Re: Low delta-T
I don't have much experience with geothermal or ground loop heat pump systems, so I'm just offering my opinion. Which is if the geothermal heat pump is sized properly for the structure's heat gain and loss load, it'll do fine. Geothermal's main advantage is a more consistent heat exchange medium (the ground) than air, which varies with ambient temperature. Ground temperatures do swing but not nearly as wide or at the same rate as air, obviously.
Originally posted by perpetual_student
Can anyone document the idea that geothermal equipment removes humidity as well as conventional AC equipment? The thing I suspect is that geo equipment in particular, will use a less-cold coil (i.e. "lower Delta-T") in AC mode than will for example, a Carrier Infinity system. Using a lower Delta-T is one way to get higher EER and SEER -- because those measurements pay no respect to humidity removal. Inferior humidity control is one of the things that I would always suspect of an extra-higher EER number.
If indeed any system uses lower Delta-T to get that last point or two of EER, they have artificially scored a big advertising number at the expense of compromising humidity removal. In some climates that is OK, in a hot-humid climate that is a very bad trade-off.
Regards -- P.Student
[Edited by perpetual_student on 03-16-2005 at 06:38 PM]
So, on a hot, humid day, when both sensible and latent heat loads are high, the ground source unit could, let's say, be set to maintain a steady condensing temperature of 110 degrees, whereas an air coold unit might be experiencing condensing temperatures on the same day somewhere closer to 130 degrees. Effectively, the geothermal unit will have more latent and sensible heat removal capacity under the same conditions than the air cooled unit.
The main drawback, IMO, of geothermal loops is the added cost of the loops and the additional maintenance factor of the loop if water is used. Of course, air cooled units also must have their outdoor coils cleaned regularly to maintain performance, but this is likely an easier maintenance item than a drain, flush, and fill of a loop.
If the geothermal OEM is trying to maintain a lower compression ratio by keeping the evap a tad warmer - hence a "lower Delta T" - (combined with reduced head pressure), then he's likely done the math to have the unit BTU rated at that evap temp to keep the building properly cooled and dehumidified.
Even a builder model 10 SEER piston flowrater system will dehumidify adequately if it and the ductwork are sized and installed properly for the expected sensible and latent heat loads of the building. It just won't be as efficient or precise. The variable speed, TXV equipped, modulating systems do the same thing with more finesse and less energy.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.