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Determining the 'unit cost balance point' for an air source heat

Posted by liameknuj (My Page) on
Tue, Apr 23, 13 at 6:53

Is this calculation/formula correct for determining the 'unit cost balance point' of an air source heat pump vs a gas furnace for my particular situation (electricity unit cost of $0.162 / KWh and gas unit cost of $1.3463 / Therm and an 80 AFUE furnace efficiency)?

KWh / $0.162 x COP[unit cost point] x Therm / 29.3 KWh = Therm / $1.3463 x 0.80
COP[unit cost point] = 0.162 / 1.3463 x 29.3 x 0.80 = 2.82

Therefore, for my particular situation, is an air source heat pump more economical on a unit cost basis, if it can deliver greater than 2.82 COP at a given temperature?


Follow-Up Postings:

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RE: Determining the 'unit cost balance point' for an air source h

Do you mind sharing your location?

Your numbers are very close as far as break even. I plugged them into a fuel comparison calculator that I like to use as a guide only. Here are the results.

80% gas furnace

Cost per 100,000 btu of useable heat
Heat pump: $1.68
Natural gas: $1.64

95% gas furnace

Cost per 100,000 btu of useable heat
Heat pump: $1.68
Natural gas: $1.38

Just for comparison purposes, I have also listed results for 95% eff gas furnace.

IMO


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RE: Determining the 'unit cost balance point' for an air source h

Thanks for the reply tigerdunes; we have conversed recently under the title "Pricing of replacement American Standard / Trane HVAC system".

The home is in the Boston, MA vicinity.

What are you using for your "fuel comparison calculator"?

I am looking for graphs of COP vs temperature lift for typical high efficiency air source heat pumps; any suggestions?

Am I correct in understanding that COP drops with increased difference between thermal source and desired temperature (increasing temperature lift)?

Thanks.


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RE: Determining the 'unit cost balance point' for an air source h

I'll just offer my opinion.

Unless you are in an all electric situation, for your location/climate, I would forget a heat pump.

I would go with a 95% eff furnace and 15 SEER AC.

For conventional air source heat pumps, COP drops off sharply around freezing. What this basically means is BTUs are significantly less.

I can't help you with graphs. Sorry. Perhaps others on the forum can point you in the right direction with link.

Trust me. My fuel comparison calculator is very accurate but as I always say should be used as a guide only. It confirms your numbers are basically a wash.

IMO


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RE: Determining the 'unit cost balance point' for an air source h

Thanks for the reply tigerdunes; please follow up to the question below.

If it is the case that the gas furnace will be 80 AFUE and without consideration of differential capital/installation cost, then is it more cost effective on a unit basis for me to run a heat pump down to about 40F?


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RE: Determining the 'unit cost balance point' for an air source h

You are looking for operating costs. On the scenario and formula you listed above, the operating costs are basically a wash.

One would have to know the sizing of the HP and its COP plus the sizing of the 80% furnace sgl stage or two stage to offer an informed opinion...that's where you would find the operating leverage of one fuel over another.

You might not like this statement and that's OK but you may be over thinking the situation and getting down in the weeds.

However, If you give me some system scenarios to work with, I will give you some guesstimates. Your electric rate though is scary.

IMO


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RE: Determining the 'unit cost balance point' for an air source h

I will assume that for whatever reason a 95% eff furnace is not a possibility.

Correct or not?


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RE: Determining the 'unit cost balance point' for an air source h

I read your other posts. I agree with the recommendations above. A heat pump in Boston doesn't make sense if you have access to natural gas.

See if you can make a 95% efficiency furnace work with your installation. The PVC piping can be run a long distance to exit the side of the house. It is possible to install single exhaust and intake port, but there are restrictions on how it can be done. Installing a flue liner for the existing gas hot water heater should not pose a big problem. If opt for a power venter water heater be aware you will have no hot water during a power outage.


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RE: Determining the 'unit cost balance point' for an air source h

Thanks for the reply tigerdunes.

Personally, I think both my electric rates and gas rates are scary.

I recognize that I am in the weeds but that is where it often gets interesting.

