Re: R V QUESTION !! Follow up!

On Tue, 29 Jan 2008 19:15:39 -0700, "Technobarbarian"
<Technobarbarian-ztopzpam@xxxxxxxxx> wrote:


Now looking at what the manufacturer has to say and those wiring
diagrams, I'd strongly suspect that fan motor relay. As Hydroflame notes: it
could be the thermostat, but thermostats are just a switch or a set of
switches. It's easy enough to see whether or not that's the problem by
taking the thermostat out of the circuit. If the fan continues to run that
likely ain't the problem. Or like the other guys says it could be water. Or
rodents. Or some other problem we can't see. But, with the wiring diagram
and a simple volt ohm meter he should be able to *see* where the problem is.
If he can't it's time to take it to the service guy.

It's possible but I don't think it will be the thermal relay. The diagram is a
little misleading. It makes it appear that both the DSI (ignition control board) and
the fan receive power through the relay. They don't. The thermostat provides power
to the relay "coil" AND to the DSI board through the limit switch and sail switch.
The relay does NOT switch the ignition voltage.

The sequence is thus:

1. thermostat calls for heat by supplying 12 volts to the relay coil and to the sail
switch.

2.After a short delay for the thermal relay's element to warm up, the fan starts, the
sail switch makes up, proving draft, and power is applied to the DSI board through
the over-temp switch.

3. After another short purge delay on the DSI board, the DSI board opens the gas
valves and fires the spark igniter.

4. The flame is proven with the flame rod and heating commences.

When the room is warm enough, the following sequence happens:

1. Thermostat opens, removing 12 volts from the fan relay and from the DSI board.

2. The gas control valve immediately closes, turning off the gas and extinguishing
the flame.

3. After a cool-down delay, the fan relay opens and stops the fan.

In my furnace, the thermal relay is located so as to be heated by the combustion
chamber heat in addition to the "coil" heater. Thus, the cool-down interval varies
with how hot the combustion chamber gets. It varies from about 20 seconds for a
short cycle to about 2 minutes after a long burn.

At this point we need a detail clarified. Lee, when you turn the thermostat OFF, a)
does the flame immediately go out and the fan keep running or b) does both the fan
and flame continue operating? I think you said b) but I want to make sure.

If it is a) then the fan relay is a likely culprit. If it is b) then the fan relay
can NOT be the problem. Something (bad thermostat, chaffed wires, etc) is supplying
12 volts to the "coil" terminal (which also supplies the DSI board and gas valve)
even after the thermostat opens.

Since this problem just cropped up when the camper was unattended (You weren't
camping, were you, Lee?), it points to a spontaneous failure. Among those are:

If situation b) above applies:

1) Thermostat stuck shut
2) Thermostat wires chaffed and shorted together between the thermostat and furnace.
3) A wire or other conductive debris has gotten between the 12 volt terminal and the
"coil" terminal of the fan relay
4) Wires connected to the 12 volt terminal and the "coil" terminal of the relay have
chaffed/melted together.

If situation a) above applies:

1) a bad fan relay
2) Wires between the 12 volt terminal and "fan" terminal have chaffed/melted
together.

I'm betting on either b1 or b2, or a1, depending on Lee's clarification.

There seems to be another anomaly in the manual's sequence of operation. Lee said:

OK, I took and put it through a heat cycle and even though I put it in
the off position and lowered the Temp the heater was still running (LP
being ignited)until I shut off the fuel then it cooled down and the
blower shut off. Would this be a good indication that maybe the
thermostat has gone bad?

If you described that accurately, Lee, then the control board also has a fan lockout
function that is not mentioned in the manual. That is, after three attempts are
made and ignition fails, the board locks out and opens a relay that stops the fan.
This is designed to prevent the "battery-sucking run-all-night" fan mode when the gas
runs out or something else keeps the furnace from lighting.

The Dino board has this function but I don't see it mentioned anywhere in this manual
we're referring to. Of course, no telling the vintage of this manual. It may
pre-date the fan lock-out mode implementation.

Lee: Please don't just run out and buy another thermostat. Blind parts changing
leads to more trouble than fixes. If thermostat wires are chaffed/shorted in the
wall behind the thermostat then moving the wires while changing out the thermostat
may un-short them. That will make you think that changing the thermostat "fixed" the
problem, only to have it re-occurred when the wires vibrate back together again.
Let's find the source of the problem and fix it.

If condition b) above applies then there is 12 volts on the thermal fan relay's
"coil" terminal even when the thermostat is OFF. After you verify that the
thermostat's contacts are opening when you turn it OFF, find the wires at the furnace
that come from the thermostat. if they follow standard color codes, they will be red
(12 volts) and white.

Disconnect one or both of the wires. If the furnace now turns off, that tells you
that the problem is in the wiring IFF you're sure the thermostat is, in fact,
opening.

If the furnace continues to run, the problem is inside the furnace. Inspect the
relay for loose/dangling wires. Also look closely for melted/chaffed or otherwise
not-normal-looking wires. If you get to this point and still haven't found the
problem, post again and we'll go into some detailed troubleshooting. I'll probably
want some digipix of the furnace internals at that point.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Save a tree, kill a beaver

.

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