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Thread: Insulated, Tiny (720s.f.) Home Needs New Boiler - Advice?

  1. #1
    DIY Junior Member DoggoneDJD's Avatar
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    Question Insulated, Tiny (720s.f.) Home Needs New Boiler - Advice?

    We removed 2/3 rds of our home to leave us with a 720 s.f., single-story cottage for our senior years. We've air sealed, insulated and replaced windows. We live in central Mass. From using http://www.loadcalc.net/load.php we think we need about 15,000 btus to heat our place, but we'll see what the pro says because we really don't know anything about HVAC but are trying not to shoot ourselves in the foot.

    Today we have the old oil boiler, a Smith Series 8 -3H, MBH 113 & Superstor 40 gal. For 25 years it has served us well. Since the demolition we have been left with 24.5 feet of SlantFin baseboard. The boiler temperature is set at 200 degrees. Don't know why. Flue temps run about 525 degrees. By our calculations, we are getting 710 btus per foot so it works out to about 15,000 btus. The current set up is heating our home just fine. At -4 degrees we are toasty at 74 degrees in here. We lower the thermostat at night and have no problem getting it up to temp when 10 degrees outside. Loads of hot water. We have used 600 gallons of oil/year since the downsizing. It doesn't go on and off constantly or anything. It just works fine, though I'm guessing from what we've been reading it could be more efficient if set differently. I was hoping we'd save more in oil than we have but Hubby says it is because we heat hot water all summer.

    We hope we are in the last year of fixing the place up and since the boiler is 25 years old thought we'd address the heating system.

    We looked at single source heating like mini split or gas direct vent stove but prefer a heat source in each of the three rooms. Due to asthma we are not considering air. We are open to radiant heat (panel radiators) or baseboard. We use oil now so we have been looking at oil boilers but can't find anything small enough. (Hubby insulated (R20) basement/crawlspace walls so he likes the heat off the boiler which keeps below grade between 64 degrees and 72 degrees even in coldest winter, and keeps floors in living area warm.) We are shopping for a backup generator so are open to the idea of using propane if necessary for boiler.

    One HVAC guy suggested Viessmann Vitodens 222-F but doesn't have pricing yet because it isn't available yet. We've looked at Viessmann but here's the rub: is it too fancy? We get nervous when a boiler is so sophisticated. Will we spends thousands of dollars to save a few hundred? If Buderus made a tiny oil boiler I'd say "paid" and have done with it. (Maybe put in Buderus radiators because I do so miss radiant heat. There ain't nothing like a wood stove, but it is no longer in the cards for us.)

    Does anyone have a clue how to heat such a tiny place?

    Thanks, Deb

  2. #2
    In the trades Dana's Avatar
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    To heat a tiny place with a reasonably open floor plan, the mini-split plus radiant cove heaters is probably the right approach, and it will cost half or less what it costs to heat with best-efficiency oil boilers (even at N-Star or Nat'l Grid pricing), at well under half the installed cost.

    As you've discovered, there is no oil-boiler made that wouldn't be at least 3-4x oversized for your actual heat load. Mini-splits are amazingly quiet, comfortable and efficient, if you size it correctly then "set & forget", letting it modulate with the load. Radiant cove heaters are also quite comfortable even when the room isn't yet up to temp, because they work by heating up the objects in the room directly (including the humans), not the air. With the doors open to the larger open space heated by the mini-split the temperature delta stays pretty small, unless you have a lot of window area or mediocre insulation in the exterior walls of that room. With the mini-split supplying the bulk of the heat, and letting the other rooms lag a bit when unoccupied it stays comfortable without using a gazillion kwh of electricity. (I have relative living in a really poorly insulated place a bit bigger than yours who heats with a 1.5 ton Mitsubishi, and uses electric baseboards in the master-bedroom only when the temps drop below 25F, which in her case isn't every day, otherwise I'd set her up with cove heaters.)

    The 15,000 BTU/hr probably a bit higher than the actual heat load, may even be on the high side, but simply multiplying by the length of the baseboard isn't going to tell you much. If you have a regular oil-filling service and they stamp a "K-factor" on the slips you have enough information to get there.

