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Can a Heat pump replace a boiler?
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Not without a bit of thought.
Maximum efficiencies are unlikely to be
achieved without a few changes to the system
Let us consider a Condensing boiler circuit
Example
Heat output approximates
to average (mean)
flow-return temperature
Flue losses
10 – 15%
85 – 90% energy
to water
56°C
56°C
(Flow-rate 1)
rt = 12°
Boiler
44°C
44°C
50°C
http://www.heatpumps.co.uk/heatpumpcalculator.html
Low return temperature helps condensing- increases efficiency
Let’s replace the boiler with a heat pump
Heat pumps are fundamentally different to boilers
Example
Heat output approximates
to average (mean)
flow-return temperature
Keeping flow temperature low
increases energy efficiency
53°C
Same heat output
As previous slide
53°C
(Flow-rate x 2)
rt = 6°
Heat
Pump
47°C
50°C
47°C
Electrical input (relative to heat output) can vary considerably
http://www.heatpumps.co.uk/heatpumpcalculator.html
The ‘flow’ is now 3° colder, and the ‘return’ is 3° hotter,
but the average temperature is unchanged.
How did we achieve these temperature changes?
In our example we have doubled the water flow rate.
Remember, our heat (kW) is the
same in every example.
Note: - To increase the flow-rate, the pipe
diameter is likely to have to be bigger so that
pumping power is not increased.
(pump energy is an energy loss).
Heat pumps are fundamentally different to boilers
Heat (kW)(Repeat
= Flow rate
(lit/sec) slide)
x 4.2 x rt
of previous
Simple formula for heat, water flow and temperature difference
Heat output approximates
to average (mean)
flow-return temperature
Keeping flow temperature low
increases energy efficiency
Same heat output
As previous slide
Example
53°C
53°C
(Flow-rate x 2)
rt = 6°
Heat
Pump
47°C
50°C
47°C
Electrical input (relative to heat output) can vary considerably
http://www.heatpumps.co.uk/heatpumpcalculator.html
You might think that the heat transfer is
better when there is a large flow-return
temperature difference.
However, it all depends on how fast the
heat is taken away.
i.e. it depends in the water flow rate.
If the heat transfer (kW) is constant, and the
flow rate is doubled, then the temperature
difference between the flow and return is
halved.
Our heat pump prefers this, it ‘sees’ a lower
flow temperature.
How can we reduce the working temperature further?
Increase the size of the radiator.
A bigger radiator will emit more heat, so the
temperatures are ‘dragged down’ to a lower
temperature.
Now with a bigger radiator
Rule of thumb: - 1° drop in water
temperature can result in about 2.5%
improvement in system efficiency.
All temperatures now 12° lower.
A bigger emitter system
reduces the working
temperatures.
This increases the COP
significantly.
Keeping flow temperature low
increases energy efficiency
Same heat output but
at lower temperature
Example
41°C
41°C
(Flow-rate x 2)
rt = 6°
Heat
Pump
35°C
38°C
35°C
(Doubling the radiator area can reduce the mean temperature from 50° to about 38°C)
(see radiator manufacturer’s data)
Could we have done anything else?
If we insulate the house more, then less heat is
needed, this can reduce the water
temperatures required.
This therefore increases the energy efficiency
of the heat pump.
What else could reduce the temperature?
Now with underfloor heating
Better still :underfloor heating
designed for
low temperatures
Keeping flow temperature low
increases energy efficiency
Example
36°C
36°C
Pipes in floor screed
(Flow-rate x 2)
rt = 6°
Heat
Pump
30°C
43°C
30°C
Note : - In general, tiles or slabs on screed give better results than wood.
http://www.heatpumps.co.uk/heatpumpcalculator.html
So, we now have an efficient heat pump system.
It took a few changes
But the increase in energy efficiency makes the
long term energy savings worthwhile
Don’t forget to check your heat pump’s settings. A simple
adjustment to reduce the water temperature in the heating
system will save energy.
This last slide is simply a summary of the previous examples,
showing approximate implications to the efficiency (COP)
Boiler
Heat pump
Heat pump
with big
radiator
Underfloor
heating
Flow to radiator
56
53
41
36
°C
Ruturn from radiator
44
47
35
30
°C
Approximate mean radiator temperature
50
50
38
33
°C
Water dt (temperaure drop)
12
6
6
6
k (°C)
-
3
3.9
4.6
SUMMARY
Approximate system COP
It should be noted that the above are mid-winter temperatures.
With weather-compensation, the temperatures can be reduced
at milder times, thus increasing the COP.