First time doing heat load calc: Whoa, is my boiler 2x oversized?
So because I have to replace my leaking boiler I intended to do a heat loss calc today which I've never done before, and was all ready for a frustrating internet search looking at different methods, followed by confusion and uncertainty — and then the post by @np_mdbr popped up with a ready made recipe:
I have questions similar to his, and it seemed like better etiquette for me to start a new thread, rather than ask questions about my situation in a thread that's supposed to be about his, even though the subject is the same.
For reference here is what I did trying to follow his steps. Skip to the result if you don't want to get a headache, but any close looks at it would be appreciated (apologies to those who just crunched his numbers too):
Total gallons #2 fuel oil past two years: g = 1211
Multiply g by 1.385 to get therms (fuel oil): T = 1677 therms
Multiply by .84 eff. for therms actually delivered: t = 1409 therms
Multiply t by 100,000 for BTUs: totBTU = 140,900,000 BTU
Sum of local degree days same period (fr. internet): dd = 11420
Divide totBTU by dd for BTU/degree-day: BTU/dd = 12337
Divide by 24 to get BTU/degree-day hour: BTU/d-h = 514
Multiply BTU/d-h by: (65F minus the local 99% outdoor design day temp. of 9 F, or 65-9 = 56 F), . so 514 x 56 = 28787 BTU/hr.
Okay, 29,000 BTU/hr, if I did all that right.
This result shocked me because my boiler is rated for over 100,000 BTU/hr., not sure if it's 115,000 or 100,000 (see photo below).
For reference, this is a two story cape, 30 yrs. old, modular, 1680 sq. ft. but with a heated garage and heated unfinished basement (where the boiler is), so I'd add 900 sq. ft. and just say it's 2500 sq. ft. Northeast climate, design temp 9 F. Four zones: 1st fl., 2nd, fl., garage, basement. The garage and basement thermostats are always turned all the way down, but the garage is leaky and I once measured it not getting above 44 degrees when it was colder outside, so I think I'm probably pumping a lot of excess BTUs into there that just go through the cracks. The upstairs rarely goes above low 60s unless we have guests, during the day it probably gets to the 50s. The boiler heats a 50 gal. indirect water heater.
I know rules of thumb are useless except maybe as sanity checks, so my sanity check is I read for the northeast for a decent house like mine might you could say 12 BTU/sq. ft., which would give 30,000 BTU/hr, so I guess I'm in the ballpark.
The above usage was for my 22 y.o. Weil-McLain P-WGO-3, oil, hydronic baseboard.
Can this really be the case? Was it common for boilers to be so oversized, if in fact mine is?
To pick a new boiler, by what factor should I multiply my 29,000 BTUs (if they're correct)? (Again, it's oil.)
Psychologically it's a bit hard to go from 100,000+ to much less, but I guess that's what I need to do? One boiler I've been looking at is 85,000 BTU, would that be okay?
And also, see the photo below, there are two numbers, it says:
"D.O.E. Htg. Cap. (Water), Btu/h 115,000"
and
"NET I = B = R RATING
Water, btu/hr ……….100,000"
What does all that mean and which is the actual rating?
Comments
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Here’s the photo with the two numbers.
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Sounds a bit low- I'd check that figure by running a heat-loss calc program and comparing the two.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting0 -
On the coldest day/night of the year, does it run constant? Or 20 min on, 20 min off?
My old furnace only ran in short spurts. Plenty of heat, a good chunk radiating to the basement at every shut-down.
It was a 100kbtu box which we used to run with a 60k nozzle if there was one on the truck, so it was more often run 80k.
I replaced with a 39k furnace. The very coldest day/night it runs 21 hours without stopping and that is not a problem. There is far less waste heat in the cellar. When I replace it I'll get the 60k rating; it is clear this 39k is an oddball, too small for the frame or fan, a bit more pick-up would be nice.
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Most residential boilers are two to three times oversized.
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There is a whole house heat loss form that I used to use Called the form 1504 WH from the Hydronics institute (now part of GAMA and AHRI). Here is a link to this form
https://centralboiler.com/pdf/FORM-1504WH.pdf
It is simple to use because it just uses the exterior Walls, Roof and floors. You select your building type, insulation amount and use the factors that apply to the actual square foot of eaxh component. It is for heating only so there are no latent heat and sensible heat that Manual J needs for cooling load calculation.
I think you will find this easy to use.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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yup this is right. You don’t have to convert to therms, just go straight to BTUs. Oil has major limitations though - it can’t be sized that low.
