Confused by Heat Loss Calc - Furnace Size

Discussion in 'HVAC Heating & Cooling' started by molo, Feb 22, 2012.

  1. molo

    molo New Member

    Messages:
    840
    Location:
    cold new york
    I had a heat loss done, and I am a bit surprised. It is an older home in the northeast so anything is possible.

    The salesman specified a 80,000btu luxaire 95% furnace for an 800'sq ft apartment.

    The apartment is on the second floor (above a heated space) and has 4" of fiberglass in the ceiling and walls. It is a 2-story building. There a 6 new windows and 2 single pane original windows.

    Does 80k btu seem to large?


    Thanks,
    Bill



    I
    Last edited: Feb 22, 2012
  2. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    It sounds HUGE for 800 square feet with any sort of insulation. That's about 60% more than what my 2800 sq. ft. home needs for 0 F. So your loss rate per square foot of conditioned space would have to be 5.6 times what mine is to justify it...and my home is not that tight, has three times as many windows, cathedral ceilings for over half of it, and 2x4 stud walls with vinyl siding.
  3. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    That is 100 BTU per square foot, which based on your description sound like double what you would need. I would look for a second bid on this.
  4. Dana

    Dana In the trades

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    That's insanely oversized unless the two single-pane windows cover half the exterior wall area, and you have a design temperature of -75F. (It's probably at least 4-5x oversized for the load.)

    Being a low-mass furnace that won't have much negative impact on efficiency but it will SURELY have a negative impact on noise & comfort, even if it's a modulating model with a 4:1 turn down ratio.

    The smallest Luxaire Acclimite is a 60K version, and is the same physical size as the 80K unit. They don't say how low the turndown is on it, but doubt it's under 15K, and 15K may be your actual design temperature heat load. There's no point to a modulating furnace if your design heat load isn't well above the minimum modulation of the burner, and a cheaper/simpler/smaller 2-stage more appropriately sized might be a better choice (or a condensing hot water heater + hydro-air coil.)

    Are there storm windows over the two single-panes?

    Got a zip code? (For design temperature and weather history.)

    Gas, or propane? If propane, a right-sized mini-split heat pump might be a better option from a total systems cost & operating cost point of view.
  5. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    I wonder if the size given in the bid is to match an existing AC coil or other part of the air handling system? Typically existing units are massively oversized with respect to the furnace anyway...and depending on your climate the AC frame size will set the minimum furnace size. (That is why my 2 stage furnace is so much larger than what I need...some oversizing of the AC forced larger oversizing of the furnace.)

    For comparison sake, the smallest 95% AFUE 2-stage Ruud/Rheem with ECM blower that I see is a 45,000 Btu/hr input on high stage. Low stage is 70% of that. http://globalimageserver.com/fetchDocument.aspx?id=b935d0d8-d769-43e0-b83b-3dda53d3e212

    If one is doing furnace replacement separate with an existing AC beware of any gotcha's in the warranties for the furnace and air handler...read through to make sure the manufacturer will back it.
  6. molo

    molo New Member

    Messages:
    840
    Location:
    cold new york
    More Info From Salesman

    I spoke with the salesman today about the choice to go with an 80,000btu furnace (natural gas) for the 800sq' area. The single pane windows are 2' x 6', and this is in western NY State. There is existing duct-work that is quite large and old (there is no AC).

    The salesman said that the heat loss showed the need for 59,000btu's and that a 60k btu furnace would only output 55k. Therefore he said that he would rather use an 80k unit to ensure that there were no problems on very cold nights.

    He said that the cost of the units was very similar, but I am worried about the operating costs. The existing unit is working, but I am trying to reduce operating costs.

    I do not mean any disrespect to the salesman. He has been doing this for 20+ years. I am only confused, because 80k btu is larger than I expected.


    Thanks,
    Bill
  7. Chad Schloss

    Chad Schloss Member

    Messages:
    329
    Location:
    USA
    i have a 95% furnace with a variable speed ECM motor. it is installed an a small ranch, about 1100 sq ft. i had an 80k put in. It runs most of the time on stage 1, which is about half output, so about 40kbtu, same as my water heater. it does seem like overkill for your smaller space, but if the 60k runs at half like this one i have does, then you are talking about 30kbtu vs 40kbtu. it keeps a nice constant temperature with the variable speed ECM motor. i also have 4 runs in the basement as well as the main floor. I have lots of windows in there, but then they are also brand new.
  8. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    I don't see how he can come up with a number that large unless he is nearly completely discounting insulation. Is the ductwork uninsulated and in an unconditioned space such as an attic? It's like he is assuming 1/2 of the output will never even enter the conditioned space. What size unit and efficiency are you replacing? What I've noticed is that many often quote the same input size for the new 95% unit as existed the old 65% or 80% unit it replaces...and that old unit is usually much larger than it needs to be anyway.

