Economic Balance Point

[lksmith]

Greetings,

Could one of you pros help me determing the economic balance point for my new heatpump/gas furnace setup?? I'm not sure what info you need so I'll just throw stuff out...

Cost of electricity 8.77¢/kwh
Cost of natural gas $1.37/therm
Furnace is a 80% AFUE V/S two stage
Heatpump is 13 seer single stage
I live in Atlanta

Also, is there any concensus (using the IAQ9000) as to where to set the compressor cut-off temperature and the Aux cut-off temp.

Thanks in advance.

Larry Smith

[deme]

Originally posted by lksmith
Greetings,

Could one of you pros help me determing the economic balance point for my new heatpump/gas furnace setup?? I'm not sure what info you need so I'll just throw stuff out...

Cost of electricity 8.77¢/kwh
Cost of natural gas $1.37/therm
Furnace is a 80% AFUE V/S two stage
Heatpump is 13 seer single stage
I live in Atlanta

Also, is there any concensus (using the IAQ9000) as to where to set the compressor cut-off temperature and the Aux cut-off temp.

Thanks in advance.

Larry Smith

I'm not a pro but the calculation is straightforward.
All types of energy can be converted from one form to another.
In this calculation I'll express electric energy in terms of THERMS.

from: http://www.onlineconversion.com/energy.htm

1 U.S. therm = 29.3kwh

Cost of 1 therm of electricity delivered by utility =
= $0.0877 x 29.3 = $2.57/therm

COST OF HEAT ENERGY SUPPLIED BY HEAT PUMP
The output of a heat pump is higher than the electric energy supplied to it. This is expressed by the coefficient of performance (COP). The COP is higher at 'warmer' outdoor temperatures and drops as the outdoor temperature drops. The heating design outdoor temperature for Atlanta is 22*F and at that temperature the COP is approximately 2.5.
This means that for every unit of energy supplied to the heat pump, the heat pump supplies 2.5 times of heat energy to the house.

Cost of 1 therm of heat energy supplied to the house by the heat pump = $2.57/2.5 = $1.028/therm.

COST OF GAS ENERGY SUPPLIED BY FURNACE
The furnace is 80% efficient
Cost of 1 therm of heat produced by the gas furnace =
=$1.37/0.8 = $1.7125/therm

SUMMARY
Cost of heat pump heat at 22*F design temp. = $1.028/therm
Cost of gas furnace heat = $1.71/therm

NOTE: The above calculations are based on the costs you supplied. The costs must include all charges such as fixed monthly charges, delivery charges, taxes, etc...

Based on the costs of energy you gave, the balance point should be based on 'comfort' or 'equipment capability' and not on 'economics'

[lksmith]

deme,

Thanks for your help. I guess the question now is how low can the heatpump pump warm air and keep the temp warm inside. Time will tell. Thanks again.

Larry Smith

[deme]

Originally posted by lksmith
deme,

Thanks for your help. I guess the question now is how low can the heatpump pump warm air and keep the temp warm inside. Time will tell. Thanks again.

Larry Smith

There are several ways to determine the setting of the balance point. Here is how I did mine:

I used the heatpump output curves to determine output for 30*F, 35*F and 40*F outdoor temperatures and, using HVAC Calc software, compared these outputs with BTU/hr required to heat the house at these outdoor temperatures.

Here are the results I got:

Outdoor temp.--------------30*F--------35*F-------40*F

BTU/Hr required
to maintain 68*F indoors---23,500-----20,400------17,320

Heat pump output BTU/hr----18,000-----19,500------20,500

I initially had the balance point at 40*F (left by contractor).
Later, I assumed that there were other heat sources in the house (hot water tank, cooking, people, lights, TV's, etc...) and dropped the balance point to 35*F. The heat pump is working well at the 35*F point.

The last thing I had to set was the defrost timer. The factory default setting was 90 minute. The contractor set it at 30 minutes. I moved it to 90 minutes and finally to the highest setting of 120 minutes. The pump works well at 120 minutes without excessive frost buildup on the condenser.

Hope the above helps.

[lksmith]

deme,

Where did you find those heatpump output curves? I'd be curious to duplicate your calculations with my heatpumps.

Thanks,

Larry Smith

[dan sw fl]

Originally posted by lksmith
Where did you find those heatpump output curves? I'd be curious to duplicate your calculations with my heatpumps. Larry,
Post model numbers.

