Subject: [Shop-talk] Power-vented water heater kicks out.

From: [email-masked]
Date: Fri, 13 Sep 2013 21:34:58 +0000 (UTC)

We have a newish gas water heater with a power vent.  We built the vent line out of Schedule 40 and minded the length and number of bends shown in the docs that came with it.

Worked fine for a couple of months, then started kicking off.  Gave a 6-2 code which implies venting was inadequate.  

Took it apart and found a diaphragm switch which measures vacuum on the upstream side of the vent blower.  Tech support says that the switch is supposed to trigger at 3/4-inch of water.  Switch tests fine by mouth test, but a human can pull several inches of water.

Measured vacuum at 7/8-inch of water.  This says the switch is bad, so I asked them to send me a new one.  Went to Door County and Road America for a week.  Came back to find a large parcel.  They sent me the whole darn blower assembly.  

I installed the whole assembly and the same thing still happens.  Get this:  It runs OK if all the house windows are closed and either the exhaust fan in the basement or the clothes dryer is running.

Can I get an auxiliary fan that goes in-line in a Schedule 40 vent line?

Phil Ethier West Side Saint Paul Minnesota USA
1973 Triumph Stag LE22439UBW "uncle jack", Sapphire Blue
2004 Suburban 8.1, Sport Red, the only automatic of the bunch
2005 Lotus Elise, Bordeaux Red Pearl
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From: Arvid Jedlicka <[email-masked]>
Date: Fri, 13 Sep 2013 22:13:58 -0500

Phil,

Doesn't that imply that it runs better when the gas is going the wrong way in
the vent? By that I mean that if you suck enough air out of the house that air
comes in the vent ... or at least does not go out as fast as it should ...
then the pressure in the vent is low enough to keep it on.

Or maybe I do not understand what is going on.

Arvid

On Sep 13, 2013, at 4:34 PM, [email-masked] wrote:

> We have a newish gas water heater with a power vent.  We built the vent line
out of Schedule 40 and minded the length and number of bends shown in the docs
that came with it.
>
> Worked fine for a couple of months, then started kicking off.  Gave a 6-2
code which implies venting was inadequate.
>
> Took it apart and found a diaphragm switch which measures vacuum on the
upstream side of the vent blower.  Tech support says that the switch is
supposed to trigger at 3/4-inch of water.  Switch tests fine by mouth test,
but a human can pull several inches of water.
>
> Measured vacuum at 7/8-inch of water.  This says the switch is bad, so I
asked them to send me a new one.  Went to Door County and Road America for a
week.  Came back to find a large parcel.  They sent me the whole darn blower
assembly.
>
> I installed the whole assembly and the same thing still happens.  Get this:
It runs OK if all the house windows are closed and either the exhaust fan in
the basement or the clothes dryer is running.
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---

From: [email-masked]
Date: Sat, 14 Sep 2013 03:52:45 +0000 (UTC)

----- Original Message -----
> From: "Arvid Jedlicka" <[email-masked]>
> To: [email-masked]
> Cc: "shop talk" <[email-masked]>
> Sent: Friday, September 13, 2013 10:13:58 PM
> Subject: Re: [Shop-talk] Power-vented water heater kicks out.
> Phil,
>
> Doesn't that imply that it runs better when the gas is going the wrong
> way in the vent? By that I mean that if you suck enough air out of the
> house that air comes in the vent ... or at least does not go out as
> fast as it should ... then the pressure in the vent is low enough to
> keep it on.
>

You do understand what is going on, and it does not make sense.

Pulling a (fantastically-slight) vacuum on the "flame" side of the fan is
supposed to indicate to the sensor that the fan is running. B If the fan
fails, there is no vacuum, the sensor thinks the fan has failed or a bird is
stuck in the pipe and we are all going to die of CO poisoning. B So the
micro-switch on the diaphragm opens and the control system shuts off the power
to the fan circuit and kills the gas flame.

Even though I measured the vacuum at 7/8 inches, which is supposed to be
enough to keep things humming, the old switch was open.

You can't just jumper over the switch. B The control system is too smart for
that. B If the thermostat calls for flame, the control system checks the
diaphragm switch first. B If the switch is closed, the control refuses to
start the flame and flashes a code that tells you the switch is shorted out.
B 
It seemed to work better when the house was closed up and there was an exhaust
fan on.  Counterintuitive, huh?

OK, so with the spec the tech guy gave me on the phone, the switch was bad.
Trouble is, they sent the whole thing.  Sigh.

My wife just drew a bath, and the silly thing was just working WITH THE
WINDOWS OPEN, so I'm going down to see how it's doing.

=====

I'm back.

It's heating, and showing the "heartbeat" (their word) code for normal
heating.  I hope this means that diaphram was stiff from being new and is OK
now.  Knocking furiously on wood.

[email-masked]