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Old 02-12-12, 10:34 PM   #1
abogart
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Default Running cost: ASHP vs. Nat Gas Furnace

I have been crunching some numbers lately, considering if it would be worth the investment to replace my outdated, undersized central AC unit with an ASHP. The idea is to use the ASHP to provide primary heating, with the gas furnace as fossil fuel backup on the really cold days. I have to find out how much I would actually save in energy costs using the ASHP to determine if it's really worth the investment. I have found that our electric company has a special rate for ASHP's, which is the winter half of the rate schedule for interruptable air conditioning during the summer months.

The attached spreadsheet details my findings. On the upper left are the electric and gas rates. The electric rate is the combined rate per kWh for interruptable air contitioning/ASHP, and the gas is February's current combined rate per therm. The reference HP is a Rheem Prestige series model 048JEC 2-stage 4 ton split system. The specs can be found here.

Basically I broke each unit (or stage) down into cost per running hour. Then, since the units and stages produce different outputs, I determined a duty cycle (% run time per hour) for different BTU/h heating requirements. Next, I converted that into a cost per hour for each unit and BTU/h. Finally, to determine at which outdoor temperature the gas furnace becomes cheaper, I figured a cost per 100,000 BTUh for each unit at each temperature.

It looks to me like at any outdoor temp below about 40F, the gas furnace is cheaper to run. That is quite disappointing, since most winter days here stay below that. Did I just pick a poor performing unit here or is it really just cheaper to heat with gas through the winter? Did I do my calculations wrong somewhere along the way?

This could be the deciding factor as to whether I upgrade to an ASHP or a high-efficiency gas furnace. Hopefully both someday.

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Old 02-12-12, 11:10 PM   #2
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I don't know what gas costs where you are, but in Aus it's _far_ cheaper to heat with natural gas than a heat pump (if you are on scheme gas anyway). From what I've observed, those that benefit most from the heat pumps are those heating with Oil. (which would be the case here if we still did that too).

Now, if you want to talk efficiency and CO2 then the heat pump wins, but nobody in their right mind pays more for less.
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Old 02-13-12, 08:41 AM   #3
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Heat pump COP degrades as outdoor temps drop. Just a result of the fixed outdoor heat exchanger surface area and limited min refrigerant temperature inside of it. Basic thermal physics I suppose.

COP also increases as indoor temps drop, ironically. Colder indoor air flowing across the indoor heat exchanger increases the rate of heat extraction there. But, of course, noone wants cold indoor air..

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Originally Posted by abogart View Post
Did I just pick a poor performing unit here or is it really just cheaper to heat with gas through the winter?
The outdoor temperature at which it's cheaper to use a gas furnace will vary with your heat pump selection. I don't know the precise dynamics of how system component properties determine COP at various temperatures, so I can't say how a system could be modified or selected to give a better COP at lower temps.

Maybe a unit with a higher load rating will have a better COP at a lower temperature. But then you don't want to oversize the unit if you're using it for cooling as well, otherwise humidity control will suffer.

I would imagine a larger outdoor heat exchanger with a given compressor would extract more heat at lower temps. And if the indoor heat exchanger surface area remains the same, it should have the same dehumidifying capability when cooling.* But unless you're building your own heat pump, I don't know how to search for a unit by size of the outdoor heat exchanger.

*Furthermore, assuming the indoor heat exchanger surface area & internal volume is unchanged, then the volumetric flow rate of refrigerant and the refr temps must also be unchanged (to provide equivalent dehumidifying ability with same compressor). To meet these requirements with a larger outdoor surface area, you would need smaller refr channels** in the outdoor heat exchanger, larger metallic fins attached to the channels, or just longer runs.

**I believe parallelizing smaller channels, using smooth branches and junctions, would permit a design of equivalent pressure drop, whereas longer runs or parallel runs of the same size will introduce additional pumping losses. Of course I have no idea how much of a load the heat exchanger presents to the compressor in the grand scheme of things. It may be quite minimal compared to the power involved in producing such high pressures.

It may also be the case that as the outdoor surface area increases, defrosting occurs less frequently at the critical outdoor temps around 32F, since lower exchanger surface temps will be necessary to meet a given load. However, when defrosting is performed, it may require slightly more energy, given more surface area is covered with ice..

You could try the AHRI directory, AHRI Certification Directory, and sort the result list by the 17F load rating, and use that to find units that are more efficient at low temps.

I hate to interject additional comments, but there is a type of heat pump system that uses less energy to perform defrosting, which does become a burden at temps hovering around 32F. I've read of at least one "reverse-cycle chiller" system (which is essentially a heat pump using water/brine to distribute heat indoors) that will direct waste heat to a water storage tank, and utilize that heat whenever a defrost is necessary, rather than resistance coils. This would only have an impact at temps floating around 32F, where defrosting occurs most frequently.

In one of Sanyo's technical manuals they specify precise losses due to defrosting, see http://www.sanyohvac.com/assets/docu...ta_2007_08.pdf page 2-7 lists coefficients. It shows between roughly 23F and 37F, a 10% loss of capacity due to defrosting. This is in addition to the linear loss of capacity shown in the graphs above those coefficients.

