Dual-Fuel Switch-Over Point


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Old 02-21-09, 11:30 AM
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Dual-Fuel Switch-Over Point

Still trying to figure out how to set our system for the most economical operation. We have a Trane dual-fuel heat pump. It is similar to a gas-pack, but has a heat pump instead of a regular air conditioner. The thermostat is set to start the backup heat (natural gas furnace) when there is 2-degrees or more differential between what the thermostat is set at, and the actual room temperature. This normally occurs only when we are bringing the house up to temperature in the morning. No matter how cold it seems to get outside, once the gas heat brings the temperature up, the heat pump can maintain the normal 72-degrees inside temperature by itself.

But in order to maintain the 72 degrees, the heat pump runs a long time, whereas the gas furnace could do it in much less time. There has to be a cross-over point (outside temperature) that can be calculated that shows when the heat pump should turn off, and the furnace turn on for the most economical use of both electricity and natural gas.

There is an option for our Trane that installs a heat sensor outside that will always switch the heat source at a given outside temperature (called Restricted mode.) If I could calculate what that outside temperature is, I think we could save on our heating costs by having the option installed.
 
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Old 02-21-09, 08:02 PM
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Heat Pump are best to set it and forget it.

The system is going to be far cheaper to run a long time in HP mode than gas running.

But, what are your rates on gas and electric?
 
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Old 02-21-09, 08:37 PM
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Originally Posted by Jay11J View Post
Heat Pump are best to set it and forget it.

The system is going to be far cheaper to run a long time in HP mode than gas running.

But, what are your rates on gas and electric?
We are on a timed meter program with the power company. Our current rate for electricity is .05 per KWH for off-peak, and .16 per KWH for on-peak. On peak is 9:00 AM to 9:00 PM Monday through Friday. Off peak is the rest of the time, basically nights and weekends.

Natural gas is currently running .66 per therm.

We run the house temp at 72 degrees from 6:00 AM to 11:00 PM. Once the thermostat drops to 68 degrees at 11:00 PM, the heat rarely runs at night. Then at 6:00 AM the gas heat starts to get the house back up to 72, then runs on the heat pump until 11:00 PM when we start the cycle all over again.
 
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Old 02-21-09, 08:47 PM
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I will agree with john that the heat pump will be cheaper even with temps in the low digits. I will add that if it was mine id set the heat pump to cut out at 25
 
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Old 02-23-09, 07:44 PM
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Well to set your outdoor thermostat. you should have one. You will need the cost of your elect vs the cost of your gas. the eff. of your furnace vs the eff of your Hp

then see the link below
http://www.nordyne.com/Literature/707765.pdf

this may help you, I can not find it right now but GA power used to have an online cal. just for that.

Also what kind of thermostat do you have , mine allows for an outside air sensor and will allow for dual fuel control from the tstat with out the need for a kit.
 
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Old 02-23-09, 09:31 PM
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Originally Posted by cyberdead View Post
Well to set your outdoor thermostat. you should have one. You will need the cost of your elect vs the cost of your gas. the eff. of your furnace vs the eff of your Hp

then see the link below
http://www.nordyne.com/Literature/707765.pdf

this may help you, I can not find it right now but GA power used to have an online cal. just for that.

Also what kind of thermostat do you have , mine allows for an outside air sensor and will allow for dual fuel control from the tstat with out the need for a kit.
Thanks. I will see what I can determine from the graph.

The thermostat is Trane XT500C Deluxe Programmable Heat-Pump Thermostat. It has provision for an outside thermostat, but none is installed. That was an option with this Trane dual-fuel heat pump. The thermostat looks like it was made by Honeywell, but with the Trane name on it.

My original question had to do with determining if I needed to buy the outside thermostat, or just let it run like it is now. The only way I can figure that out is to know the cross-over point for cost between heat pump and natural gas. The document you referenced calls it, "Calculation of the economic balance point temperature."
 
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Old 02-24-09, 08:24 AM
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here, use this tool to calculate everything when you get your efficiencies....

Warmair.com - Fuel Cost Comparisons

You will have different economic balance points for peak and off-peak times.


FYI, you'll change the COP on the heat pump section until your gas and elec costs are the same. Then use the literature for your heat pump to find the temperature at which that COP occurs. That will be your economic balance point temperature.
 
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Old 02-25-09, 07:26 PM
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Originally Posted by deweyman View Post
Our current rate for electricity is .05 per KWH for off-peak, and .16 per KWH for on-peak. On peak is 9:00 AM to 9:00 PM Monday through Friday. Off peak is the rest of the time, basically nights and weekends.

Natural gas is currently running .66 per therm.
ok it seems that on off peak times you set point would be about 12F
Your peak time set point would be 60F+

Your system should run more at nite than during the day.. colder ya know...

So I would probably set mine in my house using this data at about 32F
This taking an average of 8 cents for elect. and the 66 for gas.

hmmm.. now if we could use the off peak times and a clock to change over to peak times.. you would heat from 9-9 with gas and all other times with the heat pump.. but then we would have to find the effective temp range for you heat pump system to heat down to..so that it could change back to gas.. yes.. I could do this.. a time clock,, couple of relays, outdoor thermostat.................................................hmmmm......
 
 

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