Gas and Oil Home Heating Furnaces - Found reason for New ThermoPride Shortcycling

Gentlemen,

Overheat cycling of our brand new furnace is being caused by the top of the heat exchanger being blocked by the foil insulation that should be attached to the underside of the blower pan.

I pulled the heat exchanger access plate and used a flashlight and a mirror to make the discovery.

This is a brand new Thermo Pride 0C2-56. In use for four days!

Have any of you ideas as to what my next step should be?

I appreciate your professional advice ! Thanks ! --Hank, Edgewood, Washington

Since Thermo-Pride sells only to their dealers, I suggest you call the installing dealer. It should cost you nothing.

The Thermo Pride OC2-56 is a downflow model.

The foil insulation barrier is attached to the underside of the blower pan. It must not have been fastened very well. Gravity causes it to rest directly on top of the heat exchanger.

All downward blowing air is prevented from moving across the fan control sensor and that cycles the burner off !!!

(Before I looked in that particular place, I have been racking my brain out. Sure we have an old house, but for the life of me I couldn't figure out why our brand new furnace--which replaced our older and larger 1960's downflow--kept tripping.)

Thanks again !

Thermo-Pride makes quality equipment. I've been installing them for nearly 20 years. Once in a great while there is a factory defect but they take care of it without question.

A horizontal insulation blanket coming detached on a brand new and expensive downflow model and blocking the heat exchanger and causing burner short cycling and significant inconvenience is no small thing--to my family, at least.

I have confidence that Thermo Pride and my dealer will help me get the situation resolved.

--Hank

Service company reattached horizontal insulation blanket back to the underside of the blower pan.

Shortcycling did not go away !!!

After furnace co. left, I looked in owners manual schematic and saw that fan control (bimetal) sensor turns burner off at 190 F.

My furnace fan control sensor setting is hard-stopped in place to shut off burner at 190 F.

But the schematic pasted on the furnace shows 250 F setting.

Looks like a wrong fan control sensor....

But that doesn't explain the short cycling. It should not readily even reach 180. You still have something wrong there if it is. Either the blower is not moving the right amount of air or you have blockage.

The downflow models are critical to airflow. They make a mobile home base for that model that seems to address this problem. Make sure the unit isn't overfired. That unit should have a .50 GPH nozzle at 100 psi pump pressure.

If all this is ok, then you may not have enough supply airflow. BTW, Is this a mobile home?

In the meantime:

http://www.thermopride.com/manuals/Mo100G.pdf

DaVeBoy & HVACGuy,

Your comments are appreciated. No this is not a mobile home. Nozzle is a .50 and pump pressure is (I have not had a check on this) 120 psi.

On page 27 in the installation guide that HVACGuy sent, you can see the 190 F limit setting in the wiring diagram. I have that sensor installed.

However, an updated version of that wiring diagram is pasted to our furnace. Instead of 190 F across from the OC2 category, there is " 250 F"

That's why I think I am short cycling.

When I operated the furnace without the sensor, I got the required 70 F rise at the registers. A thermometer that I inserted into the port where the sensor came out, only showed around 170-175 F, and this was a "rough" take.

The correct nozzle is .50 & the correct pump pressure is 120 psig. I suggest you contact Thermo-Pride regarding the fan/limit control. Those devices are, on occasion, out of calibration. Sorry to hear you are having so much trouble. This is very unlike Thermo-Pride.