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"Nominal Power Rating" - defintion?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Martin A, Aug 27, 2010.

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  1. Martin A

    Martin A New Member

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    I installed a wood-burning stove in my big old house in Normandy. The manufacture says it has "Nominal Heat Output of 15kW". But I found (during the winter) that it raised the room temperature much less than 6 kW of electric heating.

    Is there an industry-standard definition of "Nominal Heat Output" or "Nominal Power Output"?

    Or is it simply what the manufacturer chooses to call the stove?

    Many thanks for any advice!

    Martin

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  2. begreen

    begreen Mooderator Staff Member

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    Greetings, or should I say Bonjour! What is the make and model of the stove?

    It's just a guess when dealing with marketing. Nominal output may be the theoretical possible, not accounting for efficiency losses? Stove location and the room itself can affect output. We have seen good wood inserts that don't put out the heat until a block off plate is installed. In some cases it even takes putting insulation behind the insert to stop the masonry from sucking out the heat. Another thing that will dramatically affect the stove output is the wood burned. We're you burning very well seasoned wood last winter?
  3. Martin A

    Martin A New Member

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    It's a Country Kiln (Scotland) Model 42 stove.

    The wood being burned was mostly oak that had been in a dry barn for at least 50 years.
  4. begreen

    begreen Mooderator Staff Member

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  5. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    The maximum KW rating is just like our maximum BTU rating. It is obtained by burning the stove very hot and constantly feeding it wood to maintain the temperature. In no way emulating real life burning conditions. I relate it to the fire tender on a train standing there throwing log after log into the boiler fire. My big wood stove is supposed to have obtained 75,000 BTU output in the lab but I seriously doubt if I ever experience more that fifty or sixty thousand BTU output during a burn. And since the output of a normal burn is on a ascending and then descending curve the average output is even less. Where the electric heat is a constant output probably right at the rating. And probably fan forced giving a more even warmth in the dwelling.
  6. Martin A

    Martin A New Member

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    Thanks for all the helpful comments.

    My primary question is *what is the definition of Nominal Heat Output*?

    This would be a big help in some discussions I am having.
  7. vvvv

    vvvv New Member

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  8. begreen

    begreen Mooderator Staff Member

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    Stated or expressed but not necessarily corresponding exactly to the real value
  9. Martin A

    Martin A New Member

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    Yes, thanks --- I already looked up "Nominal" in the dictionary.

    I want to know if there is an industry norm for "nominal output" - or is it simply the power
    output that a manufacturer thinks they can put on the publicity material without having too
    many problems from people complaining that the actual power output is way below
    what they gave as the "nominal" figure?

    (On the grounds that complaints can be deflected by saying - "we only gave it as the NOMINAL power output - not the ACTUAL output - - - Sucker!")

    cheers
    Martin
  10. begreen

    begreen Mooderator Staff Member

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    As noted earlier, marketing can make lots of claims. My car has a nominal gas mileage of over 100mpg... as long as it is going downhill. The better stove companies tend to be more accurate with their heating claims.

    Still, I am surprised that you are not able to heat the room with any basic box. Can you describe the stove installation location and the area it is heating? Is the stove out in the open or in a fireplace? How large is the area and how high is the ceiling? Are the walls or ceiling insulated?
  11. Martin A

    Martin A New Member

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    Many thanks for the helpful comments.

    Room to be heated

    The room is 7m x 5m, with ceiling about 2.5m high.

    All walls are insulated (exterior with 10cm fibreglass, interior with 5cm fibreglass). The floor is also insulated (5cm polystyrene foam) though this is not relevant because of the thermal inertia of the 10 cm of concrete on top of the insulation. The ceiling is not insulated but there is a heated living room above the room in question (a kitchen).

    The windows (three) are single glazed but are quite small (it's a 200+ year old house) for the size of room. There are also two single-glazed doors.

    The old chimney is closed off with 5 cm of fibrelass, supported (temporarily) by plasterboard.

    Informal Tests

    When it was very cold (for France) outside (-7 deg C), I tried the stove and could not get the temperature of the room above around 10 deg C, after running it all day. I discussed with the maker who told me I must have an "air lock" in the chimney. Hard to believe as the stove seemd to be burning as hard as could be expected - vigorous flames visible etc - plus the chimney is a straight vertical tube with nowhere for an "air lock" - whatever that is - to hang around). I have no means for measuring the surface temperature of the stove.

    The next day (also -7 C outside) I put on the three 2kW electric heaters and did not light the stove. They raised the room temperature to around 17 deg C. (The heat output of these was probably a bit below 6 kW, as the mains voltage was often down to around 210V, instead of its normal 230V). These informal tests made me believe that the stove was producing much less than the 15 kW it had been advertised as producing.

    I also noticed that you could stand as close as you wished to the stove, when it was burning fiercely. A neighbour's French "Insert" wood burning stove is also rated at around 15kW. When that is burning firecly, you would get uncomfortably hot if you stood directly in front of if for more than a few moments.

