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Thread: Sizing boiler allowing for future expansion

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  1. #1
    DIY Junior Member DPJ's Avatar
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    Default Sizing boiler allowing for future expansion

    I'll be installing a new boiler soon in my 1835 2,000 sq ft farmhouse in CT. House has another uninsulated two story sheep barn attached to kitchen which I plan to finish a few years down the road. (about another 500 sq ft). I'm trying to weigh my options in considering sizing the new boiler which needs to be replaced soon. On the one hand, I'm trying to gain efficiency now, but by planning for the future expansion project, I want to make sure I have enough in reserve without oversizing too much.

    Based on some online heat loss calculators, I've come up with a figure of around 75,000. So, I'm looking at the Burnham MPO IQ115 at 85,000 BTU, or going one size higher to the MPO 147 with 112,000, and adding the Alliance 35 gallon indirect HW heater.

    I've spoken to a couple of installers over the phone, met with another, and all have said based on sq feet and length of baseboard I need at least 125,000-140,000 btu's, just for my current needs????

    Figured I'd check in here to get some advice.



    It's an old house, so it's pretty loose. Was insulated during a renovation in the 60's where PO's tore out all plaster, insulated all outside walls with R11 rolls, and re-sheetrocked. Saw some of it when I replaced some clapboards a few years ago. Not too bad. I actually found corncobs in the attic soffits too. Also insulated the ceillings in 2nd floor under the floor of an uninsulated walk-up attic. Unused Center Chimney with three fireplaces, all sealed with r16 glued to plywood jammed up into the flue. Fieldstone foundation and dirt basement. 17 windows, all with storms, 6 original single pane still original to the house with wavy glass, the other 11 replaced during the 60's renovation. Two zones, 100 ft of baseboard on the upstairs zone, and 80 ft downstairs.

  2. #2
    In the trades Dana's Avatar
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    If the house is reasonably air tight, with at least code-min double-panes (or reasonably tight older windows + triple-track storms) it's unlikely that your 2000' farmhouse has a heat load of 75K. If it IS that high due to air leakage or degraded batts, it's likely that it can be brought under 50K, maybe even under 40K cost-effectively with some air sealing and spot-insulation upgrades.

    125-140K is and INSANE value for 2000 square feet of living space- even a reasonably tight tent could come in under that at 0F, and your 99% outside design temp is a few degrees warmer than that.

    It's worth air-sealing and insulating the foundation with 2" of closed cell spray foam, and putting down a ground vapor retarder on the floor. If you plan to continue to heat with oil (sounds like you do), it's also worth putting down at least R5-R7.5 of rigid foam on the floor and pouring at least a rat-slab over it. (2" of non-structural concrete is enough), but basement floor insulation is not nearly as critical as the air-sealing and insulating the foundation to at least a couple feet below grade.

    On clapboard sided building it's possible to blow cellulose in over existing batts on exterior walls, which will fill in the gaps and tighten up the place considerably. Stud spacing & depths in 1835 literally NEVER conform to 20th century batt sizes, so it's highly unlikely the R11 batts are performing better than R8-R9 as-installed. As long as there is at least some plank-sheathing under the clapboards a cellulose blow from the exterior is pretty low-risk/high-reward on oil use. Cellulose installers can either pop a few clapboards and drill the sheathing to execute the install or drill through it all & pound in a wooden plug to be trimmed flush with the siding prior to painting. Cosmetically the former looks better, but it's less labor to just drill & fill.

    If you have a zip code and a mid or late winter oil bill with a K-factor stamped on it, it's pretty easy to calculate an upper bound on the design condition heat load based on the DOE efficiency on the old beast.

    The length of the existing baseboard has no bearing on the actual heat load, but having excess limits short-cycling on the boiler. Having "too much" isn't really possible, but it affects the near-boiler plumbing, since the return water coming back needs to stay above 140F where it enters the boiler. There are various methods for achieving this.

