Up against a wall!

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  #1  
Old 12-28-11, 11:54 AM
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Up against a wall!

OK,

First off - equipment:

Armstrong Air Tech 80 furnace
Field Controls Power Vent - http://www.fieldcontrols.com/pdfs/46257600.pdf


Here's my issue. I cannot get the units to work nicely (anymore). This was installed (against my will) two years ago at an exorbitant cost to get us up to 'code'.

It worked just fine until the end of last season (2010-2011). Then, the power vent would stick on, and give me a nice "pressure switch closed - should be open" blinking error. Now, I have switched thermostats, cleaned pressure hoses, etc.

The only way I can get it to turn off is by opening up the furnace's lower cabinet and unscrewing both white wires from W. In 15-20 seconds, the power vent shuts off. Then I replace the lower cab cover, and viola, it runs...until the power vent sticks again.

Now, the power vent was NOT wired to Field Controls' guidelines. W is supposed to be routed through the PV controls and then to the furnace. It is supposed to begin pulling negative pressure before anything lights... WHICH THIS FURNACE WILL NOT DO!!! I continue to get the blinking code.

The unit is wired with both the thermo and PV white wires to W, and another PV wire to C (only wire to Field's specs). The installers left the red wire form the power vent dangling.

Any ideas out there? Is it possible to trick the furnace into operating under any condition where the pressure switch is closed? How about using the PV's pressure switch to override the furnace's? I'm really thinking aloud here... woke up to 53 degrees this morning...

Anything anyone can figure out, please let me know. I'm ready to try anything. I work in electronics, and was thinking I could separate the power vent from the circuit,and just relay it in with the 24VAC W signals... any disadvantage here (minus the obvious issues with CO )


Best,

Alex
 
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  #2  
Old 12-28-11, 12:07 PM
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it is really hard to figure out what is going on with your furnace..what is happensing/ does the power vent saty on after the burner light? does the powwer vent start and the burner does not start?
what is sticking.
please list astep by step what is happening ;example below
power vent starts
burner lights
etc etc
go to page 8 on the fielsds control and then check your wiring. it must be wired as it says in the fields control. look at page eight and then come back and tell me how it is wired in relationship to page 8
 
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Old 12-28-11, 01:11 PM
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Thanks a ton for your reply.

I'm going to break it down into the two different wiring schemes. #1 is how it was installed after the techs left, #2 is how it should be wired via Field's instructions (page 8).


#1 (post install wiring via techs)

- Thermo clicks
- Furnace inducer starts
- power vent starts about 5-6 seconds after furnace inducer
- burners light as normal
- furnace runs just fine
*MULTIPLE SHUTOFF OUTCOMES BELOW*
- during 'post-purge' odds are the power vent will not shut off ever, even with multiple clicks from the thermo relays
- if power vent doesn't shut off, the furnace will NOT turn back on until I either power cycle the power vent via the breaker, or pop the furnace covers off and pull the W wires.

- if the power vent manages to shut off properly, the system will work just fine. It's a 90/10 not working/working situation now.


#2 (page 8 wiring)

- Thermo clicks
- Power vent turns on
- Furnace flashes the "pressure switch closed - should be open" error
- Furnace sits around complaining and will not turn on. blinks pressure switch error code



Wiring to Field's specs seems to have the worst effect on my system. This furnace just will NOT turn on with the power vent doing what it was designed to do, which is to turn on before the inducer and flames to net a negative pressure.

Any ideas? I was thinking maybe a multiple stage thermo (one to control pv, one to control heater etc)... but again this isn't my domain. I dabble in AC systems (for cooling computers etc) but 24VAC heating systems are just not something I'm very familiar with.

BTW, I have tried the 'rite-temp' thermo that came with the house. Switched to a Honeywell Round (the ubiquitous....), and now I have a Filtrete non-programmable.

To make things more complicated... The Filtrete worked in wiring scheme #1 for almost two weeks. The furnace seemed to cycle irregularly (it would turn on for 5-10 seconds mid way between normal cycles, sometimes shutting off even before the purge cycle), but this irregularity would seem to shut off the power vent so the system could turn on normally. I would HAVE NO PROBLEM letting the power vent run continuously if only the furnace could turn on at the same time...


Wiring scheme #1 -

W_thermo -> W_furnace
R_thermo -> R_furnace
G_thermo -> G_furnace
W_powervent -> W_furnace
C_powervent -> C_furnace

(difference is W is not passed through the power vent back to furnace... this is how the techs did it upon install. This also worked for almost a year...oddly enough)

Wiring scheme #2 -

Linked Field Controls' manual - page 8 verbatim
 

Last edited by thenrz; 12-28-11 at 01:59 PM.
  #4  
Old 12-28-11, 07:58 PM
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Is there a barometric damper installed in the vent pipe? I think your problem is too much draft. According to the installation instructions the draft at the furnace outlet should be between -.01 & -.03 inches water column. The -.01 shoudn't be enough to operate the vent switch on the furnace.
 
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Old 12-28-11, 11:23 PM
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I do not see anything resembling what you mention. I referenced what you said in the Field manual. Hadn't even thought of that!!!


Can you explain what you mean by -.01 & -.03 inches water? Is that a pressure measurement? What is the damper doing in this case? Is it basically an overflow so the power vent can only create so much negative pressure in the system? Makes sense, bit I could be in Cleveland and you in LA.

Again, thanks to all for your help. I'm pretty handy, I just need the info


P.S. An idea popped into my head... Could the power vent be treated as a second stage? Again, over my head, but I've been racking my brain and this is what I came up with... The Filtrete thermo has a W2 port. I was thinking an isolated system that comes on at the same time. I hear two relays click in the thermostat when it cycles.
 
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Old 12-29-11, 04:27 PM
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The -.01 & -.03 inches water column are pressure measurements Actually they are very slight vacuum as denoted by the negative symbol.

The more draft thru the pipe, the more the barometric will open. Another common name for a barometric damper is "draft regulator".

The W2 terminal on a thermostat is for the second of 2 stages of heat. Why yours is connected, I haven't a clue.
 
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Old 12-30-11, 05:03 AM
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I think that grady is right....it is a draft problem...you dont have enough draft to pull the furnace pressure switch closed. the power venter cannot develop enohg draft thru the furnace. you need a barometric damper installed justlike you posted in the fields control. this will allow the power venter to start and pull the barometric damper open to develop enough draft thru the furnace. you relly need ot follwow the manufacturers install instructions. look at the drawing and you will see the barometric damper between the furnace and power venter
 
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