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I agree with @Steamhead that you calculation sounds too low. It may be accurate but I would do further calculations by some other methods to cross check
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If a house has significant solar gain the fuel use method will understate the heating load.
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@DCContrarian interesting, could you explain a bit more? (Thanks.)
@EdTheHeaterMan to the rescue again. Am taking measurements.
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If you’re replacing with oil again, there’s no point redoing the calculations. The best you can do is 80kbtu input, so 64kbtu-ish output. You didn’t have 34kbtu/hr of solar heat gain.
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@Hot_water_fan thanks. Yes, staying with oil. Not sure what you meant by those numbers. What do you mean by the best I can do is 80kbtu input? And the other numbers mean what exactly? Just need a little clarification, thanks. I am coming from a place of profound ignorance.
(I’m thinking maybe with she 80k you mean I probably won’t find a boiler with fewer btus, and the 64k takes account of efficiency? Still not clear on the 34k, but if your basic point is I don’t have to redo calcs to account for solar gain - good!)0 -
Correct. Oil’s limited to how low the fuel flow rates are. So any heat loss under ~60kbtu is covered by the same size, which means almost all homes should be getting the smallest oil boiler, but they aren’t.
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Don't know what your local energy costs are or how adventurous you are for the install. If your existing emitters are oversized (or can easaly add some more) and can run at low enough temperature at your outdoor design temp, you can look at a air to water heat pump. Your heat load and design temp put you well within the range of a 5 ton unit.
The equipment is definitely more expensive but since you are not dealing with combustion or refrigerant, you can DIY the install. Around me a heat pump operating cost is about 1/3 of oil, so ROI is only a couple of years.
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I assume you mean my point about solar gain?
When you're calculating heating load, what you're concerned about is having enough heat on the coldest day of the year — the cloudless full-moon night in late January at 3AM with a stiff breeze.
That weather doesn't come around often enough for it to be practical enough to measure it, so you estimate it using a couple of simplifying assumptions. The big assumption is that heat loss is linear compared to the difference between the inside temperature and the outside temperature — if it's 70F inside, the heat loss at 35F is going to be half of what it is at 0F. Using that assumption, if you know the total fuel consumption for the heating season and the average temperature, you can figure out the fuel consumption per hour at the average temperature and then extrapolate to calculate what the fuel consumption would be at the design temperature. And heating degree days are really just another way of expressing average temperature.
If a house has big solar gain — say, lots of unshaded south-facing windows — the consumption of fuel will be lower when the sun is shining. This will affect the average fuel consumption. However, it doesn't affect the heating load, because maximum heating demand is almost certainly going to happen at night, when there is no sun shining. So a heating load calculation based upon average fuel usage will understate the design load.
If you think that solar gain can't be that much of a factor, I recommend this post:
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Thanks @DCContrarian , I will go over your post again and try to wrap my brain around it. We do have windows with full southern exposure.
@EdTheHeaterMan I filled out your form. Very helpful to see the weight btu-wise given to different parts of my house. The rest of this post is divided into two parts: The holy cow! part and the well, does it really matter? part.
After lots of measuring and calculating and filling out the form I got about 70,000 btus/hr. So now I've pinpointed my desired energy capacity to exactly precisely somewhere between 30,000 (fuel bill method) and 70,000 btus (this method). You can imagine how confused I am. That form has a lot of room for adjustment (and user error), and I erred on the side of more btus; for example, I have no clue what the R-value of the insulation in my walls is, so I just picked the smallest option. I could play around with various categories, but by far the biggest contributors are these: 1) Windows and doors, 16,000 btus/hr. I have the most faith in this one because the heat loss factor was easy to pick: "double glass," I know I have that. "Basement walls above grade" was 12,000 btus/hr and I have faith in that too, because they are cinder block and definitely not insulated. The difference between those btus/hr and the "Basement walls below grade" (my basement and garage walls are exactly half below grade, half above) is huge: 1300 btus for below, the 12,000 for above. Food for thought.
The biggest contributor was something called "infiltration" (20,000 btus) and it is also the biggest seat-of-the-pants estimation; you pick a heat loss factor based on whether your house is a "standard house, some insulation" or "tight construction, heavy insulation," etc. I have no idea, it's a 30 year old modular, might be a tight house, might not. I chose standard house; choosing "tight construction" would get a me a few thousand btus lower, but would bring the total nowhere near the 30,000 btus of the fuel bill calculation.
Offhand, it seems to me that among all these unknown R-values and estimations, shouldn't the fuel bill method be more accurate (if the method is sound) since it knows exactly how many btus we used to heat the house comfortably over a couple of years? And yet @Steamhead and @EBEBRATT-Ed say it does sound too low — and it certainly is low compared to my current 115k unit. However, although I haven't gone over my calculations with a fine tooth comb, I believe @np_mdbr got a similar low number and people thought his calculations to be correct.
