Heil tripping hi limit - http://www.hvacmechanic.com/ Forums

Original message

oscarhobson

99.142.3.152

"Heil tripping hi limit" , posted Sun 13 Apr 12:08

I have an older Heil furnace (from around 1991-92), standing pilot, that is tripping the hi limit on the fan switch. I live in a 2 story townhouse on a slab with the furnace in a furnace closet. I've had 2 service calls in the past few months that didn't fix the problem. The first tech replaced the fan/limit switch, and this seemed to fix it while he was here, but within a day or two it started tripping again. On the second call, the tech replaced the limit switch again, and when he turned the power back on he had forgotten to change some jumper on the switch, so it fried the gas valve and the tstat, both of which he then replaced. The limit was still tripping, so he checked the manifold pressure - it was set to 4; he dialed it down to about 3 1/4, and this seemed to fix it. The next day the limit is tripping again. It's warm enough now that it doesn't seem to trip unless I try to raise the temp a couple of degrees and make it run for a while. The filter is new. The second tech cleaned the blower fan/motor, and looked at the A/C coil to make sure it wasn't blocked, though he did make the comment that he didn't like the design on the A/C coil as far as the air flow it would allow. All vents are open; all returns are unblocked. He also checked the size of the return pipes and vents and seemed to think these were adequate. Any thoughts or ideas as to what may be causing this? Any help is appreciated, I'm about at my wits and dollars end.

Oscar Hobson

Replies:

QualityHVAC

71.48.19.124

"Re(1):Heil tripping hi limit" , posted Sun 13 Apr 14:24

If coil is clean and blower wheel is clean and they changed the parts you said they did. I would have them check the capaciter on the blower motor it could be weak and causing the blower to shut off on thermal overload and that would cause the hi-limit to trip. If capaciter checks good could be intermittent prob. with the fan motor itself or speed needs increased.

marchvac

24.57.149.86

"Re(1):Heil tripping hi limit" , posted Sun 13 Apr 13:38

Did any of the mechanics check the temp rise. What you should do is check the temp in the return close to the filter and then check the temp above the coil and give us the reading. You can also remove one of the screws from the limit and check the temp at the limit to see if it is working correctly.

Be careful sometimes you don't get a second chance

tom446

66.66.203.58

"Re(2):Heil tripping hi limit" , posted Sun 13 Apr 19:20

If air flow and tempurature rise all check out, being an older furnace, I wouldn't rule out a crack in the heat exchanger.

If in doubt call a pro !