Rheem Furnace problem

MacPHJr

I am troubling shooting a problem with with a Rheem M# RGDA-100-GR, about 20 years old. I cant find a tech support number for Rheem heating and cooling products so I wanted to run it past the Wall.



There is a manual reset bi metal switch that keeps tripping. It trips at 210*f. The limit setting is 200*f.



The unit is natural gas with 2 motorized damper zones. I checked airflow and filter. The supply air temp is 175*f with 75*f return air. Seems extremely hot to me but I have no experience with these furnaces.



Any help would be greatly appreciated.

Slimpickins

something is wrong

First, did you check the manifold gas pressure? Also 75 degree's as a return temp sounds way too high, typically return air is 3 to 5 degrees cooler that the set temp in the house. No matter how you look at it the 100 degree temp rise is out of the ball park. On the furnace along with model, serial number, and more fun facts it should say what they want the desired temp rise to be, usually in the range of 40 - 75.

Does it have AC? If so, the coil maybe is clogged. Maybe the dampers aren't fully opening. Check the registers and make sure they're all open. Maybe they put a couch against the return air grilles. Be a detective! If you call support, they're gonna wanna know your manifold gas pressure and static pressure, be prepared.

MacPHJr

More Info

Didnt check gas pressure. Checked AC coil, pretty clean considering its a 20 year old unit. There are two units in the house, same model, both have same temp rise, but other unit doesnt have manual reset switch.



Stack temp is around 424*f. Seemed pretty high to me.



Made sure both zones called for heat so there was no airflow restriction. Didnt see by-pass duct work in mechanical room, but it is pretty crammed. Still tripped.



Not even really sure why the switch is there. Has flame rollout and back draft switch.



Wierd

Tim McElwain

What is the make

and model of the bi-metal switch?



By the way normal stack temperature for natural gas systems is between 350 to  450 degrees net stack (flue temp minus room temp).



What did you get for combustion analysis figures? How is the draft?



A properly operating supply duct system temperature is around 105 on the low end to 145 on the high end. This assumes a 45 to 75 degree temperature rise through the unit.



What is the B TU input to this unit? Did you clock the burner to make sure it is not over-gassed?

MacPHJr

I dont have enough info

I was orignally called in to replace a blower motor on another unit in the same home. I was not set up to troubleshoot a gas furnace. No CO anaylzer or manometer.

Im going back out with the proper equipment.

How do I check draft? How do I clock a gas meter?

Ashamed that I dont know this already but I have to start somewhere.

Thanks for your help so far.

Tim McElwain

Never be ashamed to ask for help -

If you have a combustion analyzer you can check draft with it. If not you need a draft gauge.



To clock the meter use the 1/2 foot test dial, let it make two complete rotations using a stop watch determine how long  it took to make the two rotations. lets say it took 20 seconds, you then divide 20 into 3,600 (# of seconds in an hour) answer is 180 cubic feet per hour. To convert cubic feet to BTU's multiply 180 times the heat value of the gas typically around 1000 to 1050. Lets use 1000 it makes the math easy, answer 180 X 1000 = 180,000 BTU's that is what the burner would be actually using. That should match up with what is on the equipment rating plate.



Check your gas pressure at the inlet and outlet of the gas valve it typically on the inlet will be 6" W.C. to say 10" W.C. the outlet is usually around 3.5" W.C.



What is the make and number of the gas valve installed?

MacPHJr

Check draft with Combustion Analyzer

How do you test draft with a Combustion Analyzer?

Tim McElwain

What make of

analyzer do you have? All analyzers have the ability measure draft it is included in the instructions for the analyzer. If not them get a Bachrach dedicated draft gauge.

Tim McElwain

I still need to know

what the switch you say keeps tripping has for numbers and make?

VictoriaEnergy

Move more air

If the manometer shows the correct manifold pressure from the rating plate,you need to move more air.  look for changes in the ducting, ask if any vents or return airs in the house might be covered up?  Was the fan speed previously lowered to reduce noise issues?  (Rheems definately aren't the quietest units out there)  Clean the AC coil.



As far as I'm concerened, all high limit switches should always be manual reset.  If it hits the limit; there is a problem, and that needs to be addressed.  If the unit is constantly cycling on the high limit, the little switch will eventually fry itself, and if it fries itself shut you might burn the house down.

MacPHJr

Follow-Up

Hi guys, I have more info.

Gas Pressure before the valve is 7'' wc and 3.5''wc after the valve.

Clocked the gas meter and the input is perfect.

Draft is good.

Limit switch is model # 432-463 and #47-22383-41, L230

Limit setting for the model unit is 220*f. I measured temp at limit switch, hovered around 220. Did some investigating and found one supply damper closed. Measured limit switch again with damper open and temp hovered around 210.

These measurements were taken with only one zone ( the smallest one) of a two zone system calling for heat.

Let system run for an hour and half while a replaced fan motor on other unit and had no problems.

Could it had been as simple as one supply damper closed??

VictoriaEnergy

still too hot

I'm thinking 180 as a upper limit.  At 210 you're getting way too hot.

icesailor

Consider this:

Your furnace is like your car driving down the highway. You have more power than you need. You are driving on a flat, level road. You have the gas pedal set to maintain a certain speed and you are driving it. If you don't change the gas pedal (burner input), when you come to a hill, the car slows down. If you tart down a hill, the car speeds up. The zones are like hills and valleys. You have to adjust your speed. If you don't, you get over speed.

Another example is piston powered aircraft. An important power setting is the Exhaust Gas Temperature, EGT. When the engine is idling, the EGT is at minimum. When you start a take off run and go to maximum power and fuel flow, the EGT goes way up. But you have full air into the air intake. The air cools the combustion in the chamber, cooling the EGT. Once in the air, the throttle is pulled back. As you come up to speed, the EGT goes up and you set the prop speed RPM, the throttle (gas) and the mixture. As you lean the mixture, the EGT goes up. The higher EGT shows best fuel utilization.

With your warm air furnace, you are depending on the returning cooling air to cool the furnace. Anything that slows down the return cooling air is a bad thing. Find what is causing the problem and you have your solution.

Is the new fan proper and giving the proper CFM's of air through the system? If the wrong fan was used in the replacement, it could cause your problem.

And I agree that ALL high temperature limit switches should be manual reset is spot on. 

icesailor

Problems:

Could it be that something is disconnected in the supply and the flow into the return is compromised? Just a thought.