Is there a way to manually control fan speed?

The blower motor on my propane furnace / heat pump system appears to have the potential for four speeds.

As it is now, it seems to be using only two speeds. A high speed for AC, heat pump, and fuel, and a slightly lower speed for the cool-down cycle.

I want to be able to have a fan-only mode for the purpose of air circulation that is quieter than the current speeds.

Is there a way through a certain type of thermostat or wiring the motor control circuit that I can have a low-speed setting? I would even consider a simple method of mounting a switch on the furnace itself to achieve this.

 

The answer is yes, but. It will require more than a simple switch. You will need at least one, and probably two, multiple contact relays to properly switch among the four speeds. Do you have any electrical or electronic experience?

 

I have a moderate amount of electrical / electronic experience.

Your comment:

prompts me to re-state part of my original post for the sake of clarity: Could I leave the system as it is now (I believe two speeds) and mount a switch at or near the furnace when I want fan only on low speed?

 

No.

Your blower motor has four separate windings to allow for the four distinct speeds. Energizing more than one winding at a time would destroy the motor so logic circuitry must be used to allow only one winding to be used at any one time. While you can think that you would only use the manual switch at specific times the truth is that you, or someone unfamiliar with the arrangement could energize either the cooling or heating system without first turning off the manual switch.

The relay logic necessary is not all that complicated or expensive. If you want, I can work up something in the next few days.

 

I completely agree.

I would find the capability to manually operate the fan only at low speed useful, so if it doesn't cost you much time, a schematic would be greatly appreciated. No rush at all.

 

Sorry I haven't posted lately, health issues. I have been thinking of your problem and I have come up with a couple of possible answers. One is very simple and only uses a single switch. The downside are the switch would be located at the furnace and that when the blower is on continuous air circulation the heating and / or cooling will be locked out. I also have a much more complicated system in mind that would allow continuous air circulation controlled from the thermostat location and also still allow either heating or cooling to operate.

Let me know which one you desire and I'll post a schematic.

 

I hope you are feeling better.

The simple way would not be very practical, so I would appreciate the more involved method.

Is there such a thing as a more advanced thermostat (possibly with associated optional circuitry to be installed at the furnace) to achieve this?

 

I haven't forgotten you, I will get the diagram and parts list posted either tonight or late Sunday night.

 

I made this more of a pictorial rather than a schematic. Hopefully you can read the captions, if not I will post a link to a larger image.

The letters C, G, Y and W are where the wires from the thermostat connect to the furnace board. The G lead from the thermostat needs to be removed from the furnace board terminal and connected to the relay socket as shown. All other wires from the relay sockets will connect to the thermostat connections in addition to the thermostat wires.

The C terminal most likely will not be connected to the thermostat unless you have a programmable thermostat. If you have air conditioning the C terminal will connect to the cable going to the outside unit. It is possible that the C terminal will be labeled 24 volts or something like that.

The blower motor leads will be disconnected from the furnace circuit board and connected to the relay sockets as shown. Additional wires from the circuit board blower motor connections to the relay sockets as shown. The connection marked L1 120 Volts needs to be connected to the incoming power "hot" wire.

Moving the "Auto-Fan-On" switch on the thermostat will cause the blower motor to run constantly in the slowest speed. If either the heating or cooling function of the thermostat is energized the blower will run as appropriate for the function.

Understand that with the standard four-speed blower motor running the blower continuously will use a noticeable amount of electricity.



The relays are Magnecraft part number 750XBXC-24A and are available from Allied Electronics. relay
The relay sockets are Magnecraft part number 70-464-1 and are also available from Allied. Relay socket
I have no connection to Allied except as a satisfied customer of several decades. They have no minimum order and are happy to deal with hobbyists.

 

Wow, you went to a fair amount of trouble and I appreciate it! I can read the image just fine. Your instructions are nice and clear too.

Funny thing is, I used to have two or three relay / socket sets just like specified - I don't think they are around the house any more though.

Thanks again - I'll make it a point to be sure to pay it forward here on the forums.

 

You're welcome. Actually, this was a fairly easy exercise compared to what I used to get paid to do. One caveat, since this is a heat pump system it is possible that yet another relay will be necessary to isolate the cooling function from the heat pump heating function. Keep us informed.

As for once having the parts but finally tossing them after having them underfoot for years...story of my life!

 

Hi I am using mars 90370 type hvac relays spdt, to control the fan on/off on my furnace. I have it spliced into the green line to turn on the fan as needed in conjuntion with a um506 .

Patriot Supply - 90-370

My fan is doing funny stuff, such as turning on running a minute then turning off for 2 seconds and then restarting . Kind of like the relay goes on then off again making the motor do funny sounds . Can this realy cause this kind of hysteresis ?


Google

thanks

 

I don't have a clue as to what a "um506" might be or how you have it connected to that relay. Maybe if you gave a bit more information I could help. I will say that the relay you linked to operates ONLY under control of the power connected to the coil terminals.

 

Hi Thanks , the um506 is an x10 device to close a contact or relay to activate a device from the powerline & powerline controller X-10 PRO PUM01 Universal Module UM506 BRAND NEW X10 - eBay (item 160375539940 end time Nov-29-10 18:25:34 PST)

When the control computer talks to the um506 it flips the mars 90 370 to put power , 40va 24volts on the G terminal of the furnace , ie just like the thermostat. This worked for 15 years till the old um506 and the relay were changed for the new um506 and the mars 90 370 relay. Now I get a problem were the fan motor comes on , starts to stop, ie slows down then is re-energized like a switch going off then on and "booms" makes a noise to speed up again to normal operating speed. The furance fan motor is new . hope that explains the problem better.

I was wondering if one of the relays is stuttering on activation causing this problem.

 

It could be electrical noise, most likely from the motor, that is playing heck with the X-10 interface. There are X-10 noise filters available which might help. Otherwise I'd look for loose connections between the relay and the X-10 interface or it could be a lousy relay in the interface unit itself.

 

looks like the - 90-370 relay was bad replaced it with a spare and the system is running a ok !!

I do have a question about fan speed , my fan can do 3 speeds but only one wire is hooked up medium fast.

Can I use this Carson Speed Control Electrical Switch



or

this NEW FSP ROTARY SWITCH 484308





to make a manually changeable speed controller for my furnace fan motor to vary speeds winter summer?



thanks