Correct lockout on Dual Fuel Trane

cswartzwelderAugust 1, 2010

Two years ago I purchased a dual fuel Trane system. I have a XC95 45K btu 3 stage communicating gas furnace and a X15i heat pump. The system seems to work well enough for the past two years. We didnÂt actually save in cash on the heat because we started using more heat. For the longest time we kept the house heat on 60 degrees in the winter. Now we keep the heat on 68 and the bill is just slightly lower than it was with the old system. Air conditioning is hard to compare because we havenÂt been home and there havenÂt been as many days that I need the AC running.

I assume by getting dual fuel, I would be able to adjust the system to take advantage of which ever form of heat was cheaper. For the longest time electric was cheaper so I thought I should lean a little more on electric heat instead of the 1st state gas heat. I kept the lock out on 40 degrees. That was really a guess because the dealer couldnÂt show me a formula to set the optimum lock out temperature.

Well in January 2011, electric rates are going up 20% in Pennsylvania. IÂm trying to figure out if I will need to adjust my lock out on the heat pump. Running the 1st stage gas when there is just a call for "a little heat" has to be competitive to the cost of running 20% more expensive electric.

Since the dealer couldnÂt help me, I tried calling Trane direct, they were zero help. Can anyone tell me if such a formula exists to help me set the temperature correctly?


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Try searching for 'calculate economic balance point'; also, 'calculate thermal balance point.

Try this for a quick comparison:


    Bookmark   August 2, 2010 at 10:51AM
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40 degrees is a high lockout. Thats the type of lockout I would recommend if electric is expensive. AT 40 degrees and above your not using the heatpump much. I would leave it at that. Currently I keep heatpump running down to 25 degrees. I may change to 30 since electric is going up a little where I live.

    Bookmark   August 2, 2010 at 12:42PM
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My service man set mine at 28 deg. It works well. My gas is propane which is expensive.

    Bookmark   August 2, 2010 at 12:51PM
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Are you NG or propane? I assume NG. I think 40 is probably pretty good with a 20% increase but it really depends where the increase is coming from and what you pay for NG. On a btu basis, a heat pump is roughly 3x more efficient that NG. That 3x is dependent on temperature and most people pay more for electricity on a btu basis.

I have mine on 35 degrees with KWH - $.10 and NG at $.98 a therm. So if your electric is going to $.12 - then 40 is a good starting point although that is pretty high. If your electric was $.08 and is going to $.10 - you should probably be at 30-35 assuming you are NG paying about $1 per therm.

I expect that my electric rates are pretty stable and I'll be lowering my setpoint when NG goes up.

Remember a heatpump is massively more efficient than NG - so even an increase in electric costs may still keep the heatpump cheaper to run. Certainly at 40 degrees unless you are paying $.20 a KWH

    Bookmark   August 3, 2010 at 6:05AM
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any dealer selling a dual fuel system should be able to provide his customer a temperature balance point and an economic balance point based on the system and his fuel costs. I doubt that 1% of dealers selliung these systems can do this.

with that said homeowner needs to understand how to set his lockout and switchover point.

a 40 degree lockout on the heat pump defeats the purpose of even having a dual fuel system. sorry but that's true.

if you don't know these balance point numbers, then you will just have to experiment. I would start at 32 degrees and adjust up and down as necessary.

what size HP and furnace do you have?

what mdl thermostat/control do you have?

I always like to start by comparing fuel costs. you must know the COP and furnace efficiency.

a link below is provided.


Here is a link that might be useful: Fuel Cost Comparison Calculator

    Bookmark   August 4, 2010 at 8:50AM
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I don't think 40 degrees defeats the purpose of dual fuel. It is high but NG is cheap right now. I'm set on 35 - which seems like a reasonable economic balance point. I used $200 NG for a winter and maybe $500 in electricity. If I went to 40 - I'd guess that NG would be $300 and electricity would be $430. Sure a little more but better than the $1000 NG alone would be. Ok I am guessing a lot but my energy audit suggested $1400 per winter. I upgraded insulation enough to make that $1200 but I think the heat pump is where the real savings is.

    Bookmark   August 4, 2010 at 8:54PM
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It depends on what the actual rate is going to on electricity and going rate is on NG to really know where the best balance point would be. I agree with Tiger, the only way a 40 degree lockout on the pump would make sense is if the electric price was crazy high or gas was close to free.

I would start with a HP lockout at anything below 30 degrees and the dual fuel lockout at anything above 40.

I would also recommend a device called 'the energy detective 5000' so you can look at a graph and see how much electricity the heat pump is pulling. Keep in mind the heat pump uses less energy in heat pump mode than a/c mode.

    Bookmark   August 4, 2010 at 9:51PM
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I have a co-worker (who actually designs industrial HVAC systems) who was looking at a dual fuel system for his house. It was interesting b/c he was somewhat skeptical about dual fuel being a good economical choice for ATL.

I found a spreadsheet online (forget site) to determine the balance point, and I had a hard time getting it above 30F unless NG prices were

    Bookmark   August 5, 2010 at 1:40PM
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