Is the heat loss a value of the heat I am losing on a daily basis?
Yes you could say that, based on the coldest day.nigt of the year in relation to the internal temperature.
Heat loss is based on U values of the fabric the building and whether a room has an 1 or more outside walls, with or without a window and whether the remaining internal walls in the room face a heated room or not.
Simply the lower the heat loss, the less energy is needed to heat your home v the higher the heat loss more energy is needed to heat your home.
The basis of calulating heat loss is on a room by room basis and from this you come up with a total heat loss for the home, based on the outside temperature (worse case) and heat required indoors which can be calculated on temperature requirement for downstairs and upstairs or room by room basis.
Transmission losses is based on each component of the building fabric U value (walls, roofs door, windows) where we try to find out how much heat passes through.
Transmission heat loss (area x U Value x temperature difference)
Ventillation losses is based on the number of air changes a room has i.e replacing air inside with colder air outside.
(Volume x Air change rate x specific room heat capacity x temperature difference)
Thermal Bridging (Cold bridging) happens when part of the buiding envelope (external walls) allow heat to pass through more easily i.e you have an internal wall that joins the external wall without sufficient measures to prevent a thermal bridge. (you put your finger on ice cube & it zaps heat away from your finger lol)
Thermal bridging is added to the calculation through clculating the Y value (forget the calculation sorry)
The total heat loss is calculated by adding all these together.
MCS qualified installers, should be calculating the heat loss on a room by room basis. EN12831 compliant calculator is in a link provided yesterday if you want a play.