Air temperature from register is about 140 F

[Mike Tao]

A 10-years-old town house and the HVAC is original Lennox system.
Air temperature from registers in third floor is about 140 F.
Some black stains surrounding the registers as attached image.

Do you concern this? Too hot?

Thanks

[ROBERT YOUNG]

The furnace is operated by a thermostat and automatic controls, which safely keep the house at an even temperature
I accept 120 F.

Yes that is high but;
What type of energy?
What were the thermostats set at?
What did the filters look like?
Did you dismount a register or two to feel inside?
Were all the registers 140.
Did you check the returns at the plenum.
Did you ask the home owner?

[Mike Tao]

Dear Robert:

Thanks for your reply.


The furnace is operated by a thermostat and automatic controls, which safely keep the house at an even temperature
I accept 120 F.

Yes that is high but;
What type of energy? Natural Gas
What were the thermostats set at? 65 F
What did the filters look like? Not bad
Did you dismount a register or two to feel inside? what do you suggest me to feel except for hot?
Were all the registers 140. Not all, most of register on third floor and the air handler is in attic
Did you check the returns at the plenum. 64 F
Did you ask the home owner? Planned to. Home owner was not there.

[Jim Luttrall]

That is dirt on the ceiling which collects there because the vent damper on the register is totally closed. Closing the registers will also restrict the air flow for the heating system and raise the output temperature (but lower the energy efficiency). Open those registers and let that thing breath!
Change the filters!

[Mike Tao]

Thanks. But there are open ones as well.


That is dirt on the ceiling which collects there because the vent damper on the register is totally closed. Closing the registers will also restrict the air flow for the heating system and raise the output temperature (but lower the energy efficiency). Open those registers and let that thing breath!
Change the filters!

[Jim Luttrall]

Thanks. But there are open ones as well.
Part of the dirt is from air forcing between the ceiling and the grill face due to pressures and lack of flow through the grill. Dirt will impact the ceiling and plate out on relatively cooler surfaces.
ANY registers that are totally closed will decrease the air flow through the system and raise the temperatures.

[ROBERT YOUNG]

I do HVAC inspections when I can but its been some time that this has come up.
Thanks for the exercise!

The offset would be almost 80 degrees on the return plenum. To high.
50 might be a high depending on the setup as well.

Gas, in your case, or oil for that matter, would be 140 to 160 - 170+ at the plenum supply side. Depending how and were you took the temp.

Rationally thinking, which is rare at my case ha ha ha, on the trip upstream, supply side heat loss is typical correct? Yes...

I will say it sounds like the fan limit/ing switch needs to be corrected.
Here is a link. Simple explanation at a simple system when you think about it.
How the Forced-Air Furnace Works: Putting It All Together (http://roscoebrown.com/articles/how-the-forced-air-furnace-works-putting-it-all-together)

Hope it helps!

[Raymond Wand]

1. For a ten year old unit, that should not be creating dirt accumulation at registers if the unit is tuned and function properly. More troubleshooting required beyond visual inspection and a host of speculative causes.

2. Dirt could be the result of cheap filter. Alternatively filter could be restricting air flow due to high efficiency.

3. Occupant activities creating dirt through dust, house situated near industrial or road ways.

4. Candles being burned.

5. What shape are the bearings and blower? Some older units with failing bearings, or improperly oiled (too much) can disperse oil film into air stream. However a 10 year old blower motor will likely have sealed bearings.

6. Cracked heat exchanger?

7. Occupants are smokers? Or not inclined to keep a clean house?

8. Sounds as if the unit has not be regularly serviced.

9. Yes temp differential is high, which needless to say; the unit should be serviced and or replaced due to age.

[Mark Reinmiller]

That is higher than a typical allowable temperature rise for a furnace. Something may be restricting airflow.