Rheem Criterion II Model # RGPH10EBRJR furnace in a central unit. Everything is working, but the unit runs constantly because cool air comes from the vents and the house temp never meets the thermostat setting. I have searched diligently for duct leaks but found none. Q. Does the heat exchanger get so rusted inside that heat is not transferred to the outside of the heat exchanger. Item is expensive and I do not want to buy a new one if not needed,

 

Are you sure the flame is burning?

 

Check for a stuck open limit switch or tripped rollout.

do you have a multi-meter and know how to use it?

 

Absolutely I went into the attic and watched them burn for at least 30 minutes! Strange situation! I have a VOM and do know how to use it. All the switches, limit, and flame roll-out are good, and not they would turn the burners off. In fact the flames are well into the tubes, and very stable. The Heat exchanger seems to not be heating on the outside in spite of the flames being on for an hour or more. My question is, "can rust become so bad on the inside to the heat exchanger that heat will not transfer to the outer part of the heat exchanger." I also wonder if the exhaust tube could cause that problem, it seems to be much hotter than it should be. Tomorrow I am going to take it down and look through it to check for plug up problems. I haven't done that at this point because if the exhaust was not sufficient the exhaust vacuum switch would not allow the system to turn on. It turns on, runs the vacuum exhaust fan for a couple of minutes as it should, then the igniter lights and when glowing bright the burners come on and no roll-out on the flames in either of the 4 tubes, the whole system runs constantly trying to satisfy the thermometer setting. It takes appx. an hour to increase the temp in the house one degree. It burns a lot of gas running constantly.

 

Not that I know of.
Stainless and aluminized steel exchangers shouldn't rust that much.

RGPH manual (pdf)

 

With the furnace running that long with such little house temp rise, I'd start to suspect a gas flow issue.

 

Your furnace could be severely under-fired due to low gas pressure or plugged orifices.

Clock your gas meter. Don't use the smallest dial, it's not accurate. You'll need to get the heating value from your gas supplier and turn off all gas appliances including pilots before testing.

https://hvactechhangout.com/home/sys...k-a-gas-meter/

If your furnace is underfired, you'll have to call a tech to check the gas pressure and orifices.

It could need a gas pressure adjustment or worst case the gas valve has failed.

There's a possibility the regulator in the gas meter has failed and the line pressure going to the furnace is too low.

You should also check the ducts in your attic for breaks and major leaks. A disconnected return can mean your furnace is pulling in cold and barely heating it above room temperature.

You can also check temperature rise across the furnace - https://www.hvacrschool.com/furnace-...perature-rise/

 

Is the pipe connecting the furnace to the chimney getting hot? If not, there is some blockage in the exhaust path or the exhaust blower isn't operating properly or both.

 

What’s the temperature rise across the heat exchanger?

 

The exhaust pipe is getting hot, too hot to touch. Is that a sign that the tube is blocked?

 

Never thought of a gas flow problem. The flame is nice and blue, and is strong, long, and well into the tubes.

 

The quality of combustion cannot be verified by sight.
Is this a condensing or conventional furnace?

Edit-just saw your model number. That’s a conventional 80% furnace. The exhaust should get hot.

 

Is there an evap coil for A/C attached to the furnace? And if so, by any chance is the condenser running when there's a call for heat?

 

Not familiar with the evap coil. Is it in the air flow contained in the metal housing on the downside of the air from the heat exchanger. I checked the temp of the flame with my laser detector and was well above 200F. Then checked the air coming out of the system immediately down stream from the heat exchanger where it was taped and I could get some air coming into the distribution box and that air was only 84F. Checked the temp on the metal immediately near the exhaust pipe and it was 179F. Maybe the heat exchanger is faulty and all my heat is going out the exhaust pipe. What do you think?

 

I checked the outside unit to see if the AC was coming on when the heater was on. It was not coming on with the heater, so the evap coil will not be the problem it seems to me. It appears now that something is very wrong with the heat exchanger. The Temp of the air immediately down wind from it is only 84 degrees F. The distribution box (metal) is taped to the metal container the evap coil and the heat exchanger are in, so I made a small slit in the tape that tapes them together and measured the temp of the air from there and it was only 84F. I am going to drill a small hole in the container just above the heat exchanger and measure the temp there. If the metal immediately next to the exhaust pipe gets to 179, the heat exchanger should at least be that it seems.

 

The flue temperature should be upwards of +/- 300°

 

What's the return air temperature? maybe you have a disconnected return in the attic.

Don't bother measuring exhaust temperature, clock the gas meter to see if it's under-fired.