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Embassy Onex suddenly won't heat past 67-70F
Hello all - I recently purchased a new home with an Embassy Industries Onex 160C combi unit for domestic and heat. I've already found useful information on this forum so decided to make an account and post a request for help. I honestly don't know much about heating so have been reading and learning quite a bit but still have limited knowledge. I tried to provide a decent description but let me know if you'd like more info or pictures.
The unit was installed in 2011 and appears to have been serviced, by the installers, yearly. I am waiting on copies of the previous records. It's a ~1500sqft house with two zones, controlled by taco zone valves, and baseboards for heat. We have a hydronic heater in the garage that is on an open loop, with the fan controlled by a thermostat. The unit is piped for natural gas. There is an external expansion tank and some sort of condensate neutralizer built into the system. When we first moved in, the temps dropped to -30F and the heater seemed to hit whatever temp we set it to.
Soon after the cold snap, we had a "FILL" error, which turned out to be a bad/ dirty fill sensor. We had a technician come out and replace the sensor, he also noticed a loud noise on startup and replaced the combustion fan. He left and the next day our house was up to 80F. We realized the downstairs zone valve was left open. After closing the valve the temp dropped to thermostat temp (68F-70F).
The next day we left on vacation (~3 weeks) and when we came back the house would not heat past 67F. Turning the thermostat resulted in the boiler running constantly (LCD panel displayed "c.#.#.#.#) but didn't increase the interior temps. Also, we noticed an increase in the gas bill so I assume it was running frequently while we were away. The tech came back and said everything is functioning mechanically but tried turning the unit to "high-temp" mode by flipping one of the switches, as described in the manual. He bled a tiny bit of air from the lines, but said it wasn't very much. It was in the teens but now it's getting into the high 30s during the day. The combination of high-temp mode, no air in lines, and increased outside temps allow the house to heat to 70F. Also, weather records indicate similar lows but higher high temps now then when the house hit 80F.
What I've done:
Turned up the thermostats to high 70s. Left the zone valve on 'open' overnight. Tested temps of downstairs registers, which I think are okay (~170-180 as checked with my muti-meter probe?). Increased the pressure from ~16PSI to 20-21PSI. Tried a new thermostat. Checked voltage (~27V) and operation of both zone valves when thermostats call for heat. Checked to make sure that the auto-vent on the Onex was open.
Finally, we didn't use the original installers for this work because we were given a home warranty and this new company is only option under the warranty.
Should I be able to heat my house to more than 70?
Why did the house get hot (80s) with an open zone valve but now it only reaches 70 with an open valve?
Could the described repairs have anything to do with this?
Should I take apart and clean the unit as mentioned in the yearly maintenance section of the manual?
Is it worth reaching out to someone outside of the home warranty network (i.e., original installers)?
Is there anything else I should be asking / looking at - again this is all new to me so feel free to inform me of very simple things I should have looked at prior to posting...
Thanks for any help and sorry for such a long post!