GE furnace / A/C system - changing out a 90-340 for a 2 speed fan controller

[shaneaus]

I currently have an old GE furnace in my attic.  I just had to replace the blower motor as the bearings seized up.  I did this without any problem.  However, the replacement motor has two speed available.  I was told the high speed worked best for cooling and the low speed worked best for heating.

I have a 90-340 control relay in the system. 

So, I have several questions:

1st:  Will running the fan on low be a good way to go when heating.  I would assume yes as my blower cycles a lot when when heating.  Seems to me I would get less motor electrical spike with less cycles and less heat would be wasted while the blower wasn't running.  Right now it seems like my attic heats up faster than any other location because of the waste heat emanating from the unit while the blower isn't running.

2nd:  Assuming that running the blower on low IS a good idea for heating - Can I replace my current relay with a similar relay which would turn the blower on low when heating was being used and on high for when the cooling system was being used. 

3rd: Assuming 2 is a yes - Can anyone suggest such a relay?

4th: Assuming 3 is a yes - Can someone provide either instructions or reference to a site with instructions to connect said relay?


Thank you for any suggestions/answers.  I have been searching the web for over an hour without an answer.

Shane
Austin, Texas

[shaneaus]

Just FYI - I did a search on this site for "90-340" without success.  So, if this has been answered already please just advise me of the link!

[JWWebster]

you have electric or gas heat?

[shaneaus]

Sorry, it is a gas furnace with the old pivot pilot/gas switch.   The 90-340 is a model number for a fairly common relay used in appliances, vending machines, and...  furnaces (learned that on the net today).

It is a 24V relay with eight contacts.  Here is a link to what it looks like:
http://bestbuyheatingandairconditioning.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=REL90340A&Category_Code=R

[JWWebster]

I am very familiar with these relays. I have sold a bunch of them over the years. Gas heat uses low speed and ac uses high. That is why you have that relay. The basic set up is when the relay coil is NOT powered the low speed red wire and common is on. To get to high the the coil is powered and black and common get going sending the fan into full speed ahead for ac. You don't want high speed fan for heat because if you did nothing but cold air would come out the registers. A nice slow steady heat is what is needed. That is how it is set up anyway. If you would have had electric heat it would not matter because electric heat uses high for heat and ac so that was why I asked what type heat you had.

[Repair-man]

Regarding the 90340 relay, it is wired so that the low speed winding of the motor is on a closed switch. The relay is only there to connect the high speed windings when the A/C is called for (or even the manual fan switch). The actual fan speed of the motor is supposed to be on low (red wire)for heat,  and the A/C high speed on hi (black). The fan itself is controlled by a separate fan/limit switch during the heater cycles.
There is a diagram behind the blower panel on your furnace or in the packet that came with it from the installer. The relay is marked with the switch connections in tiny letters on the front of the relay.

[shaneaus]

Thank you both for your excellent advice.  Both of your explanations regarding the low speed being on a closed switch/coil not powered and the coil only connecting for high speed operation makes a lot of sense.  If I spent enough time checking out the relay coil I could figure out what it did...  But, with almost ALL of the wires being black and not knowing where the wires go TO, without a diagram, and without knowing what you have just told me about the blower motor....   Thanks for the explanation!

Just FYI - this furnace is old and there are no diagrams, instructions, etc anywhere to be found. Most of the wires off of the relay are black and it appeared that the last technician wasn't too concerned with neat wiring!  I'll probably go up there and clean up the wiring this summer.  I don't due much work in the winter (I hate cold weather!)

Thanks again!  You placed my mind at ease that it is all connected correctly.