Which brings me to the Well, does it matter? part of this post. @DCContrarian and @Hot_water_fan put things in perspective, DC pointing out that most boilers are 2-3 times oversized, and Hot-water pointing out that most oil boilers are rated at 80kbtu or above, which would mean that whether my load is actually 30,000 or 70,000 or something in between, I'd be fine getting a 75,000 or 85,000 btu unit, and wouldn't even be able to get one lower; and even if that were oversized it would be a lot less oversized than my current 115,000 one.
Ed has pointed out the availability of the Weil-McLain 2-section WGO at 75Kbtu, and I was considering that or the 3-section one. My current old leaking WGO one is 3-sections and 115k, but I think the newer 3-section WGO is rated at 85k; I assume units with a given number of sections are now considered more efficient?
My thinking now is that just in case the 70kbtu load estimate is accurate, maybe I should go with the 85K WM rather than the 75K one, so the efficiency factor doesn't bring it below 70k on that design day? (Still looking at EK units too based on recommendations on another thread; those may or may not be right for me now, but that's another subject.)
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Yout heat loss is realistcly about 50k. The insulation in the walls if the house is 30 years old with 2 x 4 wall construction you can bet for sure that you have 3" of insulation in the walls. The ceilings are sure to be 6" minimum with that age of house.
I would go with the 3 section Weil McLain.
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@EBEBRATT-Ed I am totally leaning that way, unless somebody knocks on my door offering a free EK.
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A simple cheap blower door test WILL show where the cheapest and highest return on investments can be made.
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Yes, @pecmsg , and that's what I should have done before my boiler started going south. I sort of figure there's no time for that now, at least for the purpose of determining what btus I need for the new boiler, which has to go in soon, of course. But plenty of time after a new one goes in, especially if you are referring to using that test to tighten up the house.
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After lots of measuring and calculating and filling out the form I got about 70,000 btus/hr. So now I've pinpointed my desired energy capacity to exactly precisely somewhere between 30,000 (fuel bill method) and 70,000 btus (this method)
yeah this is exactly why you should not calculate it by the assembly lol. Way too inaccurate to use for someone who’s doing it for the first time. Why not just trust what actually happened? Otherwise you’re relying on a bunch of guesswork.
Regardless, you know what to do now. Reach out to some contractors and good luck!
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Thanks @Hot_water_fan . I might be trying to do the physical install myself (though probably not setup and testing), in which case you will soon see panicky posts asking for help, advice, handholding ….
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After lots of measuring and calculating and filling out the form I got about 70,000 btus/hr. So now I've pinpointed my desired energy capacity to exactly precisely somewhere between 30,000 (fuel bill method) and 70,000 btus (this method). You can imagine how confused I am. That form has a lot of room for adjustment (and user error),
I found the values on that form very conservative in terms of what was available. None of the values would apply to a house built to current codes.
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modular homes are built tight. Infiltration will be lower. How thick are your walls? You will be able to tell if you have 2x6 walls or 2x4 walls. So if you have 2 x 6 walls, you’ve got R-19 insulation. Can you see the attic insulation? How thick is it?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@DCContrarian thanks. I assume by conservative you mean erring on the side of leaky drafty high btus?
My house is oldish (30 years) but it was modular which I hear good things about, and the guy was building it for himself but couldn’t move in due to personal circumstances, so he probably didn’t cut too many corners. (I remember the house inspector we hired 30 years ago asking whether the builder had intended it for himself, because he noticed a lot of quality work).
@EdTheHeaterMan l’ll check and let you know. (I was too lazy to get a ladder and poke into the attic.)
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I assume by conservative you mean erring on the side of leaky drafty high btus?
Yes. I'm in climate zone 4 and the 2017 energy code is enforced for new construction. Cold ceilings have to be R60, walls require continuous insulation. Air infiltration has to be tested with a blower and has to be 3 ACH50 or less, which works out to about 0.15 air changes per hour. There's nothing even close to that on the form.
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Wow, @DCContrarian. That’s sure not my house. Though I know even here in zone 6 new construction has to be insanely tight, to the point where people worry about a house being able to breathe. My house definitely breathes.
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@Hot_water_fan is spot on. The only unknows with a fuel based calculation is your actual boiler efficiency and solar gain effects. Unless designed for something like passive solar, older houses with reasonable sized windows don't have enough solar gain to change numbers much.
