No 24v at furnace fan relay or heat pump contactor

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  #1  
Old 05-14-19, 06:44 PM
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No 24v at furnace fan relay or heat pump contactor

Purchased a mobile home last year. Furnace and heat pump where installed in 2014, Coleman model EB15 furnace and THJD36S41S4 heat pump. I think its same as a York and there are Johnson controls unitary products notations on all the manual pages. Thermostat is a Honeywell TH8320R1003

After not using the system at all other than a test of both heating and cooling at purchase last May, I turned it on for heat in November. All good for a number of weeks then one night it didnít shut off. Tstat set for probably 70 but unit was still delivering heat and inside temp was 80 I think. Shut power off to it and did a little online learning (never had a heat pump or AC system before). Opened up outside unit by removing fan and cleaned it out, blew out the electric connection area thinking that was going to clear out a bug or two that was maybe causing the issue. Put everything back together and all seemed good.

I thought.

Noticed from time to time that the tstat was reading aux heat. It was colder than usual and I just thought that it was needed to keep heat up. Paid a huge electric bill for one month and really started to pay attention to the tstat and found that the aux heat was always on when calling for heat. To be clear the tstat was set to Auto thru all this. I still wasnít bright enough to realize there was a problem, I just lowered the tstat put on a sweater and sucked it up.

Fast forward to first hot days of spring and no AC. No blower while calling for cool and watched closely as I started testing things and no blower when calling for heat (set to auto still) would only activate blower when tstat went into aux heat after a brief wait for heat pump to send heat.

No blower with tstat set manually to blower on, or circulate.

No blower with tstat set to cool or heat.

It WOULD come on if I manually selected emergency heat on the tstat.

Heat pump compressor and fan would never come on in any mode.

After spending a bunch of hours getting my hvac degree at googleyoutube university I did the following trouble shooting

Testing found capacitor at contactor and blower to be good at least testing it with digital meter, not able to test actual capacity.

Fan relay tested good.

Contactor tested good.

Sequencers tested good.

High pressure and low pressure switches each have two wires coming out of them. Of those 4 wires one from each switch are tied together in what looks like a factory connection that ends at that connection. The other two wires, one from each switch go to two separate tabs on the heat pump control board (wire diagram calls it the defrost control) marked pressure switch. Pulled those two wires from the board and they have continuity so Iím assuming both switches are closed and not causing the problem. Wire diagram also shows optional heat temp switch for some models in with the pressure switches but I donít see one.

Two blue wires from defrost sensor inside heat pump going to tabs on defrost control when removed also have continuity.

Found that with auto demand for cool or heat after reasonable wait, never get 24v at the fan relay or the contactor.

Pulled tstat and connected wires that were connected to R/RC, O/B, and Y. Wire colors are red, green and yellow.
Turned power on to furnace and Heat pump. Blower turned on instantly. Heat pump condenser and fan turned on in about 5 minutes and runs fine. Left it on for 20 minutes cooled great no issues noted.

Killed power and took the green wire (originally connected to the O/B terminal) out of the connection leaving just the red and yellow wires connected(originally connected to R/RC and Y terminals). Power back on blower runs instantly and heat pump condenser and fan turned on in 5 minutes again and began delivering heat. Left on for 20 minutes again all good.

So, is the tstat bad or is it miswired? Did I miss something else?

At the tstat
blue wire to C
red to Rc jumpered to R
green to O/B
yellow to Y
white to AUX-E

In the furnace 24v wires go as follows
red from transformer to red tstat and red wire to heat pump
black wire from transformer to blue tstat and blue to heat pump
black from transformer to sequencer then white from sequencer to brown wire that goes to heat pump
black from transformer to fan relay then green from fan relay to yellow tstat and yellow to heat pump
white from tstat to white to heat pump
green from tstat to orange to heat pump

At the heat pump
red to R on defrost board
blue changes to black and goes to C on defrost board
brown to W1/66 on defrost board
yellow goes to Y on defrost board
white to W on defrost board
orange to O on defrost board

I did put in new capacitor for blower fan with no affect before I learned how to test with digital meter vs analog.

Thanks for any help, and suggestions on new tsat choice if that is indeed the problem
 
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  #2  
Old 05-14-19, 11:03 PM
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Very hard to get thru all this.

At the tstat
blue wire to C
red to Rc jumpered to R
green to O/B
yellow to Y
white to AUX-E


What about G on the thermostat ? G is the blower. Without that the thermostat cannot turn the blower on. When you bring on the electric heat...... the sequencer relay in the heat section turns the blower on.

