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| The external compressor unit is in the foreground. The four indoor air-handlers are located in the rooms you see. Hoses run from the compressor to each air-handler. |
Over the years that probably got expensive, and the previous owner replaced the electric heaters with ducted forced-air propane furnaces and electric AC cooling.
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| Historic hours of heating and cooling in the rooms to be serviced by the heat pumps. (Data reported by Nest thermostat.) |
But the propane forced-air furnace broke that same December. (Apparently problems resulted from running a poorly designed high-efficiency furnace on dirty propane.) It was either going to be an expensive repair or replacement. The old ducted furnace and AC was also inefficient and incompetently implemented. (For example, there was only one return servicing two isolated zones!)
Instead, we opted to install a 4-zone mini-split ductless heat pump system.
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| Indoor Air Handlers heat or cool the air in the room. |
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| Outdoor Compressor services four Air Handlers |
(This whole heat pump business is a payoff for having to study thermodynamics in college.) A mini-split heater can produce 4.5 times as much heat energy as the energy it consumes in the form of electricity. i.e. 450% efficiency. Compare that to radiant electric heaters which, at best, can convert 100% of the supplied electricity into heat.
Mini-split heat pumps also function as air conditioners in hot weather. They do this by essentially operating in reverse -- removing heat from the indoors and dispersing it outdoors.
We are located in an ideal climate for air-sourced heat pumps.
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| Heating Degree Days are a simplified representation of outside air-temperature data. (Heating Degree Days measure the amount of time multiplied by the number of degrees needed to raise the outdoor ambient temperature to 65-degrees F.) The chart above shows the historical Heating Degree Days for each month at our location. Notice the correlation to the historic hours of heating in the previous chart. |
In lieu of the Nest smart thermostat (which does not work with mini-split heat pumps), I am using a set of Sensibo internet-connected smart thermostat controllers.
After a year I calculated that I saved about $1000 in propane for 2018 compared to my previous annual average. Also, I had been spending an average of $750 annually in maintenance and repairs on the forced-air furnace and AC system. The new heat pump system should have very low maintenance costs.
As my 1-year "True-Up" showed we stayed within our solar electricity generation capacity for 2018. The heat pumps caused no significant increase in electricity use compared to the old, inefficient propane furnace and electric AC.
Appendix: Calculations and Assumptions
Furnace Status 12/2017
- Poorly designed and implemented forced air HE propane furnace and electric AC.
- Furnace is malfunctioning.
- AC was incompetently implemented. A single return in one 200 sq ft room of one zone was being used for AC and heating to two separate zones covering 1200 sq ft. Existing AC was rated to SEER 10 when new 20 years ago.
- Household propane costs $2800 per year of which the West Wing heater was using $939 of propane per year.
Repair high-efficiency propane furnace - $2000-$2500.
Fixes symptom not the problem.
Doesn't address problems with existing AC implementation.
Replace with a low-efficiency 2 stage propane furnace - $4500.
Contractor recommends low-efficiency furnaces with propane because they are more reliable.
2 stage furnaces are better because they startup at 75% then eventually kick up to 100%. If they are running only for a few minutes they only operate in the 75% mode. Cost: $4500.
One stage furnace cost: $4000.
Doesn't address problems with existing AC implementation.
4 zone 37K BTU/h mini-split ductless heat pumps - $13,250
"whole-home" ducted heat pump to replace existing forced air and A/C. Cost: greater than $10K
Heat Pump
Heat energy cost per yr heat pump: $557
Heat energy cost per yr HE propane: $1400
Net heat energy cost/savings: ($843)
Maintenance cost heat pump: $0
Maintenance cost HE propane: $738
Net maintenance cost/savings: ($738)
AC energy cost per yr old AC: $271
AC energy cost per yr heat pump: $77
Net AC energy cost/savings: ($194)
Total net cost/savings per year: ($1775)
Cost difference of upgrade to heat pump vs furnace replacement: $13350-$4500 = $8850
Payoff on upgrade to heat pump: $8850/$1775/year = 5 years
Payoff on heat pump: $13350/$1775/year = 7.5 years
CO2 emissions reduction: 3.2 metric tons
Heating cost estimates:
Heat pump uses 2300 kWh/y to create equivalent heat to $1400 of propane feeding HE propane heater.
