Any HVAC guys here?

Post by **Jacampb2** » Tue Jan 20, 2009 11:21 am

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 shit, 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.

Thanks,
Jason

Post by **VW337** » Mon Jan 26, 2009 12:51 pm

i'd say contact a reseller and see if they have the overheat controller it may be whacked out or failing.

Post by **Jacampb2** » Mon Jan 26, 2009 4:40 pm

Oops, I forgot to update this. I fixed it already. The heat exchanger was just severely covered in shit. I blew it out, and ever since it has happily run and not shut down on high limit since.

Thanks,
Jason