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 ·ELECTRONICBOX
| Re-Wiring My Thermostat I have an oil furnace and currently my thermostat only turns the heat on or off. On the furnace itself there is a switch to manually turn the fan on or off. I would like to have the fan control from my thermostat instead of having to go into the basement to do this. Currently the wire from the furnace to the thermostat only has two conductors (white and black) so I would need to change the wire (easy since basement is open). My questions are:
1- The new wire should have how many conductors?
2- How would I hook it up?
Below are some of the pictures from my furnace.
Burner
Thermostat Wire
Fan control |
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| said by thestealth:I have an oil furnace and currently my thermostat only turns the heat on or off. On the furnace itself there is a switch to manually turn the fan on or off. I would like to have the fan control from my thermostat instead of having to go into the basement to do this. Currently the wire from the furnace to the thermostat only has two conductors (white and black) so I would need to change the wire (easy since basement is open). My questions are:
1- The new wire should have how many conductors?
2- How would I hook it up?
Below are some of the pictures from my furnace.
[att=1]
Burner
[att=2]
Thermostat Wire
[att=3]
Fan control
when you add AC to your oil forced air furnace they add a relay that turns on and off the fan. This relay connects to the wires at the fan limit switch and allows you to control the fan with low voltage wiring at the thermostat. Basically your thermostat cannot control your fan unless you add a relay. |
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·ELECTRONICBOX
| said by telco_mtl:Basically your thermostat cannot control your fan unless you add a relay.
Ok So that adds a relay to my shopping list. What specifications am I looking for on this relay? |
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TheMGPremium
join:2007-09-04
Canada
kudos:1
1 edit | reply to thestealth
said by thestealth:1- The new wire should have how many conductors? I'd run a cable with at least 5 conductors.
You'll need to add a relay with a 24VAC coil and contacts rated for the size of the motor in your furnace.
Connect the normally open (NO) contacts of the relay in parallel with the manual fan switch. Alternatively, you may disconnect the wires from the fan switch and just connect them to the relay, if you don't wish to retain the manual fan switch functionality. Looking at your pictures this would likely be the easiest approach, and the wires to the fan switch already have the terminal connectors on them that will fit right onto the relay.
Connect one side of the relay coil to the "G" connection on the thermostat. Other side of coil to the 24VAC transformer common.
Now what will happen, when the thermostat wants the fan to run, it will complete the circuit between "R" and "G", supplying 24VAC to the relay coil. The relay will energize, closing the NO contacts, completing the circuit supplying power to the blower motor. 
said by thestealth:Ok So that adds a relay to my shopping list. What specifications am I looking for on this relay?
24VAC for the coil.
Contact voltage rating should be 120/240V (whatever voltage your blower runs at or higher).
As for the current rating, relays/contactors are normally rated in terms of horsepower (HP) for motor loads. For instance, if your blower motor is rated 1/2HP, you should get a relay rated for 1/2HP or greater.
Alternatively, you can go by the current ratings, but a de-rating factor needs to be applied. If I remember correctly, relays should be de-rated 40% of the resistive rating when used to control motor loads. |
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 nunyaWho is John Galt?Premium,MVM
join:2000-12-23
O Fallon, MO
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| reply to thestealth
It looks like your fan setup *might* already be capable of doing this with a new t-stat wire.
I have many questions though. I'll start with an easy one.
It appears the fan should already be cycling on when there is a call for heat - correct?
It should happen in this sequence: Call for heat - furnace ignites. Reaches low limit and fan starts. Heat is satisfied, fire stops. Fan runs until cool temp is met.
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TheMGPremium
join:2007-09-04
Canada
kudos:1 | reply to thestealth
Hmmm... after reviewing the pictures carefully it looks like you might be out of luck.
My recommendation was based on the assumption that the furnace has a 24VAC transformer, but looking at the wiring on the igniter/controller, this does not appear to be the case.
Since the voltage source for the thermostat is built into the igniter, you don't have access to the 24VAC common, which would make hooking up a 24VAC relay in the way I described impossible (well, not impossible, anything is possible, but just a lot harder).
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antbhill2Premium,MVM
join:2001-02-28
Northern VA | reply to thestealth
You might want to redo some of the connections shown in the first picture. The terminal for the white thermostat wire appears to have a loose broken piece of wire, and the white wire appears to be barely held under the screw. |
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 jack bGone FishingPremium,MVM
join:2000-09-08
Cape Cod
kudos:1 | reply to thestealth
It looks like you're going to need to wire in a new 24 volt transformer and add another relay for the fan. The relay that's pictured there appears to be 120v (coil).
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| reply to nunya
said by nunya:It should happen in this sequence: Call for heat - furnace ignites. Reaches low limit and fan starts. Heat is satisfied, fire stops. Fan runs until cool temp is met.
Yes that is the sequence.
Yes from looking at these pictures it looks like the fan control is all 120VAC. Though I may have to pull out the multimeter and test to verify.
I do not have a problem of adding in a 24VAC transformer if needed. I was thinking of eventually getting one of those NEST thermostats one day and I believe those require 24VAC to operate.
