Why oversize the indoor coil?

[junkhound]

If you plot out the T-s and pressure on a Mollier diagram for a given compressor flow rate, you will see that a LARGER evap and condensor will almost always increase COP. Only exception is if the pressure drop thru longer tubing lengths compormises efficiency.

My own custom system has a 7-1/2T condensor on a 4T scroll compressor and custom tube in tube evaporator (GSHP) and has a COP of 5.6. Ends up that the bottom 2 coils of the condensor fuchtion as a receiver.

[bwalley]

Maybe, but he has a HP and and that may not be true, I have "always" heard on a HP system, the coil and condenser should match tonnage wise.

These Tax credit systems are sometimes really strange. We just installed a Ruud system, 3ton that required the 100,000 btu furnace with the Ruud coil, and a 75,000 furnace with the Aspen coil, go figure. In Ohio this might be a good thing, but in Texas, I am sure the furnace will short cycle.

It is not uncommon to have a larger coil for a heat pump, the reason being it gives more surface area to transfer heat and on the cooloing side it gives more surface area for better dehumidification.

I installed a Goodman unit for a friend of mine, he it was a 3.5 ton system, the same air handler was used on 3.5 to 5 ton units, the only difference was the expansion valve, it was a variable speed unit.

He bought it from a distributor that would sell to anyone, it was pretty cheap that is why he bought a Goodman, other than the goofy beer coozy for the TXV it wasn't a bad unit.

I am an American Standard dealer and their air handlers match up to varying sized condensing units and the TXV is the only difference.

[Delta-T]

Mr Bill, What are some pros and cons when you have high static pressure, that you've seen over the years you've worked in the field?

[Mr Bill]

Mr Bill, What are some pros and cons when you have high static pressure, that you've seen over the years you've worked in the field?

Increased static, could be a problem, if the indoor fan is not capable of handling the increased static pressure, usually results in decreased airflow across the indoor coil, and could lead to excessive wind noise from the registers. Reduction of the airflow usually, in turn, leads to lower temperature of the indoor coil, because the amount of air moving across it, and is no longer able to maintain the proper temperature. Now this is just what I have personally witnessed, there may be others with differing opinions.

[carls]

Go to the ari.org site and see what coil matches what outside unit and what the rated cfm to achieve proper efficiency is. Coils seem to be numbered to match the lowest efficiency outside unit. Increase outside coil for efficiency and the refrigerant needs larger inside.

[beenthere]

I guess I am not being clear about my "only grief" with these new high seer systems.

beenthere: the furnace has a 5-ton drive, to me that's saying 2000 cfm's, it has the X13 ECM motor. The X13 has the electrical efficiency of the ECM, but has fixed speeds like a PSC.

Ok that said here's been my point all along, how many service companies "after" the installation set the motor speeds, via "dip switches or whatever" for best sensible and latent removal?

And if they do try and set the speeds in good faith, has this done anything to the beginning efficiency?
Ok example, we have a service co. that has now set the speeds to perform the best, and in my opinion, there still guessing "somewhat" so now we have a 5-ton drive 2000 cfm furnace running at 1580 cfm's. Everything is cool and there gone, so why did I just buy a 5-ton drive 2000 cfm furnace, when a 4-ton drive 1600 cfm would have been fine, is anyone following me?

Someone said is the motor variable, well even if it was, in hot and humid Houston Texas, in the middle of the summer, when it's 100* out with 95% humidity, is it going to be running in high cfm or low? in high were kicking some sensible but no latent, in low were kicking some latent but less sensible, to me, if your losing anyone of the two, your loosing capacity. I am not condemning the new higher seer stuff, I just think the Government view of the tax credit systems are a little off.

If its a VS. You would set the blower to start with a slow ramp up, so that in the beginning of the cooling cycle it removes more humidity.. And then increases speed to remove more sensible.

Areas with both high sensible and latent loads. Benefit from a thermostat that can slow down the blower if need be to remove more moisture, and then allow it to speed back up o handle the sensible load.

The 5 ton drives X 13 motor running at a lower RPM to move 1600 CFM will use less electric then a 4 ton drives X 13 at a higher RPM to move 1600 CFM.