Quote:
Originally Posted by AC_Hacker
This is very curious...
If your logic is correct, you have just defeated heat pump heating, and all other low temperature heating strategies.
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?
If you wanted to actually enjoy the efficiency gain of a larger condenser, you would need to increase the volume of air through it, to get those BTUs into the room.
Or, if radiant floor was your heating method, you would need to increase floor efficiency through lower R-value materials above the heated part of the floor structure, and/or increased pipe density and/or increased water flow rates.
-AC
-AC
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With a heat pump the temperature rise over the coil is not as great as with a fossil fuel heater so you have to be careful about the airflow. Too fast and you don't absorb the heat and you throw cold air from the ducts and too slow and you saturate the air and needlessly wait around. The bigger surface area allows you to transfer more heat, faster.
Now, at the registers you have to find the proper balance. Too fast and you get the cold draft feel but too slow and you don't heat the room properly. Around 600 FPM is idea, give or take 50-100fpm. This gives you enough mass to get the heat into the room but it's slow enough that in the occupied zone you won't feel a draft. This is a function of the duct sizing and also the way it hooks to the register will affect how the air is distributed.
The ideal system is one that you can't tell if it's on or off.
Now with oil or gas heat you have plenty of temperature rise over the heat exchanger 40-60°f in most cases vs as low as 15°f with a heat pump. 60 ° return air + 60° temperature rise = 120° out of the heat exchanger that will be 110°+ out of the vents which will be warm enough that it will feel like warm air hitting you. Air velocities don't matter and most people will like a higher velocity that they will feel hitting them.
I haven't designed a ton of systems but I've done them both ways and in the end the house is comfortable. Coming from a forced air gas heater I'm used to feeling the blast of warm air coming out and it takes some getting used to when I'm in one of my rentals with a heat pump. The house is warm but you don't feel the airflow.
I believe there is some benefit to a slightly larger evap coil but I don't see any benefit to grossly over sizing them. I also believe the best benefit is for the air conditioning side in humid environments. The units I install that have fixed orifices typically have sizing for a 1/2 ton larger A-coil and also include the orifice for it so you get the proper superheat out of them. They typically do not have sizing listed for anything past 1/2 larger - i.e. they have the direct match and a 1/2 ton larger A-coil.
I typically install 2-3.5 ton systems and with conventional split systems the range is 1.5 tons to 5 tons, I can't speak for mini-split sizing.