25 year old forced air gas heater has odd delayed problem

JPB

My upstairs forced-air gas heater (sideways mount in low-ceiling attic; requires crawling a dozen feet or so to get to it!) has a problem I've never seen anyone mention before - before calling our local HVAC contractor, I like having some ideas for them to consider and I am hoping there are some good ideas here that could be investigated.

Six months ago, I was trying out the heater (my wife and I mainly live downstairs; the upstairs is just a master bedroom/bath). I noticed it wasn't heating and discovered the heating element (a small stick-like object that glows to ignite the main burner, I guess) was broken, and a HVAC contractor came and repaired it.

Fast forward to now: My wife and I only really go upstairs to shower and turn the heat on for 20 minutes then turn it off. A few days ago, I accidentally left it on through the day and overnight, then discovered the next morning that the thermostat was 'on' and claiming it was trying to heat, but the forced air was not blowing, and there was no heat.

So - I went up to the attic, crawled to the unit, and reset the pilot light and the system according to the manual. And wow - I got heat right away. I stayed up there for 20 minutes and had great heat, then turned it off... waited 10 minutes and turned it back on - thinking 'problem solved!' - but then, later in the day, I thought 'what happens if I leave it on a few hours.' Well, it heated the upstairs for about 3 hours, then... stopped heating. The thermostat (an old Nest unit that has worked for years) said it was trying to heat, but no luck. So - I try something easier. I just turn off the circuit breaker for the heater, wait 5 minutes, turn it on again, and the thing works like a charm. So I think 'perhaps problem solved' - but I let the thing run for a few hours, and sure enough, the unit is not heating. I again reset the entire unit with the circuit breaker, and the heat flows again.

Net-net, it seems that it can heat after a power reset for a few hours, then just stops heating. No error given. A power reset lets it heat again - for a few hours. If we only use it 15-20 minutes a day, so it never gets to the temperature I set it to (early morning shower, temp upstairs at 66, set it to say 72, by time done it's only 69) - it seems to work again and again like that day after day. It's only when it's on for a few hours and gets to the temperature set point does it act like this where the temperature drops, but the heat won't come on again (nor the blower).

Any thoughts?

Ironman

Is the board giving an error code?

If it’s a 90%+ furnace, have you checked the condensate trap and drain lines?

EBEBRATT-Ed

And it could be the Nest. See if the Nest is hooked up with 2 wires or 3 wire (if it is heat only) there should be a wire on the common (C) terminal

Jamie Hall

The Nest is the first thing I'd check -- by substituting a conventional, two wire thermostat. See what happens.

JPB

thank thank you all for the feedback. I am going to try a second Nest tonight when I get home (the furnace has 5 wires going to the nest - white black yellow green red). if that also fails, I’ll dig through the garage to find the old thermostat. As to the first comment, the furnace is not giving an error code. During a previous problem, it did - so I know how to get the error code. I’m just not getting one now. 

EBEBRATT-Ed

@JPB

I would just bypass the Nest and see if it runs they are nothing but trouble.

You must have ac on the furnace as well.

Usually (caution not always)

Red is 24 volt power to Nest "R" terminal
White starts the heat "W" terminal
Yellow starts AC "Y" terminal
Green starts fan "G" terminal
I am guessing they are using black for common "C" terminal

That's more or less the standard hook up but some techs have strange ideas. Electricity is color blind it doesn't care about wire color