Ceiling vent/heater thermal cutoff keeps failing - Page 1
Forum section: electronics / repair

author: Peabody
author_rank: Super Contributor
timestamp: January 28, 2024, 07:06:25 pm
content: My neighbor has a combo heat/fan/light unit in the ceiling of his bathroom, and when using the heater the thermal cutoff fails every few months. It's a Broan QTX100HL unit. A replacement heating unit is shown here:https://www.amazon.com/dp/B0BS6NC628/The thermal cutoff is a Microtemp G4A00, 110 degrees C. For a heater like this, that temperature seems low, and in fact in each case of failure it's the cutoff that triggers. The heating elements have never opened. Each time it fails, he has to get out the ladder, take the unit down, then apart, then replace the heating unit. The OEM price is $32, but cheaper on Amazon.Thermal cutoffs appear to cost about $1.60, but I don't see a practical way to replace the cutoff with a higher-temp version. It is riveted in place. And I assume I can't just solder a new one to the leads of the old one - because the solder might melt from the heater, or from internal heat with high current. This is a 1500W heater. (120VAC).Is there a mechanical way to connect a new cutoff to the old leads? Also, would it make any sense to insert a capacitor in series with a power line to reduce the effective wattage so things run a bit cooler? What kind of capacitor would that be? Is there any other option that comes to mind to get this to work properly while still preserving the function of the thermal cutoff?

author: floobydust
author_rank: Super Contributor
timestamp: January 28, 2024, 07:41:03 pm
content: You don't know why the TCO is tripping - it could be legit. I would not bypass or increase the temperature spec of a part with a safety function.I would suspect an airflow problem-Low pressure in the home if the furnace or kitchen fan on, strong wind with open windows etc can overpower the fan.The vent/duct has blockage etc. Years of lint and dust.The fan motor slow if the 1.8uF run cap is failing, or the cage is dirty, it needs oil etc.The TCO has brass rivets and short leads, so replacing it will be difficult. I have soldered TCO's using forceps but you gotta be fast like lightning.The entire heater assembly is selling for $15-$20 I would probably just buy that.

author: Peabody
author_rank: Super Contributor
timestamp: January 29, 2024, 01:29:36 am
content: My neighbor says he has researched online, and found that this is a very common problem with this line of ventilators - lots of complaints. So I think the most likely explanation is that it's just a bad design. But we haven't figured out a way to replace just the fuse, so he's ordered a new heating unit.But I still wonder about making the heating element a capacitive dropper, in effect, to reduce the heating current. I'm not an EE, and just don't know if that's reasonably possible at these voltage and current levels.

author: floobydust
author_rank: Super Contributor
timestamp: January 29, 2024, 02:23:19 am
content: I see the fan and heater housing are all steel construction and I'm not supposed to really offer any safety advice here, there is a risk when making any changes.The heater is high power at 1,500W which is not easily reduced. Capacitive droppers are practical for a few watts only. Probably easier to increase airflow.TheMicrotemp G4 TCOis rated for 15A/120VAC or 10A/250VAC.You could consider the G5 series TCO which is rated for higher current, is the same size, and tolerates higher overshoot temps 225°C vs 150°C for the G4 110°C part.The TCO might be too close to the resistance heating wire.I would get a replacement TCO, solder it in at the rivets- but located farther away from the heating element.Poke the leads through the rivet holes and with heatsinking on the TCO leads, solder it. This is very difficult- to not wreck the TCO from the heat.Otherwise you need crimp ferrules and somehow to get in there with the tool, it's cramped.NTEincludes ferrules.It looks like ordinary PVC wire/heatshrink tubing for the wires going to the connector. This means around 105°C is the material (plastic) limit. You can look at the wires to see if they are truly running too hot or something else is causing the TCO to fail.It can also be the shutdown process - it should turn the heater off yet run the fan for a few seconds to cool off the heater.

author: Peabody
author_rank: Super Contributor
timestamp: January 29, 2024, 03:38:08 pm
content: If this were mine, I might try a number of things. But since it's my neighbor's, I don't want to do anything that I might be held responsible for later, justly or not. My 60/40 solder melts at 186C or so, which I think is too low to be using it to make connections. Also, I don't know how well 15A travels though solder. But I don't see any way to get a crimping tool in there to make a mechanical connection.So that's why I was asking about ways such as a capacitor to reduce the current so it would run cooler. But if that doesn't work, I think he will just need to replace the heating unit when it fails.One thing that I did mention to him was to put something between the heating elements and the TCO. The body of the unit is not metal, and not conductive, so he could cut a piece out of one of the side panels of the old unit, and stick it in there to block some of the heat. But of course if that worked, it might pretty much defeat the purpose of the TCO.It's just too bad the manufacturer hasn't come up with a replacement heating unit that's not so hair-trigger, but still functions as a TCO. But I guess they make too much money on the replacement heaters to do that.

author: BrokenYugo
author_rank: Super Contributor
timestamp: January 29, 2024, 03:51:50 pm
content: Plenty of people are more than willing to complain online and project about their deficiencies in reading the manual, doing the installation correctly, using the product correctly, etc. Complaints alone is not proof of a flaw in the product. I'd also assume airflow issues without seeing anything.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 29, 2024, 04:11:11 pm
content: If it's a flaw, and if the flaw is overheating, then the last thing to do is to modify the thermal cutoff to be less sensitive. That could easily result in a house fire. If you are replacing with higher temperature switch, then you must be damn sure the product and surrounding building materials can take the heat. And remember, the cutoff isn't necessarily placed at the hottest site so even if 110degC doesn't sound too dangerous, maybe it gets to 200degC at some other location further off from the cutoff, especially during some fault condition like air blockage.Investigate if there's anything wrong with the airflow. If you modify, try to increase the airflow. You could also install a diode in series with the heating element, halving power output.

author: Peabody
author_rank: Super Contributor
timestamp: January 29, 2024, 05:05:52 pm
content: I had thought about using a diode, but based on my neighbor's comments I think that would reduce the heat output too much. But I'll go over that option with him again. Not sure what diode I would use. It would need to be mounted inline.

author: garydirkse
author_rank: Newbie
timestamp: January 30, 2024, 02:52:49 am
content: Hi, Thermal cutouts are available to over 195 degrees F. Small thermal fuses can be purchased also. First look for anything that might hinder air flo or speed. Fuzz, rodent nest?? run capacitor? frozen exit? A higher temp cutout may lead to a fire. I threw my Broan out. aldue