    I did not do any more investigation at the time because of priorities that could not be deferred. If I had had the time, I'd have plotted hourly graphs of temperature with the different forms of heating, carefully noting starting conditions etc.

    Maker's Comments

    I asked the maker if the stove had actually been tested as having a 15 kW output but got vague answers such as "I don`t know. I send them for testing, pay the bill and dont think much more beyond that".

    The stove is said to comply with EN 13240. I don't know if that standard specifies that power output should be measured or that it should agree with the nominal rating.




    Any general advice would be appreciated. I have come to the conclusion that, although many makers use the term "Nominal Power Output", it does not have any generally accepted definition and there is no legal requirment for its actual heat output to be close the the nominal figure.

    Thank you for any comments. I want either to get the stove to produce 10+ kW output or take the loss and replace it with one that does produce the heat output I need.

    Martin
  12. begreen

    begreen Mooderator Staff Member

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    OK, I will take a guess based on the information given. Can I surmise that the stove is installed inside a large old stone fireplace? And if yes, is this fireplace on an exterior wall? As noted, you are not going to see 15KW of heat coming from the stove, but it is realistic to expect to see, say 9KW. What I suspect is happening is that most of the heat is being sucked up by the mass of the fireplace. If you could post a picture of the installation, that would be most helpful.

    From the description it sounds like a temporary attempt at a crude block off plate was tried in the chimney. Plasterboard is not an acceptable material for a block off plate. My guess is that it is also poorly fitted and that a lot of heat is heading up the chimney. If this is the case, moving the stove out of the fireplace and onto the hearth plus installing a proper, insulated, metal blockoff plate that is sealed with silicone around the edges, will make a large difference in the satisfaction achieved with the stove. The hearth may need to be extended to safely accomplish this.

    Of course, I could be completely off. I'd need more details and a picture to verify assumptions. But I could see how heaters could outperform the stove if they have fans and are out in the open space of the room.
  13. Martin A

    Martin A New Member

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    Thanks for the questions.

    The electric heaters are pure convection heaters (no fans). One 2 kW heater is mounted under each window of the room.

    I shall attempt to attach a photo of the installation. Please note that the "stonework" behind the stove is constructed from lime and mortar attached to heat-resistant (French "pink" grade) plasterboard. Behind the plasterboard is 5 cm of rockwool insulation, then 5 cm air gap, then the original stone wall.

    The temporary chimney closure is plasterboard with 5 cm of fibreglass on top. The plasterboard rests on a shelf at the rear and is well sealed with fibreglass at the front. It is about 12 inches above the oak lintel that is visible (I hope) in the photo. I do not belive that much heat is being lost up the chimney - I paid special attention to getting a good seal with fibreglass pushed into all possible gaps.

    I found there is a makers plate on the back of the stove, which I removed. It is hard to read (the maker's varnish has melted and burned) but it says things like Standard number EN13240, maker's model number, weight, cast iron specification. It also it says Heat Output 10kW This is already 2/3 down from its "Nominal" rating and is presumably the result of a lab test.

    Martin

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  14. begreen

    begreen Mooderator Staff Member

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    Very nice. That is a beautiful setup, though the stove top is a bit close to the mantel. It "looks" like it should heat ok, but obviously it is not. That could be because it is very inefficient as compared to a modern stove with good secondary combustion or that it is a touchy stove to get burning hot. One thing I would get is a thermometer on the top of the stove. That will at least give you some guidance about how hot the stove is burning. The second thing would be to examine how the air controls are being used. Are the lower ones used to start up the fire, then closed down and then the upper ones used to achieve some secondary combustion?
  15. Martin A

    Martin A New Member

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    Very nice. That is a beautiful setup, though the stove top is a bit close to the mantel.
    Thank you. I built the "plinth" the stove is standing on so that in years to come, the stove could be loaded comfortably without too much bending.

    The plinth has a 12cm opening under the stove to allow a rising current of air to contact the underside of the stove and then exit as warmed air between the stove's legs.

    It “looks” like it should heat ok, but obviously it is not. That could be because it is very inefficient as compared to a modern stove with good secondary combustion or that it is a touchy stove to get burning hot.
    It is a modern stove! [Or at least, it is in current production]

    One thing I would get is a thermometer on the top of the stove. That will at least give you some guidance about how hot the stove is burning.

    Yes, a thermometer is a good idea - I wanted to measure its surface temperature but I did not have the means.

    As a general indication - with this stove burning as fiercely as I could get it to burn, you can get as close to it as you want (without actually touching it). Neighbours' "inserts" [wood burning stoves designed to be encased in brickwork], also rated at 15kW, produce enough radiant heat that you really can't stand close to them when they are burning at high output.

    The second thing would be to examine how the air controls are being used. Are the lower ones used to start up the fire, then closed down and then the upper ones used to achieve some secondary combustion?


    Here is what the makers said in the message they sent following the purchase:
    Open air vents result in faster burning, closed air vents cause a slower burn. The airwash vents are at the top and best half open once the fire has matured. Fiddle to avoid smoke omissions, as each chimney is different.

