Heat Pump

[Denali]

Originally Posted by itdave

I would think that the heat pump effectiveness would be rated the same as AC which is output air temp is 20 degrees less than input are temp. So in heat pump mode I guess it would be output temp is 20 degrees more than input air temp.

That's right.

Remember, the "input air temp" is the temperature of the room being heated, not the outside temperature. The heat pump (in either heating or cooling mode) does not draw outside air into the house.

[frankdamp]

Larry:

I can't pass on experience with RV heat pump systems, but when we were looking to move from our big house in Everett to something smaller, many of the places we looked at had heat pumps.

The main thing I noticed was that the output air was warm (maybe 15 degrees above the room air temp), rather than hot (which we were used to with a conventional furnace). I think the reaction tme needed to heat the environment to the temp you've asked for is a lot longer with a heat pump, but once it's close to the temp, the heat pump is a lot more efficient.

In the end we decided not to go for a house with a heat pump, mainly because Eileen didn't like what she sensed as "cold drafts", even though they were a few degrees warmer than the room ambient.

[SteveSkinner]

Originally Posted by Denali

That's right.

Remember, the "input air temp" is the temperature of the room being heated, not the outside temperature. The heat pump (in either heating or cooling mode) does not draw outside air into the house.

Not quite true
As above no air actually travels thru the unit but

The heat pump attempts to draw heat from the air outside and transfer it thru the heat exchanger to inside the unit. It becomes more difficult to extract heat as the temp drops and when it gets to some place above freezing it just gives up.
So turning on a heat pump in near freezing conditions will just chew electricity.
You will not get 20deg temp difference across the aircon as you would in cool mode.
Put it another way
When it's hot outside and you are in cool mode the unit is extracting heat from a hot motorhome and transferring thru the heat exchanger to the outside. There is generally a lot of heat available. In reverse there is very little heat available at 40f
There is no such thing as cold just a lack of heat
Would someone like to correct my analogy

[Denali]

Originally Posted by SteveSkinner

Not quite true
As above no air actually travels thru the unit but

The heat pump attempts to draw heat from the air outside and transfer it thru the heat exchanger to inside the unit. It becomes more difficult to extract heat as the temp drops and when it gets to some place above freezing it just gives up.
So turning on a heat pump in near freezing conditions will just chew electricity.
You will not get 20deg temp difference across the aircon as you would in cool mode....

That why I said "The colder it is outside, the less warm the output of the heat pump".