View Full Version : Dual Fuel Economic Balance Point
02-23-2007, 09:13 PM
I have a question regarding my RUUD RQPW dual fuel package unit.
My installer put in a White Rogers 1F95-377, but did not use the outdoor thermometer. When asked, he pointed out that the thermostat looks at the approach rate of the temperature vs time and when the HP is not efficient it will energize the nat. gas.
The RUUD users information manual shows the thermostat with outdoor thermometer and shows how to calculate the balance point based on price per therm vs price per kwh. Looking at the graph, if the price per therm is $1.40, the balance point would be 10F. This would probably keep the HP on until the next morning if the low was say 12F, as well as not maintaining set point. Although economically it may be the best choice, it seems like a foolish choice for comfort.
I called RUUD customer service, and all they would say is call your installer, but this is more a matter of opinion. The low last night was 36 and the gas came on at least once.
So my question is: Should I leave the system as-is? Or should I have the outdoor thermometer installed and set a balance point like RUUD recommends?
Thanks to all, I have read this forum for a couple months now and it helped me make the dual fuel choice for my home with confidence.:)
02-24-2007, 07:27 AM
I live in your neck of woods and also have a dual fuel system-albeit a Trane.
Keep in mind that there are two balance points.
1.temperature balance pt where the heat pump will maintain your inside thermostat setpoint down to a specific outside temperature based on your home's heat loss.
2.economic balance point is a calculation based on outside temperature where gas backup heat is more economical to heat your home rather than the heat pump even though the heat pump is maintaining your home's inside thermostat setpoint. Keep in mind, a heat pump loses its efficiency advantage as the outside temperature drops.
As you mention, there are two methods of handling the switchover from heat pump to gas backup.
1.the non restrictive method is performed from your home's thermostat. This is the method you have.
2. The other method is the restrictive type which involves an outdoor temp sensor and tells your ststem which type of heat to operate depending upon outdoor temperature. I have this method and I can set the outdoor thermostat where system changes between HP and gas backup. I personally like this better because I am in control. Is one method better than another? Not necessarily. That is something you will have to decide.
One caveat. It is my opinion that very few dealers know how to calculate a temperature balance point and an economic balance point. For information, I used a trial and error method of several adjustments and just decided that 30 deg fah works about right for my home , system, and climate.
02-24-2007, 01:01 PM
The thermal balance point can very in temperature depending on build of structure. Also you must have the heat loss of the structure and ari info. on the unit you have installed to plot the balance point.
i agree with the above, on dual fuel setups. i like using outdoor stats, set to 30, no problems.
02-25-2007, 09:13 PM
Thanks for all the responses. :D
So, it seems like outdoor stat is the way to go to be in control of the gas. To calculate what it should be set at, it appears that it should be the point at which the heat exchange between the home and surroundings is equal to the heat output of the HP, + 10% or so, so that the HP can actually heat the home. This I can calculate, but it is not entirely temperature dependent. I am on a hill and have considerable wind, with almost no protection from it (i.e. trees).
Or....just install it and find what works.:)
I can most certainly install the outdoor stat for the White Rogers (1F95-377)myself and hook up to the appropriate connection. I have an email in to them on how to set up the control, but does anyone here have this info?
02-26-2007, 07:22 AM
Just stating the obvious but with the high cost of propane vs electric and assuming cost of operation is important to you, I would push down the switchover point from electric to propane as low as you can and still be able to handle your home's stat setpoint and more important your family's comfort.
02-26-2007, 12:44 PM
How do you like that RQPW? It's new this year I believe?
02-26-2007, 01:36 PM
I am very pleased with it. It was priced ~15% higher than an equivalent gas pack, and has only hit the gas once when it was about 35F out. The fan is a tad loud, sounds almost like the blades are 'smacking' the air, but being a package unit the noise is acceptable.
I looked very closely between it and the Heil, and the price was better on the Ruud, The HSPF was higher (7.7 vs 8.0), and the SEER was higher (14 vs 13).
I have no regrets in purchasing this unit.
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