Two furnaces one venting system? What is the right repair?

[Matt Crowley]

I'm looking to get some input from someone with some furnace / HVAC expertise.

I am diagnosing a legal suite in Edmonton with two furnaces. Originally, I was under the impression that each furnace had its own independent duct system (one for upstairs and one for downstairs). As you can see in the picture below, it appears that both heat stacks combine into one venting system:

The problem is that the basement furnace is no longer operating correctly. It does not stay lit for very long periods and likely needs replacement. When I look at this setup it does not make much sense to me. (Note: It is a 1277 SF home.) There are two thermostats: one in the basement and one for the main floor. I believe there are two different thermocouples for upstairs and downstairs. The result is probably that the whole house is heated to the lowest common denominator of the two thermostats as the whole house is served by one venting system.

I believe that one problem is that the mechanical room is overheating because when one furnace is on, the heat blows through the other furnace and disburses in the room. This seems unsafe to me, but I am not an HVAC expert.

As I look at this system, I am considering closing off one furnace entirely and just adjusting the venting overhead so that the operational furnace warms the entire house. (I will need to make sure it has sufficient BTU's). I would decommission the downstairs thermostat and the basement tenant / upstairs tenant would need to agree on a household temperature.

What would you do? Am I missing anything?

 

[kfort]

If it fires up (pop the cover off look for nice blue flame) and quits in a few seconds your flame sensor is probably the issue. Pull it out, grab a piece of clean ish paper (years ago a dollar bill would work, now they're plastic) and wipe it off.

If the flame is yellow it's a burner issue (dirty mixing nozzle or similar).

Doesn't look like there's a barrier in the duct line but that's an easy check, when the good furnace fires up does hot air come out both the up/ down vents? If so you've got common ducting.

Don't use emery cloth to clean the sensor, it's too abrasive. (Unless you use the back side)

1277 isn't big enough to make me think 2 furnaces. Most likely the second was to allow for some independent temp control for each unit. Thermostat down will lead to one (crank it to 80* and see which kicks in) and thermostat up to the other.

Two furnaces seems like overkill. Two with a common duct line seems like half assed overkill. If it was me, I would consider decommissioning one (leave it there as a backup), move to one thermostat, install appropriate adjustable baffles to properly ration up/ down air.

All that said, a thermocouple/ flame sensor is generally a $40 part that costs $100 to install and takes 20 minutes to deal with. I've actually begun to keep a backup at each property as the last several winters I've had them crap out far too often on a Friday night at -30*C.

 

[Matt Crowley]

Thanks for the response Kristopher, this is a huge help. I'll answer some of your questions below here:

If the flame is yellow it's a burner issue (dirty mixing nozzle or similar).

Flame is definitely yellow. Is there an easy fix for a burner issue? Is this a thermocouple issue? Thanks

Doesn't look like there's a barrier in the duct line but that's an easy check, when the good furnace fires up does hot air come out both the up/ down vents? If so you've got common ducting.

Hot air comes out of both the up/down vents.

Two furnaces seems like overkill. Two with a common duct line seems like half assed overkill. If it was me, I would consider decommissioning one (leave it there as a backup), move to one thermostat, install appropriate adjustable baffles to properly ration up/ down air.

What is an adjustable baffle?

Thanks!

 

[kfort]

If flame never gets blue it is either because the thermocouple is dirty and not sending required voltage to the controller (which would allow high fire action) or else it is a burner issue (could be dirty, could be totally shot).

Most likely, this is still a thermocouple issue. Chances are it isn't actually registering low fire because it is dirty. If it doesn't register low fire, it won't high fire.

If it ignites and turns off in 5-10s and does this repeatedly, that would be my best guess without poking around.

Easiest fix for dirty burner is a service call and cleaning. Easiest fix for a thermocouple is a service call and cleaning. Generally complete unit cleaning is ~$125 and I do it annually in August.

There often is also an LED that will blink the diagnosis. Check the back of the panel you removed, it probably has instructions. Ex: 4 fast beeps = flame sensor "failure" etc.

Grab the model # & serial # for when you call the plumber, that way you won't pay for an extra 45 min "parts run" as they can bring it first try.

Last but not least, if you're handy & have a Phillips screwdriver or set of nut drivers, YouTube "thermocouple replacement"... It's usually in an awkward place and is literally 1 or 2 screws. Like I said, new part ~$40-70. And 20 min for a rookie to replace.

If it's dusty & gross in there I would book a cleaning regardless.

NOTE: do NOT stick your body parts in the furnace without ensuring the power to it is off. No need to wreck something over a $150 outsource

 

[kfort]

Adjustable baffle = a plate in the duct line, generally indicated by a small wingnut that can be opened / closed/ positioned to create back pressure & restrict flow.

These can be tricky to find if they've been dry walled over. But I've seen some fun stuff like people insulating interior basement walls etc because they had a closed baffle and didn't know it.

Only sure fire way to indicate the baffle is 100% open or closed is a flow meter. Don't buy one, just grab a Kleenex or similar paper and watch how hard it is blowing off the register as you adjust the nut. Can do the same for your bathroom fan to make sure it's sucking and not plugged.

 

[Matt Crowley]

Just thought I'd take a minute to update the thread and thank you again for your lengthy explanations. When I got down to it, I was able to detect the error code from the number of blinks of the error light. From that point, I had only a few potential sources for the error. I ended up pulling out the flame sensor and found that it was really dirty:

I gave the piece a light scrub down with a piece of sandpaper (front and backside). Then I tried to fire up the furnace again and it fired up beautifully:

 

[kfort]

Well done! A furnace guy would've charges you $125 for that same 15 minutes of work!

 

[Steve6134]

I never thought I would find this kind of useful information in the REIN forums. Thanks, good info!!