ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Been having trouble running a household furnace with a solid state flame control on standby generator power. The furnace runs flawlessly on the grid, but on standby, the flame keeps droping out, and after three tries at ignition, locks out. The generator is a 30 year old 3 phase wye 15kw. The furnace is 120v single phase. I have checked the voltage at the control, it is perfect, even under startup surge. Any ideas?

How clean is the power at the control module? Do you have access to an oscilloscope to look at the waveform?

Some electronics can be very sensitive to noise, spikes, or a distorted waveform, any of which your generator may be producing. In the case of high frequency noise, an appropriate bypass capacitor across the power input may help.

The neutral from the generator must be properly grounded and the polarity at the furnace must be correct.

Tim

Thanks for your help. I know the polarity is correct. There are 2 ground rods plus an underground copper gas line between the generator and the furnace. What do you guys think about running the control circuit with a pure sine wave inverter? If I use magnetic contactors hooked to the control board to run the furnace motors and ignitor, i can get down to about 2 amps, thus reducing the size (and expense) of the needed inverter.

I don't like it because I think that if it runs off the normal supply it should run off the generator. If the supply is correct polarity at the furnace, it makes me wonder about the grounding. Is your generator neutral bonded to the ground system? If it is, I'm out of ideas.

It could be an interaction between the genset voltage regulator and the AC-powered electronic assembly. A power-rated oscilloscope to look at line-voltage waveforms may be the next step.

ok, the genset is hooked up using a 8-4 rubber covered cord. (it is a manual hookup, not an auto start). there is a ground rod at the box. (it is located in a outbuilding) I do not have a separate one wire ground for the genset. I could try running another ground wire. Also, my house has a ground rod for the service entrance. Could you explain what you mean by an interaction between the voltage reg and the ac powered elec assy? I can tell you i understand how these contol boards sense flame. Also, if the generator output is not a pure sine wave, then what?

Due to mechanical inertias of the generator and voltage regulation that is less than the regulation of the utility grid, I suspect that voltage sags from the generator are causing the flame sensors to fail to see the flame.

See:

Sag Correction for Small Equipment Without Using UPS Systems

"The most common power quality problem encountered is that of voltage sag. All equipment is susceptible to dropping off line if the incoming voltage is low enough for a long enough period of time.

"Publications found in the trade journals give the following generalized typical equipment shut-down characteristics...

"* Boilers with flame sensors may indicate a loss of flame with voltage sags, thus causing needless shutdowns."
http://www.powerqualityanddrives.com/sag_correction_without_ups/

Frank

ok, thanks for the reply. It could be a voltage problem. I have watched the voltage at the control as the furnace cycles with a digital voltage meter. I honestly can not see the voltage drop below about 118 and that is during the blower startup surge. Do you think the voltage is sagging faster than can be seen on the digital meter? If it does, will a Ferroresonant transformer change fast enough to solve the problem? The burner will light and burn for maybe 30 seconds, then drop out and relight. thanks for any thoughts you might have.

You could spend a lot of time and energy searching and installing solutions or you could go to the local computer store and buy a UPS for under a hundred bucks.

Hi- the flame circuit needs to read a ground path through the flame-which is flame rectification. this is a mA circuit that, when not present whether because of a poor ground or a dirty sensor, will present itself exactly as you describe-3 tries and a lockout. this sensor usually resembles a spark igniter electrode and is at the opposite end of the burner assy. from the igniter. While the unit is on the grid, you could disconnect this sensor and see the same behavior. That being said, there must be something about the gen-sine. Perhaps a small ups/conditioner would do upstream from the control board, so it wouldn't need to be sized for the fan motor. Have you checked the frequency of genset output?

"High speed" {sub-cycle} voltage changes cannot reliably be measured with a voltmeter. That's what oscilloscopes and related power-quality instruments [and consequent bird-dogging] are for.

I suspect that a ferroresonant transformer will have a faster response time than a furnace and should resolve the problem. The advantage over a UPS is that the transformer doesn't need the maintenance of that a battery requires.

<< A ferroresonant transformer, also called a constant voltage transformer, will give a constant AC voltage output when the input voltage varies within the rated input voltage range. Typical response for output regulation is plus or minus 3% for input voltage swings that are within the rating of the transformer. Typical response times are 8 millseconds for a 5% line or load variation, and 16 milliseconds for a 10% variation. Generated harmonic distortion will be less than 3%. >>
http://www.powerqualityanddrives.com/sag_correction_without_ups/