correction to square footage - about 2100 sq ft downstairs

nash

as far as nat gas,I think price will be more of a concern than supply and that the price difference between the two fuel sources will only widen.

I am attaching a fuel comparison calculator for your
use.

I generally like straight high eff HPs with air handler and backup aux strips but with older homes not well insulated or tight envelope, then I think dual fuel is a better choice. Of course, you need a professionally performed heat/cool load calculation for correctly sizing new HVAC as well as ductwork to be thoroughly inspected for size, insulation qualities, leaks, etc. If you have any hot/cold spots in your home, now is the time to address these issues.

here is a quick synopsis of Nashville climate from Trane's site. I personally think it underreports both cooling and heating but that's just my idea. anyway, it gives you an idea.

Climate averages for: NASHVILLE, TN, 37203
Hot weather Cold weather
Average days per year above 75: 153
Average high temperature: 88.7
Average days per year below 40: 31
Average low temperature: 36.8

I would not have a Lennox HP because of poor performance/eff numbers.

Regardless of brand, you want a var speed blower whether air handler or furnace.

BTW, what is the model of the upstairs HP?

let me know if you have other questions.

IMO
Good Luck!

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

The model number on the upstairs unit is for the indoor unit. The unit ALONE does not have heat strips. This is standard. Heat strips are added most of the time with heat pumps to the air handler, which is the model number you provided. There may be some notation on the unit to show that it has heat strips. The heat strip model numbers will be something like BAYHTR1410.

"Electricity is 8.503 cents/kWh. Gas is $1.89/therm.
Our house is old, brick and not especially tight. We are going to have extensive duct work, including some re-designing. We keep our house about 68 in winter, 74 in summer.

Will a heat pump with heat strip be more economical to run than dual fuel?"

Nash:

Here are your operating cost numbers:

Compare the price of 1 million btu's of heat for heat pump, gas, and straight electric strips.

For heat pump w/electricity at 9 cents/kw-hr delivered, C.O.P (Coeff of perf) = 3.25 at 35F ambient:
(1,000,000 / 3413) x .09 / 3.25
= $8.79

For straight electric at same 9 cents/kw-hr
(1,000,000 / 3413) x .09
= $26.36

For natural gas at $1.89/therm, 95% efficient furnace:
(1,000,000 / 103,000) x 1.89 / .95
= $19.31

The heat pump is 300% cheaper (26.36/8.79) to run than straight electric, and 220% cheaper to run than the gas furnace.

The gas furnace is 137% cheaper to run than straight electric.

Note that the heat pump operating costs are at 35F ambient. As the outdoor temps rise from 35F, the heat pump is cheaper to run (more efficient at higher ambients). The cost of a defrost cycle for the heat pump is not included in the operating cost.

Best to you.

Thank you very much tigerdunes and garyg. It's very helpful and reassuring, especially to have the numbers. Also I looked at the fuel comparison calculator.

I am going to go for dual fuel.

I am worried about the upstairs unit. I can't find any indication that it has a auxilliary heat.
The whole thing just looks like one box to me. There is no other unit attached to it and nothing with a model number that looks like BAYHTR1410.
On the sticker it says furnace may be factory or field installed, and Field Installed is checked. But on the sticker there is a list of furnaces, starting with NONE, and then a list of numbers like BAYHTR etc. None of these options is checked!

I am going to have to ask the installer about this. No wonder it's cold up there if there is no aux heat. This installer is also bidding on my current job, so I really want to know if he's messed up.

nash

get one of the quoting HVAC dealers to check upstairs air handler. they can tell you in a matter of minutes whether you have a heat strip as well as what size it is. If you do have heat strip, more than likely it was field installed rather than factory installed. you need at least a small heat strip to temper the air if there is a defrost call as well as backup aux/supplemental heat.

IMO

Good idea, I will get one of the guys to look at the upstairs unit.

More questions:
My husband is concerned that a dual fuel unit is more likely to go wrong than furnace A/C, because of more complicated electronics. Is this a problem?

I am wondering if it's reasonable to go with an 80% furnace rather than a 90% furnace. Web sites differ, but we will probably dip below the balance point of the HP about 45-55 days per year. One issue is the price of a >90% furnace. Another is that (according to two of my three contractors) we can vent an 80% furnace into our chimney alongside our water heater exhaust.
Whereas, a 90% furnace has to be exhausted to the outside.

The reason they gave is that the cooler exhaust from a 90% furnace will condense the moisture from the water heater and corrode the chimney. Venting outside means running a long pvc pipe through our basement to an outside window, or drilling through the rock wall foundation and coming up in my flowerbed! We are on the side of a hill, so one end of the basement is underground, and the other is above ground; the furnace is in the underground part.

I don't like the thought of letting lots of lovely heat escaping up the chimney, but it may be that an 80% furnace makes more economical sense?

Before you worry too much about dual fuel electronics difficulties, consider that most companies now use a thermostat that handles all the dual fuel conditions. it's no longer needed to have a huge dual fuel control box to do the switch over.
all that's needed is the proper wires in the thermostat, and an outdoor thermometer.
the true dual fuel controllers are complicated, and they do give better control of the two systems than the thermostat can, but it's very simple to install and it works well for me.

We have a dual fuel setup with an 80% furnace. The HP came with a dual fuel kit- essentially a temperature switch in a small box at the compressor outside that locks the HP out below the set temperature.

Also have a VisionPro thermostat, and I've added the outdoor sensor to it. My plan is to take the dual fuel kit out of the circuit and let the thermostat pick whether to run HP or furnace.

My reasoning is that there were too many times last winter when the furnace was running and it wasn't that cold outside. I'm hoping that running the switchover with the VisionPro will be more accurate and save on gas bills.

"My husband is concerned that a dual fuel unit is more likely to go wrong than furnace A/C, because of more complicated electronics. Is this a problem?"

No. New thermostats like the VisionPro w/outdoor temperature sensor handle the switchover.

"I am wondering if it's reasonable to go with an 80% furnace rather than a 90% furnace."

- Good thought since the furnace is auxillary heat and not primary heat.

"Web sites differ, but we will probably dip below the balance point of the HP about 45-55 days per year."

- The balance point depends on many variables including insulation, windows, doors, sun exposure, heat pump btu output, outdoor temps, airflow, indoor temp setpoint, etc. A good starting point is 32F. My balance point is mid 20s (fairly tight construction and good windows).

Take care.

Can someone give me a desciption of how to adjust for more electric and less gas usage on a trane duel fuel system? Thanks