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Posted by Zyp on February 2, 2008, 4:22 pm
Tom Beckner wrote:
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>> I have a Carrier 58SXA that I don't know what to do with next. When
>> the thermostat calls for heat, most of the time the furnace works.
>> Sometimes though it just does nothing; no inducer motor, no noise,
>> nothing.
> Isolation relay.
> If this was already proposed, then disregard.
> I didn't read all the responses to your question.
> I recall a similar problem back in the late 'eighties when I was doing
> residential work. Same deal, intermittent no heat. Turn the power
> off to the furnace, back on, normal heat cycle starts and continues to
> cycle for undetermined amount of time until next failure. This
> problem arose when changing from a mercury switch thermostat like a
> T87F, to a solid state control thermostat like a Chronotherm or similar.
> The fix: install a 24 volt isolation relay on the W (heat) circuit
> between the thermostat (t) and the board (b). When the call for heat
> satisfies, the relay makes a clean break between Wt and Wb, same as
> the old mercury switch thermostat did, eliminating any feedback or trace
> through the solid state thermostat which they were stealing to charge
> up a memory chip or battery or whatever they do.
> Wt to one side of the relay coil.
> Cb (24v common) to the other side of the relay coil
> Rb (24v) to relay switch common
> Wb to relay switch NO (normally open).
> Tom Beckner
That's nice, there's no isolation relay needed on this model.
--
Zyp
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>> the thermostat calls for heat, most of the time the furnace works.
>> Sometimes though it just does nothing; no inducer motor, no noise,
>> nothing.
> Isolation relay.
> If this was already proposed, then disregard.
> I didn't read all the responses to your question.
> I recall a similar problem back in the late 'eighties when I was doing
> residential work. Same deal, intermittent no heat. Turn the power
> off to the furnace, back on, normal heat cycle starts and continues to
> cycle for undetermined amount of time until next failure. This
> problem arose when changing from a mercury switch thermostat like a
> T87F, to a solid state control thermostat like a Chronotherm or similar.
> The fix: install a 24 volt isolation relay on the W (heat) circuit
> between the thermostat (t) and the board (b). When the call for heat
> satisfies, the relay makes a clean break between Wt and Wb, same as
> the old mercury switch thermostat did, eliminating any feedback or trace
> through the solid state thermostat which they were stealing to charge
> up a memory chip or battery or whatever they do.
> Wt to one side of the relay coil.
> Cb (24v common) to the other side of the relay coil
> Rb (24v) to relay switch common
> Wb to relay switch NO (normally open).
> Tom Beckner