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That depends more on the size/capacity of the unit's distribution fan - not usually an issue.

Duct size doesn't change with the forced-air technology (heat pump v. furnace) used to produce the conditioned air.

It's not unusual to see a hybrid system with both a furnace and a heat pump. They use the same ducts.

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If you're referring to a change like that here in N. Illinois I can tell you without hesitation it will not work. Heat pumps in our area are not common and must have a secondary system for colder weather. Heat pumps alone simply cannot provide enough heat for our winters - period. Don't let someone snow you into this one.

Eric Barker

Barrington, IL


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Air source heat pumps are only good to around 35 degrees, then another heat source is needed. What I usually see is an electric (sometimes gas) forced air furnace. They do work and are usually more efficient than other systems, but the farther north you go (such as in northern Illinois) the less days you will have available for them to work due to the colder temps. I am in central Indiana and there are some here but they are not real common. The farther south you go the more common they are.

Water source or geo thermal heat pumps are a different story. Because they do not get their heat from the air, they can be used at much lower temperatures.

I'm not a HVAC tech, but here is my $.02....

As far as duct sizing, the ducts are sized differently for an air conditioning system than they are for a heating system. Colder air is more dense and therefore needs larger ducts for the airflow demands. Usually a furnace has a multiple speed blower motor. In the summer the blower will be on high to help with the air flow and the blower is on medium in the winter. Some say that it is because the home will feel drafty if the blower is on high in the winter, but it is more of an efficiency issue. Too much air across the heat exchanger and the limit switch will shut the blower off until it heats back up then it will quickly cool offf again. Kind of the same as short cycling an oversized a/c condensing unit. You shouldn't have an issue in newer homes because they are usually designed for a/c. Older homes become an issue because they did not have a/c available and were ducted with the smallest needed because they were more economical.

Long story short, I think that the larger ducts would be needed for the heat pump during the cooling cycle (no different than central air) and should be sized accordingly. Duct sizing is beyond our scope and you should refer to a HVAC tech if you think that they are undersized.

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Originally posted by japsee

What changes must be made to the ductwork if changing from a conventional furnace to a heat pump. Need bigger? Smaller? Can leave the same?

You'll need larger ducts.

A traditional furnace's ductwork is sized to convey warm air.

A heat pump's ductwork has to convey both warm and cool air. The cool air is denser and more difficult to move.

Was this another question from the test?

- Jim Katen, Oregon

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Very true.

Since you made no mention of A/C being an upgrade, I ASSumed that the existing service includes a split system A/C with a furnace.

If that is indeed the case, then your duct sizes will not need to be modified.

And hybrid systems are common because the heat pump cannot handle severe cold - as Eric and Paul said earlier.

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