Heater Thermostat power gets cut off

[gaminid]

Hello,

We have a Comfortmaker (Model Number NBF075F16A1) Forced Gas Furnance (6-10 years old) and have a unique problem. Everything seems to be working correct except time to time the themostat screen goes totally blank as if there is no power. When that happens I noticed that it no longer blows hot air. But after a few minutes the thermostat screen become live and start blowing the hot air. If the set temp is far away from the current temp it takes a long time to reach it or it never reaches the set temp. As a result in our house which is a two story building the upstair gets over heated while the downstair is always under heated.

We called in a technician and his diagnosis was a bad thermostat and replaced it. Guess what!! It didn't solve the problem at all. It keeps doing the same thing with the new thermostat.  Now he says the house has a badly designed vent system to which I don't agree as we didn't have any problem with it for the last 8 years.

My 2 cents is this: I think for some reason on the heating cycle the heater takes two much power and heater controller cuts off the power to the heater and the thermostat and it gets reset after a while. But don't know how to figure out or fix it.

Any advice is greatly appreciated.

Thanks

Gamini

 

 

[hvacdrd]

What brand and model of thermostat do you have? Do you have a means of checking amperage? Is the furnace equiped with a humidifier? If your meter is rated at least 2 amps we can do some simple checks right at the thermostat.

To start - remove ALL thermostat wiring at the furnace control board except the two wires from the thermostat on terminals "R" and "W" - (typically the red & white wires from the thermostat) see what happens & report back.

[dkpd1581]

Since you stated that it is a gas furnace, look to the bimetal (auto reset) over temperature switches.  One is tripping for some reason and stopping the 24 VAC contol signal.  This kills the 24 VAC to the gas valve, blower relay, and t-stat.  When the switch cools and resets, then you see the t-stat come back on and  remembers its last setpoint and tries again to  reach the heat temp..

As to WHY it is tripping we need to know which one of the switches it is.  It could be a flue limit, an exchanger limit, roll out limit on the manifold.  Another thing to look at is to watch the blower motor as it runs.  Is it going bad, is the bearing going bad, or is the capacitor out of 10% ?? This will casuse the motor to run normally for a while and then it will start to slow down and may even stop.  This in turn causes the exchanger to overheat because we are not moving the proper amount of air over it.  Again, the over temp switch opens and shuts the system down.

Is the filter clean, are all your registers open and unbostructed (supply and return), is the flue unobstructed (no bird nests, no rust piles), is the gas pressure to the manifold adjusted properly and not over firing the burners, is the motor running properly and the speed adjusted for the proper temp rise across the exchanger, is the capacitor withing 10% of the name plate, are your bimetal switches just old and getting outside of their specs?

There is one more thing to check and this is only if your furnace is a high efficiency condensing furnace.  If the PVC vent that carries the condenced water away is not pitched properly the water will lay in the lowest spot and build up.  This will in turn

block the flue and cause the air proving switch to not make and of course cshut everything down.

Grab a bucket to set on, a volt meter, and sit and watch the unit run, all the while checking the amp draw on the motor, and then finding which switch has tripped immediately on shutdown.  If your control board has a diagnostic light, read the code fault and track things down from there.  It can possibly point your right to the switch.

[gaminid]

Many thanks for the sugesstions/ advice. I looked at almost everything you mentioned and these are the things I found:

1. The reason for the shut down is the limit switch (200 degree limit) on the exhaust. As you mentioned after about 3 minutes of running the the furnace gets over heated and the limit switch causes to shut down everything except the fan. After sometime when the furnace is cooled the heating cycle starts again. When I bypass the switch the heater runs without shutting down prematurely. I removed the limit switch and tested. It is working correctly indicating that the furnace is getting overheated.

2. I changed the fan speed to HI. There are three speeds (HI, MedHI, LO) and I connected the HI wire. However I cannot really tell whether the fan is running high just by listening. Motor speed seems to be constant.

3. All the registers (vents) are open. I don't see any other blockage. Also has a new filter (I regulary changed filters)

4. I lowered the flame by simply adjusting the gas value (ball value). Then the furnace doesn't get overheated and system runs. I am running it like this now as it heats the home compared to no heat.

5. Evaporator and fan blower seems to be clean.

As you said it looks like for some reason not enough air is pushing thru the heat exchanger. I didn't measure the current draw on the motor yet though.    

I am wondering whether there is gas regulator on the furnace itself. If that is the case may be that regulator is not controlling the gas flow and running the burners at full blast and the rest of the system cannot keep up with heat generated. Is it possible?

