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Author Topic: Air handler - thermostat wiring  (Read 2850 times)

wdingus

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Air handler - thermostat wiring
« on: January 07, 2017, 05:00:41 PM »

I have an air handler and water-to-air exchanger. There are no heat strips or condenser, not a heat pump, just the sheet metal box and fan. And of course 24VAC transformer and relay... I have the 24VAC going to a thermostat to power it and one lead to the relay, currently connected to "Aux".

TH3210D:
C 24 Vac common
B Changeover valve energized in heating
R 24 Vac power
L Sends output when set to Em. Heat
O Changeover valve energized in cooling
G Fan relay
Y Compressor contactor
Aux Auxiliary heat relay
E Emergency heat relay

This probably isn't the best thermostat for this application, it's just a spare one my dad had and I borrowed. http://s3.supplyhouse.com/product_files/Pro3000Install1.pdf  I believe that it's meant only for a HP and does not directly control the fan itself. Connecting to the (G) Fan relay terminal does not result in the relay tripping, unless I manually slide the "Fan" switch from auto to on. I of course want the fan to come on when the tstat calls for heat. Suggestions? I've checked the rest of the terminals and unless I was just impatient, am not seeing 24V on any when the tstat is showing "Heat On".

Connected to Aux, it does automatically kick the fan on, when the tstat shows the temp a couple or more degrees below the set point and "Aux Heat On" displays. Works, maybe good as I'm going to get out of this tstat...

Thanks.

PS. P&M 2840 feeding into my basement as only heat source for that part of the house. Heat pump upstairs, toasty warm upstairs without HP having been on in last few days. Basement is nice and warm, heat rises, whole house is nice and warm. Single digits tonight, got it loaded up with white/red Oak.
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RSI

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Re: Air handler - thermostat wiring
« Reply #1 on: January 07, 2017, 05:13:20 PM »

You should be able to use the B connection.
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shepherd boy

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Re: Air handler - thermostat wiring
« Reply #2 on: January 07, 2017, 05:48:11 PM »

Looked at the stat manual again. The G terminal should work. however there is a delay built in the stat. so if you switch to heat the compressor will not come on for up to 5 minutes, and that may be effecting your G terminal as well. That can be changed in the stat to no delay or turn it to heat and and wait the 5min. and should run.
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wdingus

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Re: Air handler - thermostat wiring
« Reply #3 on: January 10, 2017, 06:58:51 PM »

Thanks.. Unfortunately though the B connection appears to be hot any time there's power to the tstat. Not just when it's calling for heat but the instant it's powered. Hmm... I checked all terminals with a voltmeter and G never went high, nor any of the other terminals. Except AUX heat which is where I have the fan lead attached currently. Works good, just nothing when the tstat calls for heat, and fan when it calls for aux heat. Maybe not the perfect solution, but works... Oh and there is no compressor, not a heat pump, air handler only.

Worked great over the past 3 days with daytime highs in the 20's and overnights in the single digits.. And then of course it's almost 60 today, heh.
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RSI

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Re: Air handler - thermostat wiring
« Reply #4 on: January 10, 2017, 07:27:19 PM »

That is odd. Did you try the Y terminal?
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shepherd boy

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Re: Air handler - thermostat wiring
« Reply #5 on: January 10, 2017, 07:48:55 PM »

The B terminal and the O terminal may stay energized when switching between heating and cooling but G and Y should come on in heat position after a max 5min delay. Aux heat position may not bring on G and will not bring on Y. Just acts like your heat setting is not working.
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RSI

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Re: Air handler - thermostat wiring
« Reply #6 on: January 10, 2017, 10:08:08 PM »

I was just looking at the manual and it does energize the B terminal as soon as the switch is in the heat position.
Shouldn't the delay only be for a restart, not the first time on? It looks like it can be turned off in the settings.

If the G or the Y connection doesn't work then I would guess the relay in the thermostat is bad.
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shepherd boy

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Re: Air handler - thermostat wiring
« Reply #7 on: January 11, 2017, 03:41:23 AM »

My understanding that is correct. However if the stat is turned off and then back on I think it considers it a restart, since the compressor could be running and the stat turned off and then on would hold it out to let the head pressure down.I'm not sure about first time on on this stat but do know other stats can delay on first time on. Yes, that can be programmed out to 0 on# 15 setting and get instant cut on. Than the G or the Y should run instantly if the stat is set on HEAT not AUX HEAT.
 
If placed in the HEAT mode and G and Y are never energized it is a bad stat.

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