PDD practice question about heating and cooling

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Yvette Louka

February 27, 2023 19:56

In question below I don't understand why answer D would be wrong?

Thank you

 

An architect is designing a single-family home in a location with cold winters and mild, dry summers. The homeowner is concerned about climate change and would like the home to have the lowest operating cost and the smallest carbon footprint possible. The local electrical grid is supplied with photovoltaic and nuclear power but not power from combustion sources. 

 

Which one of the following systems is most appropriate?

 

A.	Ground source heat pumps for space and water heating with reversing valve for summertime cooling.
B.	Local, sustainably harvested wood pellet space and water heating furnace with heat pump cooling.
C.	Direct-fired natural gas space and water heating with whole house heat exchanger forced air ventilation.
D.	Electrical resistance space and water heating system coupled with heat pump for summer cooling.

Your answer: D

Correct answer: A

 

CORRECT RESPONSE

Ground source heat pumps for space and water heating with reversing valve for summertime cooling.

While heat pumps require electricity to drive the refrigeration cycle, the electricity in this location is not generated by fossil fuel combustion, addressing the homeowner's concern about carbon footprint. 

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  Michael Ermann February 28, 2023 11:22

  Heat pumps are more efficient than electric resistance heaters, especially if they’re ground-source-coupled (geothermal).

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  Yvette Louka February 28, 2023 21:15

  Hello Michael, so what about "The local electrical grid is supplied with photovoltaic and nuclear power but not power from combustion sources. " in the question. Is it just a distractor?

  Thank you

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  Michael Ermann March 01, 2023 10:55

  Not a distractor at all….making electricity from burning fossil fuel is wildly inefficient. It might be that more than half the energy is wasted as heat goes up smokestacks in the power plant and as electrons overcome resistance in the lines…so in those cases, directly heating your house with natural gas may be better for the climate than using dirty electricity made from coal…heat pumps have become much more efficient, but air coupled heat pumps still only do so well in very cold weather…so given the priorities here, using clean electricity more clearly disfavors the natural gas option

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  Yvette Louka March 01, 2023 12:28

  I feel for sure I’m missing something here Michael. That’s why I chose answer D because it says but not from combustion sources. I thought this satisfies the owner smaller carbon footprint request. 

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  Yvette Louka March 01, 2023 12:40 (Edited March 01, 2023 12:41)

  Both options A and D have the heat pump but I read in one of the books , and I can search more which book, that heat pumps don’t work well in cold climates. That’s why I chose if for the cooling. 

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  Joseph Scherer March 04, 2023 18:59 (Edited March 04, 2023 18:59)

  There are two criteria given:

  1. Operating cost
  2. Smallest carbon footprint possible

  Because you are told that the electricity is produced without combustion, Options A and D satisfy criteria #2. However, Option D is inferior to Option A with respect to operating cost (criteria #1) because Option D requires the homeowner to buy more electricity: it involves electrical resistance space heaters (option A does not), and the heat pump in option D is not ground coupled, so it will require more electricity to provide the same level of heating or cooling.

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  Yvette Louka March 05, 2023 11:54

  Thank you Joseph for the explanation 

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  Michael Ermann March 05, 2023 20:44

  I wrote above “ heat pumps have become much more efficient, but air coupled heat pumps still only do so well in very cold weather”…I meant to write “ heat pumps have become much more efficient, but air coupled heat pumps still DON’T do so well in very cold weather”

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