EcoRenovator - View Single Post - Evaporator sizing

jeff5may · 02-16-14, 02:07 PM

AC and SERVICETECH,

Yee haw! You're both right.

Because heat from an oil fired heating duct can be uncomfortably warm, but does that mean that a heat pump heated house with the same inside air temperature would feel less warm, therefore less satisfying than oil heat?

Most people who have had oil or gas blast furnaces in the past tend to expect that "shot" of hot air from the heater when it first starts up. This is one of the big reasons heat pumps took a long time to catch on, especially in Northern latitudes. As with the whole short cycling fallacy, they perceive a long cycle time of less "hot" air as inefficient and costing more in the end. We know this idea is backwards as a left hand thread, but you will never convince Joe Dirt.

More indoor coil surface area is great for heat pumps in heating mode. Lower coil bypass factor will mean lower head pressure with the same discharge air tempatures. Airflow does not need to be increased to make it happen, although higher airflow rates would lead to further reductions in head pressure at the expense of lower temp at the register.

Register temp 100-110f is warm enough to reduce the "cold draft effect" but cool enough for efficient operation.

The most important thing here is delaying the blower startup until the indoor coil has time to preheat. If the blower is not delayed, and starts up with the outdoor unit, the heat coming out of the registers will take time to develop. This goes against the previously mentioned fallacy. Joe Dirt will complain about it.

Oversizing the indoor coil will prolong this lag, more so if the airflow is increased. Even though the unit may gain 10-15 percent in COP, the perceived lack of "hotter" air at the registers may become a point of contention with the homeowner. The unit may cost much less to run every month, but other than that, the unit seems not to perform any better than before.

02-16-14, 02:07 PM	#34
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	AC and SERVICETECH, Yee haw! You're both right. Because heat from an oil fired heating duct can be uncomfortably warm, but does that mean that a heat pump heated house with the same inside air temperature would feel less warm, therefore less satisfying than oil heat? Most people who have had oil or gas blast furnaces in the past tend to expect that "shot" of hot air from the heater when it first starts up. This is one of the big reasons heat pumps took a long time to catch on, especially in Northern latitudes. As with the whole short cycling fallacy, they perceive a long cycle time of less "hot" air as inefficient and costing more in the end. We know this idea is backwards as a left hand thread, but you will never convince Joe Dirt. More indoor coil surface area is great for heat pumps in heating mode. Lower coil bypass factor will mean lower head pressure with the same discharge air tempatures. Airflow does not need to be increased to make it happen, although higher airflow rates would lead to further reductions in head pressure at the expense of lower temp at the register. Register temp 100-110f is warm enough to reduce the "cold draft effect" but cool enough for efficient operation. The most important thing here is delaying the blower startup until the indoor coil has time to preheat. If the blower is not delayed, and starts up with the outdoor unit, the heat coming out of the registers will take time to develop. This goes against the previously mentioned fallacy. Joe Dirt will complain about it. Oversizing the indoor coil will prolong this lag, more so if the airflow is increased. Even though the unit may gain 10-15 percent in COP, the perceived lack of "hotter" air at the registers may become a point of contention with the homeowner. The unit may cost much less to run every month, but other than that, the unit seems not to perform any better than before. Last edited by jeff5may; 02-16-14 at 02:09 PM..