It seems to me that, if I installed a dual fuel system, then at heating for higher than 40F outside temperatures the heat pump would be economically superior on a unit cost basis and vastly superior as the outside temperature rises. Mind you, I recognize that I might likely never regain my capital investment, and that the current accepted practice in my situation and area would be to not install an air source heat pump and use an air conditioner at 15 SEER.

furnace:
Trane XV80: 80K nominal BTU, 80AFUE, two stage, variable speed
TUD2B080A9V3VB

heat pump:
Trane XR16: 3 nominal Ton, 16 SEER, two stage
4TWR6036A

Lets frame the question this way: what would be better economically; (a) to receive $200 in ten years or (b) to use a heat pump above 40F?


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RE: Determining the 'unit cost balance point' for an air source h

Thanks for the reply mike_home.

The 95 AFUE furnace will not be happening.

I am interested in installing an air source heat pump for reasons that are not financially driven (a capital/installation cost loss is expected), provided it can operate more cost effectively on a unit cost basis.

With the current state of technology, even if the air source heat pump could operate more cost effectively below freezing, I would not operate it.

So is there a temperature above which it is more cost effective, on a unit basis, for me to operate the air source heat pump?

Obviously, I also need to consider the heating load balance point.


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RE: Determining the 'unit cost balance point' for an air source h

I am sure there is an outside temperature where the COP is high enough to operate more economically than an 80% efficiency gas furnace. Getting a dual fuel system is not such a bad idea. The question is given that Boston area can have harsh winters, how often will the outside temperatures be above the balance point.

Your electric rate is lower than mine and believable, but your gas rate is very high. Are you sure the gas rate is correct? Another thing to consider is that gas rates are continuing to drop in the US. It will be mean the balance point of your dual fuel system will have to be adjusted if you want optimal operating efficiency. Typically thermostats can only set balance points at 5 degree increments.

Even though I urge people to get high efficiency furnaces, your case is an example as why I am opposed of forcing homeowners to install them when it becomes a big burden.


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RE: Determining the 'unit cost balance point' for an air source h

OK here's the deal.

I have reread your previous thread. I understand your quandary over the 95% eff furnace and of course that's your call.

A question that I wanted to ask is with your zoning system, how will the staging of this two stage 80% XV80 furnace or AmStd equivalent be controlled. Mighty important if operating costs are important to you. Be specific. If you don't know ask dealer. Normally and under best practices, a true two stage thermostat would control the staging based on demand. I doubt if this is true with your zoned system.

I see no reason not going with a heat pump as long as the additional upfront cost is not an issue. You would however want the ability to disable it altogether if electric rates made it disadvantageous.

I looked up on AHRI HP directory and came up with this system.

5021994 Active Systems HERITAGE 15 AMERICAN STANDARD, INC. 4A6H5030G1 4TXCB004CC3 *UD2B080A9V3 32200 12.50 15.00 28600 9.00 18300 1 HRCU-A-CB 260 646 Yes

There is a down and dirty formula used to convert HSPF to COP. I came up with 2.70-2.75. I will leave it up to you to draw your own conclusion. Certainly, if the HP dual fuel system still interests you, best coil model number and COP should be confirmed with dealer through Trane/AmStd. I sure wish AHRI would list COP on their directory. And keep in mind, that HSPF/COP is at 47 degrees fah.

IMO


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RE: Determining the 'unit cost balance point' for an air source h

Thanks for both your replies, mike_home and tigerdunes.

• Electric (NStar) is $0.162/KWh unit cost (delivery 0.092 and generation 0.070 costs; fixed costs not included). Gas (National Grid) is $1.3463/Therm unit cost (transmission 0.5278, distribution 0.1865 and supply 0.6320 costs; fixed costs not included).

If thought of at all, I believe that people often only think of their gas cost as their gas supply cost (I am not suggesting that either of you do), and incorrectly skew some of these considerations. Thats not what I pay on my bill. Additionally, since my gas supply cost is only about 45% of my total unit cost for gas, changes in supply cost will be muted. Further, I believe that we have seen most of the drop in gas prices already, and that they are likely to rise modestly over the next ten years as electric power production continues to migrate to using gas as its preferred fuel.

