Re: Furnace losing 24v when heat requested

mm wrote: 
On Sun, 18 Dec 2005 19:14:23 -0500, "Max Metral"
<memetral@xxxxxxxxxxx> wrote:


They were indeed remarkably rude. At least most of them, a couple were helpful.

Anyhow, the main question has now become, if my digital (i.e. needs power) thermostat says it only requires the following leads connected:

R - 24vac
G - fan
W - heat
Y - cool

Where does it get power when it demands heat, thus shorting (?) G and W to R? 


Is this a question with a built -in answer!

The way you have your question, after the chart, makes me think you
don't underwstand.

First, I'm pretty sure the thermostat gets its power from the same
source when it is "calling for heat" as any other time.** 

Assuming your colors, when you turn the fan to On, it's going to
connect the R to the G, so the fan terminal back at the furnace gets
24 volts.

But when you turn the thermostat switch to Heat, and it is cold enough
to call for heat, it's going to connect the R to the W, so that the
Heat terminal  back at the furnace gets 24 volts.   Unless the fan
switch is on fan (and if that is the case, the fan is already running)
it's not going to supply 24 volts to the G, because that would keep
the fan running regardless of what the heating circuit wants, and the
heating circuits in the last 30 years or more have wants.  They want
the fan OFF until the air in the furnace is hot, and they want the fan
ON until the air in the furnace has cooled off a lot.  The heat
circuit is in charge of the entire heat functioning, including the
fan.   The Fan is directly controlled by the thermostat, only when the
Fan/Auto switch is set on Fan.

** In my case, that is the furnace with battery backup.  I know that
because I haven't replaced the battery in more than a decade, and
indeed when I turn off the power to the furnace, the thermostat
forgets what time it is.  So I know the furnace powers it, and I'm
sure the battery would also if it weren't dead.  But I get the
impression that not all thermostats work the same way.  (I only depend
on the battery to keep the day and time.  The temperature and setback
times are set by mechanical multi-segment slide switches.)


I would assume (standard decomposition of assume applies) that it gets it from Y staying at ground, and perhaps that's where the fault has occurred. If not, where else can it pull the power?


<vmravinec@xxxxxxxxx> wrote in message news:1134937714.769477.8350@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I guess you gave up on those pricks in alt.hvac, eh?




Remove NOPSAM to email me.  Please let
me know if you have posted also.
Hi,
24V is coming from a step down transformer. Battery back up for 'stat is to keep the program settings. As long as main power breaker to furnace is on, 24V should be present. It's called logic sequence control power.
If it goes missing, something is shorting it out or the transformer is
bad. Or sosmething is loose.
Tony
.

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