Heating consumption of buildings

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Scope

Presents day degree method, which is used to define heating consumption of buildings. D↷

Definition

Heating consumption (Q) of buildings can be defined with knowledge of surface area of building, inside and outside temperature in certain time period, and heat permeability factors. Easiest way to use heating consumption is to define it as floor area weighted calculation.

Assumptions:

Af = Ar
Aw = nAf
A = Af+Aw+Ar=(n+2)*Af

Buildings overall heat transfer coefficient per floor area is calculated as:

Uf = (Af/Af*Uf + Aw/Af*Uw + Ar/Af*Ur)/(A/Af)
=[(1/n)*Uf + n*Uw + (1/n)*Ur)]/(n+2)

where A_f is area of floor, A_r is area of roof, A_w is area of walls, A is area of building, which emits heat, n is number of floors in the building and U_f is the overall heat transfer coefficient of floor, U_w the overall heat transfer coefficient of walls, and U_r is the overall heat transfer coefficient of roof. Unit of areas are m², unit of the overall heat transfer coefficients are kW/(m² K).

Building overall heat transfer coefficients are regulated in Finnish building regulations, which are presented in following table

Part of building	1978-1985		1985-2002		2003-2006		2007-2010		2010-?
,	Mass less than	Mass over	Warm area	Half warm area	Warm area	Half warm area	Warm area	Half warm area	Warm area	Half warm area
,	100 kg / m2	100 kg / m2	> 17 °C	> 5 °C	> 17 °C	> 5 °C	> 17 °C	> 5 °C	> 17 °C	> 5 °C
,	[W / m2 K]	[W / m2 K]	[W / m2 K]	[W / m2 K]	[W / m2 K]	[W / m2 K]	[W / m2 K]	[W / m2 K]	[W / m2 K]	[W / m2 K]
Walls	0,29	0,35	0,28	0,45	0,25	0,45	0,24	0,38	0,17	0,26
Roof	0,23	0,29	0,22	0,45	0,16	0,45	0,15	0,28	0,09	0,14
Floor	0,23	0,29	0,22	0,45	0,16	0,45	0,15	0,28	0,09	0,26
Part which is against ground	0,4	0,4	0,36	0,45	0,25	0,45	0,24	0,34	0,16	0,24
Window			2,1	3,1	1,4	2,1	1,4	1,8	1	1,4
Door			0,7	2,9	1,4	2,1	1,4	1,8	1	1,4

Heating consumption of the buildings is calculated as:

Qh = Uf * (Tin-Tout)dt

where Q_h is heating consumption of the buildings (kW/m²/year), U_f is floor weighted overall heat transfer coefficient per floor area of building (kW/m² K) and T_in is inside temperature (usually used 17^oC=290,15 K) and T_out outside temperature (K) in time dt.

Buildings also need thermal energy for warm tap water. This can be calculated by a following function

Qw = cp,w pw qm,w * (Twarm - Tcold )t

where Q_w is thermal energy which is needed to heat tap water (kW/m²/year), c_p,w is specific thermal capacity of water (4.1962 kJ/kgK), p_w is density of water (1000 kg/m³), q_m,w is volume of needed warn tap water, which can be estimated to be 40 % of total used tap water (in Finland 100-224 (dm³/inhabitant)/day or 3,5 - 4,9 (dm³/m²)/day), T_warm is temperature, where the tap water is heated (60 ^oC = 333,15 K), T_cold is temperature of the cold water (4 ^oC = 277,15 K) and t is time (1 year = 360 days). This can be also estimated by looking buildings thermal energy consumption in summer months, when there can be estimated to be only warm tap water use, because of outdoor temperature. Note that specific thermal capacity and density of water changes when the temperature changes. In calculations is used extrapolated value for the specific thermal capacity value (in 28^oC).

So buildings total need of thermal energy is

Q = Qh + Qw

Seasonal variation of heating consumption (Q_h) can be evaluated also with monthly mean temperature differences.

Unit

kW/m²/year D↷

References

Rakennuksen energiankulutuksen ja lämmitystehontarpeen laskenta[1]

Rakennusten lämmitys. Olli Seppänen. 2001, 2 päivitetty painos. Suomen LVI-liitto ry. Gummerus kirjapaino Oy, Jyväskylä 2001. ISBN 951-98811-0-7.

See also

Monthly mean temperature in years 1900-2000 at Helsinki Jyväskylä and Sodankylä [2].