    If you heat the main rooms with the mini-split, and temperature-balance the doored off spaces with radiant cove heaters you'll be golden. With a room-by-room heat load calculation you can size the cove-heaters reasonably, and at 600W or less they're easy to control via a combination of occupancy sensor & line-voltage thermostat so that they only come on when the room is occupied. (A manual override switch for the occupancy sensor for use in bedrooms while sleeping is easy to deal with.)

    "Well insulated" isn't a well defined description- what is the construction of the wall assemblies layer by layer, including the cavity insulation amount/type, the window U-factors (or describe them, if you're not sure), and the amount/type of insulation in the attic? (Describe how the crawlspace & basement walls are insulated too.)

    Got a ZIP code? (For fine tuning the outside design temps, etc.)

    A propane fired condensing hot water heater (AO Smith Vertex) and low-temp panel radiators with room-by-room micro-zoning could get you there too, but at 2.5-3x the operating cost of heating with mini-splits & occupancy-sensor limited cove heaters, and 2x the installation cost of the all-electric solution. But it's better to just get out of the volatility of the fossil-liquids markets- the price trends aren't looking all that good, and a the high cost of new-production oil/gas it'll clearly never go back to the relative pricing of 1970, or even Y2K. The current low price of natural gas is only due to the high prices they get out of the liquid fractions (like propane) out of those fracked wells. Fracked dry-gas is a serious money loser at the recent years' price averages. Even though it came out of the same holes in the ground, you haven't exactly seen propane prices falling with the newer extraction techniques.

    [edited to add]

    You live in a 6500-7000 heating degree day (base 65F) climate. At 600 gallons a year in an 85% burner that implies (worst case) 600/6500= 0.092 gallons per heating degree-day.

    That means you're consuming 138,000 x 0.092= 12,696 BTU per HDD source fuel...

    ...and 0.85 x 12,696= 10,792 BTU/HDD as heat that went into the system.

    With 24 hours in a day, that's 450BTU per degree hour (base 65F).


    Central MA 99% design temps run between 0F and +5F, call it 0F, and you have 65F-0F= 65F heating-degrees.

    With 65F heating degrees, at 450BTU/degree-hour, your implied heat load is 65F x 450= 29,250 BTU/hr.

    That's a lot bigger number than 15K, but also a very big number for a tight well insulated 720' house.

    But if you're using the boiler for domestic hot water with an embedded tankless coil in the boiler it skews those number considerably, since it burns a lot of oil in idle mode during the 4 months of non-heating season, which is why a K-factor on a mid or late-winter fill up would be useful.
    Last edited by Dana; 01-10-2014 at 07:31 AM.

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    DIY Junior Member DoggoneDJD's Avatar
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    Dana, thanks for the thoughtful reply. The easy stuff first. Zip code is 01005. K factor is 8.163.

    You're right about not being able to just multiple baseboard btu's to get household needs, I'm sure. If nothing else, the cast iron boiler radiates heat throughout the basement/crawl space which warms our toes upstairs The second HVAC fellow called last night to say he calculates the need at 26,000 btus. He would like to put in a System 2000 by Energy Kinetics with radiant panels. He explained that with the oil boiler they say to size it 30% bigger; maybe that is because of the hot water?

    When I read an article on how to calculate whole house load, including infiltration, I came up with 29,585 btus (figuring a differential of 77, -3 to 74 degrees -- we get ten degrees colder up here than Worcester so I used Pittsfield for the outside design temperature). When I used the computer (which forced me to use an inside design temperature of 70 degrees) I came up with 15,971 -- exactly what I had with the manual calculations for whole house load but without the infiltration numbers. So maybe 29,585 btus is correct. A little knowledge is a dangerous thing.

    And of course you are right that it is very difficult to quantify insulation and air sealing especially as we won't have a blower door test until all is done. We are NOT doing a deep energy retrofit. It was way beyond our means. We are doing the best we can with sweat equity, Wigluv tape and caulk.

