HVAC woes

[NW-Bound]

I agree with what's stated above, but without an emissivity correction on the IR gun, a quality thermocouple will still be more accurate for the purposes discussed in this thread. At the very least, an operator of an IR gun should be aware of it's limitations and how it works. I'll gladly wait 5 to 10 seconds for an accurate reading from a thermocouple over an instantaneous, potentially inaccurate reading from an IR gun that hasn't had emissivity dialed in. Looking at the IR gun shown in the picture, it appears that one doesn't have the means to adjust for emissivity.

Most IR thermometers are set for emissivity of 0.95-0.97. This covers much of the surfaces that the user encounters in real life.

The reading would be way off for shiny metallic surfaces which have much lower emissivity. Even if the user can adjust the emissivity, I wonder how he knows what value he should set it to.

And IR guns have another limitation. The way an IR works, it measures the temperature difference between the object in sight and itself. It has a cold-junction temperature compensation, just like a thermocouple does.

I have found that if I take my IR gun outdoors to take measurements, as heat is soaking into its body, the readings start to get off. This could be because of the thermal gradient within the gun itself.

If I let the temperature of the IR gun equalize with the outdoor ambient perhaps the reading will be good again, but I have not tried this.

Still, the IR gun is what AC servicemen use. And it is getting accurate enough to be used in routine medical applications.

 

[freedomatlast]

Even if the user can adjust the emissivity, I wonder how he knows what value he should set it to.

Interestingly enough, unless the user can refer to an established table, the way a user normally sets emissivity is to measure the surface temperature (or one with similar color and gloss) with a thermocouple and adjust the IR thermometer accordingly aiming it at the exact same place the thermocouple was placed.

 

[CRLLS]

There are pluses and minuses to thermocouples vs IR. I use the point and shoot IR device more oft than not. If I have shiny metal to measure, a piece of tape, like a masking tape applied will generally get close to e=95. We aren't talking about rocket science. When I was working, a FLIR camera was used almost exclusively for engineering work on various components on PC boards to ensure nothing was outside of its allowed temperatures. It was good enough for UL.

 

[freedomatlast]

Still, the IR gun is what AC servicemen use. And it is getting accurate enough to be used in routine medical applications.

Great discussion!
They are great for medical applications because the surfaces being measured are relatively consistent (ie: foreheads, etc)

HVAC service guys may use them for some applications but certainly not, for example, critical applications such as measuring the temperature of the suction line at the service fittings for the purpose of determining subcooling when charging R410A.

 

[NW-Bound]

Interestingly enough, unless the user can refer to an established table, the way a user normally sets emissivity is to measure the surface temperature (or one with similar color and gloss) with a thermocouple and adjust the IR thermometer accordingly aiming it at the exact same place the thermocouple was placed.

Come to think of it, I can see that an IR gun error will be somewhat proportional to the difference in temperature between the test object and the gun body.

This means that when measuring the temperature of an object at say 1000F, it is not going to be as accurate as a thermocouple. This is due mostly to the unknown emissivity.

For temperatures not too extreme, meaning within 100F from room temperature, an IR gun should be quite good, with the exception of shiny metallic objects of course.

 

[NW-Bound]

Back on HVAC woes, an air duct leak can cause problems. Here's an easy way to check.

With the AC running, open just one door, or one window a crack. Try to see if air is getting sucked into the house or is being blown to the outside.

The system airduct should be airtight. If you have a leak at the intake duct, hot air in the attic will be sucked in and mixed with the indoor air coming from the AC filter. The extra air is then blown out to the registers. Where does this extra air go? It causes the indoor air pressure to rise above the outside pressure. When you open a window a crack, you can feel this air being blown to the outside.

On the other hand, if you have a leak at the discharge duct from the AC, not all the air taken from the inlet filter is returned to the house. The cold air that is blown out to the attic has to be replaced somehow. What happens is that the pressure inside the house drops relative to the outside. When you open the window a crack, outside hot air is sucked in to replace the air vented to the attic.

You can check this out in 15 seconds. When the leak is bad, you don't even need to use smoke from a cigarette or an incense stick to see if there's an airflow. A slightly ajar door will be pushed close or sucked open.