Any advice is greatly appreciated.

Thanks

Gamini

[dkpd1581]

In a residential gas application your typical inlet pressure to the gas valve is 1/2 LB PSI. .  The outlet of your gas valve to the manifold is typically 3.5 IWC unless you are using LP gas and then it is approximately 11 IWC..  Therefore, your gas valve inside the furnace cabinet is your regulator.  I was surprised to hear that you are using the ball valve as a regulating device.

Using a manometer, measure your inlet pressures and outlet pressures while the unit is running.  You may possibly have a defective gas valve or an improperly adjusted one.  You may want to verify that the manifold orafice (spuds) are the correct size for the furnace.

You can check your bimetal temp sensor by checking continuity while immersing the sensing side of the sensor in a pot of hot water.  Put a pot of water on the stove and adjust the flame so that the water reaches 190-200 F.  Stir the water constantly to get an even temp and use a good thermometer to verify the water temp.  Do not dunk the whole sensor in, just the lower portion.  If after a minute the sensor opens you know its good.  Pull out the sensor, let continuity make again and repeat the process at a cooler water temp 160-170F.  This will tell you how far off the bimetal is and whether it is really the problem or a symptom of the problem.

An amp draw on your indoor motor will give you an indication of the motor at high speed vs. med high.

[gaminid]

Thanks again....I think the problem is the gas value. I the fact that the furnace is running smoothly when I lowered the flame indicates that the value is the problem. I have tested the limit switch and it opens at the correct temp (200 F). IN fact I replaced with a new limit switch (240 F) and stil the systems shuts down when I run at the full flame. Just tonight I measured the amp draw on the motor and it turned out 5.7 Amp at HI speed and 4.8 AMP at MidHI speed. I am reluctant to mess with gas plumbing so I think I will call a technician. I got three different techs (via out home warranty insurance) to come over to look at the the heater they three different reasons which I didn't believed. First guy said the evaporate installed in wrong and second guy said the duct design is bad and third guy didn't have any idea. None of them never though about the flame power. The furnance ran without any trouble for abut 7 years and suddenly it developed the problem.

Do you also think it is the faluty gas value?

Many thanks for helping out.

Gamini

[gaminid]

For a few days I thought I figured the problem but now I am totally lost again. Here is what happened.

Since the furnace is running correctly when I lowered gas supply by adjusting the ball valve I called the local gas company to come and have a look at the gas valve. The tech showed  up on time and did some measurements of the gas usage (only the furnace was running and the ball valve was fully open) and told me that the furnace is taking 40% more gas than it should (the furnace is 75000 BTU but it behaves as if was 105000 BTU) and he too suggested the faulty gas valve. His exact words were the furnace is "over gassed". I called in a heating company to change the gas valve. The one they replaced matches the specs on the one we had (regulation at 3.5 WC). Even after that I still have the same problem of over heating when I open the gas valve fully. As before I am running the furnace by lowering the gas supply.

Do you have any idea what is going on?

Many thanks in advance for your help.

Gamini 

[dkpd1581]

I ran into something of a similar situation a few days ago.  I had an over limit switch tripping and cutting off a split system that was twinned together (two air handler/furnaces glued side by side).  In the end it came down to a two inch crack in the heat exchanger.  The flue gasses were exiting the crack and filling the supply air.  This hot gas caused the temperature in the area of the OL switch to overheat.  It may be worth your while to look at the exchanger thoroughly.

Is the gas valve over rated for the system?? Is it allowing to much volume of gas through at the proper pressure (rated for more BTU's than the system is rated for).

Look at the evaporator coil's bottom side and see if its dirty and blocking air flow.

Look at the blower wheel for the same.  Is the bower motor running and slowing down after a few minutes (signs of a bad motor and/or capacitor).  Run the motor with an amp probe on the common wire and the door closed - an open door will give you a higher amp reading.

Check the orafice size on manifold spuds.

What was the inlet pressure to the gas valve.  There is a max rating that if it is exceeded requires a gas pressure regulator.

Do you have a duct issue there some duct became crushed and restricted or a piece of the fiberboard/mylar doing the same thing somewhere in the run.

How is your induced draft motor.  Is it moving enough flue gas.  If not, then you will over temp the exchanger and get the same symptoms.  Is the wheel missing a few teeth, is it slowing down over time. 

Is there rust built up in the exchanger or flue exhaust causing a flue gas flow obstruction and over temping.

We have to start over with the basics again an see what we are missing.  DOnt feel bad...it happens.  We go through A-Z only to go back through the process and see what we missed the first time...then shout "DOPE."