the main trigger to this change in equipment plans was a figure in the ashrae 2008 hvac systems and equipment manual:
it's quite apparent that control of interior humidity is essential to healthy indoor conditions. my simulations with the ornl wufi building envelope simulator about a year ago gave me familiarity with expected indoor humidity variations throughout the year in this local climate, in buildings with and without humidity control (and with or without a vapor barrier in the wall.)
i know from my readings that proper sizing of hvac equipment is stressed. oversizing is common, and this leads to the system turning on and shutting off repeatedly for very short periods. this wastes energy, puts wear on the mechanical components, and it leads to high humidity levels. because the system is usually sitting idle when it's oversized, the coil temperature warms closer to indoor temperature.
a properly sized system is usually on, which keeps the coil cold. interior air condenses on the coil and is carried away on the condensate drain. this process dehumidifies the air. from the figure shown earlier, we know there is a sweet spot for desired humidity. not too dry, not too wet. the system's ability to dehumidify depends on how long that coil is running cool.
when a system is sized, it's capacity is matched to an extremely high load condition, which is a figure dependent on geographical location, and is derived from statistical observation of local weather patterns. even a properly sized system will have a capacity greater than the given load on most days throughout the year. the system capacity is only met on those very hot or very cold days. this means on most days it's still not running optimally, but of course it's better than being drastically oversized.
many mini-split systems on the market today employ what's called an inverter compressor. this is a dc-powered compressor, and its input voltage can be variably adjusted (with an inverter) to control the rate at which the compressor runs. changes in compressor speed equate to changes in refrigerant flow between indoor and outdoor units, and ultimately to changes in rate of heat exchange for a given air speed over the coil.
this, in a sense, changes the capacity of the heat pump. the indoor coil and outdoor heat exchanger still have the same surface area exposed to local air, but the amount of heat available for exchange on those surfaces is different. this allows an intelligent controller to optimize conditions for dehumidification, and allows the system to be ramped down enough to keep it on constantly without the on/off/on/off cycling of traditional systems.
the powerful dehumidification abilities of variable compressor speed is what really attracts me to these minisplits. it doesn't hurt that they offer high EER/SEER performance for their cost, either. a few traditional split systems are on the market with variable compressor ability, but they are very expensive and hard to come by.
i scoured the internet and made a list of potential systems to purchase. most minisplits come with a wall mount indoor unit, as these are easy to install and mini-splits are usually selected for retrofit applications because of this. however, there are also indoor units that are like miniature indoor air handlers, with a blower and coil, and they are connected to ductwork. i plan to use these as it allows me to direct fresh conditioned air directly to everywhere i want it, and it presents in a traditional fashion to people (potential home buyers) as good old registers and diffusers.
i also looked for units that would closely match the load requirements of vinnie. i ran the house specs through a load sizing program, rhvac, and it gave me back precise load values for all parts of the house. total summer cooling load is 11,634 kbtu/h and total winter heating load is 16,566 kbtu/h. all system specs also list the heating capacity at low temperatures (17F) and the winter figure produced by rhvac is for 15F outdoor conditions. so i ensured the systems can meet this load at 15F. this requirement excluded a few otherwise excellent contenders..
the short list of favorites remaining:
lg lmu245hv + lmdn095hv (x2) - 10.8 EER, 8.1 HSPF, 10.8k min cool, 21.1k? low heat, $3100
fujitsu 24rmlq-c ; 24rml1 + aru12rml (x2) - 9.5 EER, 8.6 HSPF, 11k min cool, 16.6k low heat, $2500
sanyo 26uhw72r ; ch2672r + uh2672r - 9.1 EER, 9.7 HSPF, 9.5k min cool, 17.1k low heat, $3040
i couldn't retrieve explicit low heat capacity for the lg, so i may have to upgrade the two blower units from 9k to 12k units. prices are what i dug up with minimal effort through google. the fujitsu may find itself the winner. i listed minimum cooling capacities because the lower numbered systems may do a better job dehumidifying on mild but humid days. the high EER of the lg is attractive, as this could correspond to much lower bills.. almost 2 points higher than the sanyo.
however EER is cooling performance, whereas HSPF is heating performance. look at the sanyo HSPF, blows the other two away. i expect yearly heating costs to far exceed yearly cooling costs. past simulations with resfen have indicated about seven times as much energy required for heating than cooling, annually (3680kWh vs 554kWh). that's 88% of annual energy is heating. i suppose i didn't need to list the EER..
the sanyo is a $640 premium over the fujitsu. the 9.7 HSPF is 1.1 greater than 8.6, 1.1/8.6 = 12.8% better. 640 * 1/.128 = $5,000. so ~13% savings in heating would pay off the $640 cost after $5k had been spent on heating bills.. guestimate $60/mo average bill..$720/yr.. that's like 7 years to pay for itself. ah, might as well save the $640 now.. fujitsu..
decisions, decisions..
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