Question

Should I go with an air source heat pump or electric radiant floor heat?

Asked by Chris DiGiacomo
Shirley, NY

I live on Long Island in NY. I have a well-insulated, 1000-square-foot home. I have no heating system in the house. I am concerned that a heat pump will not be able to heat in the coldest winter days and the unit will rely on the back-up electric coil. I do have solar PV to offset most of the electric to run the heat. Is it more cost effective to install and run a heat pump with back-up coil or electric radiant floor heat? Ductwork would have to be installed for the heat pump.

Answer

Rick Goyette

Answered by Rick Goyette

Pawtucket, RI

Emerald Dream Builders

September 1, 2010

I always find the questions about the lesser of two evils to be the most interesting. It sounds like you have removed combustible fuels from the list of options, and given that you do not currently have heat in the home, you are searching for the best path given the constraints for this home.

Let me start by saying that neither option is optimal because both will require you to produce heat using electrical resistance given the climate of Long Island, NY.

  • Electric radiant heat produces heat purely through resistance. When you pass an electrical current across a resistor the result is: heat. That heat creates more resistance -- which creates more heat. If you remove the factor of how the electricity is produced at the power plant, you can assume that 100% of the electrical energy you are purchasing is being converted to heat energy.
  • An air source heat pump uses electricity and converts it to mechanical energy via a compressor. The compressor gathers heat for your home operating in “forward mode” and cools your home while operating in “reverse mode.”

As a comparison to the resistive radiant heat option above, you can expect the heat pump to produce 1.5 to 3 times as much heat energy as the unit consumes in electricity. Sounds perfect, right?

  • The issue with air source heat pumps in locations like Long Island is that they begin losing efficiency as the outside temperature approaches 32 degrees, as you pointed out in your question.
  • At that point, most units rely on an electrical resistive coil to continue producing heat during the winter.
  • A quick look at some climate data for your region and you will see that an air source unit will be utilizing back-up resistive heating for parts of December and March and all of January and February. You can have a look at the climate data here.

Of the two choices, I would recommend the air source heat pump purely because it will only use electrical resistance part time. In the back of my mind, I also have concerns over how the heat from an electrical radiant heat floor system would be conducted into your conditioned space.

  • Will you be installing it in an unconditioned space under wood plank subfloor?
  • Will the system be insulated?
  • Many other variables about how an electrical radiant floor heating system would perform given a limited understanding of your home environment make it hard for me to recommend it.

Lastly, please consider an Energy Star labeled heat pump. For a cost estimate of the savings this decision can provide, use this Energy Star spreadsheet to calculate estimated savings.

For more information about air source heat pumps, point your web browser to EnergySavers.gov.

You also mentioned that you will have to install ductwork for the air source heat pump.

  • If it is possible, consider installing all ductwork inside the thermal boundary. This can be accomplished in a number of different ways. A starting point for your project would be to visit the Energy Star website.
  • It's imperative to use mastic instead of tape on the ductwork. I’ve found through trial and error that it is almost impossible to accomplish duct tightness without mastic sealant.
  • Ironically enough, some duct tape actually has a warning that it is not to be used for ducts. Even tape that is sold for use on ducts will not provide you the tightness that mastic can provide.

According to the Energy Performance of Buildings Group at LBNL, each year, U.S. residential duct leakage costs consumers $5 billion. It’s that important!

Good luck on your project!

 

For more information:

Read Richard Parker's Q&A "Can you help me select a high efficiency furnace?"

Tagged In: radiant heat, heating cooling, ductwork

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