Heat source efficiency

Fig. 1 Oil boiler. The black box at the
bottom is the furnace (Statoil), and the
tall blue container is the boiler
(DFJ Salamander C). The furnace is new,
but the boiler is old and not insulated,
with low efficiency.

Almost all furnaces suffer heat losses, which is why the thermal efficiency expresses the amount of heat a furnace can provide, relative to the available energy supply.

For example, an oil furnace loses heat through the chimney, or if your burner is sooty, then it is only using part of the energy available in the oil. The efficiency rate is measure of the installations useful effect.


The efficiency rate measures the relationship between the energy produced and the energy supplied, in other words

efficiency rate = useful energy out / total energy in

The term 'useful energy' corresponds to 'living-room temperature'. It is not possible to produce more useful energy than is supplied, so the efficiency rate lies between 0 and 1, or in percentage terms, between 0 and 100 %.


1. Old oil furnace

An oil furnace from 1977 or earlier loses energy to the chimney, boiler room and from incomplete combustion. If the oil tank is stored outside, and the oil is cold, there is an additional loss from heating up the oil.

Unless you've measured the efficiency rate directly, it can be assumed to be 0.6 or 60 % (Table 1).

~hs~efficiency rate oil furnace 1977 

The green bar displays useful heating, while the red bar shows the loss. The whole bar corresponds to 100 % of the supplied energy from fuel.

2. New oil furnace

A new, good-quality oil furnace with an optimized injection pump and an insulated furnace has an efficiency rate of 0.9, or 90 % (Table 2).

~hs~efficiency rate new oil furnace~hs~~hs~ 

Your chimney sweep may have measured the efficiency rate and left a note by the furnace with the written rating.

3. Electrical heating

Electrical heating has an efficiency rate of 1.0:

~hs~efficiency rate electrical heating~hs~~hs~~hs~~hs~~hs~ 

While this might appear to be ideal, the price of elctricity is so high that in reality, this is the most expensive form of heating.

(Naturally, there are losses in the electricity transfer from the meter to the appliances, but arguably, these should not be counted. The same loss occurs for heating, and if that heating is used for the general household, then the efficiency rate becomes 1.0)


Table 1 (below) contains officiel figures from the Danish Energy Agency, while table 2 contains the unofficial historical figures from Energy Service Denmark.

Table 1. Standard efficiency rating values (Danish Energy Agency 2009)

Appliance Efficiency rate
Oil furnaces from 1977 or earlier, cast or sheet iron boiler 0.60
Oil furnaces after 1977, cast or sheet iron boiler 0.77
District heating installation 0.95
Conventional warm air gas furnace 0.74
Induced draft gas furnace 0.83

Table 2. Efficiency rates (Energy Service Denmark spreadsheet 2009)

Appliance Efficiency rate
New condensing oil furnace 0.97
New efficient oil furnace 0.90
Rapeseed oil furnace 0.85
Best wood pellet boiler 0.80
Average wood pellet boiler 0.75
Wood bricquettes 0.70
Ground source heat pumps (COP) 3.00
Water source heat pumps (COP) 2.80
Household stove 0.70
Electrical heating 1.00

Also see

Created by tanja.groth. Last Modification: Thursday 10 June 2010 16:22:02 CEST by tanja.groth.