Thread: Heating Good or Bad ?

Inspecting a Philco 2.5 ton 8 year old heat pump today, the outside temp. was 50 degrees. I usually check the temperature differential between the return and supply when testing the unit. I need to get a 20 degree differential temperature to call it acceptable. My Question is what does everyone else do ? This unit failed the test, I only got a 10 degree differential.

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Originally Posted by Sam Morris

Inspecting a Philco 2.5 ton 8 year old heat pump today, the outside temp. was 50 degrees. I usually check the temperature differential between the return and supply when testing the unit. I need to get a 20 degree differential temperature to call it acceptable. My Question is what does everyone else do ? This unit failed the test, I only got a 10 degree differential.

With a TD of 20 deg, most are satisfied the system is working properly. But just because you have a TD of 20 deg. that by it self does not mean the system is working properly. As does a TD of 10 deg does not mean the system is not working properly.
Way too many factors to consider; Air flow, Humidity, condition of ducts, and more.
That said, 10 deg TD would cause me to question it's condition also.
If you have doubts about the HVAC, it's better to call for it to be serviced and checked by an HVAC contractor.

Did you consider the 5 degree loss from one supply to the other?

Was it a heat pump and was it in defrost mode when you made your heat measurements?

An air conditioner which has a 20 degree TD may have considerably less TD in heat if a heat pump and a cold day.

As Rick said - too many other things to consider to really put any faith at all in TD measurements taken at the return and supply ... that is one of those old HI myths taught by HI schools and passed on from inspector to inspector.

An example of how using TD only can mislead you.

In your own home, open all vents and take a temperature reading at one vent. then take a reading at the return.
Now, close all other vents and take a reading at the same vent again.
Likely you got two very different readings, yet they were taken only minutes apart.

Whenever possible, I try to take my temperature readings as close to the air handler as possible. Ideally, there are already holes in the supply plenum and return trunk next to the air handler where I can insert a temperature probe. But if there are no viable areas or holes to allow this next to the air handler, I will take the temperatures at supply and return vents in the living areas. I look for a differential of 18-30 degrees in heat mode.

Using an iR thermometer is taking the temp of the metal not the air.
Also if you compare an iR reading from the vent it will differ from a pocket stem thermometer which if in the air stream will likely read higher then the iR reading.

At my home I have geothermal heat pump. The air temp at supply register is 100F using a thermometer, and with the ir thermometer a reading of 82F - iR pointing the laser beam onto the metal supply register.

Return air grate metal temp w/ iR thermometer 68F, thermometer 80F air stream temp.

Three common misconceptions about infrared thermometers | ThermoWorks

You can get very different readings from different vents. I try to check the temperature where the airflow seems good. I also check at several vents and use the highest temperature difference when checking delta T.

That being said, if 10 degrees is the highest reading I would be concerned, especially at 50 degrees outside temp. You should be getting probably closer to 25 degrees at that ambient.

So what am I missing here, other than the temp differential wasn't met or didn't meet expectations? How many register were checked? What was the distance between the two temps shown?

The outside temp is estimated at 50 deg. The inside temp measured at one register is 84 degs and 74 degs at the return. That means the inside ambient temp is somewhere in between, probably closer to the lower temp. I doubt a 2.5 ton unit is going to heat the inside ambient temp to much more than mid. 70s with those outside temps even running all day. Seems to me the system is operating within its own capabilities irrespective of temp differential.

I'm sure 'real feel' inside temp, at mid 70s, when coming in from the 50s cold feels nice and toasty. So what bearing does the lack of significant temp differential really mean under these circumstances? I would be more interested to find out if the system shut off, via tstat, once called for temp was reached and recycled back on when temperature dropped to lower setting.

General
With a TD of 10 deg, I would expect to find one or more of the following.
Undersized unit for the duct work installed. For example: Unit was replaced. Original unit was 3.5 ton. (To save money, a small unit was installed)
Severely leaking ducts, maybe a duct has come off a vent boot.
Poor air flow at compressor. ( Dirty coil, shrubs, fan blades, fan motor)
Low refrigerant
incorrect orifice, trash in orifice, or defective metering valve
Compressor failure

Some of these are beyond what a HI can determine, that's why I said call for an HVAC contractor.

