# Furnace temperature rise (delta) is too high



## jadziasman (Dec 26, 2014)

I have a seven year old Carrier 58MXB-100 furnace with an overheating problem that started during Thanksgiving 2014. It is high efficiency furnace (92%) that had a MERV 8 1 inch pleated filter installed just outside the blower cabinet in the return duct. This is the first time in the seven years it's been in operation that I've been having problems.

One of the issues with the furnace is that 1 inch MERV 8 pleated filter I installed is being bent toward the blower like a sail when the blower runs. After removing the filter from the cabinet, the panel remains bent. The blower never used to pull on the filter so strongly. What would cause this?

The furnace has been getting too hot and had been tripping the high temperature limit and had been short cycling with the MERV 8 filter. As a temporary fix, I replaced the existing filter with a Filtrete basic filter - no more than MERV 2.

The plenum temperature above the evaporator coil reaches 150 degrees F even with the MERV 2 filter, which is much higher than it should be. The supply air entering the furnace is about 70. 

I have inspected the evaporator coil from the top and the bottom and visually it appears to be clean. The blower wheel also seems to be clean.

I had an HVAC guy look at the furnace on Dec 22 and he said the supply and return ducts near the furnace were undersized. They are probably smaller than ideal but they still should be adequate for the house (barely). I'm thinking that either the airflow from the blower to the supply plenum is not adequate or there's not enough air being pulled in by the blower to be passed over the heat exchangers to drop the plenum temperature to an appropriate value - 120 degrees.

Also someone had cut an opening in the supply plenum and put a vent over it to allow some of the heated air out. This indicates there was a problem with excessive supply plenum temperature in the past, maybe even during installation of the HVAC system in the house.

The house is a two story colonial with a full basement. Lots of big windows and a total of 2500 sq ft. A 100K BTU furnace could be a little big for the house but not outrageously so.

The furnace has not tripped the high temperature limit since changing to the MERV 2 filter. I don't want to use this filter too long for obvious reasons.

If the furnace has had a problem with temperature rise (delta) from day one, would it help to add another cold air return?


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## beenthere (Oct 11, 2008)

If it never use to pull/bow the air filter in, but does now. That suggest that something is restricting return air now, that wasn't before. look for it.

Adding another return won't hurt anything.


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## yuri (Nov 29, 2008)

Check all your supply and return vents in the house for blockage/furnitore in fron of them etc. Make sure all the dampers are open too. If all that fails the tech should check the burner firing rate and gas pressure to the burner.

If the motor capacitor is weak the motor may not be turnng fast enough. Need a meter that can read capacitance or replace it as they are cheap. The motor should spin easily with one finger turning the fan. If it is stiff then it may be seizing.


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## jadziasman (Dec 26, 2014)

Thanks for your answers and suggestions. 

I have opened all of the supply vents and made sure the return vents aren't being blocked.

I believe the blower motor is operating properly. It's drawing the filter toward it with quite a bit of force.

I guess I'll have to have someone come in and measure how much air is being pulled in at each return vent to find which ones if any aren't drawing in air at all.


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## yuri (Nov 29, 2008)

The reason you are having a problem is that the limit control is getting weaker from having been tripped so many times. It may break soon and that is not uncommon. They are not designed for continuos tripping.

Get the furnace firing rate checked as it may be overfiring. There is a vacuum hose from the gas valve to the burner box that modulates the gas valve pressure and if it is cracked or off then it may overfire. Also the ductwork needs improving.


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## jadziasman (Dec 26, 2014)

Thanks for your replies, Yuri.

The high limit switch has not been tripped in the past week or so and it appears to be functioning acceptably. I bought a new high limit switch a few weeks ago and haven't needed to install it. I will if I need to. It's a little challenging to replace because there's stuff in front of it.

I neglected to mention that my house uses wall joists and panned floor joists in some areas as cold air returns. There is a 25x8 sheet metal duct fastened to the basement ceiling. It's about 20 feet in length. I need to see how the cold air returns attach to it. I installed a suspended ceiling several years ago in the basement so I'll need to remove several of them to get a good look at the return.

The panning on the floor joists was not installed properly in some places when the house was built (1997) so I will need to correct the builder's mistakes.

As I mentioned earlier, I or someone else will check all of the cold air returns to see if they are leaking or blocked. 

Once I have optimized the return air as best as possible, I will let everyone know.


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## jadziasman (Dec 26, 2014)

Okay,

As I mentioned, there are huge gaps between the main return air duct (which is 8 inches by 25 inches) and the joist panning in many areas on my basement ceiling. The dopes (crooks) who installed the return duct during house construction in 1997 did not fasten the duct to the floor joists properly and left these gaps that I need to fill in because I most likely will not be able to move the duct up to the joists now. I have duct taped the gaps as a temporary stop gap measure until I get some Great Stuff and the time to seal off the gaps properly. I have been living with this substandard condition for 14 years - I bought the house in 2000. You never realize you have a problem until you look for it I suppose.

Another thing I just noticed today was that the low resistance air filter - Filtrete basic flat panel - that I installed just two weeks ago was nearly clogged with a white powdery substance that was blocking the air flow to the extent that the temperature rise was becoming a problem again.

After replacing the Filtrete basic flat panel filter with a new one, the temperature in the supply duct two feet from the plenum dropped from 150°F to 130°F. I might need to change the filter weekly if it keeps catching gobs of this white powder - whatever it is. I have been checking the furnace temperature rise daily in two locations in the main supply duct (2 feet from the plenum and 25 feet away) since my furnace overheating woes began during Thanksgiving week (if not earlier during the 2014/2015 heating season).

So, I guess I'm making progress. The house is warmer overall now that air circulation has improved with more return air getting from the first and second floor registers back to the furnace and the temperature delta is closer to what it should be.


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## zappa (Nov 25, 2011)

The filter clogging might be caused by you sealing up the cracks. Now that more air is flowing through the return spaces, it could be picking up dust and construction debris which should eventually stop.


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## jadziasman (Dec 26, 2014)

I sure hope this stuff is removed from the ducts eventually. New light colored carpeting was installed in the bedrooms about five months ago. Maybe some of this is from the carpet?


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## zappa (Nov 25, 2011)

I suppose it could be. Carpet is one of the biggest dust generators.


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## old_squid (Oct 31, 2012)

More than likely sheetrock dust from when the home was being built and no one bothered to close off the register holes during construction. Worse yet, they used the furnace as a construction heater because it was cold out.


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## jadziasman (Dec 26, 2014)

It could be sheetrock dust, I suppose. It seemed too "fluffy" for drywall though.

We have a scanning electron microscope at work. I could try to see what elements are detected in the white debris but I'm not all that interested - unless the problem doesn't go away, which it almost certainly will over the next couple of weeks. 

It's going to get very cold here in SE Michigan this week - below 0°F overnight at least one of the nights. So I hope the furnace temperature rise stays about where it is currently (60°F) during the week.


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