    U-factor of windows and french sliding door is .27. Hubby is using Wigluv and caulk to air seal inside & out (plywood sheathing seams taped, plasterboard caulked & special electrical outlets). All walls are 2 x 6 wood framed traditional construction. Only the south wall is insulated as it will be: dense pack cellulose. Because the installer (the only one we could find that uses "healthy" cellulose) blew out one of the plasterboard walls because he didn't put in the second hole for air pressure release -- which took us 3 days to clean up (there was cellulose inside the toaster) -- we will probably redo the R19 fiberglass in the rest of the walls with R21 dense pack fiberglass rather than have him back.

    I had to look up cove heaters. Had never heard of them. Will read on. Thanks for the direction. Much to consider.

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    In the Trades Tom Sawyer's Avatar
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    I would go with a mini-split with three outlets. Mitsubishi and Fujitsu make them
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Junior Member DoggoneDJD's Avatar
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    Electric heat. We pay $.1751/kwh. Are they THAT efficient?

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    In the trades Dana's Avatar
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    If the individual room load isn't 5KBTU/hr or higher, you're better off with radiant coves. The smallest heads are rated around 7KBTU/hr. If you oversize the head for the space it'll run at lower efficiency, and it may not be worth the additional upfront expense. The multi-splits never quite hit the same efficiency as the single head versions, but they'll still do pretty well, and the better ones work just fine at -5F.

    You might be looking at 2-3 head 2-2.5 ton ductless system, or pair of 1-tons or 3/4 tons- depends on how the room-by-room loads work out. A pair of 1- tons or a 2.5 ton triple-head will still cost less to install than a System 2K (even without the new radiation), and cost less than half to run. A 3-head 2.5 ton multisplit is probably still only 2/3 the cost of a system of the all-in system replacement using a System 2K and panel radiators.

    Even the very smallest System 2K (EK1 Frontier) puts out about 82,000BTU/hr. That's about 300% of your ~26-29K estimated heat load (and at $4/gallon would be far more expensive to run than any ductless solution.) You'd NEVER upsize a boiler for the domestic hot water unless you're filling a 500 gallon super-soaker party tub on a regular basis, but for small heat loads you don't have much of a choice- they're all ridiculously oversized. Oversizing by 30% is fine- it'll hit it's AFUE numbers easily. Oversizing by over 100% is just a painful reality of the just how big the very smallest oil-burners are. The System 2K will still run at reasonable efficiency at 3x oversizing due to it's heat-purge controls, but so what? It's an expensive fuel, and WAY too much boiler, any way you cut it.

    The 99% temperature bin in Barre is about +2F, to Worcester's +5F ,and Pittsfield's -3F. The mean January temp in Barre is about 24F to Worcester's 24.5F and Pittsfield's 22F. Barre weather is a lot more like Worcester than Pittsfield, not that it makes a huge difference in your heat load.

    A 2x6 wall with dense-packed cellulose comes in at a whole-wall R of about R14 after thermal bridging, a wall U-factor of about 0.07. R19 fiberglass if perfectly installed in an air-tight cavity comes in at about R13, or a U-factor of about 0.075. Dense-packing over the fiberglass will bring it in-line with the cellulose figure, and will plug the air-leaks in the cavity with cellulose during installation, reducing infiltration.

    But if you install 2" of rigid EPS on the exterior of the sheathing under the siding it would run about R22 whole-wall, a wall U-factor of about 0.045, about half the heat loss of an R19 batt wall. That's not super-insulation, but it's sufficient for protecting the sheathing from interior side moisture drives, eliminating the need for strong vapor retarders. (With cellulose you don't want to put polyethylene sheeting on either side of it.)

    You can get 2" EPS and other foam on the super-cheap from a number of local vendors. Search the Worcester Craigslist materials section for rigid insulation, or foam insulation there is a very small operator in Winchendon who usually has reclaimed roofing foam who occasionally advertises there, as well as a few others posting more regularly. Worst case there is Insulation Depot at Waverly Street in Framingham (insulationdepot.com). At the prices these vendors charge for reclaimed & surplus foam board it's cheaper than batts, about 1/4-1/3 the cost of virgin-stock goods.

