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Heat Loss Calculations

Discussion in 'Plumbing and Central Heating' started by Longclough, 5 Nov 2013.

  1. Longclough

    Longclough

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    Hoping someone can help, I am trying to do some heat calculations for my Lounge, as I think that my current radiators are undersized for the size of the room and characteristics:



    I have looked at on-line calculators, but none seem to cover the case where a room is open to the stairs. How do you go about calculating this?

    Do I need to consider the Porch which includes the WC and Hall, or just the lounge?

    If it helps current radiators in the lounge are 1149 x 530 single with fins (I believe about 985w each), house is 10 years old. Not sure that the pouch roof is insulated, but believe the house has some form of insulation.

    As the thermostat is in the lounge, I measured the heat up times for the lounge last night. Heating turned on at 5:45 it was 13.0 degrees (6 outside) at 6:45 it was 16.0 degrees and by 19:30 it was 18.5 degrees. All radiators get too hot to touch longer than a few seconds and all over.
     
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  3. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    Do do the correct calculation you need to do some investigating, you will require to find out what the fabric of the building is constructed of, the U-valves(W/M²) converted in to resistance values, add all the resistance values together, plus the resistance value of the outside wall surface (0.04 w/m²) and the internal surface resistance (0.13w/m²).
    These totals are added together and divided by 1.
    This can then be multiplied by the square are of the wall and temperature different between outside(-1°C) and inside generally 21°C.
    Do this for each wall, the fabric of internal walls and temp difference between rooms will differ to externally.
    Then you require to do similar calcs for floors and ceilings plus air change calcs. If the landing/stairway is heated to temperature of a lesser value to that of the lounge then, this should be calculated and added to the total figure.

    The total of fabric heat loss of all walls, windows and doors are then added together with the total of the air change value plus 15% for intermittent heating and that is your total. Simples!
     
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  4. Longclough

    Longclough

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    PrenticeBoyofDerry thanks for the info.

    So for external walls I would need to count up the U values of the Plaster Board (dot and dab) + breeze block + air gap with ALREFLEX insulation (i think that is what it is) + brick skin. Does that sound reasonable.

    There is no heating on the stairs and landing. In other words are you saying when the calculate the heat loss to the ceiling I should consider the temperature difference to be less than say the area of the ceiling which has a heated room above it?
     
  5. Galoka

    Galoka

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    Yes, though if the house is of standard construction, then you should find U values for the whole element to save adding it all together - eg cavity brick with insulation, and plaster or whatever. A 10 yo house should have cavity wall insulation on construction.

    Lots of the online calculators are a rather basic. But the stelrad one here seem pretty comprehensive. But for one room it's not that hard to do it by hand

    http://www.starsapp.co.uk/

    As to the landing, if there is no heating and it is open to the lounge, then you need to include that in your calculations, as the lounge rads will be heating that space. Treat the landing area as a separate 'block' , it will probably make the calc easier, then add the results together.

    Porch - well, that is closed off by a door, as long as that is warm enough you don't need to worry about that. just allow for the temp difference across that bit of wall. (or ignore it, treat it as an outside wall, and then your rads will be a little over size.
     
  6. D_Hailsham

    D_Hailsham

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    Size, type and location of windows?

    The house will have been built in accordance with the BR requirements at the time.

    Required maximum U values were:

    External Walls - (1995) 0.45; (2002) 0.35 W/m2 K
    Ground floor - (1995) 0.45; (2002) 0.25 W/m2 K

    It might be safer to use the 1995 figures as the plans were probably approved before the 2002 figures came into force.
     
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  7. Longclough

    Longclough

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    Galoka, things for the link I will have a look at it. Are you saying that I calculate the Heat Loss for the landing separately block, i.e. Length, width and Height of landing with one external wall. This would then be added to the lounge calculation, it give the overall amount.

    D_Hailsham, all dimension are included in the drawing. The window is 1.5m2 and above radiator 1.
    Are you saying I work out the area of the 2 External walls and floor and add the window loss using the u values provided?
     
  8. Galoka

    Galoka

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    Well, when I did mine that is how I dealt with mine. It seemed easier that way. In essence that is how we do all heat loss calcs by dividing up the house into blocks - just normally these blocks are individual rooms, and might be a different temps.

    You might find this page on the uk.d-i-y newsgroup wiki useful:

    http://wiki.diyfaq.org.uk/index.php?title=Heat_loss
     
  9. D_Hailsham

    D_Hailsham

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    Didn't see the window data.

    Yes!

