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Heat demand dalculator - principle of calculation
VAlKo (10 Beiträge)
on 10.6.17
Hello everybody,

I am trying to understand the principle of operation of the heat demand calculator, especially the influence of the location-entry. Can somebody please correct and supplement my following assumption:

Heat demand calculator takes in account:
- heat loss through surrounding surfaces
- heat loss through ventilation
- heat gain through people living in that room
- heat gain through the sun shining through the windows

It calculates:
- the heat energy to enter the room to maintain constant temperature (max heating capacity), not taking heat gain into account and based on outside temperature from the location
- the heat energy to enter the room per year (heat demand), based on the avg outside temperature for that location

What I don't understand (yet):
- internal gain (people living in the room) is calculated from 0.32 * heated-volume. Does this mean that the average German room is 3m high?
- near the bottom of the heat demand calculator page, it says that internal gain is calculated from 0 W/m2 usable floor space. I am sure that I ones saw 4 W/m2 here. How is this value influenced?
- in the table heat loss/demand per month also the internal gains is shown. Why does the internal gain vary per month? I would expect it to be constant since the amount of energy produced by people living will not vary per month
- how does the entry "heat loss through... to..." outside air/attic/non-heated room/cellar/ground influence the calculation? How does it relate to the outside temperature (governed by the location)?
- in my calculations solar gains is always on zero. Must I do something special to mark a surface as glass (I used "insulated glazed unit (igu) Ug=2.8" as building material)?

Thanks for any further explanation,
AndreasTeich (1021 Beiträge)
on 12.6.17
Hi Albert
Mostly correct your assumptions,

0,32 refers to 1 m2, according to the tabel:
100 m3 gross heated volume (incl walls)
76 m3 net volume ( actual heated air volume)
32 m2 floorarea

Average height of rooms is 2,5 m

Real internal gains you find in the figures in brackets, which are quite similar throughout the year.
I only found 4 W/m2 for internal gains, vastly depending on the number of people present (per person you gain 80-100 W), also LED lighting or normal light bulbs, A+++ household devices or old ones etc.

Heat loss through...
For example: Ground temperature in cellars is roughly +10 degree, not varying much, accordingly the difference to + 20 degree normal room temperature is fairly small (10 degree)- consequently little heat loss.
Similar to the other rooms mentioned, which vary differently, but never as much as the difference of heat loss of walls to outside air, which might be 20 degree minus (i.e.40 degree difference= delta t= 40)

I am not quite sure why in your calculation solar gains through glass result in zero

Andreas Teich
VAlKo (10 Beiträge)
on 12.6.17
Hello Mr Teich,

Thanks for your clarification.

My mistake, the 0.32 relates via the height of the room to the net volume, not the gross volume. Sorry, my mistake.

Do I understand you correctly that in the "Heat loss through...":
outside air = temperature according location setting (avg temperature / month)
attic (non insulated) = ?
non heated room = ?
cellar (non heated) = ?
ground = +10 degree

How does this setting relate to the "outside" setting in the "u-value calculator (graphical)"?
direct contact to outside air = ? outside air ?
rear ventilated cladding = ?
rear ventilated roofing = ?
non-heated room = ? non-heated room ?
unheated roof space = ? attic ?
ground = ? ground ?

It confuses me that I have to enter "the other side of the wall" twice, once in the u-value calculator (where I enter the building materials) and once in the heat demand calculator.

Could the zero solar gains have anything to do with me having location set to NL:De Bilt?

Thanks for your help,
AndreasTeich (1021 Beiträge)
on 14.6.17
I'm not quite sure if I answer according to your questions:
'Heat loss through' refers to rooms adjoining to areas with lesser loss of heat. The resulting calculation should be correct anyway.

Only make sure, also windows incl relevant orientation are put in correctly.
Just try to put in another location and see if there is another result- I didn't check it.

Andreas Teich
VAlKo (10 Beiträge)
on 15.6.17
Hello Mr Teich,

To support my question with a picture, I send you an email at info@u-wert.net (I don't see a possibility to attach pictures in this forum)

Kind regards,
veverone (2 Beiträge)
on 13.11.17

Please, allow me a question: if the calculator says "max. heating capacity: 2.82 kW", does this mean that to maintain the temperature, I need, for example, an oil-filled radiator (heater) that consumes 2.82 kw electricity per hour?? I assume that that is "thermal watt"... but isn't 1 thermal watt produced by a 1 electric watt resistance?

2.82 kw of electricity, per hour, that sounds a lot! Am I missing something here ?
VAlKo (10 Beiträge)
on 13.11.17
Hello veverone,

Watt is just the unit of energy, it is equal to J/s.
How it is produced (converted would be better) is to your choice.
In my situation it is with natural gas. For natural gas is known that it produces 31650000 J/m3. So the 2820 Watt = 2820 J/s = 0.32 m3/hr.

When you know how many calories your oil delivers (in units of Joule/liter), you can calculate the amount of oil it would take per time.

But, to answer your question, yes, when the calculator says 2.82 kW, this would mean that you would need a 2.82 kW electrical heater.
veverone (2 Beiträge)
on 16.11.17
Thank you very much, VAlKo.

This is exactly the answer I was hoping for.

Have a great day forward!

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