I suspect COP decreases right along with the capacity, as at that point the compressor is maxed out and outdoor temps are the controlling factor. I suppose the heat capacity flat lines at warmer temps because the surface area of the heat exchanger become the controlling factor.. that is, fixed area, and the internal refr pressures are limited. The compressor would run slower and slower as outdoor temps rise, to prevent the internal temp from becoming unsafe..

I should really stop now because this is entirely too much conjecture. Maybe someone with more experience can chime in.
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Old 02-13-12, 09:05 AM   #4
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Once you select equipment and determine the temperature at which gas is cheaper, then you could find a website with daily average temps for your area, and use that to count days on heat pump vs days on gas, and run the numbers from there. Using averages should get you close enough.
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Old 02-13-12, 10:34 AM   #5
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I might have messed up the numbers in the HP calculations. Since the specs on the given HP didn't include electricity usage at each temp, I divided the kW heat output at each outdoor temp by the COP to find the kWh power usage. I was looking at the specs on a Goodman 4 ton HP (found here) today. They give the amp draw at various outdoor temps. Using this to determine kWh energy usage, I found this unit to be a few cents cheaper per hour on the high stage than my gas furnace is at outdoor temps down to 10F. Either this unit is more efficient for heating, or I just didn't use the right numbers on the first one. The payback period would be something like 17 years (rough estimate) at this rate, but at least I'd feel better about using a greener and more efficient source of heat. This doesn't account for varying daily and seasonal temperatures either. I'm sure that on the warm winter days when the outside temp does creep up around 35-40, I'd see much cheaper heating with the HP. Not to mention that replacing the outdated, undersized AC unit with a higher efficiency unit will save a good chunk of change on summer cooling.
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Old 02-13-12, 11:21 AM   #6
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Also take into account that at temperatures around freezing and with high humidity the HP's heat exchanger may become covered in ice, causing higher power draw, lower heat output, and/or going into defrost mode.

Also, if you hack your own HP, then you can make the HX as large as you like, increasing efficiency.
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Old 02-13-12, 12:42 PM   #7
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Here is another spreadsheet, this time comparing the Goodman 4 ton and a 96% AFUE gas furnace to my existing 80% gas furnace. It's quite clear this time that this unit would be cheaper to run than the 80% furnace even down to 0F, although heat load requirement would be higher than what the unit could put out at that temp. The changeover to nat. gas would be the outdoor temp. below which the heat loss of the house exceeds the heating capacity of the HP unit. I estimate that would be around 15F in most cases.
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Old 02-13-12, 01:07 PM   #8
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I have also been thinking about building a little "house" for the outdoor unit with vented walls set out about a foot or so from the unit and a riser vent in the top to route the discharge air farther from the intake. Replaceable black and white vent panels could be made to promote or discourage solar heating of the intake air, depending on the season. I think the walls would help keep snow and ice from the intake to help deter icing, and a cap on top of the riser tube would keep rain and snow out of the fan compartment. Also, I have noticed that when it rains in the warmer months, the dirt around my current AC unit tends to splash up and accumulate on the grille and HX. Extending the foundation and having a space between the intake vents and the unit's grille would alleviate that problem as well.
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Old 02-13-12, 03:37 PM   #9
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I've done the math for my gas rate of .75/therm last year (cheaper this year) and electricity of .11/kwh and I couldn't get it to make sense for heating with a heat pump above 40 degrees with a Carrier Greenspeed which is supposed to be the most expensive and most efficient central heat pump out there. At 40 degree daily high outside and a clear sunny day and my house is at 60 degrees before I add heat. Heat pump is a no-go where I am. Even if it was a grand for the heat pump it doesn't make sense here. Our shoulder seasons are short up here in the north so the nightly temperatures between freezing and 50 degrees would make little use of a heat pump if you are far north enough. I considered a mini-split until I got a hold of the Fujitsu service manual(hard as f#@! to get now too) and looked at amp rates and capacity below the AHRI 17 degrees. Thing gets thirsty and will crank full bore at 9.5 amps and as it gets colder capacity drops like a stone. I think the 17 degree 10k rating is very close to max compressor frequency which places it very near the 9.5 amps(2.28kw). The COP is great when the unit isn't running all out but it seems once its at max capacity heating or cooling it sucks power and COP drops. There was an independent study done on Mitsu and Fujitsu inverters and at max load they have worse COP than central a/c and heat pump units. I suppose if I build another house that is well insulated I might use two just to keep them below 70% load except for maybe the absolute worse 2 weeks worth of heating. Non-inverter units are more predictable but if you look at the Carrier Greenspeed balance chart you can see how the 17 and 47 degree points seem to be programmed in the inverter to be the most efficient and the compressor speed varies to keep the capacity nearly the same between those two points. Gives them better HSPF I'm sure but looking below 17 degrees and the capacity drops quick because the compressor was already cranking at its max.
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Old 02-13-12, 06:32 PM   #10
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I think the trick is to tune to system and changeover temp to always use the cheaper source of heat. That is one advantage of a dual-fuel system. As mentioned before, at my gas and electric rates the HP would still be cheaper to run down to 0F. The only limitation is the capacity of the HP at those temps.

I'm not sure about the inverter units, but from what I found with the two-stage units the COP is the same between the two stages. Cost per BTUh is the same, but capacity is reduced. COP varies with outdoor temperature. Also, power consumption actually drops as outdoor temp drops, so it uses less amps at lower capacities.

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