Where you using the thermostat in setback mode for the heating season ?
 
  #3  
Old 05-14-19, 11:42 PM
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I appreciate you taking the time to try to decipher.

I thought it might save time to list everything I did and it became a novel.

no wire is attached to G
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when you say "bring on the electric heat" do you mean aux or emergency? I was wondering why the blower worked then but never any other time. Obviously there had to be more than one path to energize the blower.

Not 100% sure on definition of setback mode. After the big bill I set the temp down significantly other than a few hours before evening and about an hour in the morning. Doing so didn't make any difference as to never getting the heat pump to work.
 

Last edited by PJmax; 05-15-19 at 06:09 PM. Reason: resized picture
  #4  
Old 05-15-19, 12:12 AM
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A thermostat used for a heat pump system will turn on the electric heat if the room temperature is 3 or more degrees below the setpoint. It will run the electric heat until the system catches up. If the heat pump was not performing correctly then the electric heat would be on an excessive amount of time. You have a 15kw electric furnace that uses three times the electricity of the heat pump so the idea is not to use the electric unless needed.

When the electric heat is activated..... a timer/relay called a sequencer is activated. That controls the heating elements and the blower.

In A/C mode the blower is turned on from the G terminal on the thermostat. Some Coleman furnaces have switches to put the blower in heat or cool mode., I don't think yours does so you need to find out why there is no blower wire connected.

It is possible that when that thermostat was installed it required a C connection and there are not enough wires there. So what is done is at the furnace end...... the Y (yellow) wire is also connected to the G terminal. When the heat pump or A/C calls on the compressor..... the blower is also activated.
 
  #5  
Old 05-15-19, 10:35 AM
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Don't know if this helps or hurts but first pic is how my wiring description looks like following the 5 wires in tstat that go to furnace and then on to the defrost board in the heat pump and the one wire that only goes between the furnace and heat pump

There is no board in the furnace. 24v wires from the tstat, transformer, fan relay and sequencer going to heat pump are joined by wire connectors only so there is no G terminal in the furnace.

Heat pump defrost board does not have a G terminal either.
There is no blend air control box in this installation.
The X/L tab with purple wire on heat pump defrost board was never connected to anything. it terminates inside the heat pump wiring area.

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Last edited by PJmax; 05-15-19 at 06:18 PM. Reason: resized/labeled pictures
  #6  
Old 05-15-19, 06:59 PM
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It's as I mentioned..... the Y wire powers the condenser and the blower in A/C mode.

Sorry..... where are we now ?
No blower in A/C mode ?

If you have a meter.... measure for constant 24vAC from R to C.
When in A/C mode...... 24vAC from C to Y.
 
  #7  
Old 05-15-19, 09:38 PM
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issue is no 24v at fan relay or contactor when tstat is in A/C mode or calling for heat.

The compressor, heat pump fan and furnace blower never come on in either mode.

furnace blower comes on with Aux heat.

have constant 24vAC from R to C

no 24vAC from C to Y

not following you where you drew red jumper from G to Y at tstat. G has no wire attached to it.

green wire from fan relay is connected directly to yellow wires from tstat and defrost board in the furnace.
 
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Old 05-15-19, 10:15 PM
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That was to show that normally there is a G connection at the thermostat. In your case you have Y and G combined at the air handler.

You've confirmed that you have 24v at your thermostat but nothing out on Y in cooling/heat pump mode. That would indicate a defective thermostat. Remember.... many thermostats have a 5 minute delay before energizing the Y terminal. This is to eliminate the compressor from cycling too quickly.

At the thermostat.... connect Y and R together. That should bring the compressor and blower on.
 
  #9  
Old 05-15-19, 11:56 PM
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Roger that. It all comes on wired that way as noted in original post. Will grab new tstat tomorrow.

Thanks much for your patience and help.



Pulled tstat and connected wires that were connected to R/RC, O/B, and Y. Wire colors are red, green and yellow.
Turned power on to furnace and Heat pump. Blower turned on instantly. Heat pump condenser and fan turned on in about 5 minutes and runs fine. Left it on for 20 minutes cooled great no issues noted.

Killed power and took the green wire (originally connected to the O/B terminal) out of the connection leaving just the red and yellow wires connected(originally connected to R/RC and Y terminals). Power back on blower runs instantly and heat pump condenser and fan turned on in 5 minutes again and began delivering heat. Left on for 20 minutes again all good.
 
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