Hot Dawg 75k BTU Eating Transformers

[Red89gt]

Hoping you guys can help me out or at least see if you have any idea's what's going on with my Hot Dawg 75AH0134. My wife had it installed about 10 years ago when she had the house built before I met her. The unit is immaculate and not even dusty inside when I pulled the access panel off to start what has been kind of an embarrassing trouble shoot path. Kind of long but I will spell out where I am today.

First I noticed walking by the t-stat that it was off, cheap honeywell digital heat only control that failed years ago. Okay, easy fix, figured put another on and down the road. Installed new one and nothing happened. That led me up a ladder to the main unit. Sure enough no power on the step down 24 volt transformer, thought nothing of it and ordered a replacement and put it in. Applied power and it died, let out an f bomb and scratched my head.

Didn't know the standard practice until I go online the thing to do is put an inline blade automotive fuse to protect it since the controller board on this unit does not have a reset breaker or fuse. Ordered another transformer and put in a fuse, didn't realize I didn't put in the 5 amp but a 10 amp that was in my bin that was the same color. I also changed the controller board since all the power gets routed through it and fig it had a short. Wrong again, ***** to be me and tool learning the hard way to fix a furnace. However I disconnected the harness that runs from the controller board to the gas control valve. With the gas control valve disconnected the furnace stayed powered up and didn't blow the transformer and the t stat had a reading so it was powered as well. I checked the wiring that goes to the stat with a multimeter as well before powering up and no shorts apparent. It rang out fine and had resistance.

So with the power on, controller board powered, t stat reading and working and transformer not burning up I went to the next step. It was 75-80* in the garage and I had the transformer on the lowest setting, 40* so it would not call for heat. I plugged in the C1 main harness from the controller board to the gas control valve and that is where the 10 amp fuse bit me, another transformer burned up.

What can cause the transformers to blow like that when the system isn't even calling for heat, just sitting static?

I have googled my brains out and cannot find anything. Am I way off to think it is the gas control valve that has a short that is putting the heavy draw on the transformer? I have gone over every wire and they are all like new, no bare spots etc. The freaking gas valve is a honeywell part number SV9410M2902 (modine p/n 5H76382) assume the sv on the honeywell number is for smart valve? The valve is $300 best price I can find online, very pricey and I don't want to be a part swapper but with the harness off of it the unit stayed powered up until I plugged it in. There are a few sensors that also go from the valve out on the same harness.

 

[Cruzingoose]

Typically a small 24 volt 20 watt transformer. The circuit is..... wire from transformer to furnace gas valve, from gas valve to thermostat, from thermostat back to transformer. The secondary is not grounded. Primary is usually rated at 115 volts input, but some neighborhoods run 127 give or take a few volts. Short circuit is not really possible. Don't bother with the fuse on the secondary side, it wont do a thing for you.

So, the possibilities....

[1] Cheap Chinese **** Transormer.
[2] Huge AC Mains power surges. (You will have problems in other places also).
[3] The last transformer was the wrong type, wrong voltage on primary or secondary.
[4] The transformer is used to power other devices on the furnace such as power
vent, damper motor and ignition. If this is the case, you will need a heavier duty
transformer with a rating of 75 watts or better.



A transformer in this application will usually outlast the lifetime of the furnace. Buy a QUALITY transformer from HVAC distributor.

 

[Red89gt]

Thank you for all the info.
Transformer was a Modine oe replacement in Modine box.
Knock on wood never had surge issues or damage to anything else in the house.
Transformer only powering furnace as wired from factory.

 

[Hugo Dave]

You said you replaced the thermostat. The troubleshooting section on my Hot Dawg manual states "if failed transformer - check thermostat wire gage and length." Good Luck!

 

[Red89gt]

Actually finally traced back to a short on the Honeywell smart valve. Was manufactured during a run of questionable soldering failures on the internal module. Needless to say it was 10 years old, valve was $300, xfrmr $60. I replaced the unit with a mr. Heater big maxx for only a few bucks more.