• Control of the HVAC system, including the two stage heat, will be by the EWC Controls NCM 300 zone control board (if two stage gas heat and one stage cool) or EWC Controls BMPlus 300 zone control board (if two stage gas heat and two stage heat pump).

In both cases the second stage heating is triggered by the time duration of the heat call from the dumb thermostats to the zone control board. I am certainly open and would look forward to other zone control board suggestions.

Please identify the important informational elements in the data string mentioned for the Heritage 15.

Did you or could you find anything on the Heritage 16?


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RE: Determining the 'unit cost balance point' for an air source h

There is a remarkable lack of data for air source heat pump available, even when searching; just a bunch of big mushy numbers.

It is like rating automobile gas milage with letter grades, except that it isn't even that consistent, because there are multiple competing scales. "I understand that an 'A' is better than a 'B', but how many miles am I going to get per gallon and how far am I going to get on a tank of gas."

Can you help me interpret this graph?
http://www.heatpumpcentre.org/en/aboutheatpumps/heatpumpperformance/Si dor/default.aspx

Here is a link that might be useful: heat pump COP vs temperature lift

This post was edited by liameknuj on Tue, Apr 23, 13 at 13:23


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RE: Determining the 'unit cost balance point' for an air source h

I found a unknown origin graph for a specific model heat pump that roughly indicates 2.5 COP at 30F, 3.0 COP at 40F and 3.5 COP at 50F.

Anyone know where one can find data regarding the number of days at certain temperatures for their region/area/local?

This post was edited by liameknuj on Tue, Apr 23, 13 at 21:59


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RE: Determining the 'unit cost balance point' for an air source h

Thanks for the AHRI information tigerdunes.

I have looked it up myself as well and understand it better than my previous question.

The XV80(80K BTU) and XR16(3 Ton) yield:
Cooling = 34400 BTU/h, 12.50 EER, 15.00 SEER
Heating (47F) = 32600 BTU/h, 9.00 HSPF

COP[avg] = HSPF x 1055 J/BTU ÷ 3600 J/Watt-h = 0.293 HSPF

COP[avg] = 0.293 x 9.00 = ~2.64

However this 2.64 COP[avg] is for the heat pump across a mock season, including its worst performance periods. What I am trying to do is use the heat pump in its peak performance spectrum (i.e. above ~40F).

Even still the COP[avg] of 2.64 is close to the COP[unit cost point] of 2.82, which is needed to be cost effective on a unit basis, provided I calculated it correctly.

Have I got that right?


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RE: Determining the 'unit cost balance point' for an air source h

A 2.5 COP at 0F seems a little optimistic to me…

SR


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RE: Determining the 'unit cost balance point' for an air source h

"A 2.5 COP at 0F seems a little optimistic to me… "

Gotta be a typo... maybe at 30 Deg Fah


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RE: Determining the 'unit cost balance point' for an air source h

You are correct; it's been corrected.


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RE: Determining the 'unit cost balance point' for an air source h

"For conventional air source heat pumps, COP drops off sharply around freezing."

Darn that freezing point of water bites again.

Frost formation becomes and issue for any system moving air when it starts to form on the coils.

Out comes the defrost energy.

Some larger freezer systems actually have dual evaporators, with one isolated with dampers from the cold compartment when defrosting is required.

This allows the compartment to remain below freezing (sometime WAY below) continuously.


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RE: Determining the 'unit cost balance point' for an air source h

There are methods to take into consideration COP.

Here is a link that might be useful: tigerdunes' rarely shared calculation


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RE: Determining the 'unit cost balance point' for an air source h

Funny but not certain how to take that....

Still laughing though...

IMO


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RE: Determining the 'unit cost balance point' for an air source h

tigerdunes,

I meant it with total respect! You're the man.

BTW, I received my copy of Washington (DC) Checkbook Magazine today. It has a detailed ratings and evaluations of VA/MD/DC HVAC installers. It included the cartoon.

This post was edited by saltidawg on Thu, Apr 25, 13 at 17:37


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RE: Determining the 'unit cost balance point' for an air source h

Best laugh I have had in several days.

Thx Salti!

I will copy the toon and place on my digital picture frame.

TD


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