Dual fuel heat pump efficiency - need a sanity check in my logic

8 years ago

Thinking of the HVAC options to install in a single family house in central MA. One of the "greener" options being suggested is a heat pump with fall back to nat gas. But the $-part, even excluding the installation cost difference, looks questionable. Here is why:

According to this usclimatedata.com, about 1/2 the time of the heating season in MA the system would use the HP and 1/2 the time it would fall back to combustion. While on the HP, it is about 300% efficient, yet on the gas - about 90%. But here is the trick, the electricity cost in MA is (Fig 2.) almost 5 times higher than NG per mmBTU. Meaning, I'd rather run the system on NG the whole season and still pay less.

Am I wrong?

Thanks,

-Mike

Featured Answer

mike_home

8 years ago

Many think highest efficiency means the lowest operating cost. That is not always true when comparing different fuels and/or equipment.

My definition of efficiency is output divided by input. It is independent of cost.

At warm outside temperatures a heat pump may have a COP (coefficient of performance) of 3.0 which translates to an efficiency of 300%. But the COP value decreases as the outside temperature goes down. Even at the cold temperatures the COP value will be above 1.0, but the heap pump can't provide enough heat to keep the house at 72 degrees. This is because as the temperature drops the house is loses more heat per hour. That is why heat pumps need an auxiliary heat system.

At the current gas prices it is cheaper to heat a house with a 95%+ gas furnace than a heat pump in most parts of the country. The northeast tends to have higher electricity rates and gas is more available. So I would think the choice for you would be a gas furnace.

If being green is more important to you than the lowest cost then consider this. Natural gas produces about half the amount of carbon dioxide compared to burning coal. The heat pump uses more electricity than the gas furnace. If your electric utility burns coal than it is possible supplying the electricity to power a heat pump may produce more carbon dioxide than operating a gas furnace. Installing a heat pump may actually be less green!

If you know your gas and electric rates post them and a calculation cost comparison can be done to give you an idea how much each will cost.

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Comments (9)

PRO
Austin Air Companie
8 years ago
Ok, you've confused efficiency with actual cost. Very easy to do.

Gas will always be the least efficient, because some of the heat is exhausted out thru the flue of the home. When natural gas is burned properly, the exhaust is carbon dioxide and more efficient furnaces will also produce some condensation along with that carbon dioxide. (CO2) When there is improper combustion the furnace can produce carbon monoxide. (CO) In most instances of this there is a lack of combustion air, improper installation etc. A heat pump will never produce any exhaust 'at the home' of which it is installed.

Heat Pump efficiency: there is no exhaust 'at your home' to run the heat pump. Depending on how the electricity is produced in your state. Some power plants use natural gas for power production... so in reality there is 'efficiency' losses thru waste or exhaust fumes.

Dual fuel options: Prices change over time and thru various times gas costs can run higher. But with large recent discoveries natural gas will likely be the cheaper option, especially for areas that have average daily temperatures below 32 degrees.

A heat pump loses efficiency due to running defrost mode when temperatures are less than 32 degrees out side. If your electric rate is high in your area then you would likely pay more to run a heat pump than you would a natural gas furnace.

Heat pumps mainly exist for those who do not have gas available in their area, as compared to electric strip heat the cost effectiveness can pay for the heat pump within a few seasons. Typically in a electric strip set up for an average home you could have 4 heat strips that run 20 amps per strip. Under full load that is 80 amps.

In comparison a modern day heat pump running in heat mode may use 10-15 amps under full load. Those operational savings in comparison to electric strip heat add up quick.
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PRO
Austin Air Companie
8 years ago
The other argument is that electricity is renewable and gas is not. Because gas is a fossil fuel.

Electricity can be generated thru solar, wind and in some cases water currents. Obviously this doesn't necessarily mean it's cheaper than a fossil fuel currently.
M&S
Original Author
8 years ago
I wouldn't want to start the solar power discussion here, but my quick reasoning would be that electricity is mostly renewable when I don't need the heat at all. When it's cold the days are shorter, the sun rays is low and the generation capacity drops. So I'd still be burning fossil fuels, just not close to my back yard.
mike_home
8 years ago
Getting your electricity from you own solar panels is the greenest and cheapest method.

But if you are buying the electricity from a utility or third party supplier than it is not as green as you think. Here is a US government report showing the fuel sources used in 2015.

Fuel sources for all of the US in 2015 were:

Coal 33%

natural gas 33%

nuclear 20%

oil 1%

renewable 13%

I think the trend will be coal will continue to decrease, natural gas and renewables will increase, and nuclear will be unchanged. Natural gas is a fossil fuel but it creates about half the carbon dioxide emissions than coal.

It would be an interesting calculation to see if a heat pump (air or ground sourced) is greener than a gas furnace. The answer will vary across the country and will change as the fuel mix changes.
M&S
Original Author
8 years ago
Right, that looks solid. Going to subscribe for Musk's solar roof and wait.... :)

Thanks!
tigerdunes
8 years ago
putting cost of equipment aside, you need to post your electric rate, nat gas rate along with efficiencies of both HP (COP) and furnace to give you a true operational cost comparison.

IMO
M&S
Original Author
8 years ago
Given that all those vary over time, I feel like the ballpark estimates above would be sufficient for the reasoning and unless there is a clear benefit, I feel the argument against HP holds in the foreseeable future for the northern regions of the country.
weedmeister
8 years ago
"When it's cold the days are shorter, the sun rays is low and the generation capacity drops. "

Just as an FYI, as the temperature drops, solar panels become more efficient, that is, they produce more electricity with less light. This helps to make up for lower sunlight times and amounts.