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hp_dude32
10-25-2006, 11:41 AM
Trying to get a feel for absolute dollar values in heating costs when comparing our Infinity 96 furnace to our Infinity 16 heatpump.

I realize that there are a lot of heatpump variables, but I have the Infinity 16 datasheets which specify high/low rate COP at 43F and 15F, so that's a good starting point. (I'll clamp-meter the heat pump power wires to get an accurate power consumption in low/high...)

From an electrical point-of-view, I assume that the cost of running the indoor DC fan during heat pump operation is trivial compared to the outdoor unit. (I assume you would never have a high-speed fan operation with a heat pump?)

On the gas side, our gas supplier measures gas in cubic meters, what's the conversion rates in BTUHr for cubic feet of natural gas? (I can do the metric conversion, and assume a 94% efficiency...)

Update: Got the above data... 1 Cubic Meter = 35.32 Cubic Feet = 36,409 Btu

With the above, if the furnace has a 65,000 BTUHr input on low, I assume that's simply 1.78 Cubic Meters, if it runs for an hour?

I plan on doing a simple test in similar outdoor settings (say around freezing) to time the furnace duration over a period of two+ hours, and then do the same for the heat pump. (tedious work, but the only real way to figure it out!)

Any other ideas on how to do a good comparison?

Thanks.

[Edited by hp_dude32 on 10-25-2006 at 12:37 PM]

deme
10-25-2006, 01:59 PM
I did a calculation on costs for my house some time ago. The arithmetic is pretty detailed but not complicated.
You may have to do some unit conversions because I used Candian information.
Hope it helps you.
----------------------------
Heating and Air Conditioning Results Using HVAC-CALC Software.

March 23, 2005

Heat Pump Nominal Capacity = 24,000 BTU/hr (2 tons)

Normal Design Conditions for Vancouver: Air Conditioning 77*F, Heating 19*F Outside Temps. Inside Temps: Winter 68*F, Summer 75*F, 50% Relative Humidity No Blinds, No Outside Shade, 8 People in the House.

From HVAC Calc. Heating BTU/hr required = 30,305 BTU/hr.

Air Conditioning BTU/hr required = 18,842 BTU/hr.

Extreme Summer Conditions of 92*F (Vancouver 30 year extreme max. temp and High Humidity (Toronto Design Humidity of 110 Grains of Moisture). No Outside Shading but with Inside Blinds drawn. 8 People in House.

Air Conditioning BTU/hr = 24,959 BTU/hr

Note: Since there are large soffit overhangs on house and tree and neighbour house shades, the heat pump can handle the air conditioning requirements especially since it is most unlikely that there will be 8 people in the house during the peak of a 30 year heat wave.
Note: In Canada, heat pump selection is usually based on heating requirements rather than on cooling requirements. That's why the heat pump is oversized for the little cooling it will have to perform.

Setting the Heat Pump Balance Point.
The nominal heating output of the Heat Pump is 24,000 BTU/hr whereas the peak heating requirements for the Vancouver winter design temperature of 19*F is 30,305 BTU/hr.
The natural gas furnace supplies heating requirements at temperatures below the temperature at which the heat pump operation is automatically blocked; this blocking temperature is called the balance point.

From HVAC Calc, House heating requirements for an outside temp. of 30*F = 23,503 BTU/hr. The heat pump output at 30*F is 18,000 BTU/hr (curve in the heat pump application manual). The heat pump is unlikely to be able to supply heating requirements at 30*F.
House heating requirements for an outside temp. of 35*F = 20,407 BTU/hr. The heat pump output at 35*F is 19,500 BTU/hr. The heat pump will probably be able to supply heating requirements at 35*F and above with the help of miscellaneous heat sources in the house such as hot water tank standby heat, kitchen range output, heat generated by lights and humans.
House heating requirements for an outside temp. of 40*F = 17,318 BTU/hr. The heat pump output at 40*F is 20,500 BTU/hr. The heat pump is more than able to supply all heating requirements at 40*F and higher.

Set the heat pump balance point at 40*F (4*C) (changed to 35*F recently)

Percentage of Heat Supplied by Heat Pump
From Vancouver B.C. weather statistics:
Heating degree days (D.D.) below 18*C (total heating requirements) = 2926 D.D. in deg. Celcius
Heating degree days below 5*C = 218.6 D.D.
Heating degree days below 0*C = 38.6 D.D.
Calculate approximate heating degree days below Heat Pump balance point of 4*C (40*F)
Degree days below 4*C = 218.6+(218.6 - 38.6)/5 = 252 D.D.

Percent Heat supplied by heat pump == (Total D.D. below 18*C - D.D. below 4*C) / Total D.D. below 18*C = (2926 - 252) / 2926 = 91.29 %

Heat Pump supplies approximately 90% of the total design house heating requirements

The gas furnace supplies 10% of design house heating requirements, the heat pump defrost requirements and all the extreme cold weather heating requirements.

