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Heat Loss Calculations

Post in 'The Green Room' started by jeffesonm, Nov 25, 2013.

  1. jeffesonm

    jeffesonm Feeling the Heat

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    I'm in the early planning stages of a future gasifier setup so I am starting with the heat loss calculation for the home. It is an 1800 sq ft ranch built in 1957 over 1300 sq ft of full basement and 500 sq ft of crawl space.

    The house has a fair amount of windows, all original, mostly aluminum, single pane with storm windows. As best I can tell there is no insulation behind the plaster-on-sheetrock walls. Last year I added R30 fiberglass batts to 1/2 of the attic and plan to finish the rest this year. There was some old fiberglass in there before so I'm guessing maybe R40 total now. I've also done some air sealing up there but have some more to finish, before the rest of the insulation goes in the attic.

    I put everything into this heat loss calculator and came up with a design loss of 75k BTU/hr at 20 degree outdoor temp. The info I used is below but not sure I did it correctly, especially with respect to the floors. This seems high but also consistent with what I experienced last year, where my 75k BTU insert would not keep the house warm enough when it dropped into the 20s.

    Ceilings - 900 sq ft R40, 900 sq ft R10
    Walls - 1272 sq ft R3.4
    Windows/Doors - 71 sq ft R1.8, 256 R.75
    Floors - 1800 R5
    Slabs - 0
    Infiltration - 1440 cu ft, 1 air change/hr

    How does this look in terms of accuracy?

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  2. altmartion

    altmartion Feeling the Heat

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    honestly, with the windows you describe and the insulation in the walls the load sounds low. are you putting in a boiler?
  3. jeffesonm

    jeffesonm Feeling the Heat

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    Right now I'm heating with a wood stove but I'd like to do a gasifier with storage. I've been pouring of all the threads in the boiler room here and my goal is to design a setup where I can get away with making one fire a day for most of the cold season. I'm just about off oil and wood is plentiful and free (minus time) so for me the gasifier is less about savings and more about convenience and comfort. Plus I like tinkering with things like this.

    So far I've figured out that more storage is better, a bigger boiler charges storage faster and radiant emitters allow you to get more out of your storage. But arguably the most important variable is how much heat you actually need, so I'd like to get that squared away first. I understand lowering the heat load will extend storage and overall cut down on wood usage, but windows cost money, finished walls are hard to insulate, etc...
  4. woodsmaster

    woodsmaster Minister of Fire

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    Why isn't there insulation in the walls ? Those aluminum frame windows suck, I've replaced a lot of those. They just radiate the cold right threw.
  5. woodsmaster

    woodsmaster Minister of Fire

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    I think I would upgrade the insulation first thing.
    woodgeek likes this.
  6. JP11

    JP11 Minister of Fire

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    Insulation and windows. Cheap (relatively) and payback is fast. You don't want to perpetually cut double the wood you SHOULD be using. Spend a couple years getting ahead on wood. Guys who get the wood appliance first always chase their tail bad the first winter.

    JP
  7. altmartion

    altmartion Feeling the Heat

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    as everyone above mentioned, work on your house envelope first. go with dense pack if the inside is finished. new windows are huge. and caulk is cheap. windows give tax incentives too. does the gasifier have any incentives? this will all help through the hot months too. plus it will lower your storage requirements and possibly boiler size and conserve energy.
    oh yea, seal the sill plate as well. the inside will be good but if you can dig around the outside and insulate that would be awesome. the sill plate is one of the leakiest area's in the house. I am not 100% sold on this but have heard it from a few other bpi guy's.
    woodgeek likes this.
  8. jeffesonm

    jeffesonm Feeling the Heat

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    I guess they didn't insulate much in 1957... probably because oil was so cheap. I actually got some photos of the house being built from the previous owner and it looks like they put foil over the studs... radiant barrier?

    [​IMG]

    All of the walls are plaster and I'd prefer not to cut a bunch of holes in them, so instead I plan to do blow-in insulation from the outside when I replace the siding sometime in the next few years. I read some threads around here and folks seem to be happy with this approach. Windows have to go too but there are a lot and they are not cheap. I may try to do these a few at a time rather than all in one shot. I'm also working my way around the basement, caulking between the top of the foundation and the sill plate and adding rigid foam sealed with spray foam over the rim joist cavities. Sealing up the house seems like the cheapest investment that will provide returns in the short term.