The breaker for the furnace is 15A, so I guess I could get a 15A relay.
So that brings me to my shopping list:
New wire must have a min of 5 conductors:
-2x thermostat
-1x fan control
-2x 24VAc (source and common)
I will also need one 120/24 VAC transformer and a 24VAC coil relay rated for at least 15A on the 120VAC side.
Looks fairly simple to wire up, unplug the switch, connect it to my new relay.
Am I missing anything else? |
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jack bGone FishingPremium,MVM
join:2000-09-08
Cape Cod
kudos:1
2 edits | The existing relay appears to be a DPDT and it looks like it's configured for manually running the blower (orange) at a different speed from heating (blue).
You need to be very careful rewiring this so that your new 24V relay does not allow the blower motor orange and blue leads to be energized at the same time, or else you will let all the smoke out of the blower motor.
[edit]
TheMG is on to it  ...
what you can do is parallel the 24V terminals of new external transformer with the T+ and F- terminals of the oil burner control
Then you can proceed with his initial wiring suggestion to parallel the NO relay contacts to the manual switch.
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 djrobx
join:2000-05-31
Valencia, CA
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1 edit | reply to thestealth
The docs for this igniter have a nice diagram including the 24vac transformer. Theirs includes using Y for "cooling" but you wouldn't need to do that.
»customer.honeywell.com/resources···0617.pdf
According to this you can actually get to Common ("X") by removing some sort of plug, but they still show using a separate transformer the fan.
A regular mechanical thermostat does not need a "C" wire (common) but if you get something fancier you'll want it it so the thermostat doesn't require batteries. Nest can sometimes work without this wire but it's much better to have it available.
So for your case I see:
Rh - To ignition
W - To ignition
Rc - To 24vac transformer (24vac)
G - To Fan relay
C - To 24vac transfomer (common)
If jack b  is correct and you have a two speed blower, you'll need to think a bit more about how you want it to work. Thermostats turn on the fan (G) whenever there's a call for heat. You then can optionally turn G on continuously without heat by switching the fan on (from Auto to ON). If it's a two speed blower and you want to run it in high speed for cooling you'd use the Y wire as shown in Honeywell's diagram.
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jack bGone FishingPremium,MVM
join:2000-09-08
Cape Cod
kudos:1
2 edits | For the purpose of twinning your external xformer, according to the OB control wiring diagram above, the upper T terminal on the control is hot (R) and the lower F one can be considered common (C).
EDIT: use lower terminal F as the common!!!
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| reply to jack b
said by jack b:what you can do is parallel the 24V terminals of new external transformer with the TT terminals of the oil burner control after you determine which is the "common" leg on that control.
So if I follow it would look like the attached diagram. The green connections would be putting the new Transformer in parallel with the burner. I would make this connection at the furnace level and not at the thermostat.
I'm thinking I will either parallel the current switch or remove and replace it with the new relay. So if it is a 2 speed fan control this should not cause any damage. |
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| said by thestealth:said by jack b:what you can do is parallel the 24V terminals of new external transformer with the TT terminals of the oil burner control after you determine which is the "common" leg on that control.
So if I follow it would look like the attached diagram. The green connections would be putting the new Transformer in parallel with the burner. I would make this connection at the furnace level and not at the thermostat. [att=1] I'm thinking I will either parallel the current switch or remove and replace it with the new relay. So if it is a 2 speed fan control this should not cause any damage. simplest solution in imho is call your provider if you have a service contract, they can install relatively cheaply the relay that is used on that furnace for fan control on AC. They have the parts in stock for sure and it wont affect your service contract. |
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jack bGone FishingPremium,MVM
join:2000-09-08
Cape Cod
kudos:1
2 edits | reply to thestealth
OOPS! I made a big mistake. I'm Glad you drew that up!
Connect the external transformer C terminal to the lower F terminal on the burner control! NOT THE LOWER T!!!
If you wire it just like the first diagram above, a call for heat will short out the external transformer! PFFFFFT!
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Reviews:
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| said by jack b:OOPS! I made a big mistake. I'm Glad you drew that up!
Connect the external transformer C terminal to the lower F terminal on the burner control! NOT THE LOWER T!!!
If you wire it just like the first diagram above, a call for heat will short out the external transformer! PFFFFFT!
That's what I thought. I was looking at it kind of weird.
Out of curiosity how much should I be paying for a relay? Went to my local electronic shop and they wanted $30. |
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PSWired
join:2006-03-26
Annapolis, MD | »www.ebay.com/sch/i.html?_sacat=0···LH_BIN=1 |
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jack bGone FishingPremium,MVM
join:2000-09-08
Cape Cod
kudos:1 | reply to thestealth
WOW! That's steep!
»www.amazon.com/Packard-PR340-DPD···8&sr=1-8
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| I went to my favourite electronics store (further from my home) and found a suitable relay for $3.99 (a Hasco UJ1CSAC24FT)
One last quick question, what size wire should I get for the new thermostat run? 18 AWG? |
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garys_2kPremium
join:2004-05-07
Farmington, MI | Yeah, 18 gauge would be fine. |
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