Dual Fuel Heat Pump: Why Not Air Handler Instead of Furnace?

15 years ago

I'm looking at upgrading our bottom of the line Heil furnace and have been looking at Trane and Lennox Dual Fuel HP/Furnace combinations.

Since our Seattle area climate is relatively moderate, could I install a HP + variable speed Air Handler instead of doubling up with a back-up furnace? For those very few days during the year when the temperature is below 20 F (usually no more than one week), it seems our yet to be installed 40,000 BTUH gas fireplace insert could supply sufficient heat to maintain a comfortable temperature.

What am I missing... why do I need BOTH a HP and a new furnace when a variable speed air handler would be thousands less expensive and easier to install. Is a gas fireplace insert not considered to be a proper back-up system and the HVAC guys just want to ensure we can heat the house.

Please advise. Thanks.

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Comments (2)

tigerdunes
15 years ago
Tom

I have been following your other thread and I believe I said that for your area/climate, I recommended a matching var speed air handler paired with a HP rather than backup gas furnace.

There may be two obstacles to this type configuration.

1.size of air handler and its location

2.whether your electrical service/breaker panel requires an update to handle the backup electric strips.

and yes, you do require electric backup strips both for defrost function, aux heat when needed, and emergency heat if you have breakdown situation on outside HP condenser. If the electric service update is necessary but is cost prohibitive, then go with gas furnace backup-only 80% eff var speed would be more than adequate.

now to get on my soapbox for a minute. Trane is clearly falling behind on dual fuel applications.I would not want to leave $1500 on the table due to their shortcomings. I am not a Trane basher as I have a 4 yr old Trane dual fuel system and have generally been very pleased especially with my savings over gas. To date, I believe I have used about 60 therms of nat gas backup this winter as opposed to relying almost exclusively on my Trane HP. My Trane furnace is an XV90 that has been phased out for the XV95-93% eff vs 95% eff. I have never used the second stage on the furnace and really the furnace is overkill for my home along with my SE area/climate.

on a three ton Trane XL15i HP, there were 6 configurations that qualified for the tax credit-all with air handlers,none dual fuel.

on the Lennox three ton XP14 HP, there were about 18 configurations that qualified-both air handlers and gas furnace.(No XP15 HPs qualified at all)

and for Carrier's three ton Performance 15 "H" mdl, there were over 100 configurations that qualified-both air handler and gas furnace backup.

If you require the gas furnace backup, then you must look at the Carrier systems.

the gas fireplace backup is nice but it should not be depended on as the exclusive backup/emergency heat for your home.

let me know if you have questions.

IMO
ryanhughes
15 years ago
I agree with all the above and am satisfied with my heat pump as well. A heat pump is probably the better choice for your area with those temperatures. Trane, Carrier, Lennox, and Rheem probably all have combinations fitting the requirements for efficiency in the 3 ton size. If A/C is rarely used and heat is used more then you may even consider upsizing a half ton to get a little more heat--not uncommon in areas where A/C is rarely used. By rarely I mean less than a month out of the year.
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I'd get a manual J just to see what you are dealing with. And ditto the high electric rate comment above. If Man J says you need 8 tons, then you know where you are at. But if the house stays cool in the summer and you are using NG, I'm not sure why it matters. If you are undersized on the heating size, that is fine. You have a good backup. This is the way I look at it (that doesn't mean it is right), you want to use HP, when it is above 30 degrees or so. Your 5 tons should do a good job down to that. If your backup was expensive, you'd want to HP down to 15 or so. Then, your undersized system would be a problem. But since you are just using HP for milder weather, you are fine - on the heating side. The HP gets less efficient as it gets colder regardless of size of unit. So you never really need the extra capacity, because you switch over to your good backup. As Tigerdunes said, if your electric rates are high, your backup might make sense at 40 degrees. In that case, your HP is doing very little. By my math, with my system, 35 degrees is my switch over at $.10 a KWH and $1 a therm. If you are $.15 a KWH and $1 a therm (therms are pretty constant nationally), then your switchover will be closer to 40 degrees.
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Dual Fuel Heat Pump: Why Not Air Handler Instead of Furnace?

Comments (2)

tigerdunes

ryanhughes

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