    Firewood I'm doing great on... have probably 10+ cords c/s/s, about half of which is ready this year. I understand it's not the most efficient path and not wise over the long term, but right now I am young and have way more time than I do money.

    Maybe a mod can move this thread to the green room? Might be a better fit for now.
  9. altmartion

    altmartion Feeling the Heat

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    as I said, if you do the blown in yourself, make sure you rent a dence pack machine.
  10. woodgeek

    woodgeek Minister of Fire

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    As to the original question, the 75 kBTU seems a reasonable guess. I have spent a lot of time modeling the heat loss versus temp in my house and matching it to a 'budget' like you have, and following the result through a lot of improvements. How are you handling the basement load?

    In my experience the infiltration is the biggest source of uncertainty. I understand 1 ACH_nat, but not '1440 cu ft'. What do you mean? In my house 1 ACHnat is roughly 20000 kTBU/h right there, and double that when the wind blows. I cut that figure in half by carefully airsealing my attic DIY. What if you are actually at 2 ACHnat, then that can boost you another 20 kBTU/h! Don't put too fine a point on it. Just accept the uncertainty and move on.

    Personally, I can see a gasifier+storage in Maine or Minnesota, but not in mild NJ. You or I can heat with an off the shelf heat pump for a cost per MBTU equivalent to (current) retail natgas or coal. The money you would spend on gasifier hardware + storage would pay for your wall insulation and a couple minisplit HPs. Your DIY time on the gasifier rig could go into DIY time airsealing your attic, sills and DIY window replacement. If you still want to burn your 'free' wood to offset your puny heating bill, keep your insert and burn it when you feel like it Dec-Feb.

    If you want to be 'green' or low carbon, buy green elec to feed your HPs, or plan on going solar in a few more years when it is even cheaper.

    IOW, I am living in your climate in a 2300 sq ft 1960 split level, and my BTU loss at a 'typical' average Jan temp (29°F) is ~22 kBTU/h, but was closer to 45 kBTU/h when I bought the place. My annual heating bill this year with a cheap HP will be ~$900, which I would be hard to beat using CSD wood (or pricing your wood processing time at a reasonable rate). Compare to 1050 gallons of oil for heat the first year I was in the place, at current price $3500/yr.
    Last edited: Nov 27, 2013
  11. woodgeek

    woodgeek Minister of Fire

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    Looking at your picture, its curious. 1957 was the dawn of the FG age. It was still a pricey new material. In this period, they sometimes installed thin batts, like 1.5-2" thick, one brand was 'econo-batts' (I had some in a kneewall in my attic). The foil you see could be a vapor barrier for such a product. The exposed stud depth in the photo is 'wrong'. If it were foil against the sheathing, we would see a lot of stud. If it were modern batts the kraft paper would cover the studs....I think you have a foil faced insulation product half filling your cavity. This would frustrate any foam in cavity guy, but is not a problem for densepak cellulose--that will just crush your existing insulation down.

    What is my point? I thought I had unisulated walls too, and later figured out they were insulated (but with big gaps around the outlets I peeked in at), and I had this huge loss from airleaks that were easy/cheap to fix.

    Rather than guessing your heat load on a typical Jan day, it is def more reliable to infer it from fuel usage. You should know how many cords you burned and how much oil last year, and you can look up the HDD last year at your location (wunderground). Use that info to compute an average BTU per HDD (after your airsealing and insulation work). You can also do this over a week timespan (now) during cold weather and get a reliable number. You can put a $20 'hour run timer' on your oil system to track usage and nail your BTU load per HDD to ±10% (I did this). That will tell you a decent estimate of your BTU demand at a given outdoor temp, without the ±30% error of your other approach.
  12. jeffesonm

