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Can a Heat pump replace a boiler? PowerPoint PPT Presentation


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(Press F5 if not auto start). Can a Heat pump replace a boiler?. (arrow-down or mouse-click). 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

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Can a Heat pump replace a boiler?

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(Press F5 if not auto start)

Can a Heat pump replace a boiler?

(arrow-down or mouse-click)

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)

50°C

rt = 12°

Boiler

44°C

44°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

Same heat output

As previous slide

53°C

53°C

(Flow-rate x 2)

50°C

rt = 6°

Heat

Pump

47°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) = Flow rate (lit/sec) x 4.2 x rt

Heat output approximates

to average (mean)

flow-return temperature

(Repeat of previous slide)

Simple formula for heat, water flow and temperature difference

Keeping flow temperature low

increases energy efficiency

Same heat output

As previous slide

Example

53°C

53°C

(Flow-rate x 2)

50°C

rt = 6°

Heat

Pump

47°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

All temperatures now 12° lower.

Rule of thumb: - 1° drop in water

temperature can result in about 2.5%

improvement in system efficiency.

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)

38°C

rt = 6°

Heat

Pump

35°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

Pipes in floor screed

36°C

36°C

(Flow-rate x 2)

43°C

rt = 6°

Heat

Pump

30°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)

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.


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