If you are used to the old fashioned furnaces, that kick on like a hurricane and blow out 100 plus degree temps, then a heat pump will feel very different. Geothermal heating systems can be especially hard to demonstrate more than minimal temp differences, but they do work.

One other thing about air source heat pumps, is that they perform much better if the return ducting is adequately sized and placed in the right locations. Good air circulation seems more important for heat pumps than for standard forced air furnaces, so poor duct sizing can cripple a system.

And, I echo the previous comment about being fooled by an IR thermometer, because they do measure the metal register grating, not the actual air temperature. So if you just turned the heat on, the metal register cover may not be warmed up yet. And it makes a difference if you are pointing it at reflective light colored metal or dark wood covers.

Most heat pump systems in cold climates also have a backup heat source like an electric heating strip. Then the problem I have in Montana is demonstrating whether the heat pump (compressor) is producing the heat, or is it the auxiliary heat source (again, this is especially true with geothermal units).

Originally Posted by Sam Morris

Inspecting a Philco 2.5 ton 8 year old heat pump today, the outside temp. was 50 degrees. I usually check the temperature differential between the return and supply when testing the unit. I need to get a 20 degree differential temperature to call it acceptable. My Question is what does everyone else do ? This unit failed the test, I only got a 10 degree differential.

I would have NOT failed the heat pump with the testing method employed. The others have pointed out the deficiency's with this type of testing. What if the compressor was in total failure mode and the unit was running solely on the strip resistance and you measured 20+ degree difference ... A better quick measure is at the condenser that takes most variables out of the equation.

Originally Posted by Sam Morris

Inspecting a Philco 2.5 ton 8 year old heat pump today, the outside temp. was 50 degrees. I usually check the temperature differential between the return and supply when testing the unit. I need to get a 20 degree differential temperature to call it acceptable. My Question is what does everyone else do ? This unit failed the test, I only got a 10 degree differential.

I would have NOT failed the heat pump with the testing method employed. The others have pointed out the deficiency's with this type of testing. What if the compressor was in total failure mode and the unit was running solely on the strip resistance and you measured 20+ degree difference ... A better quick measure is at the condenser that takes most variables out of the equation.

It's funny how you use some number and then realize you have never tried to verify how appropriate it is. I remember from H.I. school that an AC should exhibit an 12 - 18 degree differential when the house had reached stability to be considered satisfactory. The reasoning, as I recall, was that less than 12 degrees might mean the system was oversized for the house and more than 18 degrees might indicate it was undersized. That was taken as a matter of faith and never verified.

Simularly, another number I have in my head is that a properly operating and sized hydronic system will have about a 20 degree difference between the supply side and the return side when the system is stable at the desired operating temperature. Again, that was taken as a matter of faith and never verified.

Anybody know of an "official" and supportable source for these numbers?

Temperature differential may or may not be indicative of failure in some other aspect of the system. Using this anomaly alone to 'fail' the system is mis-stating the need for further evaluation. There are many variables which can easily lead to close temp readings not all of them indicate failure....inadequate, maybe but that too can be subjective. For example, if the client is satisfied with an undersized unit which controls airflow to a reasonable degree and saves $$$, in the short term, how is that a failure?

The numbers alone maybe symptomatic of some other issue(s). Those issues, if determined as defective and leading to the reading differential, should be reported and discussed with the client. I rarely find any heating or cooling system to have been serviced regularly and invariably suggest servicing with a view to addressing any concerns.

Originally Posted by Marshall Brown

It's funny how you use some number and then realize you have never tried to verify how appropriate it is. I remember from H.I. school that an AC should exhibit an 12 - 18 degree differential when the house had reached stability to be considered satisfactory. The reasoning, as I recall, was that less than 12 degrees might mean the system was oversized for the house and more than 18 degrees might indicate it was undersized. That was taken as a matter of faith and never verified.

Simularly, another number I have in my head is that a properly operating and sized hydronic system will have about a 20 degree difference between the supply side and the return side when the system is stable at the desired operating temperature. Again, that was taken as a matter of faith and never verified.

Anybody know of an "official" and supportable source for these numbers?

The only "official and supportable" comes from measuring pressures and temperatures, outside the scope of a normal HI. What we do is like Mom used to do; feel the forehead, look at the tongue, and if she thought you looked a little green take you to the Doctor.