    Assuming for simplicity that the room is a rectangle 5.35m x 4.45m you get

    External wall
    Area = (5.34+4.45) x 2.4 = 23.52m² less window area (1.5m²) = 22.02m² x 0.45 (U value) = 9.92 Watts per °C

    Window
    Area = 1.5m² x 3.1 (U value) = 4.65 Watts per °C

    Floor
    Area = 5.35 x 4.45 = 23.81m² x 0.45 (U value) = 10.71 Watts per °C

    Party Wall
    Area = 5.35 x 2.4 = 12.84m² x 1.33 (U Value) = 17.08 Watts per °C

    Ventilation
    Room volume = 5.35 x 4.45 x 2.4 = 57.14m³ x 0.33 (specific heat of air) = 18.86 Watts x air changes/hr Watts per °C

    External walls, floors and air changes are usually calculated for -1°C outside and 21°C inside - a difference of 22°C

    Party walls are calculated assuming a 10°C difference

    Ventilation rates can vary depending on type of room, for a living room a rate of 1 to 1.5 changes/hour is normal.

    So loss from external walls, window and floor will be:

    9.92 + 4.65 + 10.71 = 25.28 x 22 (temp difference) 556.16 Watts

    Party Wall = 17.08 x 10 (Temp difference) = 170.8 Watts

    The total loss through the fabric is 556.16 + 170.8 = 726.96 Watts

    Ventilation = 18.86 x 1.5 (Air changes) x 22 (Temp difference) = 622.38 Watts.

    Total loss = 726.96 + 622.38 = 1349.34 Watts

    As you can see the ventilation loss can be a significant part of the whole, which is why draft proofing is so important. Obviously an airtight room is not achievable, and certainly not desirable!

    Use the same method to calculate losses on the landing. You ignore any floor losses as there will be no temperature difference.

    You have approx 2kW of rads, so that is probably sufficient.

    Does you boiler cycle a lot?
    Where is the room stat located?
    Do you have TRVs on the rads (except where stat is located).
    Do you keep the doors closed upstairs?
     
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  11. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    Yes you will also have to calculate windows and doors in external walls, that can be deducted from wall area, then added as different element. Don't forget the internal/external wall surface resistance. You will also need to calculate ventilation losses due to air changes.

    Yes if the lounge is heated to 21 degrees C and the landing is not heated, we could assume it to be 12 degrees C, then temperature difference of 9 degrees C, would need to be factored in.
     
  12. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    plus 15% for intermittent heating.
     
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  13. Longclough

    Longclough

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    D_hailsham thanks for the comprehensive response. I have managed to get my head round it. In answer to your questions:
    Does you boiler cycle a lot? Not a great deal
    Where is the room stat located? Bottom of the stairs (lounge)
    Do you have TRVs on the rads? No just upstairs bedrooms
    Do you keep the doors closed upstairs? Yes[/quote]
     
  14. D_Hailsham

    D_Hailsham

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    The boiler does not appear to be oversized and the stat is in a sensible position.

    TRVs on the rads in hall, WC and the other rooms would be a sensible thing to do.

    Keeping upstairs doors closed does mean that the landing has to be heated from downstairs, so you have to take this into account when sizing the the living room rads.

    It might be worth experimenting with some of the upstairs doors open , so the heat filters out of the bedrooms to warm up the landing.

    I have re-read your opening post and noticed something which does not make sense. You wrote:

    Heating turned on at 5:45 it was 13.0 degrees (6 outside) at 6:45 it was 16.0 degrees and by 19:30 it was 18.5 degrees.

    I assume that it was +6 outside, not -6, so the outside wasn't even down to the design temp (-3 nowadays).

    That means it took one hour for the first three degrees and nearly 13 hours for the next two and a half degrees?

    Is that correct???
     
  15. Agile

    Agile

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    I don't know where Hailsham has got info on the boiler power as I don't see it quoted here.

    Regardless of heat loss calculations, there is obviously insufficient heat input to the room! Even when its still + 6 C outside! When its below freezing its going to be far worse!

    That depends on the rad size AND the flow and return temps!

    We have not been given any heating temp information. Unless its a Vaillant boiler they will need to be measured with a contact thermometer!

    Heat loss calculations are all fun but at the end of the day its the actual measured heat loss which counts. If I am ever faced with that problem I run an electric fan heater and see what temp that can create in the room compared with the outside and extrapolate from that data to see what heat output is required.

    Tony
     
  16. D_Hailsham

    D_Hailsham

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    I agree: it's not stated. But I wasn't concerned with the actual power but whether the boiler was producing more power than the rads can handle.
    There have been enough topics on this forum about boilers never getting up to temperature because they are massively oversized compared to the heat requirement and total rad output. Normally combi boilers are the culprit.

    If the OP would advise us of which boiler he has, it will be helpful; and the temperature it is set to will be even better.
     
  17. DP

    DP

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    OP, check the pipes the rad is connected to. One should be too hot to hold, other slightly cooler so you would be able to hold it.
     
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