This board is awesome. I have done some reading first on here. Here's my case:
Compr runs fine. Fan on HVAC blower inside runs only in Heat mode. Not on Cool. Not even on "FAN-ON" as opposed to FAN-AUTO.

Can this be an issue with relay and board within HVAC? Per other thread, it was talking about using EAC term, this may be relevant.

Also I found this recommended on another thread:

" take the t-stat off and jumper from r to g if still no fan then the t-stat is ok"

Should I try that? Hard to figure. Can relay/board issue affects both FAN-ON and FAN-AUTO-COOL?

Thanks in advance. What other info helps? HVAC 10 yrs old. Combo Oil/AC system
Compressor new last summer. T-stat Honeywell T8400 4-wire (W,Y,G,R) non-prog. My first thot was to just purch and try new t-stat. Then I found out about the relay/board stuff on other thread...

The fan on relay is the same as the cooling relay so yes it could have went bad.

Heating mode will have its own relay

Thanks for lightning-quick answer
That explains (both on same relay). I'll try jumping R-G - and if no joy, then the prob would seem to be relay or other component/issue for which I will call the repairman.

What brand furnace do you have? And if it is 10 yrs old you probably do not have a control board so it may just be a relay that you can change yourself

Furnace name plate= Armstrong
Installed date 9/1999

I actually have little qualm about replacing relay from an elec. Point of view. As long as it's easily accessible and I don't have to do any metalwork to locate/replace/remount. Guess I need to remove panels and try to find it.

Thanks for pointing out this option. Can you point me toward where I'd find it? Is it in a some kind of box? --

Regards!

Should be in the top door of your furnace probably on the right side.

This board is awesome. I have done some reading first on here. Here's my case:
Compr runs fine. Fan on HVAC blower inside runs only in Heat mode. Not on Cool. Not even on "FAN-ON" as opposed to FAN-AUTO.

Can this be an issue with relay and board within HVAC? per other thread, it was talking about using EAC term, this may be relevant.

Also I found this recommended on another thread:

" take the t-stat off and jumper from r to g if still no fan then the t-stat is ok"

Should I try that? Hard to figure. Can relay/board issue affects both FAN-ON and FAN-AUTO-COOL?

thanks in advance. What other info helps? HVAC 10 yrs old. Combo Oil/AC sytem
Compressor new last summer. T-stat Honeywell T8400 4-wire (W,Y,G,R) non-prog. My first thot was to just purch and try new t-stat. Then I found out about the relay/board stuff on other thread.....

In your subject line you say "New compressor works fine" when was this compressor installed and was a new thermostat installed at the same time?

You mention shorting the R wire to the G wire, if you do this and the fan runs that will mean that your relay is good but that something in the thermostat could be bad.
What kind of thermostat do you have? It could be a loose terminal in the thermostat.

On the face of the burner chamber? I found what sure looks like a sort of DPDT relay . Some soot. Maybe contacts need cleaning? Running for the shop vac with the mini nozzle

Now I wonder if I throw the t-stat to FAN-ON can I hear the soleniod coil hum...
Doesn't appear to have a PC board

Wiring interesting. 2 purple on R and 2 Black on L appear in the locations I would expect for if each color pair was for one circuit pole.

But the 2 Blues are not where I'd expect them. One is about mid way between the purps. The other is line with the top Purp and Top black.

Pu---BLU---BLK

---BLU

PU----------BLK

In your subject line you say "New compressor works fine" when was this compressor installed and was a new thermostat installed at the same time?

You mention shorting the R wire to the G wire, if you do this and the fan runs that will mean that your relay is good but that something in the thermostat could be bad.
What kind of thermostat do you have? It could be a loose terminal in the thermostat.


Comp installed 9/2008
Old t-stat
t-stat is Honeywell dig non-prog. T8400 (round)
Pulled case. 4-wire ends secure to mounting block. 4-pin SIP block makes connections to T-stats
Have not accessed interior of T-stat to chk for connections
Roger that on shorting the R-G to prove out the relay

Will report back on R-G results - Thanks!

If it is a oil furnace it will not have a board. Should have some sort of transformer relay combo. If that is the case and you are not to fimilar with wiring. You may need a repairman

OK, R-G jump no joy. No fan. I think I heard what sounded like 60Hz power supply ripple hum when I put my ear near what I think is the fan relay. I suppose if I can identify relay specs (label slip on side -- if I can find my little mirror on a stick, I can read it).. to make sure it is the fan relay. I can replace as it's mounted with 4 hex head sheet metal screw to the burn chamber --- just need to figure out it's the right relay. Suppose I can get burner model number too to make sure there's no board it sure doesn't look like it though. Thanks to all.

if it is a oil furnace it will not have a board. should have some sort of transformer relay combo. if that is the case and you are not to fimilar with wiring. you may need a repairman

Since a relay is just a set of NO contacts that get switched on via solenoid, I sure wish I could jump the right relay contacts and make that fan come on. I could throw the breaker, get my VOM and find some likely candidates. I would I think be jumping one side , one of the Blues to one or the other of the Purples or one of the Blues to one or the other Blacks, right?

That would make sure I am replacing the proper part! --- :-)

Thanks for all the advice..

If you can take it off easly and change it. I am pretty sure that is your problem. And I am almost 100% sure you do not have a control board. It should be right there. So I dought you do.

if you can take it off easly and change it. i am pretty sure that is your problem. and i am almost 100% sure you do not have a control board. it should be right there. so i dought you do.

Thanks for the enouragement. I'll sign off and rpt back in few days when or if I get this resolved. You and this board are doing a great service.

Thanks hope it works for you