"Delta T" , can someone please explain the finer points


Old 08-28-01, 06:01 PM
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never heard of Delta T until I stumbled upon this website and forum; I believe I understand but then again I don't think I do; why does a difference of 25-30 Delta T indicate an airflow problem, isn't airflow the same all the time? in my untrained mind it would seem the more difference (downward) in temperature between fan and a/c would be better because it is colder, no?

I live in west texas, 30 year old house and carier a/c and furnace is about same age, a/c cools but somewhat on the mediocre side; this house has very strong southern exposure and being a brick home with no overhang the afternoon temperature inside in summertime can be anywhere from 78-84
degrees, at times almost unconfortable; my a/c really does not give me any major problems, the big line is nice and cold and the smaller one is warm, except it seems that on a regular basis in spring and in the fall the unit will inexplicably freeze up; to fix it I turn off a/c and turn on fan for a few hours and that seems to fix it for the duration.

would like to get the temperature in the house colder though; after reading about Delta T I tried it, I placed thermostat inside my furnace supply duct, right above the cooling coils and measured temperature for fan is 67
and for a/c is 42; this was started with a house temperature of 75; do I have an air flow problem, if so, again can someone explain the finer points of this method?
in closing I should state that I have discovered a problem with my supply blower wheel, the bearings are very badly worn giving the shaft quite a bit of play; I am running the whole thing with wooden braces right now until parts arrive;

one last question please; when I moved into this house it had one return vent in the hallway at one end of the house and the other return vent in the garage which has two output registers which I however never use because I want all the cold air I can get inside the house and not in the garage; I closed off the return vent in the garage and in turn cut one on the opposite wall inside the house of equal size thinking that all the hot air from the garage going over the cooling coils wasn't helping matters any; was I right in this or should I utilize that old garage vent in addition to the new one in conjunction with the one in the hallway at the opposite end of the house to get even more air flow? this is a 2000 sq.ft. home and the which I am using now measure 48"x8" on the far side of the house and 30"x8" at the furnace, the one which I closed off at the garage side of the furnace measures probably 30"x12".

I know I got a little long winded here for my first post but thought I'd throw it all in one pot as I was going along; any help at all is greatly appreciated, thank you.
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Old 08-29-01, 03:39 PM
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wow, you were kind of long winded! ok, delta t...over 20 degrees can be evidence of restricted air flow, as the air spends more time, per say touching the coil and giving up heat. you may see a difference with the new bearing. other causes are dirty blower wheel, not enough return, dirty filter, dirty coil, etc. there is another possibility, however, and that is that you could be low on refrigerant. tep and pressure have a direct relationship, therefore if the pressure in the evap is low, so will the coil temp. this is why they freeze up. the humidity extracted will freeze the coil when the temp gets too low.
as for the ducting situation, it depends on the size of your unit. for each ton you need 400cfm, your tonnage can be determined by looking at the o/d unit model#. 24 is 2 ton, 30 is 2.5, 36 is 3 ton, etc. these numbers will almost always be found somewhere in the model#, except if it is a lennox, they are funky, find a "ductulator" on line or at the library, and this will determine if you have enough return air. if you are not conditioning the garage, and need more return, get it from in the house, do not reuse the existing garage return. talk about long winded! hope this helps, stay cool
Old 08-29-01, 05:48 PM
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thanks for informative reply; will put input to best use; new wheel & bearings arrived today, don't know about new shaft though, can't get one from local carrier dealer & have not yet inspected old one, as I said on pulley side everything is wobbly & shot to hell; one last question if you please, I looked at my model# and wasn't able to apply your information; can you help?; local supplier says it's a 4 or 5 ton unit but I'd like to know exactly; model#58GA1003-C, serial#E276487, it's a carier; thanks again.
Old 08-29-01, 06:11 PM
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sounds like the numbers off the inside unit, 100,000btu, possibly 3 ton drive....blower capacity. if your furnace is so old as to have the setup you describe, may want to think about replacement, the efficiency difference alone will pay for itself over less years than you think
Old 08-29-01, 06:31 PM
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yeah, I know what you are saying, when you get as old as me everything around you seems to get old too; probably will have to replace both units pretty soon; am thinking of carrier again or one of their subs (duco or dunco, I believe; this house is just shy of 2000 sq.ft. am thinking of 4 ton for both or should I go 5 ton considering the weather here in west texas (months on end 99-108 with no rain? any advice as to size and brand? as with everyone else, cost is a big, big factor; thanks again.
Old 08-30-01, 03:01 AM
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New shaft

If shaft and pulleys are wasted,call HVAC supply houses.Most in my area carry shaft stock and replacement pulleys.PDF

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