View Full Version : OT: Need a bit of help with my shop Unit heater

01-20-2009, 03:32 PM
I have a Modine unit heater in my shop, 75K BTU input, 60K BTU output. It is fairly old, model #PA75AB. Anyhow, I don't have the installation manual, or the owners manual and I think it has a problem. Since I put it in, it has not been able to keep my shop at 60 when the temperature falls bellow 25 or so. I always thought it was just a bit to small, but the other day while running my 55K BTU reddy heater to supplement the shop heat, I noticed that the reddy heater alone will happily heat my shop to 60. I finally took the time to climb up to the furnace and read the name plate, and lo and behold, it is rated for more BTU than the reddy heater. So, I do the temperature rise math to determine what is actually needed to bring my shop up 40 from ambient. 36x24 shop with 10' walls, comes out to right around 55K btu. The Modine furnace should do it, but it does not.

What I think it going on, is the Modine keeps shutting off the gas valve while it is running, I have noticed it happening, and thought it was normal. It will heat for 20 minutes or so, then kick off the burners and blow cool air, then kick them back on. From reading the trouble shooting info on the modine site for the new unit heaters, this is not normal behavior, and is due to the fire box sensor tripping out on high heat. They also mention that it could be a defective overheat controller. I have a good blue flame, no yellow tips, no lifting from the burners and all the flames are centered well under the HX. So, the question is, where should I go from here? I intend to try to clean out the HX today, but I suspect that it is actually a bad control. Any idea what it uses for sensing box temp?

I know a fair amount about this crap, I used to do residential HVAC with my dad before he passed away. This is the first and only unit heater I have worked on, and it is old enough that the controls are not really familiar to me.


Liger Zero
01-20-2009, 03:49 PM
You know more than I do but I'd start by replacing the overheat sensors. They might be dead due to age issues. Then replace the control valve itself.

I always start with sensors when troubleshooting anything.

01-20-2009, 03:50 PM
Belt drive or direct drive?
Have you checked the limit with a volt meter to see if it is cycling the gas valve?

01-20-2009, 04:19 PM
It is direct drive, propellar type. The fan is running great, and it moves a lot of air. As for the overheat sensor, I have not even found it yet-- Manual for the new models alludes to it being a thermal switch. I just got SWMBO's blessing to come out to the shop for a few hours. I am going to start taking it apart now.


01-20-2009, 05:09 PM
Okay, I blew the heat exchanger and burners out with compressed air. It was filthy. I let it run for about 20 minutes, and it never shut off, maybe the fire box was really over heating. So far, it has brought my shop up about 10 in the 20 minutes or so. That seems much better than in the past.

New issue after blowing it out. All of my burner flames are a blue cone with an orange tip. The modine trbl shooting info says yellow tip is to rich and to add primary air. The gas manifold was already almost back to it's maximum point, but I moved it a hair further and got orange tips still, but wavering blue flames. Wavering blue means to lean, so I put it back where it was. Any idea what orange means? Am I just burning off some crap I knocked loose blowing it out. Keep in mind that after 20 minutes, the orange is still there.


01-20-2009, 05:42 PM
I have an older "Dayton" unit heater from maybe back in the 80's that I bought for a song in the local want-ad paper. Since Dayton is Grainger's brand I was pretty sure it was made by someone else and a little investigation showed that Sterling made them for them. I have the manuals for the Sterling and I am pretty sure the manuals are available from the research I did before I owned one by going to Modine's site. They will show you the "HotDawg" series stuff in the sales lit section, but they still make the other series which goes to much higher outputs, so dig around.

I don't think you mentioned whether it was NG or LP, but if its LP, then the orange tipping is to be expected even in the ideal from what I am told. Mine does it and especially shortly after startup. The heater I have has separate air adjustment for each burner (a typical furnace burner butterfly opening). The way you describe it, it sounds like maybe yours does not. I would think this would be required for multiple burners on a single gas manifold as I would think the gas pressure would drop as you got further away from the inlet...making for a different mix at the end of the line, so to speak.

Do also make sure that you are not running an NG furnace on LP...it could certainly explain the low output. I converted mine and have been delinquent about getting the correct label for the furnace. Grainger will give you the label for free as it turns out...so I have no excuse. My point is that it would be easy for one that had been converted and not properly labelled to change hands with the wrong label on it.

Most newer gas valves have an easy conversion kit, but in my case, I found an unused Honeywell gas valve (a standard item) packaged with the conversion kit on Ebay for less than the conversion kit would cost locally. A friend in the LP business had some drive-in brass spuds with only a tiny pilot hole. You drill out the burner orifices to take the spud, press in the spud and then carefully drill it to the proper number size drill and then screw the modified burner orifice back in. A guy could easily make these too.


01-20-2009, 05:49 PM
Mine was originally NG, somewhere along the line it got converted to LPG, before we hung it, my Dad converted it back to NG. I remember we had to drill a blank orifice, because he didn't have the proper one in hand. I may have to pull the orifice back out and make sure we got the correct size in there. If I remember right, he had a chart of some sort that specified orifice size, but I have since found the manufacturer specific info on the modine site. As for the right manuals, I have found several that are close, but none for my exact model...

01-20-2009, 06:02 PM
As it turns out, the burner orifice size is fairly straightforward based on the gas used and the number of BTUs for each burner.

It sort of sounds like you have a single burner orifice? If that's so do you have just one burner?

Mine, (just as an example) is 125K btu and has 5 burners as I recall, making each burner 25K. I went to the chart (actually came out of a LP gas installer's handbook of some sort) and looked up the number size of the orifice. Co-incidentally, I called the vendor and they were prepared to sell me the orifices and indicated the same drill number on those as sort of confirmation of what I was doing. I believe it was a #54 in my case. I think I drilled one number size under (#55 is smaller) after experimenting to see what hole size I really ended up with when drilling. It doesn't take much even in a lathe to end up with an oversize hole using a tiny drill in brass...and a few thousandths is a lot when working with sizes this small. 125K BTU's was really too big in my case anyway so a bit undersize would be the side to err on.

In any case, if I recall the numbers with LP, it would be easy to have the orifice size too small and make a *substantial* difference in the BTU output.

I looked to see if I still had info on the Modine unit heaters I had gathered as I found various used units and I do not. I do remember calling about a really old one I was looking at and they claimed they were not convertable. I will poke around and see if I can find their archive of documents. I would be tempted to call them if I were you. I can't imagine they won't help since failure to do so could be a safety liability.

Edit-- this link contains a lot of documents on their unit heaters. I don't know if yours is in the list or not:


01-20-2009, 06:21 PM
Orange means you are burning up the rust you disturbed. Yellow means not enough air for proper combustion. Which is it?


Edit: Ok, reread your post and see that you are seeing orange. Don't worry about orange, it will go away eventually.