my question is I have 27.8 volts coming into the fuse holder should I have a fuse that is 3 amp with a 30 volt capacity and or can I increase the fuse

What should the device be fused at?

Does it blow with the t-stat off? Or just with cooing on?

It blows while the stat is on also the fuse that I found in the stat was a 3A 250Vdoes that matter?

I don't have the mannual but from what I hear the fuse should be 3 amps does the 27.8 currentto the fuse holder have anything to do with it blowing I thought the transformer was to make the 120v reduced down to 24v ? Is it normal to see the extra 3.8v

So you understand, I asked because if its actually spec'd for a 3 amp fuse. (Sometimes a wrong fuse gets installed). Then there is a malfuction that causing excessive current drain. Going too high in value can result in as a worst case situation a fire... or at best irreperable damage to the device and/or wiring.

smoothy,josh, I really appreciate your input! I went to the condo today bought more fuses and this is what I found out I turned the power off, I put the new fuse in, put the unit switch in the off position ,because there is a off switch between the

just when I put it in the cool position, heat and fan work in on or auto position.josh thanks I really value and appreciate your opinion!! As well as your help.thanks

lancebassett,
Control fuses can be tricky some times because the problems are usually intermediate.
However, if you have 27.8 volts for a secondary voltage you have to many volts.
Your secondary voltage should not exceed 24 volts. You could have a defective transformer. But, more likely the problem is going to be a short or shrunt in the coil of the compressor contactor some where in your control circuit.
Hope this helps
Jimmy The Air Guy

Hey jimmy I just found your response if you read my last question you already know I'm not familiar with air conditions any ideas how I can test for that and if it is not that do you recommend that I replace the trans or the thermostat wadathink I app

Could be blower or compressor relay coil, or control wiring between the 2 units, or the thermostat wiring.
Transformer or thermostat may not be your problem. Many transformers read High with no load.
See if you can follow the control wiring ran with freon lines.
Personally I run control wiring in conduit with the Supply lines to avouid damage.

Hey start could you walk me through that test I have a pancake unit in the ceiling I pulled the covers there was a relay ,trans, fuseble link,two squirl fans what do I look for

I don't recommend up sizing your fuse because you obviously have a short in your control circuit. You can diagnose your system and locate the short by isolating the circuits.
For Example:
* Take the wires off the contactor coil replace fuse and power up.
* Disconnect R and C from the thermostat replace fuse and power up.
* Locate and disconnect the coil wires on your blower relay replace fuse and power up.

You should be able to at least determine where your short is at.

If it only blows when the ac is on. Most likely the contactor outside has went bad. http://www.chinacontactor.net/Products/Contactor/MBC-1N-AC-Contactor.jpg

Hey Josh, How can check if the contactor is bad ?

Go to the outdoor unit, find the small t-stat wire. Take the 2 screws out of the door where the electric goes in. The t-stat wire will have 2 wire bugs on them. Just take the wires apart make sure they are not touching any metal. Put a new fuse in and give it a try. If the fuse blows, not the contactor, if it does not then the contactor is bad.

I believe he is saying, if fuse blows with contactor disconnected, contactor is not your short.
You could disconnect wires in air handler that go to contactor coil to see if the wires are shorted on the way outside.

This thread is getting out of hand. See this one for some ideas:

https://www.askmehelpdesk.com/heating-air-conditioning/transformer-blew-just-before-hearing-many-click-sounds-377952.html

Kiss, I have a 240v step down to 24v 40amp rating transformer, I checked the power coming out of the transformer it is 27.8v the same going to the fuse holder, would you say it's the transformer or is it normal to have a trans exceed its limit of 24v

The transformer has no regulation. Only a ferroresonant transformer does and the one inside your HVAC unit isn't one.

Here is a difinative answer: Application Notes AN-7 24VAC Transformer Output (http://www.securitypower.com/AN724VAC.html)

That basically says, it's within normal operating range of the transformer.

I'd be curious what the input voltage is. It should divide by 10.

Do the light bulb trick and have someone watch while you wiggle wires. Use intercom mode on a cordless phone. Use a cell phone.

If it's not intermittant, it should be easy to find.

The more troublesome connection is the one to the outside unit. If you disconnect wires going there at the furnace, and the light doesn't light, the problem is likely between the furnace and the outdoor unit.

One HVAC guy I knew went as far as replacing a water to air HP in the ceiling and it still wouldn't work. Using about 10 min of my time, the thermostat wire had a short. No insulators in the metal studs and the wire was yanked during a remodel. Yep, he could have save some serious bucks.

One HVAC guy I knew went as far as replacing a water to air HP in the ceiling and it still wouldn't work. Using about 10 min of my time, the thermostat wire had a short.

I would not call him an HVAC guy being so stupid to not check the low voltage wiring before replacing the unit.

25+ years of experience too.

In a walk through of the renovation, I discovered one of the HVAC water loop valve operator stuck in the firewall and drywall compounded around it so it couldn't be moved.

The contractor who built the building was a little looney too. The building called for an isolated pad probably 50' x 75'. They had a novel way of creating one. They poured the entire building floor and then jackhammered out the part that was to be isolated and then poured the isolated pad.

The HVAC system installed (new building) was looney too. In one instance some bozo installed 2 HP's in the same room with two auto-changeover thermostats in the same room. One per pump. They also installed the condensate drains into the sanitary sewer and didn't allow enough pitch. There were at least 36 water to air HP's in this building. Basic disaster.

Kiss, Also thanks for the application, that's the kind of info that I understand, simple and to the point

But still, the voltage is not an issue.

Lots of issues: # turns, core magnetization, how much load is placed on the transformer.

JimmyTheAirGuy disagrees


You disagreed on post #1 when I was asking what the device should be fused at? Have you ever seen something that someone put a bigger (or smaller) fuse in than belongs in it? Just because you took a fuse out of XX ratings you can NEVER assume that was the correct fuse in the first place.

Disagrees whould only be used in cases of factually incorrect information.

I have a degree in Electronic Engineering... and you never make assumptions that what you took out was the correct item to begin with just because it was there.