Last week my air handler quit working, I found that the 5AMP fuse for the secondary 24volt pwr supply was blown, replace and all worked fine for a week. Now I find it blown again, but each time I relace it it blows, The voltage reading I get at the fuse is 29.5 V is that to high? The transformer is 60hz/208v/230v 24 V load 40VA. Or may it be the 24V circuit board?
Bob

I would start by carefully going over all the control wiring. It could be a wire has rubbed through. I lost a couple of transformers before installing an inline fuse on my old furnace. When I replaced it, I discovered chipmunks had chewed the thermostat wring.

The wiring looks good. But was wondering if the 29.5 volts out of the transformer is to high indicating a faulty transformer

Quote:

---

Originally Posted by labman

I would start by carefully going over all the control wiring. It could be a wire has rubbed through. I lost a couple of transformers before installing an inline fuse on my old furnace. When I replaced it, I discovered chipmunks had chewed the thermostat wring.

---

All wiring looks good. Is the 29.5 volts out of the 24V transformer to high?

No 29 volts is not to high, something else is shorting out. Just a hunch, but look by the A/C unit for bad insulation in that low voltage wire.

Quote:

---

Originally Posted by frostrc911

Last week my air handler quit working, I found that the 5AMP fuse for the secondary 24volt pwr supply was blown, replace and all worked fine for a week. Now I find it blown again, but each time I relace it it blows, The voltage reading I get at the fuse is 29.5 V is that to high? The transformer is 60hz/208v/230v 24 V load 40VA. or may it be the 24V circuit board?
Bob

---

I don't think it's a short, because the blower tries to start up. I had it in heat put a new fuse in and reset the 220 bkrs. I first saw a smal spark on the sequencer termianl the fuse went. Reading 29.5 volts instead of 24 also.

That fuse controls the low voltage circuit of the control wiring, so it won't effect the blower. You should not have seen a spark at the sequencer, start there.

The 29VAC has me thinking the board may be bad or the programmable termostat. Did disconnect all thermostat wire to the low VAC , air handler side and those going to heat pump and read between all for shorts no short. The Time delay circuit is a questionable item also or is it?

29 volts is not an issue, I see 22 to 31 volts all the time and nothing is wrong. The sequencer has a heating element in it that slowly closes the high voltage contacts. The heater could be shorted out. I really don't think it is the thermostat. You can try disconnecting all the heat pump wires and burn up another fuse if you like. Or observe when the fuse blows. Ex. When t-stat is completely off? When you turn on the heat pump? Or when you turn on the emergency heat?

Okay, just replaced fuse, turned thermostat to off but fan to on. All works fine, blower was advertised. Turn to heat or AC and fuse pops.

Disconnect the sequencer in question and try again. Please be careful.

This is weired. Turned T-stat off. Fan on. Fan came on. Went to AC.. droped temp and HP came on blowing cold air. Went to heat and it produced heat. Went to recover and emergency, and both ran.. did put back in normal auto. Still running. Could the T-stat benn confused? Did bump it hard last week when it first went out. Maybe in H mode with recovery or emergency to start overloads something.

I thank you for your help. I have been in the Navy for 29.8 yrs now as an AME (aircraft climate control, and E-ject seats). I've learned to be safe. Thanks again. Master Chief Bob/KC5GOY

Secondary voltage is a product of the turn ratio between the primary and secondary side of the transformer. i.e. 240volt input with a ten to one ratio gives 24 volts on the secondary. Primary sides have "taps" they come with the primary side connected for a 240 volt input. If the voltage input from the power company is less than 240volts, the secondary side of the same 10 to 1 ratio would cause the secondary side to increase voltage. The amp carrying capacity of a transformer is the secondary is the VA rating divided by the secondary voltage... in your case... 24 into 40va = 1.667 max amps. If you have a 5 amp fuse, it should hold and not blow the fuse... look for the intermitent short where wires are kinked, pass through or by a sharp object, where water or moisture could cause a short... does the fuse go bad in the early AM when there is high humidity and dew on the ground?.

Carrier is known to have low voltage shorts on A/C and heatpumps. The low voltage wires from the reversing valve coil, high pressure switch, defrost sensor and low pressure switch will rub into the discharge line of the compressor and make the fuse blow. Turn power off to both units and check the wires around the compressor to see if they are rubbing on the refrigerant lines.

Quote:

---

Originally Posted by T-Top

Carrier is known to have low voltage shorts on A/C and heatpumps. the low voltage wires from the reversing valve coil, high pressure switch, defrost sensor and low pressure switch will rub into the discharge line of the compressor and make the fuse blow. Turn power off to both units and check the wires around the compressor to see if they are rubbing on the refrigerant lines.

---

Check the white wire coming from the defrost control board. Seems like when the system goes on defrost mode or the temp outside falls below 40F the defrost control board sends 24vac signal to the white wire to energize your aux electric heat in the indoor unit, which causes the fuse to blow, if this white wire is shorted to ground this will definitely open the 5 amp fuse.