Floor plans + R & U-Values + window areas are needed.

Set balance point to about 29'F initially.

[lksmith]

Uhhh Dan,

I'm clueless about what R and U values are. (But I did stay in a Holiday Inn Express last night...). I do have heat load calcs from HVAC-Calc if that provides any useful information. Lemme know.

Larry Smith

[dan sw fl]

Originally posted by lksmith
Uhhh Dan,

I'm clueless about what R and U values are. (But I did stay in a Holiday Inn Express last night...). I do have heat load calcs from HVAC-Calc if that provides any useful information. Lemme know.

Larry Smith

Post model numbers

Post heating requirements from HVAC-Calc based on
70'F inside temp and ___ ? outside temp

[lksmith]

Hi Dan,

Here's the info....

Heatpump (up) 2TWR3024A1 Furnace (up) TUD2B060A9V3
Heatpump (down) 2TWR3036A1 Furnace (down) TUD2B080A9V3

(All Trane products)

When I ran HVAC Calc I used an inside temp of 72 degrees and an outside temp of 22 degrees.

The heat loss upstairs was 11,490 KBTU
The heat loss downstairs was 48,270 KBTU

Thanks for your help.

Larry Smith

[cem-bsee]

U & R numbers are used to express the resistance of an object to heat flow thru it -- U = 1/R

read lots in this forum --
hp will produce some heat at OD temp near zero!

you need to "play" to get a good value for your house & life style. so, start tracking btu/DD/sf from your energy bills -- DD= degree day for the billing period sf= sq footage you are heating. btu = energy used, converted from kwh or cubic feet of gas.

BUT, subtract out the energy used for hot water, showers, cooking, etc -- eg: the energy used in May... or whenever least heating/ cooling

[dan sw fl]

Originally posted by lksmith
Heatpump (up) 2TWR3024A1 Furnace (up) TUD2B060A9V3
Heatpump (down) 2TWR3036A1 Furnace (down) TUD2B080A9V3

When I ran HVAC Calc I used an inside temp of 72 degrees and an outside temp of 22 degrees.

The heat loss upstairs was 11,490 KBTU
The heat loss downstairs was 48,270 KBTU


___Downstairs___
Heating Balance Point is ~ 40'F
Annual Heating Electric = 5,400 kW ... $475
Annual Gas = 130 Therms = $178.

5,400 $0.088 $475.2
130 $1.37 $178.1 total $653.3

2TWA3036 ( close to following)
Temp COP Heating
47'F 3.70 33,000 BTUh
35'F 2.94 25,670
17'F 2.58 21,800

Downstairs load is ~1,000 BTU per degree
(48,270 / [ 72-22 ] )

Heat pump is a bit small looking at heating needs and
the apparent high balance point, however, Atlanta only has about 200 hours below 32'F.

Hope this general summary for DOWNSTAIRS helps somewhat.
..............................................

UPSTAIRS
Balance point for upstairs would be
SIGNIFICANTLY Lower ( ~ 20' F) due to 2-ton heat pump capacity and calculated load of only 11,490 BTU.

230 BTU per degree : 11,490 / 50
At 25'F the heating load would be 45'F x 230 = 10,340 BTU

Temp COP Heating
47' _ 3.1 20,300
35' _ 2.6 15,100
17' _ 2.3 11,000

I would use electric heat strips as upstairs back-up instead of a furnace
___ IF __ the calculated load of 11,490 BTU IS Accurate.



[Edited by dan sw fl on 11-12-2006 at 06:09 AM]

[lksmith]

Hi Dan,

Thanks for doing the magic calculations. I must say I had't considered the fact that a 3 ton heatpump wouldn't have the capacity to handle 48 KBTU of heat loss. Easy to get wrapped up in the cost advantage of the heatpump but forget that it simply doesn't have the BTUH output to handle the load at the lower temperatures, even if it's cheaper to run.

We did comtemplate using electric strips upstairs, but in the end, decided that since we were already plumbed for gas, we would stick with a furnace that could maybe be used as a backup heat source if the heatpump failed.

Thanks again.

Larry Smith

[deme]

Originally posted by lksmith
deme,

Where did you find those heatpump output curves? I'd be curious to duplicate your calculations with my heatpumps.

