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Matthew
Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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air handler wont come on

Resolved Question:

air handler won't come on
Submitted: 3 years ago.
Category: HVAC
Expert:  Matthew replied 3 years ago.

Matthew :

Hello, My name is XXXXX XXXXX I will do my best to help you today.

Matthew :

Have you checked the high voltage breaker to the unit?

Matthew :

If you set the thermostat to the fan "on" position does the blower come on?

Customer :

air handler failed. system was installed in a san diego condo in 2004 and gets light use. Has never been a problem. Was getting use most of today. About 90 mins ago, the air handler came on and within 5 secs, shut down. I went to the thermostat and shut it off, then turned heat back on. All i heard was the normal thermostat click whenever the air handler starts up but this time, no air. Tried using AC and same thing...click at the thermostat but no air handler. Checked the circuit breakers in the main panel and nothing tripped. Wondering if there's one on the air handler

Customer :

where is the high voltage breaker located?

Customer :

and no, moving fan to the ON position does NOT bring on the fan.

Matthew :

the one in the main panel that is what I meant by the high voltage breaker

Matthew :

There is a 3 amp fuse located on the control board of the furnace. It will have a redish purple top and looks just like a car fuse.

Customer :

main panel in the condo is fine

Customer :

that's probably the culprit

Matthew :

Full Size Image

Customer :

the 3 amp fuse

Matthew :

in this picture it is in the top left corner

Customer :

where is it located on the hvac?

Matthew :

It is either that or the transformer in the furnace is fried due to a high voltage short. The breaker in the panel should trip before the transformer is taken out by high voltage shorts and the 3 amp fuse blows to protect the transformer from a low voltage short.

Matthew :

Take the top door off the air handler and you will see the board.

Customer :

well let's hope it's the 3 amp...guessing that's a hell of a lot cheaper than the transformer

Matthew :

you got it

Matthew :

now if it is blown and you replace it and it blows again immediately after power is restored then you have a low voltage short somewhere. Which means one of the thermostat wires has lost it's protective coating and is bare and touching something(this could be the wire leading from thermostat to furnace or the wire leading from furnace to outdoor unit.) the thermostat is shorted out, or there is a 24v part in the outdoor unit that has gone bad.

Customer :

its not a 3 amp...its a 5 amp...and it doesn't look blown

Customer :

the metal "S" looks intact

Matthew :

do you have a volt meter?

Customer :

yes

Matthew :

5 is fine some use a 5 some use a 3 just can't use bigger than a 5

Matthew :

Find the transformer here is a picture.

Matthew :

Full Size Image

Matthew :

Full Size Image

Matthew :

you need to find the two 24v wires coming out of the transformer and test to see if there is 24v leaving the transformer.

Matthew :

If there isn't then you need to test the high voltage side of the transformer and see if the lines tying into the high voltage wires of the transformer are feeding 240v.

Customer :

should there always be 24v leaving the transformer or do i have to have the fan ON when I do this?

Matthew :

should always be 24v leaving the transformer as long as there is 240v feeding it

Matthew :

the control board then decides when to turn what on based on what the thermostat is telling it.

Customer :

ok...my volt meter is too simple for this task...it simply test for elec...light comes on if the wires are hot...

Matthew :

So if there is 240 to the transformer and no 24v leaving it then you need a new transformer

Matthew :

yeah you need a multi meter that can read 240 and 24

Matthew :

Sometimes a fuse doesn't always look blown but is still bad

Customer :

and is it located close to where I found the 5 amp fuse?

Matthew :

so it's worth a shot for the 50 cents it costs

Customer :

that's what I was thinking...

Matthew :

yeah the transformer will be in the same area as the control board

Customer :

ok...will home depot have one or do I need to go to an hvac store and about how much is a fair price for one?

Matthew :

There are a lot of things that could be going on if you do not have 240v going to the transformer. Like a bad breaker in the main panel, a bad wire in the wall, a bad disconnect on the side of the furnace

Matthew :

home depot will have these fuses yes.

Customer :

actually i meant the transformer....

Matthew :

The transformer probably not. They may have one that is a universal that you would have to pay close attention to when hooking back up to get the right wires for 240v.

Matthew :

let me check

Customer :

if it's not the fuse, think i'll hire someone to do the work. 240v is a lot of juice...not a great way to start the new year if I connect with those wires

Matthew :

I was just about to say if you do not have good experience with working on high voltage I do not suggest it.

Matthew :

I looked and home depot doesn't carry a 24v step down transformer.

Matthew :

this is something any service company will have on the van.

Matthew :

one more thing

Matthew :

you can do yourself

Customer :

ok....there is a wall switch next to the hvac which killed the power to the unit when I switched if off so I guess that helps a bit but

Customer :

think I'll still defer to the experts if the fuse replacement doesnt work.

Matthew :

It is possible the thermostat is bad. So at the furnace control board take a small jumper wire and connect the G terminal with the R terminal and see if the blower of the furnace comes on.

Customer :

the thermostat has an electronic display which is all lit up...thinking it's ok but maybe not

Matthew :

normally those displays are battery powered

Customer :

are the g and r terminals marked?

Matthew :

and you may have power to the furnace but it isn't closing contacts to that power to turn stuff on.

Matthew :

yeah they should be

Customer :

ok

Matthew :

should be the terminals with the red thermostat wire going to it and the green thermostat wire going to it.

Matthew :

meant to say you may have power to the thermostat not the furnace in that last sentence.

Customer :

well that helps. and a jumper wire can be just about any wire?

Matthew :

yeah anything metal a paper clip would work

Customer :

yep...ok...got it....thanks. appreciate your help tonight. hope it's just the fuse

Matthew :

no problem

Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience: 14 years experience in the HVAC field of service, installation, Manual J & D calculations
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