(206) 949-5683, Top Rated Plumber, Seattle
Results 1 to 15 of 18

Thread: Need advice on a couple things for heating my home.

Hybrid View

  1. #1
    DIY Junior Member videopuppy's Avatar
    Join Date
    Sep 2011
    Location
    southeastern pa
    Posts
    10

    Default Need advice on a couple things for heating my home.

    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. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009
    Location
    01609
    Posts
    2,720

    Default

    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. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,401

    Default

    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Junior Member videopuppy's Avatar
    Join Date
    Sep 2011
    Location
    southeastern pa
    Posts
    10

    Default

    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. #5
    In the trades Dana's Avatar
    Join Date
    Jan 2009
    Location
    01609
    Posts
    2,720

    Default

    Quote Originally Posted by videopuppy View Post
    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
    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.

  6. #6
    DIY Junior Member videopuppy's Avatar
    Join Date
    Sep 2011
    Location
    southeastern pa
    Posts
    10

    Default

    Quote Originally Posted by Dana View Post
    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.


    >>>>>>The mini splits are pretty much out for me as there really isn't enough wall space to accommodate them without them looking horrible.



    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?


    >>>>>No, there is no interior or exterior insulation on the stone part of the house.

    >>>>>> I normally would go thru about 350 gallons, usually less, that is also with a domestic coil.



    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.

    Please see answers in the quote. Thanks Dave
    Last edited by videopuppy; 09-20-2011 at 06:24 PM.

  7. #7
    DIY Junior Member videopuppy's Avatar
    Join Date
    Sep 2011
    Location
    southeastern pa
    Posts
    10

    Default

    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?

  8. #8
    DIY Junior Member videopuppy's Avatar
    Join Date
    Sep 2011
    Location
    southeastern pa
    Posts
    10

    Default 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 by videopuppy; 09-20-2011 at 01:53 PM.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,401

    Default

    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

Similar Threads

  1. Interested in solar water heating system for my midwest home
    By Strategery in forum Solar and Geothermal Water Heating Forum
    Replies: 7
    Last Post: 04-02-2012, 02:14 PM
  2. Furnace cycles while heating the home
    By nestman in forum HVAC Heating & Cooling
    Replies: 14
    Last Post: 01-07-2010, 04:03 PM
  3. Thermostat/Heating Home?
    By rundawg325 in forum HVAC Heating & Cooling
    Replies: 2
    Last Post: 01-14-2009, 08:52 AM
  4. need advice!---new (old) home well problems
    By artsychick in forum Pumps and Tanks Well Forum & Blog
    Replies: 3
    Last Post: 05-31-2007, 06:52 AM
  5. builing a home salon, need advice ....
    By jekky in forum Remodel Forum & Blog
    Replies: 3
    Last Post: 01-17-2006, 09:51 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •