Reliability of dual fuel HP vs gas furnace package unit

[squire630]

I'm looking to replace my 16 yr old 4 ton Amana gas pac with either a dual fuel HP or another gas pac (either case Trane or Carrier). Will also install aprilaire zone system for my 2floors plus half basement home. Reliability in the heating mode is the most important criteria - lose heat, you're in trouble. Lose AC, you're just uncomfortable. It would seem to me that a simple gas pac is much less likely to fail than any heat pump based system. Is this right? Can a dual fuel unit switch automatically to a gas heat only mode? And if so, is their gas heating capacity enough to do the job?

[yourairman]

I'm looking to replace my 16 yr old 4 ton Amana gas pac with either a dual fuel HP or another gas pac (either case Trane or Carrier). Will also install aprilaire zone system for my 2floors plus half basement home. Reliability in the heating mode is the most important criteria - lose heat, you're in trouble. Lose AC, you're just uncomfortable. It would seem to me that a simple gas pac is much less likely to fail than any heat pump based system. Is this right? Can a dual fuel unit switch automatically to a gas heat only mode? And if so, is their gas heating capacity enough to do the job?


Backup heating should always be designed to carry full load in heating. You will have two sources of heat with a dual fuel system. The backup would automatically kick in when the heat pump will not keep up. Heat pumps are much more reliable than they were 15 years ago.

[Juleous]

Dual Fuel HP will be more reliable as you have two heat sources to carry the load, if the GASPAK as you call it stops then your screwed no heat. Plus it's more expensive to replace a Package System compared to a split system. You can replace the piece that fails instead of the entire unit.

[jerryd_2008]

... Can a dual fuel unit switch automatically to a gas heat only mode? ...

I believe switching to heat is a tstat function. My Honeywell IAQ allows you to set a lower temp for the HP to switch to heat from cooling, say 75 cooling and 69 heat. Don't know if you can force to gas heat except possibly due to large tstat temp change.

[Juleous]

I believe switching is a tstat function. My Honeywell IAQ allows you to set a lower temp for the HP to switch to heat from cooling, say 75 cooling and 69 heat.

Yes and no, both the heat pump and the gas will not be on together except in defrost mode, the stat will control this change over point when the second stage of heat calls the heat pump will shut down and the gas will finish the call for heat cycle off and then go back to heat pump, except when the outdoor temp is below set point in the stat for heat pump lock out then the gas is primary source of heat. No the thermostat will not know that the heat pump has stopped working and switch the heat pump off and only run gas, some newer control systems may do this I am not sure I don't work in residential anymore so I am not up to date on all the new control systems. Usually this is the emergency heat function of the stat.

[jerryd_2008]

I stand corrected. Should have said that the tstat can switch the DFHP to heat using cooling/heating settings (as long as they are far enough apart like maybe 6 degrees). Whether the HP or gas furnace engages depends on the outside temp.

Haven't been thru a heating season with a DFHP, but I believe that my York DFHP will go to furnace if you cause a big increase in the tstat set point. I wouldn't think that this is the case when switching from cooling to heat though.

[Juleous]

That confused me, if you increase the stat set point at least 2 degrees above space temp then that calls for second stage heat, which in dual fuel means heat pump shuts off and gas finishes call. Not sure what the far enough apart thing is about but the stat can be programmed to disable the heat pump bellow a certain outdoor temp when it's no longer advantageous to run the heat pump due to low heat available outdoors, when this happens gas becomes first and only stage of heat.

[jerryd_2008]

That confused me, if you increase the stat set point at least 2 degrees above space temp then that calls for second stage heat, which in dual fuel means heat pump shuts off and gas finishes call. Not sure what the far enough apart thing is about but the stat can be programmed to disable the heat pump bellow a certain outdoor temp when it's no longer advantageous to run the heat pump due to low heat available outdoors, when this happens gas becomes first and only stage of heat.

Just a HO and probably mixing 2 points. The spread on the IAQ tstat is as I recall 6 points to prevent the DFHP from going automatically in and out of cooling/heating. So, if in the collar months when you may want cooling during the day and heating at night, the tstat will switch automatically to heat when the inside temp drops at least 6 degrees and below your heating set point.

The point about a large change in the tstat setting is probably more appropriate when already heating. My understanding, given I haven't been thru a heating season yet, is that the DFHP will go to the furnace if you call for a couple of degree additional inside temp increase at the tstat. I could be wrong.:couchhide:

[Juleous]

The minimum spread typically on an Auto Changeover stat is 2 degrees. Max is what ever you set, a lot of electronic stats can be setup for what ever minimum spread you want.

[jerryd_2008]

The minimum spread typically on an Auto Changeover stat is 2 degrees. Max is what ever you set, a lot of electronic stats can be setup for what ever minimum spread you want.

Re-read my IAQ instructions. If set to manual change over to heat then you can set minimum spreads to 2-9 degrees F. If automatic change over it's not clear but looks like it will take whatever spread you program. The 6 degree spread must have been my mental note on what I would set.

Thanks, Juleous.

[squire630]

.....guys - thanks for the comments. Couple of clarifications: I'm replacing a package unit, and don't have the option of a split system. My concern is what happens when the heat pump part fails during the winter. Will the furnace keep right on going? All the time or just when the outside temp is below the setpoint?

[beenthere]

You would have to switch the stat to emergency heat. If the ouotdoor temp is above the lock out temp.

[yourairman]

.....guys - thanks for the comments. Couple of clarifications: I'm replacing a package unit, and don't have the option of a split system. My concern is what happens when the heat pump part fails during the winter. Will the furnace keep right on going? All the time or just when the outside temp is below the setpoint?

It really depends on the thermostat. Some thermostats will not allow the backup heat to come on if outdoor temperature is above the changeover setting. Other thermostats will depending on run time. But if you know you have a problem been there is right set it to emergency heat for gas only operation.