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Community and Q&A

ASHP burning dust smell

kat_h | Posted in General Questions on

We’ve had our new ASHP* up and running since November. It’s a ducted Mitsubishi system: outdoor: PUZ-HA36NKA; indoor: PVA-A36AA7. Much of the ductwork in the basement was replaced when the system was installed.

We live in southwestern Ontario, CZ 6.

We like it cooler at night, so we’ve been setting the temperature back at night by 2 degrees C. When it is especially cold (~ -16 C overnight low), we manually hold the temp to the regular daytime 20 C, otherwise it takes too long to recover in the morning.

So for most nights this mild winter, the setback has been in place and the system kicks up to high at about 7:00 am and stays there until we’re back to 20 C. (We have an IoTaWatt doing monitoring, so we can reliably see when the outdoor unit is on high.)

Several times over the past few weeks, during that recovery time, we’ve smelled
a burning-ish smell. It smells exactly like the smell you get when you turn the gas furnace heat on for the first time of the year–like burning dust.

Any ideas? Should we be worried about this? Call the hvac people out?

* Thanks so much to everyone here for advice that helped to get us to this point!

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  1. walta100 | | #1

    Does your system have resistance back up heat installed?

    Installers have been know to install the back up heat and not disclose the fact it was installed. If the indoor unit is powered by a breaker over 20 amps you almost certainly have some resistance heat.

    Resistance elements will make the smell you are describing. Many thermostats will turn on the resistance if they are more than a degree below the set point for any reason.

    I encourage you to try forgoing the nightly set back the HP will run more efficiently if you can adjust to the constant temp.


    1. kat_h | | #2

      We do have backup resistance heat, but we are sure it has never come on. (We only setback by 2 degrees and the the Mitsubishi algorithm is not aggressive. Moreover, we can clearly see through the power monitoring system that it doesn't come on in the morning.)

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