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

7 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

7 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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PRO
Austin Air Companie
7 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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No problem and I appreciate you being ok with me bumping into your exchange here- I am following this with alot of interest. Just quickly, on my issue, the installer came back out. At first he said this was normal as shared earlier and had to be told (from a non-expert) that this was not normal at all. It seems that he made some type of wiring adjustments (unknown since my wife cant follow any of this when she was watching and asking) and then adjusted settings within the setup of the Edge Thermidisstat. We are currently running oil heated-water baseboard as the backup and then the Performance series HP system (EER 12.5, SEER 15.5 and HSPF 9) cant find the AHRI# handy. Temps have been consistently at between 28 and 34 degrees here in Viginia for the last 5-7 days. We are having to manually switch from HP to the oil heat system for right now since I dont want this installer messing with anyting else to connect a relay that allows both to be working from the one thermostat. I will do that later (hence reason for following this discussion closely) I am tracking kW useage off the meter. Here's what I am seeing: Set the HP Edge at 68 degrees and it holds house temp steady wihtout having to use AUX at all. Set the oil heat-boiler at 66 setpoint(manual) It seems that only infrequently will the boiler kick on to augment the HP (running simul right now) Energy useage has been between 3.12 and 3.77 kW per hour across last 5 days (taken in 24 hr periods). VaPOwer is at about .06 cents per kW right now I think, so I'm looking at what 744 hours per month at avg 3.44 kW (.20 cents) per hour= $153.00/month. This is for a brick ranch, 1820 sq ft house
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nash as far as nat gas,I think price will be more of a concern than supply and that the price difference between the two fuel sources will only widen. I am attaching a fuel comparison calculator for your use. I generally like straight high eff HPs with air handler and backup aux strips but with older homes not well insulated or tight envelope, then I think dual fuel is a better choice. Of course, you need a professionally performed heat/cool load calculation for correctly sizing new HVAC as well as ductwork to be thoroughly inspected for size, insulation qualities, leaks, etc. If you have any hot/cold spots in your home, now is the time to address these issues. here is a quick synopsis of Nashville climate from Trane's site. I personally think it underreports both cooling and heating but that's just my idea. anyway, it gives you an idea. Climate averages for: NASHVILLE, TN, 37203 Hot weather Cold weather Average days per year above 75: 153 Average high temperature: 88.7 Average days per year below 40: 31 Average low temperature: 36.8 I would not have a Lennox HP because of poor performance/eff numbers. Regardless of brand, you want a var speed blower whether air handler or furnace. BTW, what is the model of the upstairs HP? let me know if you have other questions. IMO Good Luck! Here is a link that might be useful: Fuel Comparison Calculator
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I'm leaning towards just heat pump with heat strips. That should be fine for your area. If you plan on keeping it that warm in the house as you say in your OP, I would get the Bosch Inverter heat pump as it produces heat clear down to about 5-6 degrees ambient and very efficient at what it does. Then use the strip heat as you need it. I don't know that you'll find a contractor though for this for a new construction job as Bosch just recently came to the Houston market a few months ago. The distributor network for Bosch is in it's infancy... so there are pros and cons for choosing this brand. For what it does / how it does it --- it's a game changer for my market. (Katy, Texas area) As a comparison --- an average run of the mill single speed heat pump will probably struggle once temps drop into the 20's for any significant amount of time. They are efficient when compared to strip heat (utility bill cost wise) but there are higher limitations with those. I've gotten email notifications from my electric provider that they are estimating my next electric bill at less than $50 for November. This may wind up being my lowest electric bill ever for my power rate of 12 cents per KWH. With that said, I keep it quite chilly during the winter and my house is all electric (no gas at all). I shut it off at night and really just use it to take the chill out of the house in the morning. If it gets really cold here, I will probably just heat the main part of the house over night --- as I have it attached to a 4 zone duct system. (This is to explain the how and why my electric bill is so low -- design of the HVAC system is a big part of it.)
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Re: mike_home I see a previous post by you has been deleted or taken down. Please don’t be ‘P.C’; there was nothing wrong or hostile about your post. We’re all ‘Long Distance’ friends here. Your question was fair and I’d like to answer it. I hope the reason for it no longer appearing is that you have answered it for yourself. So… “mike_home Fsq4cw, You said: the average cost to run a 5-ton Waterfurnace Series 7 in all but the most brutal conditions would probably be ~51¢/hr. Can you show how you calculate this operating cost?” Sure Mike, I’d be glad to help. OP says he lives in Boston Mass. So from, ‘Ground Source Heat Pumps’, ‘Appendix 1 – Ground Temperatures’ (Kavanaugh & Rafferty 1997), ground water temperature in Boston is 50˚F. Makes sense to me, I’m in Montreal Canada and our GWT is ~46˚F. I think it’s actually a little colder but these guys literally wrote the book on this subject. This GWT is easily found on the web for any North American city. Next, if you download the specs for the Waterfurnace Series 7 HP and ‘Drill Down’ through the specs (there’s a lot of them), on page 38 you will find the specs for the 5-ton HP (060). OP’s contractor says a 3-ton Mitsubishi air source HP would do most of the job, so why not for argument’s sake say a 5-ton would also do most of the job too. Water furnace Series 7 (060) Ground temperature in Boston is 50˚F. So let’s say after running for a long, long time the EWT (Entering Water Temperature) returning from the ground is really cold, say, 30˚F, 2-degrees below freezing (no problem, fluids contains glycol and should easily be able to go down into the lower 20’s˚ F). The performance data (page 38) on the ‘060’, a 5-ton HP on ‘Part Load’ 50%, these things like long run times and will scroll down once the set point is met. So EWT 30˚F, 6.3 GPM flow rate from the boreholes, 850-cfm from the blower, LAT (Leaving Air Temperature) from the HP to the main plenum would be 98.6˚F, Power kW (i.e. TOTAL HEAT PUMP POWER) 2.17kW. OP pays 23.6¢/kW x 2.17kW = ~51.21¢/hr. The only thing that matters is the heat loss of the envelope, outdoor temperature can go down to -60˚F, as long as the ground temperature is stable, nothing changes for the heat pump, COP remains constant. Minor changes in ground temperatures near the surface generally lag by at least 1-season, unlike the air temperature that can change hour to hour. Below about 12-feet there is much less change. Below 200-feet the ground temperature actually starts to rise, boreholes can be 450-feet deep for residential. Hope that helps, SR
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PRO
Austin Air Companie
7 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.
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M&S
Original Author
7 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.
0
mike_home
7 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.
0
M&S
Original Author
7 years ago
Right, that looks solid. Going to subscribe for Musk's solar roof and wait.... :)

Thanks!
0
tigerdunes
7 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
0
M&S
Original Author
7 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.
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weedmeister
7 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.
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Dual fuel heat pump efficiency - need a sanity check in my logic

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

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