    The heat load of the additional 500 square feet is within your control. Insulating the walls with either blown cellulose or spray foam insulation is much tighter than batts, and going with better-than code new windows (or low-E storms over antique windows) makes a difference. And insulating/air-sealing the foundation is far more effective than trying to air-seal and insulate the floor. It won't break the bank to get the heat load of that addition down to under 20BTU/square foot (10KBTU), or even under 15BTU/ft (7.5K), given that it has at least one wall in common with the rest of the house.

    Bottom line, don't go any bigger than the smallest boiler in the lineup- at $4+ oil you can't afford the inefficiency incurred by even further oversizing, but you CAN afford to fix all of the lower-hanging fruit on the heat load. Air-sealing is the most-critical, followed by insulation. Not all air-leakage counts the same- sealing the foundation and the upper-floor-ceiling/attic-floor interface are the most-critical, along with any plumbing & electrical chases or flues/chimney-chases that extend from basement to attic, since that quells stack-effect leakage that runs 24/365, and runs ever-faster with decreasing outdoor temps. Balloon framed walls can act as stack-effect flues too, so it's important to have some air-retardency to the cavity insulation. R11 batts too low-density- little more than an air-filter in the wall cavity from an infiltration point of view, but putting even low-density cellulose in the cavities reduces that by 90% or more.

  3. #3
    In the trades Dana's Avatar
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    It occurs to me that depending on how much radiation you put in the new addition it'll short-cycle the boiler for LOUSY efficiency. It's probably worth using a "reverse-indirect" as a central buffer to buffer the entire heating system and provide the domestic hot water function and slaving the boiler to the buffer's aquastat as it's only "zone". You can bump the temp of the tank up to whatever it needs to be for the baseboard to deliver enough heat in winter, (and drop it back to 140F for the summer. The zone controller for your heating zones can just activate the pump(s) &/or zone valves pulling water from the tank, and the boiler loop pump can be controlled by the tank's aquastat. It's approximately (but not exactly) this:




    There are a few vendors of this type of indirect, ThermoMax (<<they may have moved out of that product line- couldn't find 'em on the web), Ergomax, Everhot (EA series only), etc. They're a few hundred more expensive than other indirects because the size of the internal heat exchanger needs to be a bit bigger, but you can't dump 75,000BTU/hr of boiler input into a 500' zone with 1/10th the heat load without either RIDICULOUSLY oversized radiation for the zone or a serious short-cycling problem, which wears out the boiler and ruins it's as-used operating efficiency. Even a ~25 gallon version makes a significant difference on both the number of ignition cycles and the length of burns.

    While ErgoMax has ports for all connections built into the tank, the EverHot EA (and ThermoMax, if you find them) will need tees placed close to the tank to achieve a similar configuration. (Heating pros might recognize that this is basically a primary/secondary configuration, where the reverse indirect tank is just a fat & massive hydraulic separator. The twist is that boiler loop need on run on every zone call, and can be slaved to the tank's aquastat.)

    If you don't go with the buffering-indirect approach it will be necessary to add mass when you add the new zone, which can be done with a cheap electric HW heater (not wired up). To avoid a huge slug of too-cold water stressing the boiler it's best if it's plumbed similarly to the upper diagram, but with the indirect being a separate zone not running off the buffer. If the tank is not maintained at temp, there needs to be separate return-water temp protection plumbed in.

  4. #4
    DIY Junior Member DPJ's Avatar
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    Dana - Thanks for the detailed response, I'm still digesting it. I replied with some more info yesterday, but apparently it didn't go through, so I'm trying again.

    Zip is 06524. Can't find K factor, but looked back through five years of bills, and winter months averaged between 120-180 gal/month of oil. Totals ranged from 790 gallons to 1140 gallons for the year during that time. Old boiler is rated at 129,000 btu

    House is Timber framed, and 3/4 of it has no sheathing beneath the clapboard. It goes clapboard/insulation/sheetrock. The other 1/4 is a circa 1900 addition that has sheathing beneath the clapboards.

