New here; boiler question

Discussion in 'Boiler Forum' started by iamthevieve, Sep 21, 2012.

  1. iamthevieve

    iamthevieve New Member

    Messages:
    2
    Location:
    South Dakota
    Hello out there in DIY land! I've been trying to find a new boiler for the baseboard heating in my house. Through my google searches, I found this site.
    The boiler we have now is about 40 years old and gets really expensive to run on propane during the winter. I'm looking for something more energy efficient, but not too pricey upfront. I have no idea what I should be looking for. Help!:confused:
  2. Dana

    Dana In the trades

    Messages:
    2,785
    Location:
    01609
    The first and all important thing to do is to determine the actual design condition heat load at your house. If you have a history of fuel use at this house it's possible to use that to put a firm upper bound on it. Use a realistic 99% outside design temperature too, not some historical-low, since oversizing the boiler cuts into efficiency pretty badly.

    Got a zip code and an annual fuel use number, and the BTUs in & out on the existing boiler's nameplate?

    The LAST thing you'd want to rely on for a heat load number is a crummy "35BTU/foot" type of heat load per square foot of conditioned space rule of thumb. That's commonly done even by heat pros, and it reliably oversizes, often by 2x or more. While it may not have much of a n upfront cost/efficiecy impact for hot air furnaces, it's a cost and efficiency disaster for boilers.

    The other thing you want to avoid is doing a BTU/foot calc on the baseboard for sizing the boiler, which is also a common but very BAD practice, even amongst "pros" who should know better. While the length of the baseboard is an important factor in the system design, it has no bearing on the size of the boiler. If you have the number of zones and the feet of baseboard per zone, let's have 'em, but it won't affect the boiler sizing.

    At the current price of propane the pay-back on going with a more expensive modulating-condensing boiler is pretty short. The up-charge for a mod-con relative to a right-sized atmospheric-drafted cast iron unit may be close to a couple grand, but it'll use 15-20% (maybe even 25%) less fuel. If you're burning 2000 gallons/year of $3/gallon propane and say a right-sized cast-iron boiler cuts that to 1500 gallons, a mod-con that cuts it to 1200 gallons is still "paying" $500-600/year or more, so you get back the upfront cost delta in 3-4 years.

    If you don't already have air conditioning, (or even if you do) heating at least partially with a ductless air source heat pump (mini-split/multi-split) cost well under half, and maybe under a third per BTU as with a condensing propane boiler. Even if it craps-out and doesn't meet the full load at your outside design condition any part of the heat load that can be supported with a mini-split will be cheaper than heating with propane. (Some are rated for 100% of the nominal heat output at +5F, many will still be putting out at least a decent amount of heat at -20C/-4C. Mitsubishi H2i series spec 70% of the nominal heat rating at -25C/-13F). Payback on a high-efficiency (HSPF>8.5) 2-ton mini-split against propane or oil heating is usually under 5 years even in high-priced electricity areas, and can be under 3 in low priced electricity areas.
  3. iamthevieve

    iamthevieve New Member

    Messages:
    2
    Location:
    South Dakota
    Is there a way to find my current fuel consumption aside from estimating based on last year? I can check the boiler's nameplate when I get home for the BTUs.

    I think last winter from October-April, we used about 1000 gallons of propane. If we buy it in the summer, the cost is less than $2 a gallon. Of course, we had a very mild winter last year.

    Also, I checked that chart you linked me to and my location was a -5 (Heating 99% Dry). I'm not sure what that means, but that's what it said.

  4. Dana

    Dana In the trades

    Messages:
    2,785
    Location:
    01609
    "Dry" means "dry-bulb temperature", which is the outdoor air temp. "Wet-bulb" temperature is a measure of the absolute-humidity, and irrelevant from a heat load point of view, but important for cooling load calculations. The 99% design temperature is the 99th percentile temp from compiled binned-hourly temperature data over a coupla decades. While 1% of the heating season hours in a decade might be below that temp, even if your heating system had EXACTLY the output that would keep up at that temp, you won't get cold, since it never stays below that temp for more than a few hours at a time, and those hours usually occur while you're in bed (unless you're an up at 4AM dairy farmer.)

