Manual J

[help3]

I had another company do a Manual J(the newest version).
I have a 2.5 ton HP. The Manual J was 45,000 heating and 40,000 cooling. This included 5,000 btu loss for heating in the ductwork and 2,500 loss for cooling. This had duct sizes on it and they were different sizes depending on the room, I have all flex the same size.

What can be done to fix this problem?

[pstu]

Please allow me to quiz you some more (I am a homeowner and won't give advice except for maybe tentative guesses). Is this an installed system with some history behind it? If so, does it do a fair job of heating and cooling? Does it fail to heat enough or cool enough? Can you detect cycling on and off during either heating or cooling season?

It would be very helpful to know the breakdown into sensible and latent load (humidity removal) for cooling, from your Manual J calcs. If you don't pay attention to that when selecting equipment, you could go wrong.


Here is my observation: that 2.5 ton heat pump is approximately 30,000 BTUH. Unless I am just making an arithmetic goof, that is nowhere close to your heating or cooling load. As to the duct sizing, I am betting one of the experts would say dampers might reduce flow to those rooms with a too-large duct (a too-small duct simply must be replaced with a larger one I believe).

Best of luck -- P.Student

[Edited by perpetual_student on 02-28-2005 at 07:25 AM]

[Shophound]

Originally posted by help3
What can be done to fix this problem?

What exactly is the problem? Inadequate heating and cooling? Poor airflow? Both?

More info, please...

[help3]

Both, its cold and limited air flow from registers. What is sensible and latent to the Manual J and how are they different?

[pstu]

Sensible heat is the obvious cooling air in degrees. That would be all you need if there were no humidity in the air. LATENT heat is the energy required to pull humidity out. My Manual J run gave separate BTUH values for latent and humidity, if I ignored the two types I would run the risk of picking an AC which is good at temperature change but poor at humidity removal.

Humidity removal has large benefits, your house at low humidity will feel comfortable at a higher thermometer setting. In extreme cases of high humidity, there can be problems with condensation and mold growth in the house. According to a dehumidifier maker (Aprilaire), holding humidity below about 50% will inhibit dust mites in the house, otherwise they might grow and produce allergies.

In hot-humid climates such as mine in S. Texas, this is a source of some problems and discomfort. Dry climates would tend not to have this problem. Cold climates are dry.

That's a first cut at explaining why you want to care about latent load.

Hope this helps -- P.Student