Old 04-07-01, 10:02 PM
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I have a Evcon unit in the attic, model BGU07512A. This is a summer home and is not used very often in the winter even thought it does have heat.

What happens is that the high temp limit switch opens and I get a status of 4 blinks, the gas shuts off. After a short period of time the unit cycles and I get heat again. I replaced the high-temp sensor (L175-30F 2702-316/A) and the unit operates the same so it was not a faulty high-temp limit switch. Why is the unit overheating? I verified all vents are wide open and the intake filter is new and I do get flow at the vents so I dont thing I have blocked air flow. Does the unit/blower assembly need cleaning? And where/how do you get access to this?? Any Suggestions?
Old 04-08-01, 04:52 AM
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Try this

With power off remove panels to unit.Use a good flaslight and examine the interior,the coil and make sure the blades on the blower wheel are not caked up.I suspect an air restriction somewhere.PDF Also make sure that the cabinent insulation has not come loose and is blocking air flow.
Old 04-08-01, 06:08 AM
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My Two Cents

Hello: JSG

If the letter 'A' in the model number stands for air conditioning, which I suspect, since the unit is in a summer home, the air conditioning coils could be the direct or indirect cause of the condition you stated.

I would suggest you do as PDF stated. Besure to remove the panels around the A/C coils. These coils will be located directly above or to one side of the heater unit, depending on the design and air flow of the heating unit.

Chances are the coils have become embedded with excess lint and dust. This condition will restricted the free flow of air through the coils causing the firebox to overheat. Which in turn will trigger the high limit switch to open.

Another remote possibility is the volume of air flow through the registers. You stated you get air flow. This is subjective to several factors. The question is how much air flow is coming through the registers and is it about equal at each register?

No reply to that question is needed, since the amount of flow cannot be discribed in text only. The method to test, without equipment, is to access the attic and verify that no ducting tubes are crushed, bent, kinked or internally restricted somehow.

Also be sure there isn't any diverter baffles closed or partially closed, if the ducting has any.
Old 04-08-01, 06:20 AM
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Just a thought.When was the last time you used heat?It is possible that a bird has built a nest in your flue while you have been away.You will have to climb on roof to check it out.PDF
Old 04-10-01, 12:27 PM
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Ok I started with the easier item first...

I climbed on the roof and checked the flue. It has a cover and is clear, no nests.

Yes the unit is also air conditioning.

The unit is mounted horizontal. I removed the two side access panels and exposed the burner and the control box/blower side.

No obvious blockage and I have not yet pulled the motor/blower out, as I have to move the circuit board out of the way first. But a quick look at the overall inside of the cabinet is not dirty at all and the air intake is open.

Is there a way to inspect and clean the heat exchanger? Is this a sealed unit? As all I could do is to try and peak in the burner holes. I do not see any access covers to remove. Would removing the exhault fan give access to the heat exchanger?

Inspected the ducting tubes that are accessable from the attic and they look ok. None were crushed or bent.

The point I am at is to open up the access panel for the AC coil. Could the AC coils really restrict the air flow that much??

Old 04-10-01, 01:58 PM
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You are doing a good job

Yes,do not tear anything else apart until you inspect a/c coil.PDF
Old 05-31-01, 09:33 AM
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Ok, finally an update. As I had said this is a second house and I finally got back there and checked out the A/c coil. Opened up the side access covers and looked across the fins. Its CLEAN and the metal is shinny. No dust accumulation on the coils that I could see. I also tried to look for any duct blockages, none noted.

Now what! Whats wrong with this units that it overheats? Any suggestions would be appreciated. Is there a way to measure the blower flow rate? Might this be the problem?
Old 06-01-01, 03:10 AM
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Buy a digital thermometer

the pocket version.Make a 1/4" whole in the supply duct 3' from unit.Place fan to on for 5 minutes.Take a temp reading.Start heat and watch thermometer.The temp should go up to around 130 degrees.If it goes up to 160,170,180, air flow is problem.Take a "DELTA" T and give us a holler back.PDF
Old 08-28-01, 08:30 PM
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Well heating season is just arround the corner and I need to get this unit working correctly. The original problem is unchanged. The unit when heating overheats and the high limit switch cuts out. The unit does do a good job of cooling the house!

I took the temperature measurements requested.

The room temp was 75degrees F and the unit is in the attic, several degrees hotter.

Run 1: Digital thermometer 3feet from exit of heat exchanger and the exit side of the A/C coils.
Time Temp degrees F
0 69.5
1 min 80.7
2 min 94
3 min 107
4 min 120
4 min 43 sec 126
Unit cycles off, led indicating high limit switch

Run 2: Digital thermometer at air intake to heating unit.
Time Temp degrees F
0 82
1 min 83.6
2 min 87.5
3 min 90.7
3 min 5 sec 91
Unit cycles off, led indicating high limit switch
Note unit is now warmer after the first run and shuts off quicker.

