Replies

1. GBA Editor
   Martin Holladay | Sep 09, 2011 03:47pm | #1

   Julie,
   1. It's impossible to compare fuel costs unless you tell us the local electricity cost and the local cost of natural gas.

   2. A ductless minisplit system is a type of air-source heat pump, so when you talk about air-source heat pumps, your question is confusing. Perhaps you are comparing a Japanese-made ductless air-source heat pump with an American-made ducted air-source heat pump. If that is what you are talking about, the former will generally be significantly more efficient than the latter.

2. 
   user-939142 | Sep 09, 2011 04:31pm | #2

   a 'mini-split' is an air source heat pump; the typical model has no ducts
   keep in mind you may not have local hvac people familar with the japanese models; a broken down unit might involve a full replacement

   at this time, natural gas costs win out, even for the foreseable future, especially if you already have natural gas plumbed into the building and venting established, or will already be using it for water heat, which means you have paid the monthly service charge.
   and you'll have a hard time heating during jan-feb-mar on a heat pump without lots of backup electric resistance heat, unless you build to near passive haus standards.

   i'm pretty sure comed got rid of electric heat resistance rates for new customers, so you can't save like you could in the past; ameren downstate did. plus the costs of seperate metering begin to eat any savings anyhow on a small unit.

   the best bet is a shared system especially if utilities are included in rent, like a large boiler providing hot water and heat to all the units. renters can then supplement if they want, with small electric heaters or window a/c units.

3. 
   jklump | Sep 09, 2011 04:38pm | #3

   Martin
   Sorry about that. We pay $0.12/kWh and $0.91/therm and yes i am referring to the Japanese mini split system. I guess another question is if i have to use ducts with a mini split am i losing efficency?

4. GBA Editor
   Martin Holladay | Sep 09, 2011 04:54pm | #4

   Julie,
   1 therm of natural gas = 100,000 BTU
   If your furnace or boiler operates at 92% efficiency, that much gas yields 92,000 BTU of heat.
   If your mini-split heat pumps have a year-round average COP of 2.0, you'll need 13.5 kWh of electricity to supply the same amount of heat (92,000 BTU). The electricity will cost you $1.62 -- more than the $0.91 you would pay for the same amount of natural gas.

   If you choose a ducted air-source heat pump, your efficiency will definitely be lower than if you choose a ductless system.

5. 
   LKudBDqV6C | May 17, 2012 04:47am | #5

   If you choose a ducted air-source heat pump, your efficiency will definitely be lower than if you choose a ductless system. Why?

6. GBA Editor
   Martin Holladay | May 17, 2012 05:15am | #6

   Jardon,
   The reason that the efficiency of a ducted system is lower than the efficiency of a ductless system is that it takes a considerable amount of electricity to propel air through ductwork.

   If the system is perfectly designed, with all ductwork located inside of the home's conditioned space, then the electrical draw of the blower motor helps heat the house during the winter. However, the same blower motor adds to the cooling load during the summer.

   Even during the winter, the blower motor heat has a COP of 1.0 -- significantly less than the COP of an air-source heat pump. That lowers the average COP of the system.

   Of course, many HVAC systems have sections of ductwork that aren't located inside the home's conditioned envelope. In a house like that, you see major a degradation of the system's efficiency.