    jeffesonm Feeling the Heat

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    I'm not sure how to calculate this... it's about 1300 sq ft full basement and 500 sq ft crawlspace, both with concrete slabs. Probably 70% of basement walls are below grade, and there's zero insulation anywhere down there (except for what I've started added to the rim joists). I have no specific need to heat the basement other than when it's ~55 down there, it makes the wood floors on the main floor cold.
    Sorry that should have been 14400, which is just 1800*8.
    I understand heat pumps in concept but nothing about practical application of them for home heat/cooling with respect to size of units, cost, placement, etc. The house has no A/C so that is an appealing benefit there too. Lot to learn in that area.
    In all honesty this is not a motivator for me. It's more about being cheap and spending time doing things I enjoy, which include processing firewood, burning it and tinkering with complex systems. I do realize 20 years from now cutting firewood might get old, but I'll worry about that in 20 years.
    It's really hard to say what's back there, and short of opening up a wall, not sure how to figure that out. The foil looks to me like it's one continuous piece across the studs, so not sure how that would work if there was FG attached to it. Here are a few more photos but none clearly show what's behind the walls. I do know it's about 3/4" of plaster over 1/2" of blue sheetrock type backer board, so there's plenty of thermal mass there.
    house1.jpg house2.jpg house3.jpg

    I like this approach. Unfortunately I did a bad job of measuring how much I burned last year but I can measure for the next week or so. I also know from the oil company that the previous owner burned 1000-1200 gallons of oil per year. The insert last year cut that down to ~450 gallons, and the majority of that was DHW and keeping the boiler warm.
  13. woodgeek

    woodgeek Minister of Fire

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    Ok. Say the previous guy used 1100 gallons of oil, and ~200 was standby loss during the summer and DHW. That leaves 900 for seasonal heat. That works out to about 95-100 Million BTU per year. At 5000 HDD, this is 20 kBTU/HDD. 20 kBTU/HDD * 1/24 day/hour * 40 deg Delta T = 33 kBTU/h at 30°F outside and 42 kBTU at 20°F outside, and def spiking well above 50 kBTU/h on the coldest nights of the year or when it is windy.

    I looked up the F55, and got the impression that 75 kBTU might be the **peak** heat output. On a sustained basis, a figure closer to 35-40 kBTU is likely, and this seems to jibe with the estimates above IMO. Perhaps others with that stove can offer an opinion.

    If the attic airsealing is not done yet, the R-30 insulation you put up there might be only saving a minimal amount (relative to the PO), (cuz the attic is still too warm from air leaks). So I think you DO have wall insulation, based on the (lower) usage, house vintage (many were insulated in 1957) and the mystery foil in the pictures. I do not know of any practice where sheets of foil were placed in empty wall cavities. It seems more likely that it is a backer for some sort of insulation board or other product (perhaps not FG). (I am looking to the right in the first picture)

    I've learned a lot from poor man's thermography...my IR thermometer. Available for $20 if you shop around. I measure the temp diff between the inner wall/window surface and the air temp (by IR'ing a nearby piece of furniture). I figure the thermal resistance (R-value) of the inner air film is about R-0.5. So if the whole wall cavity were R-10 (insulated) the inner wall surface should run about 5% below the air temp. IOW, if it is 50°F colder outside than inside, an insulated wall should run about 0.05*50 = 2.5°F colder than the air temp. For an uninsulated wall (R-2 to R-3), the inner surface should run more like 10°F below the air temp when it is 20°F outside. I have used this method to estimate cavity R-values all over my house, to see that my old window+storms are R-2, and my old windows+new low-e storms are R-3, etc. With care, you can even 'see' the thermal bridging by studs in the insulated walls.
  14. woodgeek

    woodgeek Minister of Fire

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    There are many threads in the Green Room about folks getting mini-split heat pumps (point heaters that hang on the wall) or conventional split HPs (that look like central air AC with ducts, but also heat). I've got the latter, along with begreen, firebroad and maverick06 (?). Several other members (in colder climates) have minis and seem to like them. Look around.

    If you have no ductwork, the minis might be cheaper than getting new ductowrk installed in your attic. HD sells some Chinese made ones that IMO would work ok in our climate and cost a bit less than the fancy mitsubishis that folks north of us would go for. If your demand were 75k, it would be a no go. At 33k, one big mini or two small would be fine.

