Posted By Pokerface:
Since the balance point is the temperature outside at which the unit cannot maintain the set temperature without using the auxiliary heat, lowering the outside temp will raise the balance point. Right?
No. The balance point is a single point. We know that capacity of the heat pump to provide heating DECREASES as the outside temp goes down (less heat, or enthalpy, to pull from the air. So, to determine balance point you plot the structure heat loss over temperature. This line will slope down and to the right indicating that the higher the outside temperature the less heating you will need to maintain space temp.
On the same graph, plot the heat capacity of the HP. This will be a line going from left to right and sloping up. This show that as outside temp increases, HP capacity increase. There will be only one temp where these lines intersect; that is the Thermal Balance Point. Temps above this point and the heat pump has enough capacity to satify the load. Below this point, aux heating is required. To do this correctly a Man J needs to be done and you have to know the HP ratings.
BTW - Once you have this data, you can size the aux heating properly. When using electrical heat, you don't want the strips to come on until, theoretically, the balance point is reached. Remember a HP COP is always greater than 1 (heat strip is 1) and WILL always be more efficient than strip heating. In reality you would set the heat strips to come on arounf 2-3 degrees BEFORE the thermal balance is reached.
Posted By CT2:
have to vote for "D" because I havnt learned about calculating the ratio yet and I dont understand how lowering the oat would raise the compression ratio
Just think about your freezer. It is pulling heat out of the freezer air to maintain a temp below 32 degrees and has to reject the heat into a house at 70 degrees.
comp ratio = head pressure in PSIA/low side pressure in PSIA.
It takes more compression ratio to get a lower pressure. A compressor has to be designed for the temp rating that you want to use it for. Be it low, medium, or high temp rating (comfort cooling). The lower the temperature you need to achieve, the higher the compression ratio needs to be.
This is easy to prove. Run the compression math for what a comfort cooling compressor has and a freezer has. HP is like the freezer when in low temp heating.
Shophound explained it exactly while I was typing this. So I am being redundant.
[Edited by on call on 08-09-2005 at 07:48 AM]