Author Topic: DC90 Ultra High Efficiency quit  (Read 5074 times)

Offline Curlybandit

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DC90 Ultra High Efficiency quit
« on: February 22, 2016, 11:03:13 PM »
Hello everyone.

My wife and I bought our home in July 2014. It has a DC90 Ultra High Efficiency furnace and a heat pump/exchange system. I'm pretty sure it's from the early 90's. It's beige in colour. Shortly after moving in we replaced the original thermostat with a NEST unit.

This evening I noticed that the home was starting to feel cold (north of Toronto, Ontario Canada). I checked the NEST and it showed that the start-up of the furnace to return the home temp to the set temperature was "delayed" by 3 minutes. There was a message on the screen that said the "boiler" was shutting off frequently during the heating cycle. It advised that this may be an indication that something was wrong with the furnace. Sure enough, once the timer elapsed, the furnace can be heard coming on but the fan would not. It did this a couple of times and then it stopped coming on all together.

At this point the NEST had a fault notification that there was no POWER coming to the NEST from Rh wire. I ran the troubleshooting (breaker, power switch, etc.) but these didn't correct the problem. 

I turned the NEST off for a couple of minutes. When I turned it back on, the furnace turned back on and the fan kicked in. But this only lasted a couple of minutes before it shut off again. It's done this a few times now. I've now turned it off to be safe.

Any ideas?

Thanks.

Offline Admin

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Re: DC90 Ultra High Efficiency quit
« Reply #1 on: February 23, 2016, 12:27:19 PM »
Did you setup the Nest as a duel fuel system, to control both a furnace and heat pump?

It's likely there's something wrong with the furnace causing a lockout.  That could be interrupting the 24V supply to the Nest's RH terminal.

I would have someone look at the furnace's heat exchanger as that model is old and prone to failure.

There could be a Honeywell Smart gas valve and control board that might have an LED light that will flash a code so we know what the lockout is.

Those smart gas valves are also prone to failure.

Take advantage of the Ontario Power Authority $250 furnace rebate and look at replacing the furnace versus repairing it.

Offline Curlybandit

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Re: DC90 Ultra High Efficiency quit
« Reply #2 on: February 23, 2016, 01:13:27 PM »
Thank you for the excellent response!

The Nest is set up for both the furnace and the heat-pump. My understanding with this system is that the heat pump is used for the AC in this home. But since July 2014 when we bought the home and installed the Nest, it's successfully controlled both.

I have a technician coming today at to take a look at it. I'll keep this information in mind.

Thank you.

Offline Curlybandit

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Re: DC90 Ultra High Efficiency quit
« Reply #3 on: February 23, 2016, 06:51:39 PM »
An HVAC technician came to take a look at the furnace. He said its a Carrier unit (DC90 UHE). He couldn't say for certain how old it was but he said it's past its lifecycle. I'm going to guess it's likely from the 90's.

He was able to determine that the cause of the "no power" to the Rh wire was due to a faulty kill switch on the lower panel door. He directly connected the wires to bypass the switch. He also determined that the flame sensor was old and dirty. He cleaned it up but advised that it would need replacing. He also explained to me that the heat exchanger was failing. He showed me how the flames had some blow-back when igniting. He explained that this is an indication that the heat exchanger is failing. He suggested that there may be a small amount of CO escaping from the exchanger, too small to be detected by our CO detectors. The only information I can see from the previous owners is that the two heat exchangers were replaced in 2008. He suggested cracking a window in the home slightly as a precaution. I'll look at options to see how we can tested for CO.

His overall analysis was that we will ultimately need a new unit. I'll also be considering replacing the heat pump which was also rewired for cooling only in 2008 with a new AC unit.

Having a 7-month-old at home is good enough reason to consider this as soon as possible.

Offline Curlybandit

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Re: DC90 Ultra High Efficiency quit
« Reply #4 on: February 24, 2016, 10:12:35 PM »
I wasn't satisfied with the assessment of the situation regarding my furnace that I had yesterday... I feel that the tech that came rushed through the examination. He didn't utilize any devices to measure the function of the components of the furnace and he essentially guessed that the heat exchanger may not be functioning properly. His solution to the problem was to bypass the blower door safety switch and roughly clean off the ignitor/flame sensor. The worst part about it was that he never resolved the issue. Instead, the furnace continues to cycle on and off, staying on no more than a minute at a time and then delaying the restart by several minutes.

So, I had another tech come to take a look. He assessed every functioning component of the system. He tested the blower motor output, the various switches, he actually tested for CO and found ZERO ppm. He also replaced the ignitor/flame sensor with a new unit and took out the burners to clean them. Based on his more thorough examination, he believes that the control board is faulty and needs replacing. He'll be coming tomorrow to replace the unit.

I've since learned that the furnace is a Keeprite DC90 Ultra High Efficiency from 1998.

Offline Curlybandit

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Re: DC90 Ultra High Efficiency quit
« Reply #5 on: February 26, 2016, 11:53:05 AM »
Control board was replaced yesterday. The problem has been resolved.

Offline Admin

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Re: DC90 Ultra High Efficiency quit
« Reply #6 on: February 26, 2016, 12:33:17 PM »
Great news!  Thanks for the feedback.

Offline Sergroum

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Re: DC90 Ultra High Efficiency quit
« Reply #7 on: March 02, 2016, 09:13:46 AM »
0 ppm on an 18 year old furnace? Does that happen often in your experience, guys? I would expect 'minimum' of 3-7 ppm at the exhaust.

Offline walker

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Re: DC90 Ultra High Efficiency quit
« Reply #8 on: March 02, 2016, 09:47:38 PM »
you will never see 0ppm in the exhaust.

Offline Ilija Atanackovich

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Re: DC90 Ultra High Efficiency quit
« Reply #9 on: March 25, 2016, 05:42:59 PM »
A customer comes here and starts bashing a HVAC technician on a HVAC forum with an 18 years old junk with 0 PPM . I don't buy this BS