EcoRenovator - View Single Post

jeff5may · 02-16-17, 12:12 PM

Slippy,

Ok, here comes a bunch of hodgepodge quick answers to get you started in the right direction:

1. Cooler supply air and longer run times beat warmer supply air and short cycle times. Heat pumps take some time to "ramp up" at the start of each cycle. This is opposite from gas burners, which preheat pretty quickly.

2. In cooling mode, air handler hx freezing is caused by load mismatch. Usually it is a shortage of airflow, but if the evaporator is not sized correctly, the same thing happens. If a cap tube system is low on charge, the air handler freezing is usually the first thing that casual users notice going wrong.

3 and 4. A TXV should be matched to the capacity of the compressor. The sensing bulb should be attached to the suction line close to where it leaves the heat exchanger. Load matching needs to be done on the water side of the hx to prevent freeze up, and many systems employ thermometers and/or flow sensing to detect possible freezing conditions.

5. An unloaded a/c compressor draws less current than when it is fully loaded, but not THAT much less. Most constant-speed scroll and rotary hermetics use about 1/2 to 2/3 of full power when they are unloaded. They are designed for maximum performance within a fairly narrow range of operation. Due to their constant displacement, BTU throughput (mass flow) varies directly with suction pressure and inversely with compression ratio. That being said, even at very low compression ratios, the compressor can only move a certain amount of gas with every revolution. Performance doesn't magically skyrocket when the delta P decreases.

Another thing to consider here is that heat pump systems are not particularly well suited for temperature setbacks. Where a gas burner blasts out many times its "normal" load while gas is burning and then "coasts" for a while before the blower shuts off, a heat pump moves only its rated load (more or less) when the compressor is running. When the thermostat is set back, both types act the same: they don't run. When set "forward", the gas unit quickly heats up the space, where the heat pump recovers much more slowly. This can cause big time comfort problems.

02-16-17, 12:12 PM	#1929
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	Slippy, Ok, here comes a bunch of hodgepodge quick answers to get you started in the right direction: 1. Cooler supply air and longer run times beat warmer supply air and short cycle times. Heat pumps take some time to "ramp up" at the start of each cycle. This is opposite from gas burners, which preheat pretty quickly. 2. In cooling mode, air handler hx freezing is caused by load mismatch. Usually it is a shortage of airflow, but if the evaporator is not sized correctly, the same thing happens. If a cap tube system is low on charge, the air handler freezing is usually the first thing that casual users notice going wrong. 3 and 4. A TXV should be matched to the capacity of the compressor. The sensing bulb should be attached to the suction line close to where it leaves the heat exchanger. Load matching needs to be done on the water side of the hx to prevent freeze up, and many systems employ thermometers and/or flow sensing to detect possible freezing conditions. 5. An unloaded a/c compressor draws less current than when it is fully loaded, but not THAT much less. Most constant-speed scroll and rotary hermetics use about 1/2 to 2/3 of full power when they are unloaded. They are designed for maximum performance within a fairly narrow range of operation. Due to their constant displacement, BTU throughput (mass flow) varies directly with suction pressure and inversely with compression ratio. That being said, even at very low compression ratios, the compressor can only move a certain amount of gas with every revolution. Performance doesn't magically skyrocket when the delta P decreases. Another thing to consider here is that heat pump systems are not particularly well suited for temperature setbacks. Where a gas burner blasts out many times its "normal" load while gas is burning and then "coasts" for a while before the blower shuts off, a heat pump moves only its rated load (more or less) when the compressor is running. When the thermostat is set back, both types act the same: they don't run. When set "forward", the gas unit quickly heats up the space, where the heat pump recovers much more slowly. This can cause big time comfort problems.