Is this furnance operating normally or do I have a problem?

At the workshop, there is this gas furnace/heater hanging from the ceiling that was there were I moved in:

I don't know much about it, but it typically does a pretty good job of keeping the place warm.  We typically keep the thermostat set to 50 degrees and then turn it up when we're actually there.  Typically, when I arrive at the shop and open the door, the place is at the afore-mentioned fifty degrees and the furnace is not running.  Once I go to turn it up to 65-70 degrees, it seems like it never turns off, but the place does warm up.  One thing I noticed is that it seems to constantly cycle the gas.  By that, I mean that the blower is constantly on, but there's a very frequent "click-whoosh" of the gas being turned on.  Looking in through the bottom of the thing, I can see that there's flame for a while then the burner goes dark then the flame comes back with the "click-whoosh" and so on.

Is that normal behaviour?  While it's a completely separate beast, it seems like my gas furnace at home comes on, burns, then shuts off and doesn't appear to keep re-igniting the burner.  I can't recall if it's always done that or if this is new, but it seems like last year the unit as a whole would cycle on and off as opposed to the constant running.  As far as controls, it's a very simple thermostat with no seperate fan switch or anything, just the temperature setting.

No experience with unit heaters like that, but I wonder if it might be cycling due to tripping the high limit due to a clogged filter. (if there's not enough airflow, the heat exchanger will get too hot and  the safety high limit switch will shut off the burner until things cool down.) Typically, a furnace will run continuously until it reaches the set point. 

That seems wrong.  There might be a high or low limit switch out of range, but that is a WAG.

No comment on your current trouble other than I've had several thermostats go wonky in my commercial space.

A ceiling fan or even a floor fan pointed up will do wonders if your ceilings are as high as they appear in the pics.

That is exactly how it's suppose to work. As long as thermostat is calling for heat, the fan will run. The unit will fire and run until it hits the high limit. If the thermostat no longer calls for heat, the fan will continue to run until the unit cools down. It's possible that the fan is blowing on the thermostat making it read higher than the rest of the room.  That will cause it to short cycle. Thermostat should be located on an inside wall away from the flow of warm air. Cleaning the unit periodically is a good idea as well.  Make sure you clean the flue. 

That's pretty much how my house furnace works.

How long are the burner duty cycles?

First possibility:  It's doing its job just fine.  The blower kicks on and the airflow pushes a switch.  Once the sensor verifies that the air is flowing, it ignites the flame.  It heats for a bit until the stat turns it off and the blower continues for a while to extract the remaining heat from the exchanger and cool down the unit.  It's possible that the stat is simply calling for more heat before the blower finishes its cooling cycle.

Second possibility:  If it's every 10 seconds or so,  the thermocouple isn't working.  There is another safety in the form of a temperature sensor.  The only way the furnace knows it has successfully lit the flame is with a temp sensor.  If that temp sensor doesn't show an increase in temperature, it assumes that it failed to ignite and shuts the gas off so it doesn't flood the space with explosive gas.  In that case it is still heating a little because it keeps trying.  

I took another look at this yesterday and it turns the burners on for about 90 seconds initially, then they go out for a few seconds.  After a short wait, the burners come back on for about a second and then off again.  It'll cycle like that for a while - rapid on/off/on/off/on/off - and then stay on for a bit before going out again.  There's no filter, and it's all one big room, basically, with the thermostat pretty well out of the direct flow of the air coming from the furnace.  I tried blowing the system out with some compressed air, but it didn't result in any change.  I'm going to call the landlord today, but I can't imagine that it's proper operation for it to be cycling the burner every second or two.

Yep, my guess is thermocouple.  It's not able to verify that the flame ignited so it shuts off the gas.

The initial 90 seconds is the processor giving it the old college try.  Then it shuts off and subsequent attempts are shorter because it already expects a fault.  SWAG

Oooh... another SWAG.

Improper carburetor setting?  When it warms up, the mix is wrong and the flame "backfires" and puts itself out?

Definitely not normal.  If the flue is clear and the fan is running, I'd suspect bad T'stat or gas valve.  That thing looks like it has some age on it.  Not much to it.

Update:  It is not running normally.  I've had to relight the pilot several times a day.  The Guy just left here and said that the problem (he thinks) is the 24 volt transformer.  He put a new thermocouple in (and the pilot light went out again about 30 minutes later) and said he'd have to come back tomorrow.

Furnace: 2.  HVAC guy: 0

Final results aren't in yet, but it looks like limit switch will be paying out.  But what's weird is that he mumbled some stuff about the limit switch being the problem and used that as the reason to come and look at the breaker box and have a conversation about if I'm using all the circuits in there or not.  I was under the impression that the limit switch would be something small and relatively inexpensive and fairly common across time and space.  But he left me with a broken furnace again today and said he'd have to call the landlord and see what they wanted to do about it....

Gas fired unit heaters aren't all that expensive.  Mr. Landlord might do well to just pony up for a new one.

In reply to wae :

Unit heaters are a pretty simple appliance in the gas fired world...   If it has a thermocouple (you said tech replaced that) it's a standing pilot and likely 20 plus years old (looks at least that old).   If the pilot keeps going out it could be because a hole in the heat exchanger (clamshell looking part) is allowing a draft from the flue to blow the pilot light out.  If there's a hole in the heat exchanger it's time for a new one, not worth the labor to change it based on the age of the equipment.    Beyond that the typical sequence of operation on a unit heater is call for heat / light burner / make fan limit switch / start prop fan to heat the space / shut burner off when the thermostat is satisfied / shut prop fan off when the limit switch cools.   There should be a high temp limit safety switch in the internal wiring to shut the burner off in the event the fan motor is not running and you're correct that if that is the problem it's an inexpensive part.

As stated above, not a terribly expensive piece of equipment.  New ones would have electronic ignition but should still run on the 115 V electrical feed that's running the existing one.  Not sure why your breaker box would be in the conversation.

Heater service guy wants to install a mini-split.......

He put a new transformer on it and at least the pilot light stopped going out, so that's a mild improvement.  It is still having a hard time keeping the burner turned on, though, so not fixed all the way yet.   I got the call this morning that the guy can't find parts for the old unit and he's going to come out to install a new one next Saturday.

winner winner, chicken dinner.

Well, not so fast!  The landlord called another guy for a quote on a new heater.  This new guy took a quick look and just straight-up fixed the thing, no big deal.  Apparently the transformer that the other guy put on was not the correct size (I don't recall of it was a question of voltage or amperage) and instead of needing to (unsuccessfully) scour the internet for the right part, he just sent his assistant out to the truck to get a limit switch.  Beats $3,200 for a new 100k BTU unit heater!  Now we just see if it stays fixed.

But for all you people from the future that are searching for answers:  High limit switch.

Streetwiseguy said:

That seems wrong.  There might be a high or low limit switch out of range, but that is a WAG.

Just going to put this here, because I'm that sort of shiny happy person.

Streetwiseguy said:

Streetwiseguy said:

That seems wrong.  There might be a high or low limit switch out of range, but that is a WAG.

Just going to put this here, because I'm that sort of shiny happy person.

Remote tech service diagnostic FTMFW!

Other than the Internet Mystery Tools, I don't think there's anything GRM can't solve!