temperature rise

[David Nasser]

New to the business. On my inspection report it has a field
refering to temperature rise. How should this be inspected ?
And is it necessary ?

David
Pyramid-Home-Inspections

[JLilly]

Temperature rise is the air vent temperature minus the return air temperature. It is often called Delta T or (change in temperature). Most all furnace data plates tell you what the max acceptable temperature is. If you results are higher than that you may have a dirty filter, undersized duct or some other deficiency. 30 to 50 degrees is typical. Depending or the type and age of furnace this can vary.

[David A. Andersen]

And is it necessary ?

contrary to popular belief: no!

In accordance with the is standard operating procedures of home inspection engineering evaluation is not required. Attempting to do psychometric evaluation of an air-conditioning unit through the supply and return is like trying to determine the horse power of your truck using exhaust pipe temperature.

Can it be done? Yes!

With the use of the psychometric chart and several thousand dollars worth of test equipment and permission from the homeowner to do intrusive boring through the duct system to insert test equipment, evaluation can be made.

If you wish to get caught up in the supply/return air temperature rise/fall, your opening yourself up to lawsuit and paying frivolous repair bills which your inspection report indicated was necessary(but was not, really). There are about 1 in 100 HVAC contractors that can accurately do these evaluations and it is outside the scope of home inspection.

HVAC equipment is to be evaluated by operating "normal operator controls".
If you have an EPA certification card in your back pocket, you can put on a set of refrigeration gauges which will tell you a little bit more about the equipment operation. The readings on this gauges (like taking temperature readings of the air duct system) do not tell you whether the system is empty or full. Refrigeration pressures will change hourly and with indoor and outdoor changes in property of air. They give you a closer proximity of whether the system is operating than using a thermometer however.


In the standards of practice of home inspection you are required to report if the equipment operates, not evaluate its performance.

[Bruce A. King]

Good Post David, I agree

For those who still don't get it, here is a situation that is normal:
Closed up vacant house, 90 deg plus outside, 80 deg inside, hvac sized barely adequate but adequate under normal conditions ie. occupied house with thermostat kept in typical ranges.
Temp. diff will indicate a problem!
Now if you have time to let that house cool down to at least 75 the diff. will likely read ok.

[David A. Andersen]

One of the contributing factors that we have not gone into too much is the difference between latent and sensible heat. This is beginning to get into the psychrometric thing. As Bruce has indicated, when we take measurements has a whole different bearing on the result.

Sensible heat; is the heat (btu/lb.) in the air that can be measured with a plain old (dry bulb thermometer). This is the temperature that your test equipment and the thermostat measures. It is only a small fraction of the "total heat" in the air. sensible heat is the heat that causes a change in temperature in a substance.

Latent heat; is the heat (btu's /lb) in the air they can only be measured with a combination wet bulb and dry bulb thermometer. This is the heat that brings about a change of state (liquid to vapor) with no change in temperature.

When we turn on the air-conditioner for the first time during our inspection, and the house is not at set point, it's hot in the attic, it's hot outside etc. Your total heat is much higher than what it appears at the thermostat thermometer. In order for the dry bulb temperature to lower at the thermostat or on your test equipment the latent heat must be removed first. This can take a considerable period of time, depending on the air supply system and proper sizing of the HVAC unit (not too big).

It might be easier to understand this if you consider boiling water.
Can water be at 212°F and not boil (and I'm not playing with altitude here)? Yes it can.
How much heat (btu's) does it take to make water at 212°F boil? 970 btu's per pound water. This is the amount of heat required to change water from a liquid to a vapor state.

Air-conditioning is the reverse process of taking water vapor and condensing it back to a liquid. Water vapor = humidity = heat. Before you can lower the sensible temperature at the thermostat you must remove a proportionate amount of water vapor from the air. At 7760 btu's per gallon of water, that can take a substantial amount of time.

Dew point temperature; the temperature at which water condenses from a vapor to a liquid. This is proportionate to the relative humidity of the air (the amount of moisture one pound of air can hold at a certain temperature). The apparatus dew point temperature of an air conditioner is always below the dew point temperature of the air if operating properly. Therefore, the sensible temperature of the air cannot fall until the corresponding amount of latent heat is removed.

Without the use of a psychrometer, used to take air measurements inside the air duct within a few inches of the air coil on the supply and return, and plotting the results on a psychrometric chart, you can not know what work is being performed by the HVAC system.