EddieWalker said:
Pat,
I've always heard that it's better to put a smaller sized HVAC unit than commonly used. A simplified example is for a 2,000 square foot house, you'd usually use a 4 tonne unit. But that would mean it's cycling on and off all the time to keep the tempature at the desired setting. A friend told me that for a home 2,000 sq ft home, a 1.5 tonne unit would keep the house just as cool, but run longer and be more efficient.
He knows allot more than I do about these things, but he seems to be in the minority. If I understand what you just wrote, you feel the same way. Is this true in your experience?
Eddie
Smaller than is commonly used is often good because commonly units are seriously oversized. I can't comment on replacing a "typical" 4 ton with a 1.5 ton in a given size house because there are too many vairables from one 2000 sq ft house to the next.
The real problem is that few sellers and installers of HVAC really know how to do the engineering and at best typically let the MFG or wholsaler tell them what size to use if they go beyond their own sphere of knowlege. Same market forces at work, bigger is more $ profit.
Nothing wrong with experience on the part of the "designer" to help modulate the results of his heat load calcs. You don't just say x # of sq ft gets y # of tons of cooling even in a specific climate zone. Infiltration and actual R-value (performance) of the building envelope is very important. Traditional stud walls do not yield overall R-values of the R-value of the F/G batts (or whatever insulation is used.) R-19 batts in the walls will give walls performing way poorer than R-19 due to heat being conducted through the studs plus whatever infiltration there is plus loads from fenestration.
The ICF portion of my house can be just about heated with a candle and cooled with an ice cube but the traditional stud walls (2x6 on 16 inch centers) of the rest of the hoiuse don't do as well.
So first thing we have seller/installers who make more if they sell a bigger unit and don't have angry customer callbacks. Of course the unknowlegeable customer has a unit that will short cycle itself to death prematurely and not humidify very well and never run long enough to get up on top of the efficiency curve but it will cool the house. Might feel cool AND clammy but it will get cool.
Good dehumidificatioin increases summer comfort and lets you have comfort at a higher thermometer reading (smaller delta T indoors to out) An undersized (or right sized) A/C unit is a terrific dehumidifier. It will have long efficient runtimes and will circulate and recirculate the air through the evaporator, dehumidifying on each pass, before it satisfies the thermostat and shuts off. With a variable or mulltispeed compressor and air handler you can run the A/C almost continuously when it is hot, getting good efficiency and dehumidification and the compressor and air handler only throttle up when needed. The compressor speeds up or goes to full speed when it can't satisfy the thermostat at a slower run speed and the air handler speeds up as required to avoid freezing up the evaporator (moving the air through the evaporator faster doesn't let it chill so much and reduces freeze up so these units often run slow till they start to freeze up and then run faster for a while to fix the problem and then throttle back again.
Houses traditionally got there fresh air from random infiltration due to sloppy building practices in the envelope. Better to make the house tight (pop your ears when sonmeone slams an outer door, OK mild exaggeration) and then use an engineered on purpose ventilation system to supply fresh air and exhaust the stale air. Today's modern materials have so much formaldahyde and such you need to get a good supply of fresh air. An ERV or HRV (Energy Recovery Ventilator or Heat Recovery Ventilator) is a great way to get fresh air and not run up the heating or cooling bill. Boitgh ventilator systems handle sensible heat but only the one handles latent heat. I'm on the border climatically where either is OK.
I get fresh air 24-7 for less than an amp and a very small impact on heating and cooling costs. Normally you can't hear the unit run but when the humidistat hits its set point it goes to high speed and then you can just barely hear it. I have a booster fan in the exhaust side for when the exhausted air from the shower is high humidity it kicks in till the humidity drops back down. It only runs when you take a long hot shower or there are two showers in a row and then for just a few min.
For interesting reading, look into heat pipes used as adjuncts to an evaporator in a freon/compressor type A/C application. Great increase in dehumidification for almost no increase in run costs (just a little more resistance to air flow through the evaporator when equipped wilth heat pipes.) It is almost like getting something for nothing.
Pat
