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Heat loss in pipe insulation.

On a cold day, I was on the (non heated) attic of my house where the central heating pipes are running.
When I touched the insulation around the pipes, I noticed they were feeling warm.
This means despite the pipe insulation, warmth is escaping from the central heating pipes.

Via some measurements I try to find out how much energy the central heating pipes are approximately losing.
I focus on the following two aspects.
- Heat loss via the insulation around the pipes
- Heat loss via the pipe brackets, through which the pipes are mounted to the wall.

The measuring set up

The components used for the measurements:
- A copper tube, 28 cm long and 22 mm diameter, the bottom side of the tube is sealed.
- A thermometer
- Two resistors of 12 Ω (series connected) which I use as heating element.

Set up for measurement 1.

The resistors are put into the tube>
The tube is filled to the edge with water, the thermometer measures the water temperature.
The thermometer is put into the water through a piece of EPS (expanded polystyrene), which covers the top side of the tube.
The bottom side of the tube is standing on a piece of EPS.
For the rest, the tube is not insulated.
The resistors are connected to a 10.0 Volt power supply.


Set up for measurement 2.
The tube is insulated with a piece of pipe insulation with 9 mm wall thickness.
Set up for measurement 3.
The tube is insulated with 50 mm thick glass wool.
After applying the glass wool, and winding tape around it, the glass wool was flattened to a thickness of 20 mm.
Set up for measurement 4.
A pipe bracket is attached to the tube.
The pipe bracket is mounted on a brick
The tube insulation is the same glass wool insulation as in measurement 3.

The set up for measurement 5 looks the same as the one for measurement 4, so also with glass wool insulation around the tube.
Only in measurement 5 an insulating cloth is applied between the pipe bracket and the tube.
The cloth can't be seen, while it is covered by the glass wool.
But on the picture below, you can see what I mean.
I used a piece of a cleaning cloth with 1.5 mm thickness.

The measurement.

Before starting the measurement, the temperature of the tube must be equal to ambient temperature.
The water used for filling the tube must also be of ambient temperature.
The begin temperature is noted, and the resistors connected to the power supply.
The tube filled with water now will slowly warm up.
On regular times the water temperature is noted.
One measurement can take some hours.

The measurement results

Table 1.
In the table below, the temperature increase of the tube with regard to ambient temperature is given.
The temperature is measured with 0.5 C accuracy.
If no value is noted, no measurement is done on that time.

Measurement 1
Without insulation
Measurement 2
With 9 mm pipe insulation
Measurement 3
20 mm glass wool insulation
Measurement 4
20 mm glass wool insulation
+ non insulated pipe bracket
Measurement 5
20 mm glass wool insulation
+ insulated pipe bracket
0 0 0 0 0 0
5 4 2.5      
10 7.5   6 5 6
15 10 8.5   8  
20 12.5   11.5 10 11.5
25 14 13      
30 16     15  
35 17.5 17.5     18.5
40       19 20.5
45   21 23    
50       22 24
55 22   27.5    
60     29 25 27.5
65   26.5      
70 24     27.5 30
75     34    
80 25     29.5 32.5
85   30      
90   31 38.5 31.5 35
95 26        
100   32     37
105     42    
110 27     35 39
120   34 45   40.5
130 28     37 42
135   35 48    
140       38  
150 28.5 36 50 38.5  
160   36.5   39 45
165     52    
170       39.5  
180     53 40  

De values from the table shown in a graph.

Graph 1:
Warming up of the tube.
Line 1 = without insulation around the tube.
Line 2 = with 9 mm pipe insulation around the tube
Line 3 = with 20 mm glass wool insulation

Graph 2:
Warming up of the tube.
Line 3 = with 20 mm glass wool insulation
Line 4 = with 20 mm glass wool insulation and non insulated pipe bracket
Line 5 = with 20 mm glass wool insulation and insulated pipe bracket

Formulas for calculating the heat loss

The power of the "heating element" is (10 volt) / 24 Ω = 4.166 Watt, this power I call: P(in).
One part of this power provides the warming up of the tube with water, by which the temperature of the tube increases, this part of the power I call: P(heat).
An other part of the power will get lost by heat loss, this part I call: P(loss).

At the beginning of the measurement (t0 = time zero) the heat loss will be zero, because the tube has ambient temperature, all added power is then used for warming up the tube.

What I want to know is the heat loss in Watt per C.
For the heat loss applies:
Heat loss
(in W/ C) = P(loss.) / T1                   (formula 1)

T1 = measured temperature of the tube with regard to ambient temperature at point of time t1.

