Heat Pump or Gas Furnace

dakfloridaAugust 23, 2011

I live in Orlando, FL and I am considering replacing my 11 yr old Rheem system with either a Lennox XP21 or Lennox XC17 with G61V gas furnace. We have a 2 story, 6,000 sq ft house that is 125 yrs old. No insulation in the walls and original single pane glass windows. The house is on the historic registry so we will never have insulated walls or windows.

We are putting a lennox XP21 (heat pump) for the upstairs. We are leaning towards the Lennox G61V gas furnace for the downstairs for comfort. From my experience, a house like ours cannot be warmed up with a heat pump when the temps are 40 or below and 10-20mph winds. The past two winters had several weeks below 40 and windy and the old Rheem heat pumps would run 24hrs to keep the house at 60.

Any opinions on installing a heat pump vs gas furnace in my situation?

My Electric is 0.125 per KW and natural gas is 1.17 per therm.

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Not a fan of Lennox heat pumps and it's possible dual fuel may even be overkill for your area/climate.

However I don't think a high eff furnace is necessary because of the usual moderate climate.

I would go with the 80% two stg var speed SL280V furnace or similar of another brand. Link attached.


Here is a link that might be useful: Lennox SL 280v furnace

    Bookmark   August 23, 2011 at 3:43PM
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The US DOE has worksheets that can help you calculate relative costs with your current prices of energy.

    Bookmark   August 23, 2011 at 8:01PM
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normally I would say heat-pump (since a heatpump doesn't cosst much more than straight A/C), but since you don't like the comfort because its a leaky house I can understand getting the furnace.

Heatpumps don't do as well in leaky older homes.

    Bookmark   August 24, 2011 at 8:08AM
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Will you explain the logic behind this statement:

"Heatpumps don't do as well in leaky older homes."

No heating or cooling equipment will "do as well" in older or leaky homes as in well-sealed homes. Heat pumps are not unique in that respect.

    Bookmark   August 24, 2011 at 5:18PM
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True, no heating or cooling system does as well in a leaky home. However since more btu's are required to heat a leaky home, a heatpump may be running constantly to maintain desired temperatures in colder situations. It sounds like the original poster is experiencing this situation already. For them I recommend the gas-furnace for there leaky home.

    Bookmark   August 25, 2011 at 9:17AM
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A BTU is a BTU. It does not matter if it is generated from gas or from a heat pump. If a 50,000 BTU furnace is running constantly to heat a home so will a 50,000 BTU furnace. Why are you writing these misleading things?

    Bookmark   August 25, 2011 at 10:42AM
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Well furnaces are cheaper to oversize and also OP lives in an area where cooling dictates HP size. So the furnace may make more sense despite the fact that a BTU is a BTU.

    Bookmark   August 25, 2011 at 12:43PM
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Is this correct? A leaky house will require lots of cooling capacity. Since cooling dictates HP size in that climate, there will be excess heating capacity if the correct size HP is installed for cooling.

HP or gas, all other things being equal, should be chosen primarily on current and projected fuel costs. The latter will be hard to do. Is your crystal ball working?

    Bookmark   August 25, 2011 at 2:48PM
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Leaks matter a whole lot more in the cold. The ACH is higher in the cold and the delta T is a whole lot higher.

    Bookmark   August 25, 2011 at 3:32PM
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I guess I am stupid. I am still not seeing why BTUs change depending on temperature differential or ACH. Maybe someone can explain in simple terms.

    Bookmark   August 25, 2011 at 6:31PM
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A gas furnace will put out (lets say) 50,000 BTUs at whatever temperature the outside air is. 50k at 70f. 50k at 40F. 50k at 20F.

A HP will not do this. The lower the temperature, the lower the BTUs produced. At some point it cannot produce enough BTUs to heat the house. Leaky houses are worse in that the point is reached sooner.

    Bookmark   August 25, 2011 at 6:46PM
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Also it isn't that BTUs change but I'll give an example. A leaky construction might have a 5,000 btu heat gain in the summer but a 15,000 loss in the winter.

Best construction assumes .2 ACH in the summer and .3 in the winter. That is a 50% increase in air exchange. Then if in summer it is 90 degrees outside and 78 inside, that .2 ACH an hour will not gain that much heat. But have it be 30 degrees outside ... that is a lot of loss.

That is why it matters more for heating and then what weedmeister said.

    Bookmark   August 25, 2011 at 8:13PM
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I guess I misinterpreted, "Since cooling dictates HP size in that climate". I read it to mean that if the HP is capable of cooling the house, there will be more than enough capacity to the house. I guess what it really means is that the HP will be big enough to cool the house, but when it comes to heating, you might have to design in supplemental heat.

Have I been fooled? I thought that the best quality modern low temperature heat pumps can deliver upwards from 90% of their rated capacity at 5 F. What % of heating, at average winter Orlando lows (50 F), do "standard quality" heat pumps deliver at record lows (20 F) in Orlando?

    Bookmark   August 26, 2011 at 11:36AM
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It would be nice to hear back from OP dakflorida.

Because of OP's location and climate plus the fact that the cooling season can be long while the heating season is much shorter, I think an 80% eff two stg var speed furnace paired with a high eff AC condenser sgl or two stg is the best choice.

For the previous comment, AHRI HP directory lists heating BTUs at both 45 degree and 17 degree for all certified matching systems.Suggest you go to AHRI HP directory and look at some of the better HP models like Trane and Carrier to get an idea of both performance and efficiency. It's definitely not 90% of rated capacity at 5 deg fah.


    Bookmark   August 27, 2011 at 7:44AM
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I am surprised that there is such a concern about the cold season given that the poster lives in Orlando, FL. I would think the winters are moderate at worst and the 40 degree weather would not last for too long. While a heat pump may be less efficient during those few weeks of 40 degree weather, I expect that would be more than offset by its greater efficiency during the rest of the year, not to mention the cost savings relative to separate furnace and A/C units.

Here is a link that might be useful: HVAC Site

    Bookmark   August 27, 2011 at 10:20AM
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