    Basement clean-up and tightening up is definitely on teh short list of things to do. Thanks for the all the great advice about what to focus on there.

    Don

  5. #5
    In the trades Dana's Avatar
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    For Bethany it's safe to use Hartford's 99% outside design temp of +6F. It might be slightly cooler than that, but it's probably a few degrees warmer than Waterbury (design temp= +2F).

    If we assume a 1000gallon/year as an average, and Hartford's ~6150 annual heating degree days , and assuming your boiler is running at 85% efficiency (it's almost certainly less, since it's oversized and aged) the simple math breaks down this way:

    1000 gallons/6150 HDD is 0.16 gallons per HDD

    There are 138,000BTU/gallon so that's a source-fuel use of 0.16 x 138,000= ~22,000 source-BTU/HDD.

    But in an 85% burner only 85% of that heat ended up in the heating system water, so that's 18,700 BTU/HDD.

    There are 24 hours in a day, so per degree-hour that's a heat load of 18,700/24= 780 BTU per degree-hour

    ... all using base 65F as the heating/cooling balance point (pretty close for most houses).

    Using a design temp of +5F, that's 65F-5F= 60 heating-degrees

    so with a maximum of 780 BTU/degree- hour, that means your heat load at + 5F is less than or equal to 60 x 780= 78,000BTU/hr as an absolute max, which is pretty close to your online heat load numbers, but in reality, a significant over-estimate, especially if you're heating your hot water with the boiler, and the fact that it's roughly 2x oversized.

    If you're not heating water with the boiler (and letting it go cold for the warmer 4 months) you're probably looking at 75% as-used efficiency and the heat load is actually (75%/85%) x 78,000= ~69,000 BTU/hr.

    If you ARE heating water with the boiler, at least 20% of the fuel use is for heating hot water, so the raw calc for just the space heating load is only 80% of that:

    0.8 x 78,000= ~62,500BTU/hr

    But since the boiler is still ~2x oversized and old, figure it's running 75% not 85%, which cuts the heat load number to:

    (75/85) * 62,500= ~55,000BTU/hr.

    These are VERY realistic numbers for that level of fuel use for an air-leaky somewhat insulated house that size, and depending on the actual condition of the boiler the heat load could be even lower.

    If you want further verification of where your as-used efficiency & heat load there's a reasonable boiler-modeling tool that can take either gallons/year or K-factor as an input, that was developed at Brookhaven Nat'l Labs, that's downloadable here. Scroll down to the links for the FSA Calculator and it's user manual. It's not bug-free, and isn't very flexible on design temps (it seems to use numbers closer to the absolute lows than the current 99% weather data numbers), but delivers fairly accurate results, with bit of interpretation.

    Based on your fuel use numbers and the fact that you can probably knock off 20-25% cost effectively with air sealing and some insulation upgrades, even allowing 10,000 BTU/hr for the addition you'd safe going with the MPO IQ115.

    If oil prices don't drop dramatically by the time you build it out, it may be cheaper/easier to heat the addition with a 3/4 ton or 1-ton ductless mini-split rather than extending the hydronic system, which would also have the benefit of high-efficiency air conditioning. These things have gotten INSANELY more efficient over the past 15 years, and the better ones still put out a lot of heat even at -20C/-4F. (The Mitsubishi MSZ-FE series still put out full-rated heat at +5F.). The seasonal average coefficient of performance (COP) of these will be about 2.7 in this climate, maybe a bit more, but for the sake of the financial argument lets assume it's only 2.5, and that electricity costs $0.20/kwh (10% more than the current CT average residential retail rate.)

    At a COP of 1 you get 3412BTU/kwh, so for a COP of 2.5 you get 2.5 x 3412= 8530 BTU/kwh

    Say the new oil-boiler really delivers 87%, that means for each gallon you get 0.87 x 138,000= 120,000BTU/gallon

    So for comparison's sake, it takes 120,000/8530= ~14 kwh of power use to equal one oil-gallon's worth of heat.