P(in) = P(heat) + P(loss).
P(loss) = P(in) - P(heat)                                    (formula 2)

P(heat) = P(in) . dT1 / dT0                               (formula 3)
dT1 = speed of the warming up of the tube (in C / minute) at point of time t1.
dT0 = speed of the warming up of the tube (in C / minute) at point of time t0.

Combining formula 2 and 3 gives::
P(loss) = P(in) - P(in) . dT1/dT0
P(loss) = P(in) . (1-dT1/dT0)                            (formula 4)

Combining formula 1 and 4 gives:
Heat loss = P(in) . (1-dT1 / dT0) / T1               (formula 5)

Results of the calculations.

With formula 5 the heat loss is calculated for the measured set up's.

Measurement P(in)
C / W
C / W
W / C
1 4.166 0.75 0.0375 130 28 0.1413
2 4.166 0.5666 0.0666 135 35 0.1050
3 4.166 0.6 0.1 165 52 0.0668
4 4.166 0.5 0.05 160 39 0.0961
5 4.166 0.6 0.1 145 43.5 0.0798

Table 2.
Calculating the heat loss for the 5 measurements.
The tube length is 28 cm, so the given losses are also per 28 cm tube.

For measurement 1, 2 and 3 we can calculate the loss per meter tube, by dividing the results of table 2 by 0.28m.
This gives the following results:
Measurement 1: loss is 0.5046 W / C.m  (non insulated tube)
Measurement 2: loss is 0.3750
W / C.m  (pipe insulation 9 mm)
Measurement 3: loss is 0.2386
W / C.m  (glass wool insulation 20 mm)

The difference in loss between measurement 4 and 3 is the loss caused by one non insulated pipe bracket,
this is 0.0961 - 0.0668 = 0.0293 W / C

The difference in loss between measurement 5 and 3 is the loss caused by one insulated pipe bracket,
this is 0.0798 - 0.0668 = 0.013 W / C

A practical example

The central heating pipes on the attic of my house have a length of 16 meter, so 16 meter flow pipe, and 16 meter return pipe.
The flow pipe is 80 C
The return pipe is 50 C
The temperature on the attic is 10 C

The difference with ambient temperature is: 70 C for the flow pipe, and 40 C for the return pipe.
By multiplying the heat loss in W / C.m by the pipe length and temperature difference, we get the energy loss of that pipe.
The following table shows how much energy we lose with different kinds of insulation.

Kind of insulation length
flow pipe
temp. difference
flow pipe
energy loss in
flow pipe
return pipe
temp. difference
return pipe
energy loss
in return pipe
total energy loss
No insulation 16 70 565 16 40 323 888
9 mm pipe insulation 16 70 420 16 40 240 660
20 mm glass wool 16 70 267 16 40 153 420

Table 3:
Example of energy loss at 16 meter pipe length.
We see, the loss can be reduced by hundreds of watt's with a good insulation.

Both flow and return pipe are on my attic fixed by 10 pipe brackets.
The following table gives the energy losses in the pipe brackets.

Kind of pipe bracket number of brackets temp. difference
flow pipe
energy loss
per bracket
 in flow pipe
energy loss
in 10 brackets
in flow pipe
temp. difference
return pipe
energy loss
per bracket
in return pipe
energy loss
in 10 brackets
in return pipe
Total energy loss
flow + return
Non insulated 10 70 2,05 20,5 40 1,17 11,7 32,2
Insulated with 1.5 mm thick cloth 10 70 0,91 9,1 40 0,52 5,2 14,3

Table 4:
Example of energy loss in ten pipe brackets.
Insulating the pipe brackets also gives a small saving.

Let's say we can reduce the heat loss by 250 watt through a better insulation.
If the central heater is heating for 500 hours per year, this leads to a saving of 125 kWh.
At an energy content of 8.8 kWh per m gas, and a heater efficiency of 85 % , the saving will be 16.7 m gas per year.

Below some pictures of the insulation I applied in connection with the measurement results.
I choose to keep the original pipe insulation intact, and apply an extra layer of glass wool around it.
I removed the glass wool (with permission) from a house which was being demolished.
It was free, and it also reduces waste, these are also kinds of savings.

If you choose, you can also apply an even thicker layer of insulation, and reduce the heat loss even more.

Insulating cloth between pipe and pipe bracket.
As we see, the pipe insulation wasn't fully applied on all places.
On other places, the pipe insulation was applied over the pipe brackets, but because of the extra thickness of the brackets this led to a chink in the insulation.
By applying the extra glass wool insulation, these points are now also warmly packed.

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