    HPs work well with wood burning in that F55. The HP covers you (and are super efficient) in the shoulder season. And if they can't keep up in the coldest weather, the stove covers the difference. The cost per BTU is ~50% that of oil (or less) during our Jan weather, maybe 35% during the fall and spring. You still get to cut as much wood as you like.
    Last edited by a moderator: Nov 29, 2013
    Highbeam likes this.
  15. Highbeam

    Highbeam Minister of Fire

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    My 1963 house in the northwest has a fiberglass R-5 batt that is 1.5" thick. The glass is yellow and the facing is foil. It was installed pretty sloppily probably because energy was cheap back then.

    Does the dense pack cellulose blow out the sheetrock?
  16. jeffesonm

    jeffesonm Feeling the Heat

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    I bought an IR thermometer for measuring my stove (and then everything else in the house once I had it). I just walked around and sampled a few different rooms:

    upload_2013-11-27_12-33-49.png
  17. woodgeek

    woodgeek Minister of Fire

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    Averaging your numbers, sounds like the wall temp is ~3.5°F below air temp, when the air temp is 25°F warmer than outside. Or 14% colder than the air (scaled by the outside temp). This would correspond by my approximate formula above to R-3.5 or so (like you said). So provisionally a wall with thick plaster, blueboard and sheating with no insulation, or maybe some thin insulation and air infiltration. Who knows?

    Let's do a check. Are your windows single or double pane? If your windows are double pane (nom R-2) then your data is still consistent with an R-4-ish wall.

    Also, for accuracy, your readings should be on surfaces with the same IR emissivity (I use blue tape on my walls, windows, 'air temp reference surfaces', etc) and should be at the same height off the floor (since there is a vertical gradient in most houses). :)

    Also, I do this when the outside temp has been steady, and there is no sun....both the case in NJ at this time.
  18. woodgeek

    woodgeek Minister of Fire

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    Usually not....unlike foam it is not pressing against the whole wall. Just a little pushing near the hose end and the air pressure during installation (readily released).

    With econobatts in there....you would want to find/talk to an experienced installer and look him in the eye. When I researched this a few years back, I read they like to cut a hole through the batt (if coming from outside) and blow between the foil and sheetrock, rather than tear/bunch up that stuff.
  19. jeffesonm

    jeffesonm Feeling the Heat

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    I did measure at the same height of the floor, however I did not use blue tape... walls are cream color and the furniture is brown. Bedroom air temp measurement was via thermometer that I've been keeping in there recently. Bedroom and stove room are single pane windows with storm windows from 1957 with the plaster/blueboard walls. Dining room was part of 1963 addition... window is double pane (from 1963) and walls are sheetrock, but without the plaster on top.
  20. woodgeek

    woodgeek Minister of Fire

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    Ok. The bedroom data (-5°F) might be less reliable (cuz I don't trust IR thermos absolute, but they are ok for temp differences). The original rooms are -4°F, consistent with R-3 or R-4. The dining room is only -2°F. IF the measurement is repeatable and not an error of some sort, it would imply that wall is closer to R-7 or R-8, consistent with a stud wall with R-5-R-7 insulation (like a 1963 econobatt). Maybe they were more concerned with insulation in 1963 than 1957?

    Of course, all this needs to be taken with a grain of salt, but shows the principle. A real energy audit would be a lot more informative. Or like me, you can obsessively repeat these sorts of experiments with blue tape when it is colder outside. :)

    You might also take the switch plates off in the different rooms, and if there is a gap around the J-boxes, poke around in the cavities with a non-conducting stick like a skewer.
    Last edited: Nov 27, 2013
  21. woodgeek

    woodgeek Minister of Fire

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    Ok, a few minutes of googling around revealed that foil, sometimes multilayer WAS used in residential wall cavities in the 1950s. Learn something new every day.

    If the R-3.5 value for the 57 part of the house is correct, it is hard to understand....too high for an open cavity, too low for an econobatt......

    You've convinced me...the 57 part of the house might just have some reflective foily stuff in the cavity, and an effective R-value of 3 to 4.

    I still think the 63 part of the house has something more effective.
    Last edited: Nov 27, 2013
  22. jeffesonm

    jeffesonm Feeling the Heat

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    Before this house I didn't know people put plaster over sheetrock. I emailed the previous owner to see if there are any more revealing pics of the walls. Also don't read too much into the R3.5 number... on the heat loss calculator I clicked "help on R values" and then used the numbers they suggested for uninsulated walls. Somehow 3.4 turned into 3.5 when I typed it in. I will definitely stick some blue tape on the walls and furniture in those same spots and repeat the experiment later this week when it drops another 20 degrees. One of these days I will order some heat/humidity sensors for my Arduino and start logging all this stuff on the regular.

