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Fundamentals: thermal mass/mass/decrement delay/thermal inertia

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    • CommentAuthorGreenfish
    • CommentTimeJan 20th 2015
     
    Like Paul I have a room sealed WBS with own air supply and have not noticed any coldness when the stove is not lit. Measured it yesterday morning after the coldest night so far, OK so around 2C outside in balmy Cornwall but did have a sprinkle of snow and a little ice on the pond/puddle. Temp was same as the walls of the room at 18 to 19C. Vent was in damped position, maybe that contributes? Anyway does not seem to be the heat sink others have warned about but will keep an eye on it.
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      CommentAuthorSteamyTea
    • CommentTimeJan 20th 2015
     
    Posted By: GreenfishOK so around 2C outside in balmy Cornwall but did have a sprinkle of snow and a little ice on the pond/puddle
    Your the wrong side of Carn Brae :cool:
  3.  
    Posted By: Ed DaviesThis is a slightly different view of the problem they had to what I understood from talking to somebody from Bioregional Development who worked (but wasn't a resident) in BedZED.
    What he told me the problem was was that when residents felt a little bit warm, or thought they might later because it was a sunny day, they opened the windows and doors between the sunspaces and the main rooms during the day with the result that the main rooms heated too much then didn't cool in the night. They wrote a leaflet saying to keep those those doors and windows closed during the day in hot weather and open them at night for cooling. This fixed things. No particular security problems on those as they're essentially internal.
    Hi Ed, I chat with Bill Dunster regularly, he wants to use the Passive Slab for a housing development in Devon. Bill mentioned "The high Thermal Mass walls in BEDzed often become heat saturated in summer, exasperating the overheating problem, the occupants didn't know how to use the building properly".
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      CommentAuthorSteamyTea
    • CommentTimeJan 20th 2015
     
    Posted By: Viking Houseoccupants didn't know how to use the building properly

    Sounds like something Bill would say, he was pretty blunt with me a few years ago when I asked a tricky question. No satisfactory answer from him though. :wink:
  5.  
    Posted By: GreenfishLike Paul I have a room sealed WBS with own air supply and have not noticed any coldness when the stove is not lit. Measured it yesterday morning after the coldest night so far, OK so around 2C outside in balmy Cornwall but did have a sprinkle of snow and a little ice on the pond/puddle. Temp was same as the walls of the room at 18 to 19C. Vent was in damped position, maybe that contributes? Anyway does not seem to be the heat sink others have warned about but will keep an eye on it.
    Can you test it tomorrow morning please with the baffle open? I'd be interested in your result as I've been advising clients against stoves for years! I read a Finish study on supply air stoves, the cold external air reduced the efficiency of the burn, it concluded that the external air supply wasn't needed!
  6.  
    Posted By: Viking Housethe cold external air reduced the efficiency of the burn,


    Sorry, I don't believe this for a moment. The delta T between the fire temperature and external air is almost the same as between the fire and internal air. Cold air is also more dense and so contains more oxygen per unit volume.

    Paul in Montreal.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 20th 2015
     
    Hence intercoolers on turbocharged cars, to lower temp of, hence densify the combustion air. And aircraft engines only work at all because high-altitude cold air (densified) partly compensates for the air's scarcity. Carnot theory says max theoretical attainable efficiency of a heat engine depends on the difference of intake vs exhaust temp, related to abs zero. As metallurgy limits combustion temp, the only possibility is coldest possible intake temp.
    • CommentAuthorcjard
    • CommentTimeJan 21st 2015
     
    Forget a candle in the WBS, a butterfly valve in the supply duct is something you only purchase once!
    • CommentAuthorGreenfish
    • CommentTimeJan 21st 2015 edited
     
    With an air tight build WBS definately need a separate air supply!! Open the door the stove can not suck air from the room despite a warm well drawing flue, needs positive pressure or air in from outside to draw on.
    • CommentAuthorGreenfish
    • CommentTimeJan 21st 2015 edited
     
    Posted By: Viking HouseCan you test it tomorrow morning please with the baffle open?

    Pretty similar result this morning, stove similar temp to everything else. It has been warmer overnight at 5C, the baffle was open but not at full. Clearly the flue and air supply pipe form thermal bridges, but the WBS seems not to cool dramatically because of it. It is not ornamental, I did burn yesterday, but checked the temps before starting too. Maybe left baffle open, unused for days in cold weather it would have an effect, but not seeing it on a daily basis. Perhaps by baffle is acting like the butterfly valve cjard suggests?


    With an air tight build WBS definately need a separate air supply!! Open the door the stove can not suck air from the room despite a warm well drawing flue, needs positive pressure or air in from outside to draw on.
    • CommentAuthorrhamdu
    • CommentTimeJan 21st 2015
     
    Posted By: fostertomCarnot theory says max theoretical attainable efficiency of a heat engine depends on the difference of intake vs exhaust temp, related to abs zero.


    True for the working fluid of an engine (and hence relevant for CHP) but how does this apply to a stove?
    • CommentAuthorrhamdu
    • CommentTimeJan 21st 2015 edited
     
    To go back to Tony and the original post.

    "I also like the comfortableness of very slow temperature swings which also only fluctuate over periods longer than days."

    I think this is very true in, say, a Mediterranean stone farmhouse in summer.

    However, I once rented a stone cottage in Dorset for a week in spring, and shivered for 5 days while it warmed up.

    As others in this thread have observed, thermal mass is good in a hot summer if you use it right, not necessarily quite so good in typical UK conditions.

    I live in a hybrid dwelling: externally insulated brickwork ground and first floors, timber frame loft conversion insulated to > building regs. The most comfortable part*, year-round, is the top floor. But the weather rarely gets hot in Brighton - sea breezes take care of that.

    *obviously thermal mass is not the only difference between floors:bigsmile:

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