    You need at least R7.5 on the exterior to be protective of the sheathing, and you have to de-rate polyiso to about R4.5-R5/inch for this type of application. EPS is cheap, runs about R4/inch at any density. If they have XPS it's rated R5/inch when new, but figure on R4.5/inch after 25 years of service. For reclaimed goods, call it R4/inch, just like EPS.

    Hopefully you did the foundation insulation at least partially as foam? (If not, depending on how it was done there may be some re-working to do, to avoid future mold risk. It's a messy subject which I've covered multiple times on the remodel forum on this site if you want to search it out.)

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    DIY Junior Member DoggoneDJD's Avatar
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    Thanks very much Dana. Since there are two votes for mini split, we'll get the name of a Mitsubishi dealer to see if there is some business in the area that is heating with the units. Nothing like touching/feeling it in the dead of winter to get a real sense of how it feels. But we will have to look very hard at the numbers. Hubby is aghast at the idea of electric heat. We are of an age that we remember our parents being told to go electric because it was so good and cheap, and the aftermath of ripping out those systems because we couldn't eat AND heat the house.

    Hubby thinks he has a feel for the water heating vs. house heating numbers. In a given year 1/2 to 2/3 is heat and the rest hot water so we will be very careful about the hot water heater too.

    Thanks for the words of wisdom about insulation. Basement/crawlspace is foam insulation so no mold, hopefully.

    We are eager to finish this labor of love… The house smells of Ben-gay. This is a young person's game. Being 70 and acting like a pretzel in attic and crawlspaces is for the birds.

    Muchly appreciated.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The energy factor for a good mini-split heat pump is a significant multiplier...as opposed to electric radiant heat, a heat pump can be in the area of 3x more efficient. WHen you compare cost per BTU, then you'd get a feel for the ultimate cost. And, since few heating systems will work without electricity, the issue about no heat with no power is somewhat mute. It would require a bigger standby generator, if that was critical to your plans than a conventional boiler.
    Jim DeBruycker
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    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by jadnashua View Post
    The energy factor for a good mini-split heat pump is a significant multiplier...as opposed to electric radiant heat, a heat pump can be in the area of 3x more efficient. WHen you compare cost per BTU, then you'd get a feel for the ultimate cost. And, since few heating systems will work without electricity, the issue about no heat with no power is somewhat mute. It would require a bigger standby generator, if that was critical to your plans than a conventional boiler.
    If he's dead set on forced hot water a small combo unit like the Baxi Luna series might be the way. Go along with wall hung radiators but I'm not sure if you can get a unit small enough to run efficiently.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    In the trades Dana's Avatar
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    Quote Originally Posted by DoggoneDJD View Post
    Electric heat. We pay $.1751/kwh. Are they THAT efficient?
    In a word, "yes". In a central MA climate with a right-sized single-head Mitsubishi -FExxNA or Fujitsu Halcyon (or Halcyon XLTH) you will have a seasonal average coefficient of performance of about 3.0 or slightly better. With a better-class multi-split you'll run a COP of 2.7-2.8.

    Even at the lowest end estimate with a COP of 2.7 you get 3412 BTU/kwh x 2.7= 9212 BTU/kwh and with 17.51/kwh electricity that's 9212/$0.1751= 52,610 BTU/$ At a COP of 3 that would be 58,466 BTU/$

    At 85% combustion efficiency with an oil burner you get 138,000BTU/gallon x 0.85= 117,300 BTU/gallon. At this weeks average of $4/gallon that's 117,300/$4= 29,325 BTU/$.

    That's pretty much a 2:1 ratio. If you really expect oil to hit $2/gallon any time soon, you have to have been drinking the frack-water! :-)

    Similarly, if electricity doubles and hits 35 cents/kwh, it would cost-effective to buy batteries and a bunch of solar panels and go completely off line and cheaper still to go grid-tied. The lifecycle cost of grid-attached solar is under 10 cents kwh even without subsidy at recent MA grid-tied system pricing. Most analysts are putting it at 8 cents. ANY money spent on oil or propane burning appliances would be far better applied to heat pumps + grid-tied solar at this point. (You might even consider a heat-pump water heater, which also qualifies for subsidy for oil-heated homes in Barre under the MassSave program.)

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