    Probably the wisest thing to do as a check at this point is to pull up your gas usage for last winter and compare it to the heating degree days. This can be used to estimate the actual energy loss rate of the space per degree delta F. You will have to subtract out any gas water heating, gas dryer, or gas range/oven from the monthly total gas use, but with a little figuring based on summer months (and applying a larger factor for winter water temps, such as 1.5 for my home) this is manageable.

    Unless I'm missing something even with two 2'x6' single pane windows (R1) and a design delta of 70 F that only adds: Q= U*A*delta T = (1/1 ft2 F hr/Btu) * (2*2ft*6ft) * 70 F = 1680 Btu/hr. The other modern windows probably don't exceed this total, so double this to represent window losses.

    Total wall area is probably on the order of 1200 sq. ft or less and if flat attic about 800 sq. ft. of attic space. The space below you is conditioned. If the wall R value is assumed to be low for 3.5" insulation it still will work out to around R11. And the ceiling should be around R9 at worst (3.5" blown insulation). That yields 7,640 Btu/hr wall losses and 6,220 Btu/hr for the ceiling.

    So unless I'm doing this wrong I'm coming up with ~17,000 Btu/hr. That doesn't include duct losses or air changes. Assume a leaky residence with 1 air change per hour and I'll throw in high ceilings and guess the air volume as 10'x800 sq. ft = 8000 cu. ft./hr. Spec. heat of air is 0.019 Btu/cu. ft. With a 70 F delta that works out to another 10,600 Btu/hr. Add this to the other and you have on the order of 28,000 Btu/hr, and I've tried to throw in quite a bit of padding.

    If it is a 95% the 60k will be 57k output and there is already enough fat in his estimate that the 2k Btu/hr difference is inconsequential. (I don't know why they even bother to quote the input rating as it isn't useful for load sizing when comparing 65%, 80%, and 95% units, but that is the way industry does it...) Unless it gets down to -70 F outside I doubt a 60k input 95% unit would have any trouble holding 70 F inside. The 80k should be good for -115 F though (Vostok station, Antartica.)

    There are two primary things you are after: comfort (first) and effiency (second). Obviously an undersized unit will not be comfortable on very cold nights. However, an oversized unit will be less comfortable than a well sized unit all of the time, because an oversized unit will result in wider effective temperature swings in the space than a properly sized unit would. This is one reason why units that can use reduced fire rates during normal operation are desirable.

    If you are getting a 95% efficiency unit that is not vastly oversized (cycling losses), then you are doing the best that you can with respect to the unit itself. The other side of this would be improving the envelope for the home: sealing leaks, replacing/adding weatherstripping, adding insulation to the attic, replacing the single pane windows, insulating/sealing ductwork, etc.
  9. Dana

    Dana In the trades

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    Unless one of those windows is litearally stuck open or the ducts run outside on the roof , have no insulation, and have big holes in them, there's no way an 800' apartment would have a heat load as high as what he came up with. A lot of old-schoolers have their own sizing methodologies, but even most of those guys with the BTU/ft coarseness never use anything more than 35BTU/square foot of conditioned space, even in buildings with literally NO insulation, (and no heated apartment below them.) Even if he used the record low temp in your area as his outside design temp (rather than the ACCA 99% value ) it's on the face of it an insane heat load.

    His heat loss calculation is simply wrong, and not by just a little bit- even an 800' tent might not have that big a load. A crusty old-schoolers 35BTU x 800' only comes in at 28K, and is nearly guaranteed to be higher than reality. A heat load of 59K it's more than 2x THAT number, and the 55K output number is nearly 2x that.

