DIY Chatroom Home Improvement Forum (http://www.diychatroom.com/)
-   HVAC (http://www.diychatroom.com/f17/)

 zootjeff 01-16-2010 06:47 PM

Heatpump vs Natural Gas - Energy Cost Spreadsheet

45 Attachment(s)
I was curious how much my heat pump costs compared to Gas or resistive electric heat (space heaters, etc). So I decided to do something with the heating tables that came with my heatpump and coil.

This exercise really helped me to understand the principals and trade-offs of what the factors are in using the various methods to heat my house. Hopefully you find it usefull too.

I got my electric and natural gas utilities. I calculated the price per kW based on my bill total and usage.. I used a small and a large bill for comparison.

The next step was to put the gas and heat into the same units for energy. Electricity comes in price per KilloWatt Hour, and Gas comes in Therms. A therm is 29.3 Killowatt Hours.

I ended up with a price of 10.2 cents per kWh for Electricity and 3.9 cents per kWh for Gas.

This shows that if I was heating with purely resistive electric heat, I'd be spending over 2x what I pay for gas heat.

The next step was to figure out how this compares to a heat pump. The Tables that came with my heatpump show the heating parameters so I can calculate how much heat it will move for how much energy goes in (work) at a given temperature. All the numbers are based off an internal house temperature of 70 degrees. By using conversions from kW to BTU I found that at 65 degrees F outside, the heat pump uses about 2.3 cents per kWh, and at 40 degrees F it uses about 2.9 cents per kWh.

It actually is cheaper than gas until about 13 degrees F based on my calucations. To do the comparison, I calculated the Output heat of the furnace at 92% of the input power. The Furnace allways costs 4.2 cents per kWh no matter what the outdoor temperature.

See the spreadsheet below.. The calcuations are based on my 1900 sq/ft house in Portland OR with a 92% 80,000 BTU furnace and 2.5 ton 14SEER HeatPump. My heat load is a pessamistic approximation, but shows the concept.

Then the other thing this spreadsheet did was help understand the Heat Load of the house. This shows how much power is lost from the house from the difference in indoor vs outdoor temperature.

The heat pump is able to transfer less and less heat as the temperature outside goes down. But the house leaks more and more heat. These two factors are why you allways see backup heat with a heat pump system.

This graph shows this relationship of heat transfer output of the heat pump compared to that of my 2 stage gas furnace vs temperature.

http://www.mpegbox.net/hvac/Heatpump_out_vs_temp.gif

This makes it very clear that for my system, even though it is cheaper to use the Heat Pump all the way down to 10deg F outside, it won't be able to keep up. If this is my real heating load from my house then my Heat Pump will only keep up until about 37 degrees. This means that when it is 40 degrees it will be running 92% of the time. So I might want to bump up my backup heat switch over to more like 45 degrees to give the heat pump the option of staying in it's defrost cycle a bit longer if it wanted, etc..

This also shows about how often my furnace runs. If it is 50 degrees outside, then my furnace could stay in the first stage and run 33% of the time to keep up.

Bottom Line Summary:
Electric space heater heat: ~\$0.102/kWh
Gas heat: ~\$0.042/kWh
Heat Pump Electric heat: ~\$0.026/kWh

My next step is to put a data logger on my furnace and actually measure how long it runs in it's various stages to validate my load assumptions..

 Marty S. 01-16-2010 07:15 PM

very nice!

 beenthere 01-16-2010 07:21 PM

Best way to determine heat pump balance point. Is to lock out the furnace until a temp you know the heat pump can't heat the house at.
Say 10°.

Then wait until you get a day that the heat pump is running continuous and is unable to maintain your 70° temp in house(best to do this at night time.
Then you know what temp is your heat pumps thermal balance point. As long as its above your economical balance point. You can use that temp as your switch over temp.

 zootjeff 01-16-2010 07:48 PM

45 Attachment(s)
Quote:
 Originally Posted by beenthere (Post 384173) Best way to determine heat pump balance point. Is to lock out the furnace until a temp you know the heat pump can't heat the house at. Say 10°. Then wait until you get a day that the heat pump is running continuous and is unable to maintain your 70° temp in house(best to do this at night time. Then you know what temp is your heat pumps thermal balance point. As long as its above your economical balance point. You can use that temp as your switch over temp.
Cool! Thanks! Is there a problem with the HP running 100% Duty? Do you ever want to give it rests by design? How long are the typical defrost cycles?

 beenthere 01-16-2010 07:55 PM

The compressor is designed for 24/7 run times.
Never needs a rest per say.

Defrost time varies with area. IEEC requires that all defrost times be less then 15 minutes. So all defrost boards will have a timer that limits the time to less then that. Many boars are 10 minutes max time.

 Rollie 01-08-2011 06:17 AM

Something to consider. One reason a heatpump efficiency is lower in winter is because of the defrost cycle and having to use a backup heat source. Most heatpumps have a defrost timer board that looks for an input from the defrost thermostat at a given time interval i.e. 30,60 or 90 minutes. The thermostat dosen't take in account humidity which is the main factor in producing the ice formation. I have disabled the defrost thermostat on my unit a few time when I know the humidity level will be low. If one could wire a humidistat is series with the thermostat (I haven't done this yet). I believe the efficiency rating would jump in winter. Just food for thought!

 beenthere 01-08-2011 06:27 AM

Better to just install an on demand defrost board. Since the outdoor coil can become oil logged if it hasn't gone into a defrost in 6 hours or so at outdoor temps of 40°F or below. And cause the compressor to seize. Costing far more to replace the compressor then what you would have saved by using a humidistat.

 PRAY77 11-19-2011 10:43 PM

Heatpump vs Natural Gas - Energy Cost Spreadsheet

Could you provide your excel file so I can use it on my home

 PRAY77 11-19-2011 10:45 PM

copy of excelfile desired

 rgrsayer 06-21-2015 05:44 PM

gas furnace vs heat pump

the most sensitive part is the cost of power, and of gas.
In North America, with a surplus of gas and a lack of water (hydro power) or gas fired power generation,
it will become difficult to compare.
Regarding the spread sheet (excellent presentation) I suggest the cost of electricity (\$/kwhr) and gas (\$/therm) be used as inputs.
Then it would be easy for others to use their local cost schedule.
Regards, rgrsayer.:thumbsup:

 user_12345a 06-21-2015 11:00 PM

How does your furnace handle extreme cold? Does it cycle between low and high?

Looks like it would in your case, but most dual fuel t-stats don't have the ability to stage fossil fuel heat.

Would probably shut it down at 40f and run the furnace.

---------
beenthere, why do heatpump coils become oil-logged?

Evaporator coils and straight a/c units don't get oil logged even though there's 100% vapour at the end of the coil.

 beenthere 06-22-2015 03:59 AM

The colder it is outside, the slower the liquid boils off in the outside coil. Velocity becomes very slow in the coil, along with the oil becoming thicker, so the oil isn't forced out of it.

 All times are GMT -5. The time now is 03:43 PM.