasons too.
    With R2-ish exterior walls you really can't heat doored-off rooms mid-winter with a mini-split in the main areas- the partition walls will have a similar or higher R value. But when it's 50F outdoors it won't matter as much, and the COP of a mini-split will be greater than 4. It'd knock better than 75 gallons/year off the annual total oil use, and add maybe $150 to the power bill, but it would also serve to keep the humidity & temp at near-perfect comfort levels in summer a much higher efficiency than any window unit.

    If you find a good deal on a nearly new used triple-pass boiler with 100K or less output you can save maybe $1500 on the installed price, but the installation won't be free- you'll be into it for at least $5K by the time you're done, especially if you're including an indirect tank type hot water heater which is a far better option than an embedded coil for both efficiency and first-hour gallons. See: http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf
  13. videopuppy

    videopuppy New Member

    Messages:
    10
    Location:
    southeastern pa
    Dana, thanks for all the info (thanks to all that replied), thanks for taking the time to answer me so in depth. Definitely more to it than I ever imagined. That boiler was in my house for around 18 to 20 years and did a good job until I broke it. I very mechanically inclined, not a dumbass, but I certainly did a number on one of the exchangers. It's an old 68 series (I think) Weil Mclain. The side of the unit is tagged 151,000.

    Have you heard of measuring rads, counting the number of sections in the rads as a way of measuring for BTU's needed?

    I do have an HVAC mechanical friend to help me on this, but he mostly does industrial boilers so not sure how up to speed he is on residential units.

    I do have a mini split now, a 2 ton unit, but it's not a heat pump version. I use it in the upper level and the cool falls and I pull it around with a fan or two here and there. The walls are not insulated on either side, but I have thought about doing the outside. That is something I need to research.

    Not even sure I'll keep this house too much longer so that also throws a wrench into my plans as to how much I want to spend on it. I want to do it right, but no necessarily top end.

    Thanks
    Dave
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    There are lots of WAG or rule of thumb ways to estimate heat needs, but none of them will produce an efficient system for your lowest operational costs. One reliable way is to use the heating degree day info and the oil use (and I think Dana did that for you already). Most of the better manual j calculators overestimate the need for heat, but some aren't too bad.
  15. videopuppy

    videopuppy New Member

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    Location:
    southeastern pa
    Thanks guys. I will definitely clear my head and sit down and try to figure out all the info in Dana's reply.

    Thanks much.
    Dave
  16. Dana

    Dana In the trades

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    Counting the total surface area of radiators IS the right way to figure out the sizing of a STEAM boiler, but it's a terrible methodology for sizing hydronic systems.

    The math on source fuel BTUs is fairly straightforward. You're trying to figure out from gallons per year what how many BTUs per hour it takes to stay warm under "design conditions" (the outdoor temp about the 99th percentile coolest hours of the winter). In your neighborhood that temp is about +10F (in mine it's about 0F.)

    The information you know is in gallons per year (~350)and the heating degree-days (HDD) per year, which ~5200, in your neighborhood. The heating-degree-days of any particular day is how many degrees the average outdoor temp was below 65F (the rough average break-even point for heating/cooling for most single-family homes.) Count them all up for a season, and in your neighborhood it'll be around 5200, give or take a few percent, rarely differing by as much as 10% in any single year.

    From gallons/year and HDD/year you can calculate gallons/HDD.

    Great, but you still want to get to BTU/hour, right?

    Since there are 24 hours in a day you can calculate gallons per degree-hour. (That is, gallons per degree below 65F). So divide the gallons/HDD number by 24 hours and you get gallons/degree-hour.

    To convert the gallons to BTUs, the generally accepted approximation for #2 heating oil is 138000/gallon, so multiply gallons/degree-hour by 138,000, and you have BTU/degree-hour.

    Then to get to BTU/hour at +10F, multiply time the difference between 65F and 10F (55 degrees), and you'll have BTUs/hour of source-fuel consumed.

    But in an 85% burner (most flame-retention burners of 20 years ago at least started out in that range, when properly adjusted), 15% of the source fuel energy went up the flue, and only 85% of the BTUs went into heating up the boiler & heating system water. So the actual heat load (the amount of heat necessary to keep the house warm) would be 0.85 times the source-fuel BTU/hour number.

    You can pad it 5-10% if you use deep night-time setbacks, but since you're also heating hot water with it odds are the summertime & hot water heating BTUs that went down the drain far exceeds any setback savings. Take that 21,450 source BTU/hr number multiply by 0.85 and you're in the 18.25K range, and that is a firm upper bound. Being that the boiler is way oversized for the load and heating domestic hot water, 18.25KBTU/hr is probably at least 10% to the high side of reality, and may be more than 20% above what would actually keep you warm.

    Which is the long-winded rehash of the conclusion that there aren't any truly right-sized oil boilers out there for heat loads that low. Some modulating condensing propane units can go about that low at lowest fire though. With oil ~55-60K (output) would be the very smallest, and probably not in a chimney-ventable unit. The 3-plate Buderus is 74K out, it's (relatively) cheap, and a reasonable compromise.

    No matter what you install for an oil boiler, the bulk of the price is going to be the installation and system (re)design not the boiler. The difference in price between the smallest chimney-ventable Buderus and a Craigslist-scavenged boiler will be less than $1500, maybe $2K if you put in an oversized beast with an internal HW coil. A brand new G115WS3 is only a couple grand- add another $600-1K for an indirect HW heater, but then there's resizing the flue with the right-sized stainless liner (another grand), and a couple of plumber-days, maybe three by the time the system is purged and tweaked. Plus there are pumps/valve/fittings and small amount of design & tweaking of the bits of plumbing necessary to keep the return water temps in the non-destructive range and you'll be talking $6-8k anyway you cut it with a professional installation. Reselling the house with a brand-new brand-name boiler with a warranty may be easier than with a hacked-in-DIY craigslist-special of unknown history/longevity.

    Insulating from the outside is possible, but it'll be expensive, not a short-termer's project under any circumstances.
  17. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    Location:
    Minneapolis
    There are several condensing propane boilers on the market at 50mbtu input and matched with a 30-40 gallon indirect, would likely be a perfect match for your radiator heated home. All good hydronic heating systems start with a heat load analysis. Degree days are of marginal use, but fuel bills can help. We use Wrightsoft manual J* in conjunction with blower door test where appropriate. We also measure radiation where competitors insist on over-sizing boilers, since it makes no sense to install more boiler than the radiators can handle. This may help.

    http://www.badgerboilerservice.com/contractor.html
  18. Dana

    Dana In the trades

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    Yabbut Morgan, on the 21st he posted:

    "I'm ready to buy now, I've decided to stay with OIL for now." (emphasis mine)

    I'd agree that he would likely be able to max out the efficiency with a propane mod-con with his radiation (ower his heating costs overall) but the up-charge for switching to a propane mod-con vs a smallest-in-class cast iron oil burner probably isn't going to pay for a short-termer like him. His as-used peak heat load will be pretty close to the min-mod of a propane mod-con and if the radiators can deliver design-day heat with 150F water out of the boiler he'd be in condensing mode even at design condition. Assuming the radiation was as oversized as the existing boiler, that is a likely scenario, and the average AFUE would be in the 95% range. With a smallest in class oil burner he's looking maybe 80% after cycling losses are factored in, but it's cheaper per source-fuel BTU.
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