2 Identical Hydronic Furnaces, 2 Different Problems - HVAC - DIY Chatroom Home Improvement Forum

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Old 11-02-2010, 10:23 PM   #1
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2 identical hydronic furnaces, 2 different problems

own a duplex, pretty comfortable working on most things myself...
both sides have same natural gas hydronic heating system

side #1:
fan comes on, purges air and starts vacuum, ignitor glows, gas comes on, ignites, flame sensor puts out proper mA, then valve shuts off, within 3-10 seconds usually. starts over. after a few attempts it may stay on a few minutes at a time.

each time it shuts the gas valve, it must be resetting the ignition control system. it will do this a lot of times, and it's not locking the ignition with a fault code. i have swapped the ignition control unit with the same unit from furnace #2 and same results. i suspect it's with the 120 to 24v transformer or other device next to it. i suspect it's a safety relay? it is hot to touch and will make a arcing sound or buzzing when the gas shuts off. what situations will cause this, and what can i do to pursue problem further?

furnace #2 is a very different problem.
starts great, temp starts to rise, then the temp will stop rising (around 140-150) and the pressure will begin to rise, from 12psi at rest to the 30psi the release valve is set at. there was little or no air in the radiators, there is a 2.5g expansion tank that is set at or near 12psi. i know no way to diagnose this problem, and lots of time sitting in front of furnaces to no avail.

i have no formal training on the subject but am comfortable with everything else i've had to fix... just a little stumped.

sorry for lengthy first post, like many others i come here in times of need...



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