When you are doing a man J or any other heat loss, you have a whole pile of thing to guess or estimate. One estimate on top of another guess and before you know it, you can make that sheet show any heat loss number you want.
For giggles, lets work backwards. Assume 75000BTU is correct. You might have 6000HDD based on your design temp.
So same 56F delta, so 75000BTU/56F=1340 BTU/degreeF
so 6000HDD to hours 6000*24 to therms 6000*24*1340=1929therms.
Assuming 85% efficiency, you need 1929/0.85~2300 therms or 2300/1.35=1700 gallons of oil. At say $4/gallon, that is $6800/year. I think at that cost, you would be rethinking your housing and heating choice.
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@seized123 , if you're going with Weil-McLain, I'd use their 3-pass Ultra Oil model UO-3R. The efficiency of this unit is slightly better than the Gold series, but its 3-pass design makes it much, much easier to service. Basically, you open a large door on the front where the burner is mounted, and this gives you access to pretty much everything that might need to be brushed and vacuumed. No more removing the flue collector and brushing from the top to clean the little pins. OK, it probably costs a bit more, but the extra bucks are well worth it.
Some other good 3-pass boiler series are the Burnham EMP and MPO-IQ, Biasi, Buderus G115WS, ForceOil (Ferguson's house-brand of the Burnham EMP, price might be better), Solaia (similar to Biasi) or Trio (similar to Biasi also).
The most important factor is a good installing contractor. Go here to find one near you:
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting0 -
Come on @DCContrarian
that form is over 50 years old. The state of the art in building homes today and those built over 50 years ago are quite different. I have used that form for the last 30 years when designing a new system for a replacement heating system (meaning the home was at least 15 to 20 years old if it needed a new heater). Those homes were in some cases 40 to over 100 years old. Many times I would take a switch plate or outlet cover off of the wall and stick a coat hanger bent into a hook and feel around for insulation in the walls. Then based on the age of the home, I would make an educated guess as to what the common practice of home building may have been at the time of construction. R-7 wall insulation for the 1940s and 50s. No insulation in 1900 homes. R-11 in the walls of 1960 and 70 homes and R-13 in all homes after that with 2x4 walls, R-19 in homes with 2x6 framed walls.
You can usually see the foundation wall in a crawl and basement, and poke your head in the attic for ceiling/attic insulation. Not until the 1970 was a blower door test even remotely used, and not until the 2000s when building departments started requesting them on new construction. Considering that most of my calculations were liberal with wall dimensions and being a little light handed on insulation when I could not verify it visually, Most if not all my calculations were coming in with new equipment being at least one size smaller heating equipment, if no more.
Professionals using computer programs with software that costs more than $3000.00 in order to select the proper size based on a set of blueprints is the standard today. A paper form with an adding machine (that is what they were called before they were called calculators) was how I learned to do load calcs. I remember using the ARKLA SERVEL form for AC and the Hydronics form for heating systems. and my forms were filled in with a number two pencil with a worn down eraser. Yea, sometimes I got it wrong and needed to change a number from time to time. But the customers were impressed with my explanation of how heat travels through a wall based on the U value of the building's different materials. Looking behind light switches and showing the actual numbers, along with a lower estimate because the equipment was a size or two smaller, then my competitor.
I would also back up my estimate with a guarantee that I would swap out the equipment at no additional charge if my equipment was too small. So they would be getting. the bigger furnace or boiler at the lower cost if needed. How could you pass up such a deal?
That form is just fine for a homeowner trying out the Load Calc on their own for the first time. If they want to run the numbers by me, they can PM me with the particulars and I will be glad to do my thing. It is a lot better than the SWAG method that many contractors use.
But I do agree with you that I would not expect an engineering firm or architect to use it for new construction or a complete building gut and renovation. They can afford the expensive software. (And they still get stuff wrong sometimes)
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@Steamhead thanks, I took a quick look at it; at SupplyHouse it looks to be the same price. Ease of cleaning would be great. It does look oversized at minimum 100kbtus for what I’m looking for, though. Another thing is it looks in the manual like the chimney flue has to come out the back, which would mean it would have to extend into my basement more than I have room for, same for piping which looks like that has to be in the back too? I only took a quick look. And also it looks like the one on SupplyHouse (107kbtu) weighs 850 lbs! I’m planning to try (and I do mean try) to install this myself, and have never moved a boiler, so if the 600 lb. WGO terrifies me, you can imagine what the 850 lb one does ….
But I’ll look at the UO-3 more closely tomorrow.
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Actually it's the UO-3RE. Slightly lower rating than the UO-3, Net 86,000 BTU/hour.
