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Old 07-19-2020, 11:15 AM   #1
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AC blows Hot air at night - Stop and Start fixes it for a while


Hi,

I have
Ameristar 13 SEER 3 TON Model#M2HP3036A1000A heat pump unit with
Honeywell RTH7500 thermostat.

It works fine during day time. temp stays 79 through out the day.
At nights suddenly it starts blowing real hot air, in minutes temp goes up to 84.

But if i turn off the unit and restart in cool mode it works fine for sometimes and hot air issue reappears again.

This is happening for few nights. I'm not sure what is the issue with unit as I can't find anything wrong when i check in day time.

Last week weather
============
Date High Low
13 85 65
14 89 66
15 91 64
16 88 74
17 93 73
18 95 70


any suggestion what to check or look for?

Thanks,

Last edited by advaitian; 07-19-2020 at 11:34 AM. Reason: added more info
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Old 07-19-2020, 11:49 AM   #2
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


Is the unit switching to heating mode? Or is the electric heat coming on? And is it getting a call to do so from the thermostat?
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Old 07-19-2020, 11:59 AM   #3
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


Thermostat stays on cool.
But now you mentioned, rapid rise in temp from 79 to 84 may be because of Electric heat!?!
It is hard to pay full attention as these happen at or after midnight.
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Old 07-19-2020, 12:04 PM   #4
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


So the actual space temperature rises to 84?
Is this at the same time?
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Old 07-19-2020, 12:44 PM   #5
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


yes.
I leave temp set at 79 through out 24 hrs.
When this happens and I feel heat, thermostat and room temp gauge both are at ~84.
I can turn off and on thermostat. AC comes back normal for another hour or two. room temp drops to 79. and when it happens again and by the time I notice it is again ~84.

It only occurs at night, so not sure it is just a coincidence or something else?

Last edited by advaitian; 07-19-2020 at 01:01 PM.
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Old 07-20-2020, 02:01 PM   #6
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


It happened again last night/early morning.
Thermostat was on Cool.
Outdoor unit, Fan was not working.
Might need to replace Fan Motor.

Still do not understand why issue only occurs at night and no issue during day?
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Old 07-20-2020, 03:51 PM   #7
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


Quote:
Originally Posted by advaitian View Post
Might need to replace Fan Motor.
Absolutely not.
The unit needs to be diagnosed first.
Was the compressor running when the condenser fan was off? Was the contactor closed or open? Was the unit getting a call to run?
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Old 07-20-2020, 04:22 PM   #8
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


AC was blowing super hot air and heat pump fan was off.

funny thing is it happens late in the night. it was 3 am.
I will try to check all these over the weekends.

roughneck, Thanks for your input.
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Old 07-25-2020, 09:45 AM   #9
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


When it happens at nights, compressor is always running. contactor is on. but fan is not.

It is running fine at the moment in day cooling home normally.

Here is the circuit diag:


When I press the contactor manually, should the Compressor and Fan both turn on?

Last edited by advaitian; 07-25-2020 at 09:47 AM. Reason: fixed image link
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Old 07-25-2020, 03:50 PM   #10
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


Quote:
Originally Posted by advaitian View Post
When it happens at nights, compressor is always running. contactor is on. but fan is not.

It is running fine at the moment in day cooling home normally.

Here is the circuit diag:


When I press the contactor manually, should the Compressor and Fan both turn on?

When contactor pulls in (turns on) the compressor and outdoor fan should both turn on. Could be a capacitor issue, or a fan motor issue. Have you checked the capacitor for visual signs of damage, or measured it with a meter?
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Old 07-28-2020, 07:45 PM   #11
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


Issue occurred in day time today.
Compressor and out door fan was off.
Indoor unit was on with heat. ( it seems emergency heat kicked in but Thermostat display was on Cool ).
Remove AUX/W1 wire from Thermostat, immediately AC is started working normally.

I believe it is AUX signal turning emergency heat ON and compressor and external fan OFF.
issue somewhere in control.

I will monitor and see if issue reoccurs with removed AUX cable from Thermostat.

Any and all suggestions are welcome to help pinpoint the issue.

Thanks

Last edited by advaitian; 07-28-2020 at 07:47 PM.
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Old 07-28-2020, 07:52 PM   #12
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


sounds like a failing t-stat bringing on the aux heat when it shouldn't. reproduce the issue before changing the stat.
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Old 07-28-2020, 08:25 PM   #13
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


Any chance you have a set back thermostat that you installed yourself?
It sounds like you have a timed setback issue on a thermostat that is wired incorrectly. There is a wiring malfunction somewhere.....or you have a bad thermostat.
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Old 07-28-2020, 10:01 PM   #14
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


Thermostat is Honeywell RTH7500D and it was installed by HVAC tech.
It was running fine for years. This issue started two weeks back.
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Old 07-29-2020, 08:32 AM   #15
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Re: AC blows Hot air at night - Stop and Start fixes it for a while


It's not that. Issue reoccured again at 2am.
something else is triggering emergency heat.��
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