Run 3: Digital thermometer at air exit from heating unit and before the A/C coils.
Time Temp degrees F
0 87.5
1 min 114
2 min 136
3 min 148
3 min 40 sec 155
Unit cycles off, led indicating high limit switch
Tried to quickly move the thermometer and get more readings:
At far side of A/C coils (same as run 1): 125
At air into heating unit (same as run 2): 92

So Delta across unit is 155-92= 63

Well I can see from the numbers above that the high limit switch is doing what its suppose to do!!!

Do these temperature numbers give a clue as to what is wrong??

Also just to say something about an early item. I did inspest for cleaness in the unit, blower motor and the exit side of the A/C coils. The only place that i could not see it at the connection of the heating unit and the A/C coils.

Could the input side of the A/C coils have dirt?? And can the A/C coils be seperated from the heating unit for inspection and cleaning??

Thanks for your help.

Old 09-06-01, 03:31 PM
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How big is your home? Maybe your furnace is oversized, and/or you don't have enough vents for your furnace to "cool" off while it's running in heating..

How many supply air vents you have? Returns? Look at your furnace plate in the burner area, and look to see how many BTU's it is.
Old 09-07-01, 06:07 AM
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I am not currently at the house but as I remember (and by the model number) its a 75000bts unit. The house is a small summer home about 800sq feet. With no insulation! The house is located in NJ.

For Vents:
bedroom #1 1
bedroom #2 1
bathroom 1
Kitchen/living room (studio style) 3
TV room 1
Total of 7 vents.

Only a single return in the Kitchen/living room that uses a filter 14 x 20.

If the unit is oversized what size would you expect installed? Is there any way to decrease the btu output?

Old 09-07-01, 11:40 AM
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I just moved, so i don't have my books handy right now on how big of a unit is should of been. To me, 75,000 sounds pretty large... My home is a 1,300 sqft, and i have a 72,000 btu furnace. When I get to my box with that books, I'll get back to you!

There really no way to down size the system right now, other than having a smaller orface put it.
Old 09-08-01, 04:12 AM
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Your "DELTA T"

Sounds okay.High limit should not trip at 126 degrees.Unit was probably oversized due to lack of insulation.As Jay11J stated you can decrease the orifice size to decrease BTU input.How many burners are in unit?3 or 4?You can always blank off an orifice to cut down on BTU input.I forgot ask when you posted original thread....had system been installed and running good for several years?PDF
Old 09-08-01, 06:19 PM
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The 126 was measured after the A-coil, I measured 155 between the A-coil and the heat exchanger. I then tried to quickly measure the air input to the unit and measured 92. That gives me a Delta T of 63 degrees. From the numbers there is a large temperature drop across the A-coil, is this normal?

<----3 Feet----------->
Old 10-08-10, 05:00 AM
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I was following this topic as I'm having the same exact problem with my heater popping the top Limit Switch. This topic goes from 1 to 14 but ends without a resolution. Is there more to this topic and how do I get to it?
Old 10-08-10, 03:45 PM
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Make sure furnace filter is not too high a restriction variety. If not sure, try to run furnace with filter removed (temporatily for testing purpose.).

Run furnace with blower door open (tape the safety switch closed) to see if that changes things. If it does, you may have a cold air return restriction.

You can drill 1/8 hole right above limit switch (must be in vertical alignment so not to drill through heat exchanger! All you have to do is penetrate the sheetmetal and perhaps a fraction inch insulation layer, then another sheetmetal layer) and insert one of those small-dial (1 inch dia.) thermometers, (with 1/8th x 6 inch rod) that go up to about 220F, to see if your limit switch is close to accurate.

Make sure registers are all open.

Make sure duct dampers are all open, if you have them. If you have them, the slot in the adjustment screw points with the duct, when open. Cross-duct when closed. Don't go by which way a handle is pointing. Go by the slot.

If you have central a/c, remove blower and inspect secondary heat exchanger if your furnace is of the high efficiency variety. You would be looking to see if the fins are caked with dust.

Ditto for the A-coil above the furnace, in the plenum above. Careful work must be done to gain access without bugering up the sheetmetal seams.

IF you have slotted (or with tiny holes) burner tubes, remove them and wire brush clean them. A hot spot at the front of the exchanger, below the limit switch, will occur and can channel most the heat at the limit switch, rather than cooler uniform heat throughout the heat exchanger. I recently had that on a rental service call, and by me cleaning them, I increased the time of high limit by 2-3 times!

If you have the gun-type burner that shoots out burning gas like jet engine, make sure heat deflector is intact and you have nice blue fire.

Make sure that blower vanes are not caked with dust.

To improve on high limit time, if you have multispeed motor, and a/c, you can use the high speed a/c wire at your control board rather than the medium or medium high speed for heat. Remember to swap it back again for the cooling season. Make note on furnace!

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