    The crude heat load calc, assuming a 23 x 35' floor aspect, 10' tall exterior wall, 10 square feet per window, and a 0F outdoor, 70F indoor design temp:

    Windows:

    6 x 10'= 60' of U0.5 window(minimal double-pane or single-panes + storms) you get 60' x 70F x 0.5BTU/hr-degree-ft= 2100 BTU/hour

    plus

    2 x 10'=20' of U1 window (single-pane), you get 20' x 70F x 1 BTU/hr-degree-ft= 1400 BTU/hour

    total windows: 2500BTU/hr

    Ceiling, assuming 3.5-4" of fiber insulation, 20% framing fraction (it's probably a lower fraction) you buys you R10 for whole-assembly R. R= 1/U, so the U-value of that ceiling will be 1/R10 or U0.1.

    70F x 800' x 0.1= 5600 BTU/hr.

    Walls: total area is 10' x (23 +23 + 35 + 35) - (8x 10' windows)= 1080' of U0.1 wall so:

    70F x 1080 x 0.1 = 7560BTU/hr.

    Your conducted losses are then 2500 + 5600 +7560= 15,560 BTU/hr, plus infiltration losses.

    Are we then to assume that you have a wind-tunnel blower pumping air through the place that the infiltration losses add up to another (59,000 - 15,560=) 43,440 BTU/hr?

    That's what they're asking you to believe, with that 59K number. I'm betting it's more like 18-20K, depending on how leaky the apartment and ducts are, and you can probably heat the places with a hot water heater or a 1.5 ton mini-split.

    [edited to add]

    BTW: Is the furnace in the apartment, or somewhere else, (like the basement of the building or something?) Being in a remote un-conditioned space there will be other losses, but it's typcally not more than a 20% load adder. But sealing the ducts would be in order. A baseboard (or fan-coil) heating system running off a loop from gas-fired hot water heater would have far lower distribution losses than old leaky ducts outside of conditioned space too. Unless the ducts are well designed, pressure-test at very low leakage, and insulated the net efficiency of an 80% water heater + baseboards could yield a higher net system efficiency than a 95% AFUE furnace on a leaky duct system when the ducts are outside of conditioned space.
    Last edited: Feb 24, 2012
  10. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA
    At least with 80k BTU, he could cut some holes in the floor/walls and heat the other apartments too, eh? ;)

    I don't believe the 59k BTU number either. To give you something to compare to, my old electric furnace was 82k BTU. At some point, someone replaced 3 of the 6 kW elements to 5 kW elements. This knocked it down to ~72k BTU. 72k BTU was plenty. This is for ~3400 sqft (about 1/3 of that as finished walkout basement). 44 year old house, single pane windows, 3 fireplaces, etc.

    I can't see anything larger than the smallest nat gas furnace or maybe a small mini-split to cover your actual load.
  11. Dana

    Dana In the trades

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    nukeman: Your design temps in VA might be 20-25F warmer than in the cooler parts up upstate NY, but the principle is the same. Even the smallest gas furnace is probably oversized for this place.

    Operating cost wise a 1.5-2ton mini-split could be either comparable to or a bit more expensive than condensing gas, depending on the local rate structures, but it would also give them high efficiency air conditioning, which would be good to have if they have only 4" of insulation in the ceiling.
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    One thing to keep in mind, too, is that having a slightly small unit (and the smallest would work and provide plenty in your situation) doesn't mean that the house is going to turn into an ice cube. Say it can keep up at -10 degrees outside (the design temp) and it gets to -11 for a hour or so, at the most, it might drop to 69 degrees inside instead of the 70 you had set on the thermostat. And, consider that those truly cold times are usually short-lived and you do have insulation that slows it penetrating (well, actually, it's the heat escaping, not the cold entering). The area you might see having an issue is with a just enough sized unit is if you set the thermostat back - it would take longer for it to recover. The same thing is true with a/c as with heating with one big exception. A smaller unit there will likely still make you comfortable on a really hot day where it is running constantly since by doing that, it is dehumidifying the air more...dryer air is more comfortable when it's hot than damp air because any sweat evaporates, cooling you.

    It is sad, but many of these guys are blowing smoke. The tools they have, if they know how to use them, often oversize things, sometimes by a lot. As mentioned earlier, if you can take your actual gas usage and the degree-day information available free, you can get a really good estimate of how much heat you are actually using to keep the place warm...much more accurate than an estimate with the (sometimes quite crude) heat loss programs some use.
  13. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA
    Very true, but it gives him another data point to compare to. Even with it being a bit colder there, it isn't enough to offset the 4x smaller space, having basically no loss out the bottom (don't know if he has anyone on the sides), and he probably doesn't have 3 fireplaces and is probably newer/better on windows, etc.

