Useful Calculations
Delta T
Delta T (or ΔT) is a difference in temperature.
When sizing radiators this is the difference between the Mean Water Temperature (MWT) and the air onto the heat exchanger temperature (generally the
room temperature).
When calculating the water flow rate this is the difference between the flow temperature and the return temperature.
The important thing to remember is that just because someone says the radiators are sized to suit ΔT10 it does not necessarily mean that they need to be
sized at a correction factor of 0.200 (assuming dynamic). It could be that they are looking for a temperature drop of 10ºC between the flow and return (as in
75/65). It is unlikely (but not impossible) that a ΔT higher than 20ºC is anything other than the difference between the MWT and the air on temp, but anything
less than 20ºC could be either.
Mean Water Temperature (MWT)
Mean water temperature is the average (mean) temperature of the flow and return water into the radiator (or across the heating or cooling system).
Mean water temperature is calculated by adding the flow temperature to the return temperature and dividing it by 2.
e.g
75/65 would be (75 + 65) ÷ 2 = 140 ÷ 2 = 70ºC
82/71 would be (82 + 71) ÷ 2 = 153 ÷ 2 = 76.5ºC
Correction Factors
Correction factors are calculated as follows
Factor(heating) = (∆T/50)
n
Factor(cooling) = (∆T/15.5)
n
*Where ∆T is the calculated difference between MWT and air on temperature, and n is the factor that varies depending on the construction of the radiator.
Every product has a different n value, but for the Jaga Low-H2O products, plus Mini Canal DBE, Knockonwood Freestanding DBE and Mini Freestanding DBE
the typical n value is 1.344, and the correction factor table can be found on page 353 of the 2013 Jaga UK catalogue
For Dynamic products the n value is 1.000 (NOT Mini Canal DBE), and the correction factor table can be found on page 366 of the 2013 Jaga UK catalogue
This calculated factor is to be applied to the ∆T 50 output in Watts.
Jaga Heating Products Ltd
+44 (0)1531 631 533 – jaga@jaga.co.uk – www.jaga.co.uk
Water Flow Rate
Mass Flow Rate
Volumetric Flow Rate
The mass flow rate can be calculated as
The volumetric flow rate can be calculated as
m = H / (cp × (tf - tr))
q = H / (cp × ρ × (tf - tr))
Where
Where
m = Mass flow rate [kg/s]
q = Volumetric flow rate [m³/s]
H = Output of radiator [W]
H = Output of radiator [W]
cp = Specific heat capacity [J/(kg·ºC)]. – This varies dependant on the actual
water temperature but can be approximated as 4187 for the purposes of
calculating water flow in radiators (assuming the heating fluid is water)
cp = Specific heat capacity [J/(kg·ºC)]. – This varies dependant on the actual
water temperature but can be approximated as 4187 for the purposes of
calculating water flow in radiators (assuming the heating fluid is water)
tf = Water flow temperature [ºC]
ρ = Density of water (or heating fluid) [kg/m³] – This varies dependant on the
actual water temperature but can be approximated as 982 for the purpose
of calculating water flow in radiators (assuming heating fluid is water)
tr = Water return temperature [ºC]
tf = Water flow temperature [ºC]
After calculating this you will need to multiply it by 3600 to give you kg/hr
to use on the pressure drop graphs for Jaga radiators.
tr = Water return temperature [ºC]
Conversion Factors
Volumetric Flow Rates
1 m³/s = 1000 l/s = 219.97 gallons (UK)/s
1 m³/hr = 0.277 l/s
Heat Output
1000 Watts = 3,412 Btu/hr
Pressure
1 bar = 100,000 Pa = 100 kPa = 14.5 PSI
Jaga Heating Products Ltd
+44 (0)1531 631 533 – jaga@jaga.co.uk – www.jaga.co.uk
Heat Loss
Overall Heat Loss
Fabric (transmission) Loss – Ht
Overall building heat loss is the sum of all of the aspects detailed below
H = Ht + H v + Hi
Fabric or transmission loss is the amount of heat lost through any surface
and can be calculated as
Ht = A × U × (ti - to)
Where
Where
H = Overall heat loss [W]
Ht = Heat loss due to transmission through surfaces or fabric loss [W]
Hv = Heat loss caused by ventilation [W]
Ht = Transmission loss [W]
A = Area of exposed surfaces [m²]
U = Overall heat transfer coefficient or U Value [W/(m²·ºC)]
Hi = Heat loss caused by infiltration [W]
ti = Inside air temperature [ºC]
to = Outside (or other side) air temperature [ºC]
Ventilation Losses – Hv
Infiltration Loss – Hi
Ventilation loss is the amount of heat lost from either a mechanical or a
natural ventilation system and can be calculated as
Infiltration loss is the amount of heat lost from natural air movement such
as opening doors or windows. This can be calculated as
Hv = cp × ρ × qv × (ti - to)
Hi = cp × ρ × n × V × (ti - to)
Where
Where
Hv = Ventillation loss [W]
Hi = Infiltration loss [W]
cp = Specific heat capacity of air [J/(kg·ºC)]
cp = Specific heat capacity of air [J/(kg·ºC)] – This varies dependant on the
actual air temperature but can be approximated as 1005 for the purpose of
calculating infiltration loss (based on external air temp of -5°)
ρ = Density of air [kg/m³]
qv = Air volume flow rate [m³/s]
ρ = Density of air [kg/m³] – This varies dependant on the actual air
temperature but can be approximated as 1.3 for the purpose of calculating
infiltration loss (based on external temperature of -5°C)
ti = Inside air temperature [ºC]
to = Outside (or other side) air temperature [ºC]
n = number of air changes per second
(= number of air changes per hour ÷ 3600)*
If a heat recovery system is to be used, then the formula should be changed to
Hv = (1 - β/100) × cp × ρ × qv × (ti - to)
Where
β = Heat recovery efficiency [%]
V = Volume of the room [m³]
ti = Inside air temperature [ºC]
to = Outside (or other side) air temperature [ºC]
* Typical number of air changes per hour would be:
Living Room – 1; Bedroom – 0.5; Bathroom – 2; Classroom – 2;
Store Room – 0.5; Office – 1
Jaga Heating Products Ltd
+44 (0)1531 631 533 – jaga@jaga.co.uk – www.jaga.co.uk