    The typical or average base 65F heating degree-days in most of SD is ~7500HDD, give or take. Given that it was a relatively mild winter I'll do the example-math on 7000HDD. (see: http://www.builditsolar.com/References/hddmap1.jpg )

    In the absence of an input/output boiler nameplate info I'll assume it's steady-state efficiency is 85% (it may in fact be lower), so with water heating & other uses this will be a clear overestimate of the heat load at -5F.

    1000gallons in 7000 HDD is (1000/7000=) 0.143 gallons per degree-day.

    A gallon of propane has about 91,600 BTUs, so that's (91,600 x 0.143=) 13,100 source-fuel BTUs per degree-day

    Burned in an 85% boiler means only (13,100 x 0.85=) 11,135 BTU per degree day ended up inside the house- the rest went up the flue.

    There are 24 hours in a day, to that means for every degree-HOUR it took (11,135/24=) 464 BTU per degree-hour.

    With a base balance point of 65F, the heating-degrees at an outside design temp of -5F are (65- -5 =) 70 heating degrees.

    So your heat load at design condition is less than (70F x 464=) 32,480 BTU/hr.

    If you were using deep overnight setbacks etc you might have saved ~10%, but that's likely to be more than offset by hot water usage etc.

    If the nameplate efficiency is 80% rather than 85% (divide the DOE BTU-out by BTU-in for the steady state efficiency), multiply that 32.5K number by the ratio: 80/85 x 32,480 = 30, 570 BTU hr.

    That's the range you're looking at. Don't be surprise if the name-plate output is 2, 3, even 4x above that.

    Even a smallest-in-class 50-60KBTU/hr-input mod-con can deliver that kind of heat with room to spare and would still modulate decently. The amount of condensing efficiency you'd get out of it would depend on how many feet of baseboard you have. If it's cut up into zones you may have to add mass or combine them to keep the boiler from short-cycling at low output temp (lower temp == higher condensing efficiency), but you'd be looking at at LEAST 90% average efficiency, and if you tweaked the temperature curves for the outdoor reset control carefully you could hit 95%.

    If you think $2 propane will continue to be available (I've not seen it that cheap locally for a decade or so) and want to go with a cheaper boiler, there are some $1600-2000 mid-efficiency boilers out there with output ranges that aren't ridiculously oversized. (eg the smallest Lochinvar Solution or the Burnham P203-LP But a Solo-60 or PF-50, would only set you back about a grand more, and use 15% less fuel. So even at $2/gallon and 850 gallons rather than 1000 gallons/year the simple-payback is at most 5 years, and if propane goes up it'll be even quicker.

    Even a $2/gallon @ 95% efficiency it's still a a bit cheaper to heat with a ductless heat pump even with 15 cent /kwh electricity. In SD the average seasonal coefficient of performance would be between 2.0-2.5. Assuming it's only 2.0, for every kwh you get (3412 x 2.0=) 6824 BTU of heat into the house. With propane at 95% for every gallon you get (91,600 x .95= ) 87,020BTU into the house. So it takes (87,020/6824=) 12.75 kwh to deliver the same amount of heat as a gallon of propane, but at 15 cents/kwh it's like heating with (12.75 x $0.15 =)$1.91/ gallon propane in a high-efficiency boiler. But whereas propane prices are highly volatile, electricity prices are regulated. During the shoulder seasons with temps in the mid to high 30s or warmer a ductless would deliver heat with a COP of 3.5-4.0, at which point it's about half the cost of heating with propane. If your electricity prices are 10 cents it almost a no-brainer to drop in a 1.5-2 ton high-efficiency ductless ($4-5K) since it would cover the load for all but the coldest of days/nights. When it's under +5F outside the COP drops to about 1.5, and heating with $2 propane would be cheaper until it warms back up to a balmy +5F.
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