Heat Pump Defrost Timer Setting
Heat pump outside coil becomes frosted when operating in cold weather. Since the heat pump does not operate below 4*C (balance point) there will be little frost accumulation. Set the defrost timer at 90 minutes (factory setting). (Changed to 35*F recently. Works well)

The heat pump will test for outside coil temperature every 90 min (changed to 120 min) of heat pump operation and if the outside coil temperature is 30*F (-1.1*C) or lower, a defrost cycle will be initiated. (defrost timer changed to 120 minutes recently)

Energy Costs Comparisons
Electric Energy Cost Including Taxes but Excluding Transit Levy Cost /Kwh = \$0.07
Gas Costs Including Taxes Cost/Giga Joule (GJ) = \$13.53/GJ (1 GJ = 9.48 U.S.therms)

Annual Energy Consumption
Heat losses calculated with HVAC-Calc.
To maintain constant indoor temperature of 20*C (68*F), the heating system must supply 1,114 BTU/hr (HVAC Calc result for my house) for every degree *C outdoor temperature below indoor temperature.
Heating energy required per degree day = 1,114x24 hours= 26,736 BTU/degree-day(Celcius)

Heating with gas only
Total heating degree-days for Vancouver= 2926 degree-days C.
Heat energy required = 26,736 x 2926 = 78,229,536 BTU/ heating season = 82.537 Giga Joules(GJ). [1 GJ = 9.48 U.S.therms]
Furnace efficiency = 80%. Gas input energy required = 82.537/0.8 = 103.17 GJ
Annual cost of heating with gas only @\$13.53/GJ = 13.53x103.17 = \$1,395.89/year (Canadian dollars)

Heating with heat pump with gas below balance point (neglecting defrost gas)
Heat pump supplies ~ 90% of heating energy.
Heat pump output in GJ = 0.9 x 82.537 = 74.28 GJ
Heat pump output in Kwh = 277.77778 x 74.28 = 20,633.33 Kwh
With temperatures above the balance point of 40*F (4*C), the heat pump coefficient of performance (COP) is approximately 2.5 or higher.
Electrical energy supplied to heat pump = 20,633.3335/2.5 = 8253 Kwh
Cost of electricity supplied to heat pump @ \$0.07 =
= \$0.07 x 8253 = \$ 577.71
Gas Energy at 80% furnace efficiency used below heat pump balance point of 40*F = 0.1 x 82.537 /.8 = 10.317 GJ
Cost of gas for heat below heat pump balance point = 10.317 x 13.53 = \$139.59
Total cost of heat with heat pump = electricity + auxilliary gas =\$577.71 + \$139.51 = \$ 717.22

Annual Saving with heat pump = 1395.89 - 717.22= \$678.67
Subtract Additional Annual Maintenance Cost for heat pump of \$100 (higher maintenance than HVAC only)
Net annual savings = 678.67-100 = \$578.67

Net cost of heat pump with taxes = \$5617.50-Carrier rebate \$175-Federal Grant \$1096= \$4346.50

Assuming stable fuel prices, Payback Period at 6.5% Interest approx. 9 years

In our neck of the woods Electricity price is regulated. Gas prices fluctuate. Gas prices will increase faster than electricity prices. Payback period will be less than 9 years.

[Edited by deme on 10-25-2006 at 10:55 PM]

hp_dude32
10-25-2006, 02:59 PM
Your electricity cost seems very low, after delivery charges and other fees, Ontario Hydro is almost double!

A \$150 Ontario electricity bill has about \$70 in "fees" over and above the actual electricity cost (6c/kWh)! (including \$7 in GST)

In your example, add our electricity prices and throw in a 92%-94% furnace and the cost saving will nearly vanish!

Also, the heat pump will need a lot more help in central Ontario than in BC! :)

See my other post (Heat Pump pointless...) about the absolute costs of each kind of energy, expressed in BTU. It's a bit shocking!

Thanks.

deme
10-25-2006, 07:38 PM
Originally posted by hp_dude32
Your electricity cost seems very low, after delivery charges and other fees, Ontario Hydro is almost double!

A \$150 Ontario electricity bill has about \$70 in "fees" over and above the actual electricity cost (6c/kWh)! (including \$7 in GST)

In your example, add our electricity prices and throw in a 92%-94% furnace and the cost saving will nearly vanish!

Also, the heat pump will need a lot more help in central Ontario than in BC! :)

See my other post (Heat Pump pointless...) about the absolute costs of each kind of energy, expressed in BTU. It's a bit shocking!

Thanks.

I am aware of the high price of electricity in Ontario. You poor people are paying for the sins of several past governments and of Ontario Hydro.

The cost per kwh I mentioned was the total cost for the year divided by the total kwh consumption in 2004-2005.

My latest B.C. Hydro bill dated Sept. 29, 2006 lists the following charges:
Basic charge per day = \$0.12490
Energy charge/kwh = \$0.06330

The total bill for 32 days including GST and transit levy for a consumption of 520 kwh = \$41.26.
P.S. The transit levy is \$0.06240/day.

As far as natural gas is concerned, the latest bill for the period August 23 to Sept. 25 , 2006 was \$31.73 for 1.7 GJ including GST or overall cost of \$18.66/GJ.
Breakdown of costs:
Basic charge \$11.16 (independent of consumption)
Delivery charge \$2.904/GJ
Commodity charge (cost of gas) \$8.137/GJ.

The furnace was already in the house and originally I was only going to add air conditioning. I chose the heat pump because the extra cost over air conditioning only was only \$1,200. That's why I only counted the 'additional' heat pump. If you include the cost of the total package, the payback, if any will take much longer!

P.S. Checked my spreadsheet for energy cost for 12 months ending in June 2006:

Electricity consumption 10,230kwh for a total of \$708.59 including taxes or \$0.0693/kwh.

Gas consumption, including hot water, 48.7GJ for a total of \$718.63 including taxes or \$14.76/GJ.

[Edited by deme on 10-25-2006 at 11:30 PM]