Ice detection in furnace exhaust pipes

Looking for suggestions from the community. My house has 2 high efficiency furnaces. These are located in my basement and exhaust out the roof on the 2nd floor. They have worked flawlessly for about 10 years, but this year during a particularly cold period of time (-35C), one of these exhaust pipes froze up. When that happened, the furnace detected the back pressure and shut down. It then tried again about a minute later, failed and shut down again. Repeat forever. This happened to me while I was in another country and I only detected it because the floor that was heated by that furnace was not heating even though the furnace was on.

Fortunately this was the 2nd floor and I was able to adjust the other furnace (heating the lower floor) to heat more and let that heat drift up. I use Honeywell redlink thermostats and lots of temp humidity sensors that allowed me to figure out that something was wrong and investigate. I called a furnace tech who was preparing to replace all the sensors in the furnace - when he realized they weren’t faulty and looked outside and saw the ice buildup.

I’d like to figure out a way to detect this condition. I could put a probe from a temp/humidity sensor in the exhaust point of the pipes - but I can’t figure out any readings that would let me know this was happening. I could also mount cameras on my roof to watch the pipes - this would help me when I know to look - but they couldn’t act as a method of creating an alert.

Some sort of an air flow sensor that I could put over the opening? - Problem with this is that it could act as a grid to increase the chance of a freeze up.

Perhaps I could build some sort of a pressure sensor that I could install in the exhaust pipe and detect the pressure buildup (these are high efficiency furnaces so the exhaust gas is warm - but not excessively hot)

Any thoughts?

I think the best option is to talk to a builder what you can do to prevent this.
I’m not at all qualified but my opinion is that the pipes should/could be bent down more to protect them more from weather.

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Also some further insulation to the ends of them to retain the exhaust heat which should help prevent it freezing up… although -35C :cold_face:

I second @sparkydave and @Hellis81 , extreme weather insulation is the answer.

They also make caps that drain the water away so there is no condensation build up

The vents should generally facing down

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You could just use that furnaces sensor output as well.

Anyway, you really need to insulate the whole exhaust, it’s a small miracle that didn’t happen before. Exhaust is mainly water vapor, try to imagine what happens when it reach uninsulated pipe at -35’C.

I was wondering if I could tap into that sensors output. Not sure about how to do that. I believe there are 3 wires coming off the back of it. Is this an esp32 app or is there a simpler way of detecting changes in these wires? I have a upb low voltage detector (used for detecting a door bell ring,) - maybe that?

Regarding insulating the pipes. Very rarely do people insulate these pipes in Calgary - but they are usually accessible from ground level so people knock off the ice as it freezes.

A couple things were different this last year.

  1. a roofer had removed the black foam insulation and then couldn’t find any new insulation for a 2" pipe so used 3" - thus a sizable gap between the pipe and the insulation. And

  2. I was away from home for 5 months so I dropped the inside house temp for 21 to 11. As a result the gurnace would not be as active as it would have been in previous years and rarely would come on full force (high speed fan etc) - thus maybe not creating as much pressure to get moisture out the end of the pipe.

I have since replaced the foam insulation (proper pipe size, much thicker than the original and it comes right up to the first junction).

I am a bit skeptical about the value of the insulation near the pipe exit. Given that (maybe) half the time the furnace isn’t running, the ambient air will be in the pipe - thus the pipe temperature will be -35c. Insulating the outside of the pipe won’t have much affect when the source of the cold is already inside the pipe. It would only help at the moment air is being exhausted.

On the concurrence of the gas inspectors, I turned the pipe exits to point down wind - this allowing the prevailing wind to attempt to creat a vacuum and potentially “suck” air from the pipe. Not sure how much that will help.

Doesn’t help much. You could try to identify vcc and gnd wires and measure output of the third one. If it’s analog sensor, should not be difficult to use it.
Could also be “alarm signal” output or digital output, then it’s necessary to find some info about the sensor.

When it’s off you have dry air inside, it doesn’t make ice. But when running you have water vapor inside and for sure it will condensate and freeze if you don’t insulate 100%.
The last inches are the worst.

My concern though is that, regardless of how well insulated the outside of the pipe is, the temperature of the plastic pipe on the inside will be -35 when the furnace is idle. Once the furnace starts and the warm moist air hits that plastic, the moisture will immediately become ice. More others if the furnace runs long enough it can heat the plastic to above 0 and remove the initial ice layer. Being a high efficiency furnace though, the temperature of that exhausted air is not particularly warm. Combine this with the house temperature being turned down to 11, and the run times for the furnace are pretty short.

While experimenting with this, I did find that turning the house temperature back up eliminated the problem. But a primary objective of my home automation is to reduce fossil fuel usage and my impact on climate, so am looking for a way to still make this work.

I definitely appreciate all the ideas about eliminating the problem, and I have been implementing changes to the furnace exhaust in an effort to do this.

I’m curious though if anyone has any more ideas regarding my question though - how can you detect this is happening?

I have passed the question to yolink and they are adding it as a potential use case for future hardware development opportunities.

I started to wonder about using a light beam pair (transmitter and receiver similar to a garage door sensor). I could mount a transmitter outside the pipe shining a light inside the pipe, and a receiver inside the pipe. When ice formed, that light beam would be blocked. To keep power usage low I’d only turn the light on once an hour, and the mounting would have some challenges. The location of this isn’t easily accessible, so I wouldn’t want to change the batteries very often - so perhaps something solar powered.

I’m also considering trying to tap into the furnace sensor and need to investigate that option further.

I would think that a generic ice detection solution would also have wider application - particularly in detecting ice dams on house roofs.

Any other thoughts are appreciated!