Thoughts on dual fuel HVAC system
#9
On our property we have a main house and a guest house. The guest house is two stories with my woodworking shop on the first floor and a one bedroom, garage apartment on the second floor that I use for my office since I work from home. The HVAC system has a gas furnace that still works but the air conditioner just can't keep up any longer. The system is 27 years old so I would say I got my money's worth out of it!

I've had a couple different HVAC companies quote a replacement and the guy that came out yesterday quoted a standard gas furnace but also quoted an option for a dual fuel system. This type of system uses a heat pump for primary heating but will switch over to the gas furnace when the temp drops below the threshold where the heat pump becomes inefficient. I'm considering this option because it would require much less propane (wouldn't have to fill the tank as often) and I would think heating primarily with electricity and occasionally with gas would be cheaper than heating 100% with gas.

The average winter temperature in this area is 38-58 degrees but it can drop down in the 20's and teens sometimes. Anyone here have experience with this type of system and if so, what are your thoughts?

TIA!
Frank
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#10
I have (and I do). When we moved to this area in 1988, i had to replace the system in the house we bought. I wanted a high effeciency gas furnace (LP) to replace the heat pump/electric furnace we had. While doing one quote the guy said basically the same thing to me: consider the gas furnace with an air source HP. This wasn't common back them and I thought he was nuts, But eventually had that installed. We stayed in that house another 22 years and the system was fine, besides saving us a fair amount of money on LP. But being air source, with the then technology it cut over to gas somewhere around 30° and I kinda wanted that to be a little lower. The units they make today do go down below that. But for comfort, when it was cold the thermometer would say the house was at the same temp all the time, but it always felt just a little cooler to me when the heat pump was doing the heating. Even so, that was 2 houses ago, and in each subsequent house I had the same setup installed, except for one point. My last HP ad the current one are ground soruce hea pumps. But our winters are much colder than yours, I think the air source will do just fine. One thing that has happened around us to consider: the electric rates have really went up in recent years, I think I'm still savings money versus LP, but those savings are a lot less now. BTW, I talked to the perwson that bought our first house I mentioned earlier, that first system was replaced after it was 26 years old, the owners went with the same set up in the new system.
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.
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#11
You might want to check for rebates. The government seems to like heat pumps and it may offset some of the extra cost.
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#12
Thanks Fred. I could swear I replied to you comment earlier today but it doesn't seem to be here now.

Thanks Brent. I hadn't thought of that. I'll see what I can find out.
Frank
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#13
I'm finishing up on a 28 x 38 second story addition. We/they went with a Mitsubishi mini-split heat pump with three heads. Back-up heat is F.H.W. baseboard by oil. I'll let them determine when, and at what price point to use which system. The pump can pull heat out of really cold air but consideration must be given to the electricity used to defrost the coils. At what point does running an electric heater outdoors in sub-freezing temperatures become uneconomical?
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"
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#14
If the heat pump is hooked to a gas furnace, it doesn't use electric to defrost the coils. It fires the furnace and pumps furnace heat out to the heat pump. I believe that's they way it works with an electric furnace (strip heaters) as well.
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.
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#15
(06-20-2025, 10:01 AM)fredhargis Wrote: If the heat pump is hooked to a gas furnace, it doesn't use electric to defrost the coils. It fires the furnace and pumps furnace heat out to the heat pump. I believe that's they way it works with an electric furnace (strip heaters) as well.

Mine is all electric and when in the defrost mode it turns off both the fans on the inside and outside units, reverses the refrigerant flow so the hot gas goes to the outside coils to defrost it.   If additional heat is needed inside the house the aux heat and blower comes on inside.   Yes the coils inside become cold but without air flow or if aux heat is on you dont notice it.  My aux heat is after the A coil inside so only the heat (over 100 degrees) from compressing the gas goes to the outside coils Roly
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#16
(06-19-2025, 07:41 AM)FrankAtl Wrote: On our property we have a main house and a guest house. The guest house is two stories with my woodworking shop on the first floor and a one bedroom, garage apartment on the second floor that I use for my office since I work from home. The HVAC system has a gas furnace that still works but the air conditioner just can't keep up any longer. The system is 27 years old so I would say I got my money's worth out of it!

I've had a couple different HVAC companies quote a replacement and the guy that came out yesterday quoted a standard gas furnace but also quoted an option for a dual fuel system. This type of system uses a heat pump for primary heating but will switch over to the gas furnace when the temp drops below the threshold where the heat pump becomes inefficient. I'm considering this option because it would require much less propane (wouldn't have to fill the tank as often) and I would think heating primarily with electricity and occasionally with gas would be cheaper than heating 100% with gas.

The average winter temperature in this area is 38-58 degrees but it can drop down in the 20's and teens sometimes. Anyone here have experience with this type of system and if so, what are your thoughts?

TIA!


I live in ATL and have a couple of systems , one near you in Jasper.   My thoughts:

We have access to cheap electricity and cheap gas/LP - as long as you own your tank and can buy/fill up when LP is cheap. Like now.  It’s pretty easy to “run the numbers” based on your home’s btu needs, the equip. efficiency, and the various fuel costs/ per btu from your providers.    
Last time I did it it was pretty close with the heat pump being a bit less $ BEFORE factoring in the equip. cost.  

A dual fuel system is nice in the abstract as long as the infrastructure is already there. If you’re adding gas/lp just for the backup heat or even the main furnace - it’s going to take a while to recoup the capital cost of that.  

Let’s say you’re gonna use gas for other things too. Now the meter/tank/piping costs are spread out over a broader range. But , if you have heavy demand like multiple hvac units, a block sized BBQ, patio heaters, a generator, or pool heater - then your meter or tank cost is going to increase fast too and the piping size is gonna need to be upsized as well.  1000-2000gal propane tank pricing can be eye watering along with the fill up  cost which is due in full at filling if you’re going for the cheapest pricing.  

The dual fuel setup adds more complexity to the system and my research lead me to believe it was a crap shoot on reliability.  Might be fine, and might be a nightmare and no brand particularly distinguished itself . This is where the dealer/contractor/ servicer REALLY becomes what you’re purchasing.  If they don’t do lots of dual fuel or are small / less than competent then you’re really taking chances.  Helps for them to have a dedicated manu. rep’s # they can call when needed. A single / real person , not a “special” number that 10 other companies in area have access to. 

If you go with an all gas furnace , you can spend a couple of hundred bucks more in simple wiring and a  backup battery power supply which will then give you heat during a power outage.  If you go with a heatpump, you are facing tens of thousands of $ for a generator setup large enough to run your heatpump.  

A dual fuel system adds height or length -or- both to the dimensions of the HVAC unit. Not always an issue , but it definitely was in one of my mechanical rooms and wouldn’t fit , so be aware.
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