Have HVAC questions? Ask an HVAC Expert for answers

Ask an Expert, Get an Answer ASAP!

hvac topic

Unit Heater Problems

What is a unit Heater?

A unit heater is a heating device designed to provide heat for a specific area. They do not usually require ducting and can be installed on a wall or ceiling. Unit heaters can be electric, natural gas, propane or hot water / steam powered. Fans in the heater blow hot air out to warm the area the unit is placed in. Read below where Experts on JustAnswer have provided answers on various unit heater installations, operation and repair issues.

I had to shut down my new Modine natural gas unit heater model hd75a50111 to repair a small gas leak and now when I turn it on it works for a few seconds and goes out. The cycle repeats 4 times and the unit goes into lockout and the green LED flashed 4 times, what could be the cause of this?

The 4 green LEDs could indicate a problem with the main limit or the roll out switches. There should be 1 or 2 manual reset roll out switches to the right of the burners. Many times the owner’s manual will give you the exact location if so reset the switch. Another thing to check is the automatic hi limit switch that will be between the burner tubes. Please also check that the gas pressure is correct and the vent pipe is of the right size and is not blocked.

What could be the problem if 3 control transformers on a Modine propane unit heater have burnt, if . I have burnt 3 control transformers in a row. I have 125 volts on the primary and 24 on the secondary. What could the problem be?

Since of of the issues could be a wire shorting, the first thing that would be needed is to install a transformer with a circuit breaker so that when the short happens it will just tip and need resetting and the transformer will be safe. The most likely cause of your problem is the gas valve. Unhook the gas valve and see if the transformer trips. If not, you know what the problem is. If it still trips, check for any wires in the control circuit that could be pinched and may be grounding out.

If a unit heater will heat the room to about 70 degrees and goes off and on limit, even though the gas pressure and blower is working, what could cause this problem?

Unit heaters are designed to operate with a defined temperature difference between the air entering and leaving the heat exchanger. If this is exceeded, the unit will stop heating. One of the first things to try is to check is if you have the correct limit for the unit – the rating is on the basis of temperature. If the limit is okay than check that the fan is running at full speed and that it stays on without intermittent cut outs which could cause the problem. Also check for obstruction in the heat exchanger that could restrict air flow and cause the input / output temperature differential to be exceeded.

Is it possible to connect an occupancy sensor to override the setback temperature on a gas fired unit heater?

Most of the time you can use an occupancy sensor to operate a relay. The wiring will be the same as wring a light to the sensor. Get one with a red LED so you will get a clear indication that it is working. , also get the “sub base” for the sensor. Next you will need to buy a 2 stage thermostat, and set the first stage to the empty room temperature you want. Set the 2nd stage to the room temperature you want when it is occupied. Now run a wire from the 2nd stage through a set of normally open contacts to the relay. Now you can set the thermostat as you want it.

How do I bleed air from my Carrier hot water unit heater model 42 AB2R-1?

Bleeding air from the system is a simple process. Use a slotted screwdriver to loosen the slotted fitting in the nut. Loosen it slowly until you hear the air come out and stop turning it. Wait till the water starts to come out and then tighten the fitting.

Unit heaters are very effective in heating a defined area or room. Because of the dangers of electric shocks, gas leaks and hot water burns, follow the instructions in the owner’s manual when troubleshooting problems. If the information there does not resolve the issue, it is advisable to get Expert assistance to help answer any questions that may be needed to get the problem resolved.
Please type your question in the field below

3 verified HVAC Technicians are online now

HVAC Technicians on JustAnswer are verified through an extensive 8-step process including screening of licenses, certifications, education and/or employment. Learn more


Mechanical Engineer

Degree, Mech Engr Technology

7976 positive reviews
Rick Mather

HVAC Technician

Vocational, Technical or Trade Scho

3133 positive reviews

HVAC Technician

Bachelor's Degree

2014 positive reviews
See all HVAC Technicians
JustAnswer in the news:
Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... Justanswer.com.
JustAnswer.com...has seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.
Web sites like justanswer.com/legal
...leave nothing to chance.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.