Measurements for Jan 16 through Mar 15 = 13.5 kWh/day = 400 kWh/month
2300 kWh = $557 at $0.25/kWh.
AC energy cost estimates:
Based on Nest records
AC use in 2016: 253 hours
AC use 2017: 367 hours
Average AC use: 310 hours/year.
old AC:
assume 3.5 kW * 310 hours/year
= 1085 kWh
= 1085 kWh * $0.25/kWh
= $271
heat pump:
assume 1kW * 310 hours
= 310 kWh
= 310 kWk * $0.25/kWh
= $77.5
heat pump system is rated 550-3420W
old AC SEER: 10 (20 years ago when unit was new; unit was discontinued in 2004 -- 2 years before it was installed in addition)
heat pump SEER: 17.7
17.7 SEER vs 10 SEER = 1.77x
CO2 emissions reduction:
heat pump:
2300+77.5 kWh electricity/year
= 2377.5 kWh * 128 lbs CO2/MWh
= 304 lbs CO2
= 0.14 metric tonnes CO2
old AC:
1085 kWh electricity/ year
= 1085 kWh * 128 lbs CO2/MWh
= 139 lbs CO2
old furnace:
$939 propane
= $939/$3.25/gal propane
= 288 gal propane * 12.7 lbs CO2/gal propane
= 3657 lbs CO2
= 1.66 metric tons CO2
https://www.eia.gov/environment/emissions/co2_vol_mass.php
Heat energy cost per yr heat pump: $557
Heat energy cost per yr HE propane: $1400
Net heat energy cost/savings: ($843)
Maintenance cost heat pump: $0
Maintenance cost HE propane: $738
Net maintenance cost/savings: ($738)
AC energy cost per yr old AC: $271
AC energy cost per yr heat pump: $77
Net AC energy cost/savings: ($194)
Total net cost/savings per year: ($1775)
Cost difference of upgrade to heat pump vs furnace replacement: $13350-$4500 = $8850
Payoff on upgrade to heat pump: $8850/$1775/year = 5 years
Payoff on heat pump: $13350/$1775/year = 7.5 years
CO2 emissions reduction: 3.2 metric tons
Heating cost estimates:
Heat pump uses 2300 kWh/y to create equivalent heat to $1400 of propane feeding HE propane heater.
Measurements for Jan 16 through Mar 15 = 13.5 kWh/day = 400 kWh/month
2300 kWh = $557 at $0.25/kWh.
AC energy cost estimates:
Based on Nest records
AC use in 2016: 253 hours
AC use 2017: 367 hours
Average AC use: 310 hours/year.
old AC:
assume 3.5 kW * 310 hours/year
= 1085 kWh
= 1085 kWh * $0.25/kWh
= $271
heat pump:
assume 1kW * 310 hours
= 310 kWh
= 310 kWk * $0.25/kWh
= $77.5
heat pump system is rated 550-3420W
old AC SEER: 10 (20 years ago when unit was new; unit was discontinued in 2004 -- 2 years before it was installed in addition)
heat pump SEER: 17.7
17.7 SEER vs 10 SEER = 1.77x
CO2 emissions reduction:
heat pump:
2300+77.5 kWh electricity/year
= 2377.5 kWh * 128 lbs CO2/MWh
= 304 lbs CO2
= 0.14 metric tonnes CO2
old AC:
1085 kWh electricity/ year
= 1085 kWh * 128 lbs CO2/MWh
= 139 lbs CO2
old furnace:
$939 propane
= $939/$3.25/gal propane
= 288 gal propane * 12.7 lbs CO2/gal propane
= 3657 lbs CO2
= 1.66 metric tons CO2
https://www.eia.gov/environment/emissions/co2_vol_mass.php