    I found that the fire seemed to burn most fiercely with all the controls fully open. It would be interesting to measure the surface temperature of the stove with different settings of the controls.


    Thanks for all the suggestions. I remain sceptical that the stove can produce 10-15kW output under any normal conditions, although the makers have produced a test certificate that apparently confirms "15kW nominal output" . (The certificate was produced with great reluctance, after some months of delay. They only allowed to be inspected at their solicitor's office by a trading standards officer and they refused permission to have the certificate photocopied.)

    I plan now to meaure its internal dimensions and compare with the dimensions of French stoves rated at 15kW. If they are roughly the same, I'll persist with efforts to get it to produce the thermal output I need.

    If it turns out to be appreciably smaller in its internal dimensions than similarly rated stoves, then I'll assume it has been over-sold and I'll donate it to my neighbour who moonlights as a scrap metal collector.

    Many thanks for all the helpful advice and comments.
    Martin
  16. begreen

    begreen Mooderator Staff Member

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    A brand new stove does not have to be modern in its fire burning design. There is a significant difference in the interior fittings and combustion methods used in many of the better stoves. This results in lower emissions and much greater efficiency which in turn results in greater heat returned to the room. From the description, this stove may have a design that went out in the early 1980s in the states. In a bried seach I couldn't find any documentation on the CK42. Does it describe how to achieve best secondary burn?

    Burning with the air intakes wide open is also part of the problem. This may be sending more than heat up the flue than to the room. The massive inrush of air is actually cooling the fire to some extent. Coupled with the very dry wood, it may be leading to rapid fuel consumption. Try burning as directed with a stove top thermometer for guidance. Once the stove is burning well, the lower intake ports should be closed off and the upper air intakes only partially open. That should help to bring about a significant rise in stove top temperature.

    The differences between older style stoves and a high efficiency heater are not just the firebox dimensions. Internally a modern stove has specially located secondary combustion feed tubes or ports and a baffling system. The fire box itself will be insulated so that the fire burns hotter. While out comparing stoves, look at a Jotul F500 "Oslo" for comparison. This stove is a good modern heater with a Heat Output: Min 3.5 kW. Nom 9 kW. Max 13.5 kW. Other brands to look at might be Dovre 760CB, or Franco Belge Ardennes.
  17. branchburner

    branchburner Minister of Fire

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    Is this wool-type fiberglass insulation? Because even though it appears to seal gaps, it does not stop airflow. As BG said, without a sealed impermeable barrier such as a metal blockoff plate, you could still be losing significant heat up the chimney.

    I'm wondering about the rest of the setup: does the newer flue continue to line the old chimney to the very top, and is the chimney-top sealed with a cover plate to restrict airflow? If not fitted with a tightly-sealed top plate your chimney may have significant airflow instead of an insulating dead air space.
  18. Martin A

    Martin A New Member

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    Does it describe how to achieve best secondary burn?

    The documentation that was provided is minimalist, to say the least. Here is what it says about setting the air inlets:

    Open air vents result in faster burning, closed air vents cause a slower burn. The airwash vents are at the top and best half open once the fire has matured. Fiddle to avoid smoke omissions, as each chimney is different.



    Thanks to all for the good advice. I will do the following as soon as the weather gets cool enough to make lighting the fire worthwhile:

    1. Install a metal blocking plate with silicone sealant at all gaps (and rockwool insulation on top). This was planned to do in any case.

    2. Get a surface thermometer

    3. Experiment with air inlet settings to find what maximises the surface temperature. [Plus measuring the temperature of chimney pipe to provide an indication of how much heat is being lost via the chimney for each inlet setting].


    In a few months, when I've done these things, I'll report back with the results.

    Martin
  19. branchburner

    branchburner Minister of Fire

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    The other question I had, is there any kind of in-line damper in the stove or venting? If your only control is air supply, and you have no control of exhaust, I would guess BG is correct in that your very dry wood is simply burning too quickly and sending much of the heat right up the flue.

    The stove seems to be without advanced secondary combustion capability. I'm not sure what its airwash design is, but it may be keeping the glass clean at the expense of heat retention. If the stove design involves little more than a firebox and simple baffle system, a damper may be helpful with such dry wood. The raging fire is one that is actually going to throw less heat into the room, compared to a controlled, sustained burn.
  20. Martin A

    Martin A New Member

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    Thanks for the question. Yes, on the advice of a friend, I installed a damper in the exhaust tube at the time of installing the stove.

    Varying the setting of the damper is another variable to adjust in finding the optimum set of settings. I'll now be trying to find the maximum temperature of the stove surface acheivable by varying:

    1. Opening of lower vents
    2. Opening of upper vents
    3. Setting of the chimney damper

    Even if I try just three settings for each variable, that is 27 settings altogether to try!
  21. branchburner

    branchburner Minister of Fire

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    And how about the chimney top? Do you know if the piping extends all the way up and out, and is the original chimney opening tightly sealed with a top cover plate so the only possible airflow is through the stove pipe itself?
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