HVAC Question

[db5]

There is an expert here on almost every electric/mechanical equipment made. Here's my issue.
Turning the thermostat to HEAT could take minutes or never to come on. To get it to come on I had to turn it to OFF, disconnect power to the furnace, reconnect it and then turn to heat and it worked. I connected red and white wires on the thermostat and it called for heat, therefore a bad thermostat.

I replaced the thermostat and everything has worked since, except once a few days ago and today. Disconnecting the power...etc. started it. So, what is the next step to diagnose and fix this problem?

[twistsol]

Not an HVAC pro, but have had similar problems. First, check the controller board in the furnace. There is usually an LED or set of LED's that blink out an error code. Turning the power on and off may clear the error, but not resolve the problem. Based on the error code it will give you a hint as to your next step. In my case there was a birds nest (with associated birds dead from CO poisoning) in the exhaust vent causing low exhaust air flow. The error code was "Vent issue" and blocked exhaust was number 4 on the troubleshooting list.

Just to make things fun, the controller in furnaces is just a computer, albeit a simple one and sometimes they crash and usually without error codes.

[davebodner]

I'm also not an HVAC pro or even all that knowledgeable. But, I assume your thermostat is something from this century, so there's some kind of computer in there. That may be one reason your heat doesn't come on immediately. Maybe the thermostat's computer and the furnace's computer have to achieve some sort of a handshake before you see anything happening.

If the thermostat's really not not working, perhaps a re-boot is in order. Disconnect it for a few minutes and then reconnect.

[drashop]

This sounds like a problem I've had before.
Do you have a heat pump?
It sounds like the heat ? cool switching valve is sticking needing replacement. It needs to be removed and a new one soldered in Job for a HVAC tech to do.

[JPG]

Back in a simpler time, the thermostat contacts made thus energizing the gas control valve(assuming the bonnet temp cutoff is 'cool' and the thermocouple is hot).

Now days not so simple.

Added stuff.

Pressure sensors interlocking the sequence.

The sequence now includes running a flue fan and pressure sensors detecting the air flow.

Igniter rather than standing pilot/thermocouple.

More sensing(igniter hot) and timing of it being energized.

Flame detectors around air/gas entry to combustion chamber(s).

Add in delays introduced by the controller(in the furnace) and the thermostat itself and instant response to thermostat simply does nor occur. If it 'never' comes on one of those sensors is a likely culprit.

[jsburger]

Back in a simpler time, the thermostat contacts made thus energizing the gas control valve(assuming the bonnet temp cutoff is 'cool' and the thermocouple is hot).

Now days not so simple.

Added stuff.

Pressure sensors interlocking the sequence.

The sequence now includes running a flue fan and pressure sensors detecting the air flow.

Igniter rather than standing pilot/thermocouple.

More sensing(igniter hot) and timing of it being energized.

Flame detectors around air/gas entry to combustion chamber(s).

Add in delays introduced by the controller(in the furnace) and the thermostat itself and instant response to thermostat simply does nor occur. If it 'never' comes on one of those sensors is a likely culprit.

Yup, had that problem last month. The thermostat called for heat, the flue fan came on but the pressure sensor was defective so the igniter and gas valve would not come on.