Question about gas furnace behavior

Wiki seems to feel that the term “furnace” is the general term for any mechanical heating system, and that “boiler” is a subset of the furnace family. I’ve never heard a plumber refer to a boiler as a furnace, however, or vice versa. None of it matters, of course; it’s just something that bugs me.

A boiler does not have to make steam. I have worked on many hot water boilers, and steam boilers.

A fire tube boiler normally has a round outside (shell) with flat ends (tube sheets). Inside the shell are tubes connected to the tube sheets. Water is on the inside of the shell and out side the tubes. The fire is inside the tubes. The boiler can make either steam or hot water. There are some variations to this that could take pages to describe.

A water tube boiler has the water inside the tubes with the fire outside the tubes and is constructed completely different. But depending on the design can be either steam or hot water. I would consider neither of these a furnace.

But a boiler can have a furnace inside it. That is where the combustion takes place.

I experienced a similar problem the first winter we were in our house. I discovered that the air intake pipe from the outside had been plugged with a rag. The screen on the vent was broken so I assume the previous owner plugged it to keep birds out. I don’t know enough about furnaces to know why that made a difference, but after I took the rag out, the furnace ran normally.

Yes.

It’s the thermostat.

Weird. I’ll check that.

Gas-fired units need air for proper combustion. It’s a good thing you unplugged it, as you could have ended up with a lot of toxic gas buildup in your home. Every year in Anchorage there was a tragic story of a family dying in their sleep because some idiot plugged the fresh air vent.

I don’t know if this behavior is normal but I’m pretty sure I know what you’re talking about because my (hot air) gas furnace does the same thing.

The thermostat is in my bedroom and there is a return air vent right above me so I can hear what is happening and here’s how I’d describe it:

  1. click from thermostat to start a normal cycle
  2. exhaust motor/blower comes on for about 10 secs
  3. gas burner comes on (I can hear it over the continuing exhaust blower) and burns for 5 to 10 secs
  4. main blower comes on, the house heats up, and eventually
  5. normal cycle ends after tempertature is reached and it all shuts off off temporarily but shortly after
  6. exhaust motor/blower comes back on for about 10 secs
  7. sound of gas burner on top of that for another 5 to 10 secs and it sounds like another normal cycle about to start but
  8. distinct sound of gas burner going off just as the
  9. main blower comes on for about 30 secs then goes off
  10. cycle thru steps 6 to 9 several times then
  11. different click from thermostat (a bit louder and sounds like it has some pop to it) when everything finally goes off until the temp drops enough to start it all over at step 1

It doesn’t always do the weird cycling part - it usually stays off after step 5 until the house temp drops enough to trigger the thermostat again. I know when it’s going to do the weird cycling thing at step 8 when I hear the gas burner go off right as the main blower comes on. I have only noticed this happening when it is pretty cold, like 30-ish and below.

I do not think this is related to the sensor that detects whether the gas has been lit - like when the ignitor has gone bad or low gas pressure or whatever. My ignitor recently crapped out and was replaced a few weeks ago so I’m pretty sure my gas is lighting. But that is what it sounds like: as if the unit thinks the gas is not lighting. But that wouldn’t explain why it cycles normally at temps over 30 and why it eventually shuts off and comes back on when it needs to and works fine until the desired house temp is reached.

If it gets cold enough tonight to start acting up I’ll try to visit the attic while it’s happening to check whether the gas is actually being lit during the weird cycling.

Also, one of my buddies is co-owner of a HVAC company and I’ve been meaning to ask him about this. If someone doesn’t check in to diagnose it based on the symptoms I’ll let you know what I find out - at least about my system - in the hopes it helps.

Seems to me it would be the thermostat malfunctioning, it’s the only thing that can make the furnace come on or go off automatically. It could be misreading the temp. or just be mis-adjusted if it has any adjustments. Possibly wired incorrectly.

I agree with glenn.

I have seen similar problem with gas units that seem to function just fine on moderate temp days, then when the temp drops out come the supposed quirky
problems.
When the furnace works overtime to combat the deep freeze like we are in right now the overall temp of the entire furnace goes up above where it would/should never rise, that is why there is a HIGH Limit, but rising up in that ballpark is where igniter’s and other components can malfunction.
Case A
My daughters rental house;
Cold week, furnace is out and I get called and tell her that I do not like to mess with Gas units and it’s not my property.
She finally gets through to her LL and a service repairman arrives and its the Igniter. He replaces it and fires up unit. I take electric space heaters home.
that same week furnace is out again and back come the space heaters.
I look furnace over and see that the filter is very dirty. (She has a dog).
I go and buy a sixpac of filters and change out the filthy filter. Service arrives again and replaces the same part, (Igniter).
I only now have to make sure and change the filter and taught my Grandson how its done. Now if I could just get him to change out the water filter without breaking something.
Case B
Old Church;
Heat is out on cold spell and service is summoned. They replace igniter and as soon as I hear about this I check the filters and they are clogged.
I have been taking care of all 6 furnaces in the new church and I still call service for repairs but I am over their shoulder always.
I have one unit that is a Lemon.
I also have Water column gauge connections on all units to monitor the filters.

Sounds like the thermostat is functioning properly as you do not hear the telltale “click” when the furnace cycles the second time. That “click” you hear is a switch in the thermostat sending a massage to another switch inside the furnace that activates the heating cycle and I suspect the malfunction is in that furnace switch.

Newer gas furnaces have circuit boards controlling this and other functions but repairman tend to replace the entire board as opposed to attempting to repair them. Repair is usually not within their level of expertise.

Will your landlord repair this? Even though you didn’t ask, your lease dictates the landlords responsibility for repair to the premises and if the landlord is required to maintain a functioning heating unit, I think an argument could be made that the landlord has done this because you have heat. So long as you have sufficient heat without additional malfunctions, I wouldn’t worry about that 15 second run time during the second cycle.

I’m mainly concerned because my gas bill was $200 last month, which makes me wonder if the furnace is wasting gas. The burner runs a long time during the buildup to that 15 second blower run, so I know a lot of gas is being used. I know it was cold last month (I’m in NH), but that was higher than expected (or advertised). If this is normal operation, so be it, but I’d hate to think that I’m paying double what I should be paying because of heater problems.

I’ll probably just change the filter and check the air intake for blockage, and hope for improvement. At this point, I doubt I’ll be able to prove that there is a problem, even if there is one.

Thanks to everyone for their input and suggestions! :slight_smile:

I see your dilemma. Even though I think your thermostat is okay, for peace of mind, a quick method to rule it out as a culprit might be to try this:

The next time the initial heating cycle ends and before that 15 second followup cycle begins, turn the thermostat down several degrees below the actual indoor temperature. If the furnace cycles again, I think you can rule it out as a cause. A surefire method would be; Instead of lowering the temp after the initial cycle, turn the switch on the thermostat from ‘Heat’ to ‘Off’. If the second cycle repeats, your know darn well the issue is within the furnace. However, turning it to "off’ may simply stop the cycle but I think it’s worth a try especially if the furnace cycles again.

At that point you will have to decide if your out of pocket cost for unnecessary gas/electricity usage justifies further conversations with the landlord and whether the language in your lease gives any ammunition to your situation. You may want to tell the landlord that the furnace issue has the potential of developing into a hazardous situation. He might just start thinking about his liability exposure.