In short, if you have a leak at the intake duct, you gain hot air in the attic, and lose cool air through door and window gaps.

If you have a leak at the outlet duct, you lose cool air to the attic, and gain hot air through door and window gaps.

 

[travelover]

They all run the program here, AFAIK. They can even do it on the tablet computer that they carry with them. Also, people here say that New Orleans is the country's biggest small town... because everybody here knows everybody and reputations matter. Frank's family has been here forever, so he knows everybody and found out who to get for HVAC work. We are very happy with the HVAC systems they installed for us.

If they actually run a Manual J, I am very impressed. The HVAC sales people I've run into had a very dumbed down program that generally just recommended replacing the AC with one the same as the old one or a little larger. I ran my own Manual J and had a smaller AC installed than was recommended. It worked out great.

 

[sengsational]

I was going to add the "exterior door test" but NWB beat me to it. That would be test 1. Test 2 would be the inlet outlet delta T test twice. One after you've turned the unit off while you were away for the day, and the second at the situation where it's not holding temperature during the heat of the day. If it's less delta T when not holding temperature, you probably have a freeze up issue. But I would have thought that the first time the tech came out, they would have discovered that (it's very obvious from reading the guages). Test 3, easy enough to do is climb up and hold your hand next to every vent, and make sure you feel air moving. The result should agree with the exterior door test.

 

[NW-Bound]

There's another possibility about air duct leaks.

That is, you have a leak in both the inlet and outlet ducts of the evaporator, and the airflow in/out of the duct system just balances out. The "exterior door test" will not reveal this.

Ah, but then going up in the attic you will not miss it. How? With cool air blown out to the attic via an air duct, then some air sucked back in via the inlet duct, your attic is just like another room in the house, with its own register and a return vent to boot. It will be nice and cool up there.

 

[bobandsherry]

If you haven't done so, verify the model that was installed. I know it sounds silly, but my son just had his HVAC replaced. He checked the model of the AC unit, it was different, lower price model than he agreed to as specified in the contract. They came out to replace within a couple days, but to add insult to injury they had a lower priced model ready to install. They finally brought the right model. So honest mistake or just trying to pull a fast one?

The new model seems to be cooling much more efficiently. So just mention this as a possiblity.

 

[Piper20]

Could be a bad expansion valve. It if the old unit had no issues keeping the OP comfortable the new unit should work just as well or better.

 

[NW-Bound]

If they actually run a Manual J, I am very impressed. The HVAC sales people I've run into had a very dumbed down program that generally just recommended replacing the AC with one the same as the old one or a little larger. I ran my own Manual J and had a smaller AC installed than was recommended. It worked out great.

Ah, y'all get too fancy. To replace an existing AC, what's wrong with staying with the same cooling capacity if the old one worked? And if it's not enough, just go up another notch.

I love variable-speed ACs. It's easier to get a larger unit to be sure you have sufficient cooling power if needed. The unit will downshift to a lower speed if it's not too hot. And the fan speed is also variable. What's not to like?

 

[swakyaby]

Another vote for having the duct work inspected. My duct separated between the 1st and 2nd floors, unbeknownst to me, to the point where I couldn’t keep the 2nd floor cool in the summer, no matter how low I set the thermostat. The HVAC salesman convinced me that my 15 year old unit needed replacing with a larger more powerful more efficient unit, and I would see the savings in my electricity bill. Nope. Didn’t fix a thing. An independent duct cleaner service guy saw a 2 foot separation of the duct inside the wall and wrapped it as best he could. Finally, cold air to the upstairs.

 

[NW-Bound]

Another vote for having the duct work inspected. My duct separated between the 1st and 2nd floors, unbeknownst to me, to the point where I couldn’t keep the 2nd floor cool in the summer, no matter how low I set the thermostat. The HVAC salesman convinced me that my 15 year old unit needed replacing with a larger more powerful more efficient unit, and I would see the savings in my electricity bill. Nope. Didn’t fix a thing. An independent duct cleaner service guy saw a 2 foot separation of the duct inside the wall and wrapped it as best he could. Finally, cold air to the upstairs.