Thanks,

Larry Smith
Larry,
I got my curves from the Carrier Product data booklet for the Carrier 38YXA heat pumps. Mine is a 2 ton unit and the following curves give information for 2, 2.5, 3, 3.5, 4 and 5 ton heat pumps. By browsing on your HP manufacturer's website, you should be able to get similar curves for your unit.

http://img295.imageshack.us/img295/780/hpbalanceptnd3.jpg

As far as the proportion of energy supplied by the heat pump and the gas furnace, you need climate data for your location.
In my case, Vancouver, B.C., Canada, I got the following results:

Total heating degree-days Celcius for Vancouver (degree-days below 18*C) = 2926.5 degree-days (dd) Celcius below 18*C.
Heating dd below 5*C = 218.6 dd (initial balance point set at 40*F or 4*C. 5*C is close enough)

Percentage of heat supplied by gas furnace=
= (218.6/2926.5)x100 = 7.46%
Percentage of heat supplied by heat pump =100-7.46= 92.5%

In calculating the heat pump payback years, I used a figure of 90% of heat supplied by heat pump.

[lksmith]

Thanks Deme,

I'll poke around on the Trane website and see what I find.

Larry Smith

[Freezeking2000]

Originally posted by dan sw fl

Originally posted by lksmith
Heatpump (up) 2TWR3024A1 Furnace (up) TUD2B060A9V3
Heatpump (down) 2TWR3036A1 Furnace (down) TUD2B080A9V3

When I ran HVAC Calc I used an inside temp of 72 degrees and an outside temp of 22 degrees.

The heat loss upstairs was 11,490 KBTU
The heat loss downstairs was 48,270 KBTU


___Downstairs___
Heating Balance Point is ~ 40'F
Annual Heating Electric = 5,400 kW ... $475
Annual Gas = 130 Therms = $178.

5,400 $0.088 $475.2
130 $1.37 $178.1 total $653.3

2TWA3036 ( close to following)
Temp COP Heating
47'F 3.70 33,000 BTUh
35'F 2.94 25,670
17'F 2.58 21,800

Downstairs load is ~1,000 BTU per degree
(48,270 - [ 72-22 ] )

Heat pump is a bit small based on heating needs based
the apparent high balance point, however, Atlanta only has aobut 200 hours below 32'F.

Hope this general summary for DOWNSTAIRS helps somewhat.
..............................................

UPSTAIRS
Balance point for upstairs would be
SIGNIFICANTLY Lower ( ~ 20' F) due to 2-ton heat pump capacity and calculated load of only 11,490 BTU.

230 BTU per degree : 11,490 / 50
At 25'F the heating load would be 45'F x 230 = 10,340 BTU

Temp COP Heating
47' _ 3.1 20,300
35' _ 2.6 15,100
17' _ 2.3 11,000

I would use electric heat strips as upstairs back-up instead of a furnace
___ IF __ the calculated load of 11,490 BTU IS Accurate.



awesome Dan

[wyounger]

It's easy to calculate the economic balance point here in Atlanta.

You take our electric rates, the efficiency of heat pumps in this climate, and the cost of natural gas, and oops... I don't have to do the math. There isn't an economic balance point in Atlanta. Run the heat pumps for absolutely as long as they can keep up.

Depending on your control system, you may not have to specify a balance point at all. Some can just run the heat pump until it's not meeting demand, then switch. I like this system because it can react a little better to other variables than outside temperature- like the state of your fireplace, solar gain, waste heat from lights, etc. If you have all the lights on and a turkey in the oven, the balance point won't be quite the same as it is when everyone is asleep.

[lksmith]

Thanks for your comments. I think, as sw fl dan has noted, that my issue not so much an economic balance point, but a capacity issue. On my upstairs heatpump, capacity is not really an issue because the heat loss is so small. Downstairs I'm using a 3T heatpump to deal with about 48KBtu of heatloss. The capacity break-even is about 42 degrees. Below that temp the heatpump puts out less than the required heat to keep the temp from falling.

I do have one of those fancy thermostats and I have the heatpump lockout set at 35 degrees and the aux heat lockout set at 50 degrees. I realize I could set the heatpump lockout lower but if I wait for a 2 degree droop I will have an unhappy family or the gas fireplace will be running at full blast and the rest of the house will begin to cool off.

It's kind of a no-win deal.

Larry Smith