Can a Heat pump replace a boiler?

Not without a bit of thought.

Maximum efficiencies are unlikely to be achieved without a few changes to the system

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Boiler

Heat output approximatesto average (mean)flow-return temperature

56°C

44°C

56°C

44°C

50°C

Flue losses10 – 15%

t = 12°

Low return temperature helps condensing- increases efficiency

Let us consider a Condensing boiler circuit

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Example

85 – 90% energy to water

(Flow-rate 1)

Let’s replace the boiler with a heat pump

HeatPump

Heat output approximatesto average (mean) flow-return temperature

53°C

47°C

53°C

47°C

50°Ct = 6°

Heat pumps are fundamentally different to boilers

Keeping flow temperature lowincreases energy efficiency

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(Flow-rate x 2)

Electrical input (relative to heat output) can vary considerably

Same heat outputAs previous slide

Example

How did we achieve these temperature changes?

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).

Remember, our heat (kW) is the same in every example.

In our example we have doubled the water flow rate.

The ‘flow’ is now 3° colder, and the ‘return’ is 3° hotter, but the average temperature is unchanged.

(Repeat of previous slide)

HeatPump

Heat output approximatesto average (mean) flow-return temperature

53°C

47°C

53°C

47°C

50°Ct = 6°

Heat pumps are fundamentally different to boilers

Heat (kW) = Flow rate (lit/sec) x 4.2 x t

Keeping flow temperature lowincreases energy efficiency

http://www.heatpumps.co.uk/heatpumpcalculator.html

(Flow-rate x 2)

Electrical input (relative to heat output) can vary considerably

Simple formula for heat, water flow and temperature difference

Same heat outputAs previous slideExample

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.

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.

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.

HeatPump

A bigger emitter systemreduces the working temperatures.This increases the COPsignificantly.

41°C

35°C

41°C

35°C

t = 6°

Now with a bigger radiator

Keeping flow temperature lowincreases energy efficiency

(Flow-rate x 2)

(Doubling the radiator area can reduce the mean temperature from 50° to about 38°C)

38°C

Same heat output butat lower temperature

Rule of thumb: - 1° drop in water temperature can result in about 2.5% improvement in system efficiency.

Example

(see radiator manufacturer’s data)

All temperatures now 12° lower.

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?

HeatPump

Better still :-underfloor heatingdesigned forlow temperatures

36°C

30°C

36°C

30°C

t = 6°

Now with underfloor heating

Keeping flow temperature lowincreases energy efficiency

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(Flow-rate x 2)43°C

Pipes in floor screed

Note : - In general, tiles or slabs on screed give better results than wood.

Example

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.

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.

This last slide is simply a summary of the previous examples, showing approximate implications to the efficiency (COP)

SUMMARY Boiler Heat pumpHeat 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)

Approximate system COP - 3 3.9 4.6