Wow, the HVAC guy installed a new unit without inspecting the duct work!

Totally amazing.

Off with his head!

 

[travelover]

Ah, y'all get too fancy. To replace an existing AC, what's wrong with staying with the same cooling capacity if the old one worked? And if it's not enough, just go up another notch.

This was in Michigan. I suspect you are looking at this through Arizona sunglasses. Installers tend to oversize ACs because then they never get "it won't cool" complaints on hot days when momma decides to do her baking. In Michigan, dehumidification is very important and big ACs do a poor job of that.

I love variable-speed ACs. It's easier to get a larger unit to be sure you have sufficient cooling power if needed. The unit will downshift to a lower speed if it's not too hot. And the fan speed is also variable. What's not to like?

Yes, variable speed ACs are the cat's pajamas. The downside is, of course, higher initial cost and much higher repair costs. Here in the PNW, I'd have a hard time spending that extra money for the occasional days the AC is actually needed. I do have a two stage gas furnace with variable speed blower and it does do a very nice job of keeping an even temperature through out the house.

 

[NW-Bound]

This was in Michigan. I suspect you are looking at this through Arizona sunglasses. Installers tend to oversize ACs because then they never get "it won't cool" complaints on hot days when momma decides to do her baking. In Michigan, dehumidification is very important and big ACs do a poor job of that...

I have not looked inside my central HVAC, but did the previous unit. Its blower could be wired for 3 different speeds. So, there's some leeway there for the installer to select a lower speed if necessary.

I have not lived in a humid climate to experience the difficulty of dehumidifying. I thought that when the dew point is as high as 80F while the AC is blowing air at 60F, the moisture in the air would easily condense and fall out like rain.

 

[travelover]

......... I thought that when the dew point is as high as 80F while the AC is blowing air at 60F, the moisture in the air would easily condense and fall out like rain.

The moisture does condense, but only when the AC is running. A big AC just runs a short time, then shuts off in moderate temperatures. Thus the problem with dehumidification.

 

[NW-Bound]

I remember Trombone-Al had quite a bit of problems with humidity in northern coastal CA. I guess if it was too cold for an AC, and he used a dehumidifier.

I recalled once staying in a hotel in Lafayette, and the room had a dehumidifier. Thought to myself, an AC would do the same job and cool the room at the same time, but perhaps the humidity would be bad in the winter too.

Where I am, the winter is even drier, and some people use an ultrasonic humidifier that puts out a fine mist.

 

[Souschef]

Another vote for having the duct work inspected. My duct separated between the 1st and 2nd floors, unbeknownst to me, to the point where I couldn’t keep the 2nd floor cool in the summer, no matter how low I set the thermostat. The HVAC salesman convinced me that my 15 year old unit needed replacing with a larger more powerful more efficient unit, and I would see the savings in my electricity bill. Nope. Didn’t fix a thing. An independent duct cleaner service guy saw a 2 foot separation of the duct inside the wall and wrapped it as best he could. Finally, cold air to the upstairs.

Same thing happened to us. We were having some work done under the house, and the repairman asked if we knew that the floor ducting in one bathroom fell down. It was hard to notice because of the register.

 

[eytonxav]

Duct work may have an air balancer that closed up thereby not routing enough cold air to the hot area of your home. Definitely something to check. Do you have thermostats in both basement and main level?

 

[Aerides]

Where I am, the winter is even drier, and some people use an ultrasonic humidifier that puts out a fine mist.

When I stay at hotels in AZ, I request a humidifier and have it blowing mist next to the bed all night. It helps a bit.

 

[travelover]

When I stay at hotels in AZ, I request a humidifier and have it blowing mist next to the bed all night. It helps a bit.

This brings back memories. When I worked for MegaMotors, we did testing near Phoenix and stayed in motels there. We soon noticed that our skin was cracking from the dryness. We figured out we could fill a wastebasket with water and then drape a towel over the AC grill and down into the water. Worked like a charm. I'm sure the maids thought we were nuts.