    My winter design temp is 16F.
  14. Dana

    Dana In the trades

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    2,804
    Location:
    01609
    What kills me it that it doesn't take even 15 minutes of analysis based on the minimal information we have to come up with the right order of magnitude (however imprecise) heat load, no rokkit syense necessary.

    If a sales-droid can state that number with a straight face it's either ignorance or apathy (or both), or an agenda (like a better commission for this equipment vs. something other.) Whatever the calculation he was performing it's not likely that is was about the heat load, or the best interest of the client.

    55K is about 1.5x the heat load at my 99th percentile design temp of 0F (comparable to western NY locations), with more than 4x the conditioned space, and not much better insulation levels, with mostly 90 year old single-pane double-hungs + storms for windows. That makes the smallest furnace in that line is on the large side even for my house, but consistent with AFUE oversizing factors. For an 800foot apartment in western NY the 60K furnace is like using a sledgehammer for a flyswatter, and the 80K is like using a 20lb maul.

    But the commission is probably better than a 21K direct vent wall furnace, like the ones I see heating mid sized ski-condos and small vacation homes all over NH, VT, & ME. A friend of mine doing a full gut rehab on a 3-family near me pulled 3 of those out of the previously uninsulated apartments, but will be replacing them with 1.5 ton mini-splits post-rehab. They had been keeping up with the load on these ~600-750' apartments, despite some big air leakage factors and no wall insulation.
  15. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA
    It is too bad. The worse thing is that most homeowners don't have a feeling for the size or any other numbers with HVAC. They figure that the guy selling the stuff knows what he is talking about. Here, the OP had a feeling that something wasn't right and asked the question. Most won't know enough to ask the question, though.

    Just as bad, if not worse, is on the A/C side where they say: "Well, you had a 3 ton unit, so that's what we will go with." or "Well, your 3 ton unit was probably okay, but we can stick a 4 ton unit in there to keep you nice and cool on the hottest days."

    If this guy did do a calc, he must have some dimension(s) in there that is/are way off.
  16. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    I think one of the most important things for us to do when people ask these questions is to ask the poster: how is your current system running? The decision tree is different depending on whether they say it "can keep up on the hottest/coldest" days or "it can't keep up during winter" or summer.

    It's like the question a few days ago about replacing the duct work. The vendor was telling them the AC was oversized, and it sounded plausible until we asked why the new unit was needed, we found out that it couldn't hold 82 F in summer or 72 F in winter. Further questions revealed lack of wall insulation and that the unit had already been replaced once and had never kept the home comfortable.
  17. molo

    molo New Member

    Messages:
    840
    Location:
    cold new york
    Getting more info

    I'll be back with pics of the unit and it's ductwork and more info.

    Thanks,
    Bill
  18. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    Location:
    Minneapolis
  19. molo

    molo New Member

    Messages:
    840
    Location:
    cold new york
    Pictures

    It's an older furnace. It may have heated more space (then the 800' it's heating now) at one point.

    The large diameter duct-work is all return air.

    The supply air is all 8" duct-work (like the duct-work in the foreground)

    It keeps the space warm, but the pilot alone is like having a burner running all of the time

    Attached Files:

  20. Dana

    Dana In the trades

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    Location:
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    I'll bet it keeps the basement warm too, with all of that uninsulated duct work.

    I'm assuming that the apartment is 2 or 3 floors up from the basement? (In which case losses from the heating system for the apartment are heating the intervening space.)

    Is the hot water heater for the apartment in the basement or in the unit, and is it gas-fired? For heating loads as low as this one likely has it's pretty cheap & easy to heat with baseboards or fan-coils running off the water heater, sealing off the supply & return ducts at the registers. With leaky uninsulated ducted systems like that the duct losses and air-handler-induced infiltration alone could be as high or higher as the actual heat load.

    Alternatively, installing a right-sized mini-split heat pump would get you both air conditioning and high-efficiency heating, with comparable (sometimes lower) heating costs to natural gas. With a leaky uninsulated duct system outside of conditioned space it's probably cheaper to operate with a mini-split than the proposed condensing gas furnace. A 1.5-2 ton mini-split is on the order of $4-5K, installed, But if you spent a grand on tighting up and spot-insulating, adding storms to the single-panes, etc. you might get the heat load down to the 1-ton range for a similar installed cost, and even lower operating cost.
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