    I did air seal the areas of the attic I insulated, before putting the insulation down.... I've just only made it halfway across the house with the whole process. The 4:12 roof is nice when you're cleaning the gutters, but really sucks to get out near the eaves. The short L part over the bedroom is closer to 3:12 and damn near inaccessible... might have to blow some insulation in there. I was also struggling to find lots of areas to seal... there is no ductwork and with the exception of a ceiling light in a few of the rooms, all of the electrical is run through the basement. There are no kneewalls, dropped ceilings/soffits or recessed lights and just a few plumbing penetrations. The existing insulation goes all the way out to the eaves and I have to really wedge the foam soffit vent baffles down in there. I've read the value of air sealing the top plate, but because there's so little space I just don't see how that's possible here.

    Any ideas on the floor calculations?
  23. woodgeek

    woodgeek Minister of Fire

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    On the airsealing....do the plumbing stack, interior chimney if you have one, attic hatch, and all the top plates interior and exterior. The top plates are the horizontal 2x4s at the top of walls...there is usually a gap on both sides to the drywall that you need to seal with sprayfoam or caulk. I couldn't reach mine under the eaves, and called in a sprayfoam outfit for those, but I did all the rest...about 100' feet in all.

    On the basement calcs, the pros disagree..basements are complicated and I don't have any answers. All the more argument for determining load from fuel usage.
  24. DBoon

    DBoon Minister of Fire

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    Hi jeffesonm,

    I took a look at your calcs (I've been doing the same exercise for a house remodel).

    Try re-doing your calcs without including the floors and slabs in the ceiling/wall/window/door calculation. That will give you 43.37 kBTU/hour.

    Then, run a separate calculation for the floors with a design temperature of 45 degrees - the likely worst case temperature for your basement, and that temperature (and not the outside temperature) is what you should base this on. Ignore the slab - you'd be double counting the losses if your counted the floor and the slab. That adds 9 kBTU/hour. The total is 52.37 kBTU/hour. Still a lot, but not 75 kBTU/hour.

    As always, insulation in the walls and storm windows will go a long way. I retrofitted insulation in my outside walls. The contractor removed two pieces of clapboard (top and bottom) and cut a hole to pump the insulation in, then put the siding clapboards back on. I had some repainting to do. but the overall visual effect at the end was very good.

    Also, Central NJ is about 5000 HDD, so you might want to change the default 6000 HDD to 5000 to get a better read on the total yearly heat loss.

    Infiltration is tough to predict. In any case, the (lack of) insulation is going to dominate your heat loss.

    I have found through my own calculations that if you are burning hardwood in a boiler with a 5 cubic foot capacity, then the boiler in one burn could potentially generate enough heat for >1000 gallons, and what you need will depend a lot on your delivery temperature requirements. See this thread here
    http://www.hearth.com/talk/threads/storage-tank-size-planning-advice-welcomed.116191/#post-1556027
  25. woodgeek

    woodgeek Minister of Fire

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    Nice DBoon, I think we're converging here.

    Based on usage it sounds like he is ~42 kBTU/h at 20°F with average or less wind. The fact that the model comes in a little above that is not unexpected....there is a lot of double counting in the model. With a large infiltration, a lot of the heat ends up in the attic, reducing the conductive loss (the OP can verify that his attic runs 10-20°F above outdoor temp), some goes out storm windows/doors, reducing conductive loss on the inner pane, or warming wall cavities. If a lot of the air is coming in the basement, it is getting warmed by the heat flowing out the floor. You are leaving off infiltration, and assuming it is all double counted?

    This is why getting at loads from usage is so much more reliable than modelling.

    Also: I doubt his basement gets to 45°F....the ground temps around here are pretty high AND he has an oil boiler down there shedding heat (I think).

    At the OP's typical end of January temp (~30°F daily average), his load will be ~33k, he would need to burn 33*24 = 792 kBTU/day. Or about 130 lbs of wood/day.

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