humidifier cuts off furnace burner prematurely

11 years ago

After I installed the copper water supply lines and PVC drain lines, I had a contractor install two Honeywell TrueEASE HE200 humidifiers on my two older SnyderGeneral furnaces. They are simple humidifiers with the only electrical component being the water solenoid. One humidifier works fine. The second one works but it consistently cuts out the furnace burner before the blower even comes on (less than minute). So there is a never-ending short cycle of one-minute burner only (with humidifier water solenoid energized), two minutes fan only, everything off for two minutes, then gas valve and humidifier solenoid both energized again until the gas valve prematurely closes and the burner turns off, etc. As soon as I disconnect the solenoid to the humidifier, the furnace will run normal cycles of 5, 10, 15, 20 minutes or whatever it takes to satisfy the thermostat. I have 27.2 volts to both gas valves when calling for heat. Both humidifier solenoids cause about a 1.5 voltage drop. The problem furnace's 24 volt circuit (gas valve and water solenoid) pulls a little more amperage than the combo that works well but ohms law tells me I am still only using about 14 watts between the valve and solenoid on the problem furnace and the 40VA transformer should handle that fine. (.2 amps gas valve plus .33 amps by the solenoid times 27.2 volts = about 14 watts).
Why will the gas valve not stay open when sharing its 24 VAC with the water solenoid? Am I barking up the wrong tree by thinking it is an electrical problem?
thanks,
cofloater

Featured Answer

Mini-Split Warehouse

3 years ago

https://www.heatair.org/

CAUSES OF SHORT-CYCLING

Clogged Air FilterCLOGGED AIR FILTER

This is one of the most frequent causes of trouble in an HVAC system. If the filter is heavily clogged with dust and debris, it may cause the furnace to shut off as a safety precaution. Fortunately, this is a basic problem to solve—just put in a clean filter. We recommend changing the HVAC system filter every 1 to 3 months depending on how often you run the heater and the strength of the filter.

MALFUNCTIONING THERMOSTAT

If the thermostat in your home is sending incorrect signals, it may be be the reason your heater keeps turning on and off. The thermostat might be reading temperatures incorrectly or have a bad connection to the heater’s control board. Technicians can recalibrate the thermostat or take care of other repairs so it operates the heater correctly.

CORRODED FLAME SENSOR

This applies to gas furnaces, which are the most common type of heating system. The flame sensor determines if the burners are on when the gas valve is open. The sensor will shut off the gas if it doesn’t sense flames—this prevents a buildup of unburned gas in the combustion chamber. Since the flame sensor is not able to detect fire when it’s covered in dirt or debris, it will continuously close the gas valve and cause the furnace to keep shutting off.

OVERSIZED HEATER

This is the most serious issue that will cause a furnace to keep shutting off is an improper installation. When a heater is too powerful for a home, it will heat up the living spaces so fast that the thermostat will shut the heater off prematurely.

All you can do in this case is arrange for a new heating system installation—this time from a qualified professional who will make sure the heater is the right size.

Comments (8)

mike_home
11 years ago
What are the voltages on each of the transformers when the humidifier and furnace are both running?
Tinmantu
11 years ago
Since you said older furnaces, are they both on digital stats? If not, it will mess with the heat anticipator if both are running at the same time. If digital, then I'd suggest an independent transformer and have a current sensing relay.
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cofloater
Original Author
11 years ago
Mike_Home,
Each transformer measures 27.2 VAC when the furnaces are off. The furnace that cycles properly is 25.9 with burner on and water solenoid open. The furnace in which the burner will not stay on measured 25.7 one day and 25.9 another with both gas valve and solenoid energized.

tinmatu,
Both are on separate mercury stats. These furnaces are completely separate from each other. They are fed by separate breakers and serve separate areas of the building. Their control circuits are on independent transformers. I only mentioned the second one to point out that the same model humidifier and same model furnace work fine together in one situation. In the second application, the voltage drops to zero to the gas valve after a minute if I leave the humidifier solenoid wired in.

thanks guys, cofloater
Tinmantu
11 years ago
A mercury stat on the same transformer as the gas valve will cause problems. They have the heat anticipators that I referred to. I'd consider upgrading to a digital stat, rather than going thru the extra relays. I didn't mean the two different furnaces, meant that running the gas valve and humidifier at the same time doesn't work well with mercury stats.
cofloater
Original Author
11 years ago
tinmantu,
You nailed it!! The heat anticipators were the first thing I thought of right after problems started following installation; but setting them a little longer made no difference and I went about further troubleshooting and my mind got lost in other stuff. I changed the T-stat out on the problem unit to a digital T-stat and it immediately solved the short cycle problem. I will now change out the second T-stat as that seems to be a bit irregular also and perhaps short cycling some also.
thanks much!
cofloater
Bill Andrews
3 years ago
last modified: 3 years ago
So I’m having the same problem. I have a Honeywell digital stat, two zones and hz322 controller hooked up to the EIM.

long story short, exact same issue but I’m with a digital stat .... As soon as I disconnect or turn off the humidifier, the furnace will run normal cycles. i love my humidifier bc we live in Colorado where there is no humidity snd really want to be able to run it.

please note I have disconnected the zones as I want heat going to the entire house. Even with the zones on, same issue...

I have also replaced my O2 sensor and control board on the furnace ...

can I adjust any settings on my digital tstat
mike_home
3 years ago
What is the model of the thermostat? I don't think there are any adjustment setting that will solve this problem. but in the installation manual may have something that could be changed. If this a WiFi thermostat without C wire connection, then that could be a source of the problem.

You could try a simple experiment with the thermostat wiring. Disconnect the R and W wires from the thermostat and tie them together. This would initiate a call for heat. If the furnace runs properly then that would suggest a thermostat issue. If you still observe the problem, then it is something else.

I would offer the same advice as I did 8 years ago of measuring voltages as method of trouble shooting.