The Solaia SL375B has a lower rating than that- Net 79,000 BTU/hour.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting0 -
@seized123 … this is called overthinking…. You have all the information you need…. a WM or Williamson will use the same footprint as the old one. The pipe connections will be near identical. You want to save on installation costs and you can DIY this one. You don't expect to be there after 4 years so any extra you spend to get a lower operating cost will be for someone else's benefit. So what is holding you up? After you get sticker shock from the EK estimate. Order the boiler and put it in. By the time anybody needs to make a warranty claim, you will be long gone and the plumber that the new homeowner calls will do all the necessary paperwork to get the warranty claim handled. No one will ask if a homeowner installed it in 5 or 10 years.
If you have a DOA boiler or accessory part, when you open it up, then you go to the supplier and work with them. It is against the law to sell you something that is clearly non functional without some kind of guarantee by the manufacturer that it will operate when installed according the the manufacturer's instructions. Just be sure to follow the instructions. They are online, so you can get familiar with them before you even purchase one.
If you show us a photo of your existing WGO, we can tweak any changes you might want to make. You can decide to replace all the accessories or use all the accessories that exist and replace only those that you may find defective as time goes on. I believe the new WM will come with the HydroStat with the built in LWCO so you done even need to get a new one of those. (LWCO is the only thing I would replace when doing a "push pull" even if it was only a few years old).
Hope this helps to push you off the fence.
Mr. Ed
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@EdTheHeaterMan you nailed it all! As the comedian Demetrius Martin has said: “I think I tend to overthink things….Or do I?”
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As a point of reference, I maintain a 4-unit condo building in the Boston suburbs. We have two WGO-5 boilers that burn a total of about 1200 gals/yr, which happens to be almost exactly double your consumption rate. I've calculated our building heat loss 4 different ways, 3 of which used actual oil consumption or boiler run times at known outdoor temps, and all the heat loss numbers agree within 10%, so I have high confidence in them. Our heat loss is 90,000 BTU/hr, and since the math is basically linear, by comparison your heat loss (in a similar HDD zone) would be half, since you consume half the amount of oil. Which puts your heat loss at 45,000 BTU/hr, quite close to the 50,000 BTU/hr that @EBEBRATT-Ed predicted earlier.
We also have massively oversized boilers, and I plan on downsizing to WGO-2's when the time comes. But since you say you're doing the install and weight matters, you might want to look at the Biasi B10/3 or B10/4. Those are around 300 pounds, much smaller and easier to wrangle than the 800-pound monster UO-3. I do not have any experience with the Biasi, but I have done some reading on it in the course of looking for alternatives to our WGO-5's. There are a number of people here who have experience with Biasi's, and there are a number of threads discussing their reliability, etc.
The downside of the Biasi is that the plumbing is all different from your existing WGO setup, so if you can wrassle a new WGO in there like @EdTheHeaterMan said, that would be simpler plumbing-wise.
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Thanks @jesmed1 , that’s very interesting. If my heat loss really is about 50k I could get the WGO-2, even more so, I figure, if we got mini splits to share the heating burden, which we’ve thought about, but who knows if we’ll do that.
For reasons Ed said the WGO is the way to go for me. 600 lbs is intimidating, but not to the pro’s here who have done this dozens of times, and if I do it once I’ll know I can do it again. (Plus, I have no staircase or steps.) And I have an idea how to do it because @EdTheHeaterMan zeroed right in on that in response to my first post: rock it back and forth while someone removes the blocks to remove the old one, reverse for the installation. It’ll all come down to how much 600 lbs “weighs” when one is only tilting it back and supporting it. Guess I’ll find out.
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Invest in a hand truck with pneumatic tires.
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@DCContrarian I thought I’d get an appliance hand truck (or “dolly” or whatever) with the straps etc.: $135 or so at Harbor Freight, a bit more at Home Depot.
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Yes, a WGO-2 will be plenty for your application. We have double your heat loss, and I plan on eventually downsizing to two WGO-2's. Those will still be oversized, but not as badly as the WGO-5's we now have.
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@EdTheHeaterMan not to keep overthinking, but I am wondering if I maybe would be better off getting the the WGO-2 (75K btu net) rather than the 3 based on the numbers from both heat loss methods, and as @jesmed1 suggests. It’s cheaper obviously but that’s not a factor; correct sizing is. The tech who came to look at the leak said Oh, no, don’t get a 2-section, but he had no idea of my house’s heat requirements, saw a 3-section in front of him that had been fine until it leaked,, and he may be part of the chronic tendency to oversize.
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