    At 20cent/kwh, that's like heating with $0.20 x 14= $2.80/gallon oil.

    Seen oil that cheap lately? (Me neither, but it's been less than a decade...)

    A mini-split heat pump isn't super cheap, but you can get a decent 1-ton for under $4K/installed, and a 3/4 ton for under $3.5K/installed. (I had a 1.5 ton Mitsubishi installed at my mother's place last winter for under $4.5K, and she got a $1K kick-back from the utility. A buddy of mine installed three 1.5-ton units to heat/cool a 3-family building he rehabbed in Worcester MA at about $4.1K/per-each) A 3/4 ton unit is rated 9000BTU/hr for cooling but will usually put out over 10,000BTU/hr at +5F. The Mitsubishi MSZ/MUZ-FE09 is rated for 10,900BTU heating @ +5F, and is still putting out over 7500BTU/hr @ -13F, but there are Fujitsu and Daikin 3/4 ton units that are as-good at +5F. You can buy a 3/4 ton high-efficiency Mitsubishi or Fujitsu for under $2K online, but it's better to let a qualified tech install it unless you're really into it. Look for heating HSPF ratings of over 9, and cooling SEER over 16.

  6. #6
    In the trades Dana's Avatar
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    As long-winded as that was, I forgot to mention:

    The circa 1900 addition with sheathing under the clapboards can be safely blown tight with cellulose, but the older part that would likely create some issues, unless the roof has big overhangs to prevent rain penetration behind the clapboards. Cellulose would wick-up rain penetration and store it, causing the paint to fail, and possibly creating rot conditions at the clabboards, since the back sides are likely unpainted, and need to dry into the cavity. Without a site-inspection it would be tough to say for sure what the right approach would be. (A deep energy retrofit, pulling or foaming over the old siding and adding sheathing cavity-insulation and foam insulation under the new siding probably isn't in the cards unless subsidized heavily.)

    You can probably get away with blowing in new-school fiberglass at 1lb density to fill in any gaps & voids on the un-sheathed sections, but tightening up the air leakage will be most-safely done on the interior side of the wall, not by dense-packing the cavities. But going at every plumbing & electrical penetration with caulking & 1-part foam is worth it, as is spray-foaming the interior side of the foundation sills & band joists with 2" closed cell foam. Fix any of the mortar issues with the fieldstone first, but 2lb polyurethane does a GREAT job of sealing & insulating. In a heated home there is zero risk to the foundation, and it in fact lowers the risk of frost-heaving by keeping the basement warmer near the base. (In unheated barns, etc insulating the foundation requires some amount of exterior treatment or it can frost heave.)

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    DIY Junior Member SD4US's Avatar
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    I'm trying to follow Dana's very nicely detailed (many thanks for it) heat load calculation in post #5 (using my own numbers) but there is a step there, where something seems wrong:

    "so with a maximum of 780 BTU/degree- hour, that means your heat load at + 5F is less than or equal to 60 x 780= 78,000BTU/hr as an absolute max, which is pretty close to your online heat load numbers"

    Either you factored in something unknown into the above equation or it could be a simple mistake, as 60 heating degrees x 780 BTU per degree-hour = 46,800 BTU/hr, which is no longer very close to DPJ's online heat load figure (75,000).

    You felt (I guess correctly) that 78,000 BTU/hr was a significant over-estimate and further lowered it based on DHW and old boiler efficiency and came up with an end result of 55,000 BTU/hr (which you felt was still on the high side). If the quoted calculation was indeed incorrect, that final number would be only 33,000 BTU/hr.

    Does this make any sense?

  8. #8
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Quote Originally Posted by DPJ View Post
    Based on some online heat loss calculators
    Better than nothing but nearly useless in renovation work, as the variables are many and the options with such software few. Let's start with a proper heat load. You have enough baseboard to support the larger boiler but that is only a one factor. The indirect water heater should not be added in the the heat load and a 40 is probably the same cost as 35-see MegaStor or Trin&Stor indirect water heaters. Foamed your rim joist yet?

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