Dual-Fuel Heat Pump Mode of Operation


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Old 01-15-07, 07:16 AM
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Dual-Fuel Heat Pump Mode of Operation

We have a Trane single package dual fuel heat pump that was installed last year. It is a combination electric heat pump and natural gas heater.

These units are shipped in what is called Non-Restricted operation. In this mode, when the second stage of heat is called for, the heat pump is cut off and the gas furnace operates until both stages are satisfied. In the optional, Restricted Mode, the heat pump operates only at an outdoor temperature above a set point.

We live in north central Arizona where it can be over 100F in the summer and well below freezing in the winter. It is currently 12F. At these temperatures, the heat pump runs almost continuously as it puts out just enough heat to keep the second stage gas heater from coming on. There has to be a 3-degree temp differential between the indoor thermostat and actual room temperature for the gas heat to kick in. Unfortunately, the Trane is just good enough to keep this from happening except when doing the initial warm-up in the morning.

In these times when the heat pump just keeps trying to maintain temp, it would seem to make sense to let the gas heater come on for a while and get it over with. I'm thinking I should have the dealer set the Trane to the Restricted Mode so that the gas heat always comes on below say 32F. Does this make sense?
 
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Old 01-15-07, 09:10 AM
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I'm thinking I should have the dealer set the Trane to the Restricted Mode so that the gas heat always comes on below say 32F. Does this make sense?

I would for sure. That way you dont even have to think about it going into defrost. Dont know why you have gas and electric heat there??? Go to http://warmair.net there you can compare fuel cost for where you are. Might pay to go electric all the way
 
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Old 01-15-07, 09:52 AM
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Originally Posted by Ed Imeduc View Post
I'm thinking I should have the dealer set the Trane to the Restricted Mode so that the gas heat always comes on below say 32F. Does this make sense?

I would for sure. That way you dont even have to think about it going into defrost. Dont know why you have gas and electric heat there??? Go to http://warmair.net there you can compare fuel cost for where you are. Might pay to go electric all the way
OK. I will have a chat with the HVAC engineer at the Trane dealer.

We went this way because natural gas kept rising in price, and we have a timed electric meter where we get our electricity at 1/3 the cost between 9 at night and 9 in the morning weekdays, and all day on the weekends. Previously, we had a Day/Night gas pack, with A/C and natural gas. We have had several heat pumps in the past with electric backup. When the temp was below freezing, you needed a loan at the bank to pay for electric use. That was why we wanted the dual-fuel unit.
 
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Old 01-17-07, 06:54 AM
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Now that I realize I need to change the mode of the Trane HP so that auxiliary (second stage) heat comes on by itself below a certain temperature, I would like to learn more about heat pumps and temperature differentials. Specifically, I would like to know at what temp differential it makes sense to turn off the HP and turn on the aux heat.

This morning it is 10F outside. The thermostat is set for 65F. When it calls for heat, only the HP runs unless there is more than 3-degrees difference between the thermostat setting and actual room temp. So when the HP kicks on at about 64F, it has been able to maintain a ceiling duct temp of 64F at the 10F outside temp. Consequently, the HP justs runs and runs trying to get up to 65F, but doesn't get the assist of second stage heat because it is able to keep within 1-degree of the thermostat setting. Chews up a lot of power.

My question is, what positive duct temperature differential is desirable as a minimum? Even with 9-degrees over the thermostat setting it takes a long, long time to bring the room up to temp if the thermostat is set to 72F. Of course when it is going from 65F (night setting) to 72F (daytime setting) the second stage heat does come on because that is more than the 3-degree difference required for second stage heat.
 
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Old 01-17-07, 11:29 AM
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Lets just say it would take the whole page here to go into it. Most units will give you 3 times more heat for your $ than the electric elements can at 30o. I find that its just best to go to the other heat at about 20o outside. Gas, oil or the EME back up.
 
 

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